MongoDB – Sharded cluster tutorial - Percona Europe 2017

MongoDB – Sharded Cluster
Tutorial
Antonios Giannopoulos and Jason Terpko
DBA’s @ Rackspace/ObjectRocket
linkedin.com/in/antonis/ | linkedin.com/in/jterpko/
1

Introduction
www.objectrocket.com
2
Antonios Giannopoulos Jason Terpko

Overview
• Cluster Components
• Shard Key
• Shard Key Selection
• Balancing
• Chunk Management
• Troubleshooting
• Zones
• Backups & Disaster Recovery
• Use Cases
• Labs
3

4
Labs
1. Unzip the provided .vdi file
2. Install and or Open VirtualBox
3. Select New
4. Enter A Name
5. Select Type: Linux
6. Select Version: Red Hat (64-bit)
7. Set Memory to 4096 (if possible)
8. Select "Use an existing ... disk file", select the provided .vdi file.
9. Select Create
10. Select Start
11. CentOS will boot and auto-login
12. Navigate to /Percona2017/Lab01/exercises for the first lab.
https://goo.gl/ddaVdS

Key Terms
5
Replication:
Replica-set: A group of mongod processes that maintain the same data set
Primary: Endpoint for writes
Secondary: Pull and Replicates changes from Primary (oplog)
Election: The process that changes the Primary on a replica-set
Partitioning:
Partition: A logical separation of a data set (may also be physical) on a server
Partition key: A field (or a combination of fields) used to distinct partition
Sharding:
Similar to partitioning with the difference that partitions(chunks) may located on different servers
(shards)
Sharding: A field (or a combination of fields) used to distinct chunks

Sharded Cluster
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s1 s2

Sharded Cluster
Deployment ● Deploying
○ Shards
○ Configuration Servers
○ Mongos
● Major / Minor Upgrade
● Major / Minor Downgrade
7

Deploying - Recommended setup
8
Minimum requirements: 1 mongos, 1 config server , 1 shard (1 mongod process)
Recommended setup: 2+ mongos, 3 config servers, 1 shard (3 node replica-set)
Ensure connectivity between machines involved in the sharded cluster
Ensure that each process has a DNS name assigned - Not a good idea to use IPs
If you are running a firewall you must allow traffic to and from mongoDB instances
Example using iptables:
iptables -A INPUT -s <ip-address> -p tcp --destination-port <port> -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -d <ip-address> -p tcp --source-port <port> -m state --state ESTABLISHED -j ACCEPT

Deploying - Config Servers
9
Create a keyfile:
● keyfile must be between 6 and 1024 characters long
● Generate a 1024 characters key openssl rand -base64 756 > <path-to-keyfile>
● Secure the keyfile chmod 400 <path-to-keyfile>
● Copy the keyfile to every machine involves to the sharded cluster
Create the config servers:
- Before 3.2 config servers could only run on SCCC topology
- On 3.2 config servers could run on either SCCC or CSRS
- On 3.4 only CSRS topology supported
- CSRS mode requires WiredTiger as the underlying storage engine
- SCCC mode may run on either WiredTiger or MMAPv1
Minimum configuration for CSRS
mongod --keyFile <path-to-keyfile> --configsvr --replSet <setname> --dbpath <path>

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Baseline configuration for config server (CSRS)
net:
port: '<port>'
processManagement:
fork: true
security:
authorization: enabled
keyFile: <keyfile location>
sharding:
clusterRole: configsvr
replication:
replSetName: <replicaset name>
storage:
dbPath: <data directory>
systemLog:
destination: syslog

11
Login on one of the config servers using the localhost exception.
Initiate the replica set:
rs.initiate(
{
_id: "<replSetName>",
configsvr: true,
members: [
{ _id : 0, host : "<host:port>" },
{ _id : 2, host : "<host:port>" }
]})
Check the status of the replica-set using rs.status()

Deploying - Shards
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Create shard(s):
- For production environments use a replica set with at least three members
- For test environments replication is not mandatory
- Start three (or more) mongod process with the same keyfile and replSetName
- ‘sharding.clusterRole’: shardsrv is mandatory in MongoDB 3.4
- Note: Default port for mongod instances with the shardsvr role is 27018
Minimum configuration for shard
mongod --keyFile <path-to-keyfile> --shardsvr --replSet <replSetname> --dbpath <path>
- Default storage engine is WiredTiger
- On a production environment you have to populate more configuration variables , like oplog size

Deploying - Shards
13
Baseline configuration for shard
net:
port: '<port>'
processManagement:
fork: true
security:
authorization: enabled
keyFile: <keyfile location>
sharding:
clusterRole: shardsrv
replication:
replSetName: <replicaset name>
storage:
dbPath: <data directory>
systemLog:
destination: syslog

Deploying - Shards
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Login on one of the shard members using the localhost exception.
Initiate the replica set:
rs.initiate(
{
_id: "<replSetName>",
members: [
{ _id : 2, host : "<host:port>" }
]})
Check the status of the replica-set using rs.status()
Create local user administrator (shard scope): { role: "userAdminAnyDatabase", db: "admin" }
Create local cluster administrator (shard scope): roles: { "role" : "clusterAdmin", "db" : "admin" }
Be greedy with "role" [ { "resource" : { "anyResource" : true }, "actions" : [ "anyAction" ] }]

Deploying - Mongos
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Deploy mongos:
- For production environments use more than one mongos
- For test environments a single mongos is fine
- Start three (or more) mongos process with the same keyfile
Minimum configuration for mongos
mongos --keyFile <path-to-keyfile> --config <path-to-config>
net:
port: '50001'
processManagement:
fork: true
security:
keyFile: <path-to-keyfile>
sharding:
configDB: <path-to-config>
systemLog:
destination: syslog

Deploying - Mongos
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Login on one of the mongos using the localhost exception.
- Create user administrator (shard scope): { role: "userAdminAnyDatabase", db: "admin" }
- Create cluster administrator (shard scope): roles: { "role" : "clusterAdmin", "db" : "admin" }
Be greedy with "role" [ { "resource" : { "anyResource" : true }, "actions" : [ "anyAction" ] }]
What about config server user creation?
- All users created against the mongos are saved on the config server’s admin database
- The same users may be used to login directly on the config servers
- In general (with few exceptions), config database should only be accessed through the mongos

Deploying - Sharded Cluster
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Login on one of the mongos using the cluster administrator
- sh.status() prints the status of the cluster
- At this point shards: should be empty
- Check connectivity to your shards
Add a shard to the sharded cluster:
- sh.addShard("<replSetName>/<host:port>")
- You don’t have to define all replica set members
- sh.status() should now display the newly added shard
- Hidden replica-set members are not apear on the sh.status() output
You are now ready to add databases and shard collections!!!

Deploying - Recommended Setup
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- Use Replica Set for shards
- Use at least three data nodes for the Replica Sets
- Use three config servers on SCCC topology
- Use more than one mongos
- Use DNS names instead of IP
- Use a consistent network topology
- Make sure you have redundant NTP servers
- Always use authentication
- Always use authorization and give only the necessary privileges to users

Upgrading - Categories
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Upgrade minor versions
- Without changing config servers topology
- For example: 3.2.15 to 3.2.16 or 3.4.6 to 3.4.7
- Change config servers topology from SCCC to CSRS
- For example: 3.2.15 to 3.2.16
Upgrade major versions
- Without changing config servers topology
- For example: 3.0.14 to 3.2.12
- Change config servers topology from SCCC to CSRS
- For example: 3.2.12 to 3.4.2

Upgrading – Best practices
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Best Practices (not mandatory)
- Upgrades heavily involve binary swaps
- Keep all mongoDB releases under /opt
- Create a symbolic link of /opt/mongodb point on your desired version
- Binary swap may be implemented by changing the symbolic link
> ll
lrwxrwxrwx. 1 mongod mongod 34 Mar 24 14:16 mongodb -> mongodb-linux-x86_64-rhel70-3.4.1/
drwxr-xr-x. 3 mongod mongod 4096 Mar 24 12:06 mongodb-linux-x86_64-rhel70-3.4.1
drwxr-xr-x. 3 mongod mongod 4096 Mar 21 14:12 mongodb-linux-x86_64-rhel70-3.4.2
> unlink mongodb;ln -s mongodb-linux-x86_64-rhel70-3.4.2/ mongodb
>ll
lrwxrwxrwx. 1 root root 34 Mar 24 14:19 mongodb -> mongodb-linux-x86_64-rhel70-3.4.2/
drwxr-xr-x. 3 root root 4096 Mar 24 12:06 mongodb-linux-x86_64-rhel70-3.4.1
drwxr-xr-x. 3 root root 4096 Mar 21 14:12 mongodb-linux-x86_64-rhel70-3.4.2
>echo 'pathmunge /opt/mongodb/bin' > /etc/profile.d/mongo.sh; chmod +x /etc/profile.d/mongo.sh

Upgrading - Checklist
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- Configuration Options changes: For example, sharding.chunkSize configuration file setting and --chunkSize
command-line option removed from the mongos
- Deprecated Operations: For example, Aggregate command without cursor deprecated in 3.4
- Topology Changes: For example, Removal of Support for SCCC Config Servers
- Connectivity changes: For example, Version 3.4 mongos instances cannot connect to earlier versions of
mongod instances
- Tool removals: For example, In MongoDB 3.4, mongosniff is replaced by mongoreplay
- Authentication and User management changes: For example, dbAdminAnyDatabase can't be applied to
local and config databases
- Driver Compatibility Changes: Please refer to the compatibility matrix for your driver version and language
version.

Upgrading – Minor Versions
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Upgrade minor versions - Without changing config servers topology
Upgrade 3.2.x to 3.2.y or 3.4.x to 3.4.y
1) Stop the balancer sh.stopBalancer() and check that is not running
2) Upgrade the shards. Upgrade the secondaries in a rolling fashion by stop;replace binaries;start
3) Upgrade the shards. Perform a stepdown and upgrade the ex-Primaries by stop;replace binaries;start
4) Upgrade the config servers. Upgrade the secondaries in a rolling fashion by stop;replace binaries;start
5) Upgrade the config servers. Perform a stepdown and upgrade the ex-Primary by stop;replace binaries;start
6) Upgrade the mongos in a rolling fashion by stop;replace binaries;start
Rollback 3.2.y to 3.2.x or 3.4.y to 3.4.x
2) Downgrade the mongos in a rolling fashion by stop;replace binaries;start
3) Downgrade the config servers. Downgrade the secondaries in a rolling fashion by stop;replace binaries;start
4) Downgrade the config servers. Perform a stepdown and downgrade the ex-Primary by stop;replace binaries;start
5) Downgrade the shards. Downgrade the secondaries in a rolling fashion by stop;replace binaries;start
6) Downgrade the shards. Perform a stepdown and Downgrade the ex-Primaries by stop;replace binaries;start

Minor Versions Upgrade (3.4.x to 3.4.y)
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s1 s2
3. Upgrade mongos
2. Upgrade config servers
1. Upgrade shards
Stop the balancer
Start the balancer

Minor Versions Downgrade (3.4.x to 3.4.y)
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s1 s2
1. Downgrade mongos
2. Downgrade config servers
3. Downgrade shards
Stop the balancer
Start the balancer

Upgrading - Major Versions
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Upgrade major versions - Without changing config servers topology
Upgrade 3.2.x to 3.4.y
2) Upgrade the config servers. Upgrade the secondaries in a rolling fashion by stop;replace binaries;start
3) Upgrade the config servers. Perform a stepdown and upgrade the ex-Primary by stop;replace binaries;start
4) Upgrade the shards. Upgrade the secondaries in a rolling fashion by stop;replace binaries;start
5) Upgrade the shards. Perform a stepdown and upgrade the ex-Primaries by stop;replace binaries;start
6) Upgrade the mongos in a rolling fashion by stop;replace binaries;start
Enable backwards-incompatible 3.4 features db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )
It is recommended to wait for a small period of time before enable the backwards-incompatible feautures.
Includes:
- Views
- Collation option on collection and indexes
- Data type decimal
- New version indexes (v:2 indexes)

Major Versions Upgrade (3.2.x to 3.4.y)
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s1 s2
3. Upgrade mongos
1. Upgrade config servers
2. Upgrade shards
Stop the balancer
Start the balancer

Downgrade - Major Versions
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Upgrade major versions - Without changing config servers topology
Rollback 3.4.y to 3.2.x
2) Downgrade the mongos in a rolling fashion by stop;replace binaries;start
3) Downgrade the shards. Downgrade the secondaries in a rolling fashion by stop;replace binaries;start
4) Downgrade the shards. Perform a stepdown and Downgrade the ex-Primaries by stop;replace binaries;start
5) Downgrade the config servers. Downgrade the secondaries in a rolling fashion by stop;replace binaries;start
6) Downgrade the config servers. Perform a stepdown and downgrade the ex-Primary by stop;replace binaries;start
If backwards-incompatible 3.4 features are enabled before downgrade:
- db.adminCommand({setFeatureCompatibilityVersion: "3.2"})
- Remove Views
- Remove Collation Option from Collections and Indexes
- Convert Data of Type Decimal
- Downgrade Index Versions

Major Versions Downgrade (3.4.x to 3.2.y)
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s1 s2
1. Downgrade mongos
3. Downgrade config servers
2. Downgrade shards
Stop the balancer
Start the balancer

Downgrading - Major Versions
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Remove Views
Find views on a sharded cluster:
db.adminCommand("listDatabases").databases.forEach(function(d){
let mdb = db.getSiblingDB(d.name);
mdb.getCollectionInfos({type: "view"}).forEach(function(c){
print(mdb[c.name]);
});
});
In each database that contains views, drop the system.views collection to drop all views in that database.

30
Remove Collation Option from Collections and Indexes
Find collections with non-simple collations
mdb.getCollectionInfos( { "options.collation": { $exists: true } } ).forEach(function(c){
print(mdb[c.name]);});});
Find indexes with collation specification
mdb.getCollectionInfos().forEach(function(c){
let currentCollection = mdb.getCollection(c.name);
currentCollection.getIndexes().forEach(function(i){
if (i.collation){
printjson(i);}});});});

31
Remove Collation Option from Collections and Indexes
Indexes: Drop and recreate the indexes
Collection: Rewrite the entire collection:
- Stream the collection using $out
- Stream the collection using dump/restore
- Change the definition on one of the secondaries

32
Convert Data of Type Decimal
Diagnostic utility: db.collection.validate(full)
Could be impactful as it performs a full scan.
"errmsg" : "Index version does not support NumberDecimal"
How you are going to convert the Type Decimal depends on your application.
- Change it to string
- Change to a two fields format (using a multiplier)
- Change to a two fields format (store each part of the decimal on different fields)

33
Find version 2 indexes
mdb.getCollectionInfos().forEach(function(c){
let currentCollection = mdb.getCollection(c.name);
currentCollection.getIndexes().forEach(function(i){
if (i.v === 2){
printjson(i);}});});});
Must be performed on both shards and config servers
Re-index the collection after change the setFeatureCompatibilityVersion to 3.2
- _id index in the foreground
- Must be executed on both Primary and Secondaries

34
Upgrade major versions - Change the config servers topology - 3.2.x to 3.4.y
1) Use MongoDB version 3.2.4 or higher
2) Disable the balancer
3) Connect a mongo shell to the first config server listed in the configDB setting of the mongos and run rs.initiate()
rs.initiate( {
_id: "csReplSet",
configsvr: true,
version: 1,
members: [ { _id: 0, host: "<host>:<port>" } ]
} )
4) Restart this config server as a single member replica set with:
mongod --configsvr --replSet csReplSet --configsvrMode=sccc --storageEngine <storageEngine> --port <port> --
dbpath <path> or the equivalent config file settings

35
5) Start the new mongod instances to add to the replica set:
- must use the WiredTiger storage engine
- Do not add existing config servers to the replica set
- Use new dbpaths for the new instances
- If the config server is using MMAPv1, start 3 new mongod instances
- If the config server is using WiredTiger, start 2 new mongod instances
6) Connected to the replica set config server and add the new mongod instances as non-voting, priority 0 members:
- rs.add( { host: <host:port>, priority: 0, votes: 0 } )
- Wait for the initial sync to complete (SECONDARY state)
7) Shut down one of the other non-replica set config servers (2nd or 3rd)
8) Reconfigure the replica set to allow all members to vote and have default priority of 1

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9) Step down the first config server and restart without the sccc flag
10) Restart mongos instances with updated --configdb or sharding.configDB setting
11) Verify that the restarted mongos instances are aware of the protocol change
12) Cleanup:
- Remove the first config server using rs.remove()
- Shutdown 2nd and 3rd config servers
Next steps are similar to slide Upgrade major versions - Without changing config servers topology

Change from SCCC to CSRS
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1) rs.initiate
2) Restart with --replSet
3) Start 3 new cfgsrv
4) Add new cfgsrv to RS
5) Shutdown a cfgsrv
6) rs.reconfig
7) rs.stepdown and restart
8) Restart mongos
9) Cleanup

38
Downgrade major versions - Change the config servers topology (3.2.x to 3.0.y)
1) Disable the balancer
2) Remove or replace incompatible indexes:
- Partial indexes
- Text indexes version 3
- Geo Indexes version 3
3) Check minOpTimeUpdaters value on every shard
- Must be zero
- If it's different to zero without any active migration ongoing, a stepdown needed
4) Keep only two config servers secondaries and set their votes and priority equal to zero , using rs.reconfig()
5) Stepdown config server primary
db.adminCommand( { replSetStepDown: 360, secondaryCatchUpPeriodSecs: 300 })
6) Stop the world - shut-down all mongos/shards/config servers

39
Downgrade major versions - Change the config servers topology(3.2.x to 3.0.y)
7) Restart each config server as standalone
8) Start all shards and change the protocolVersion=0
9) Downgrade the mongos to 3.0 and change the configsrv to SCCC
10) Downgrade the config servers to 3.0
11) Downgrade each shard - one at a time
- Remove the minOpTimeRecovery document from the admin.system.version
- Downgrade the secondaries and then issue a stepdown
- Downgrade former primaries

Change from CSRS to SCCC
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1)Remove minOpTimeUpdaters
2) rs.reconfig() set priority=0
3) rs.stepdown()
4) Stop the world
5) Start cfgsrv as standalone
6) Start all shards
7) Start the mongos with SCCC

41
Lab 01 1. Unzip the provided .vdi file
2. Install and or Open VirtualBox
3. Select New
4. Enter A Name
5. Select Type: Linux
6. Select Version: Red Hat (64-bit)
7. Set Memory to 4096 (if possible)
8. Select "Use an existing ... disk file", select the provided .vdi file.
9. Select Create
10. Select Start
11. CentOS will boot and auto-login
12. Navigate to /Percona2017/Lab01/exercises for the first lab.
Setup A Sharded Cluster

Shard Key
● What is it?
● Limitations
● Chunks
● Metadata
○ Change Log
○ Collections
○ Chunks
42

Shard Key
43
This is how MongoDB distributes and finds documents for a sharded collection.
Considerations of a good shard key:
• Fields exist on every document
• Indexed field or fields for fast lookup
• High cardinality for good distribution of data across the cluster
• Random to allow for increased throughput
• Usable by the application for targeted operations

Shard Key Limitations
44
There are requirements when defining a shard key that can affect both the cluster and application.
• Key is immutable
• Value is also immutable
• For collections containing data an index must be present
• Prefix of a compound index is usable
• Ascending order is required
• Update and findAndModify operations must contain shard key
• Unique constraints must be maintained by shard key or prefix of shard key
• Hashed indexes can not enforce uniqueness, therefore are not allowed
• A non-hashed indexed can be added with the unique option
• A shard key cannot contain special index types (i.e. text)

45
Shard key must not exceed the 512 bytes
The following script will reveal documents with long shard keys:
db.<collection>.find({},{<shard_key>:1}).forEach(function(shardkey){
size = Object.bsonsize(shardkey) ;
if (size>512){print(shardkey._id)}
})
Mongo will allow you to shard the collection even if you have existing shard keys over 512 bytes buton
the next insert with a shard key > 512 bytes:
"code" : 13334,"errmsg" : "shard keys must be less than 512 bytes, but key <shard key> is ... bytes"

46
Shard Key Index Type
A shard key index can be an ascending index on the shard key, a compound index that start with
the shard key and specify ascending order for the shard key, or a hashed index.
A shard key index cannot be an index that specifies a multi-key index, a text index or a geospatial
index on the shard key fields.
If you try to shard with a -1 index you will get an error:
“Unsupported shard key pattern. Pattern must either be a single hashed field, or a list of ascending fields”
If you try to shard with “text”, “multi-key” or “geo” you will get an error:
“couldn't find valid index for shard key”

47
Unique Indexes
Sharded collections may support up to one unique index. The shard key MUST be a prefix of the
unique index
If you attempt to shard a collection with more than one unique indexes or using a field different than
the unique index an error will be produced:
”Can't shard collection <col name> with unique index on <field1:1> and proposed shard key <field2:2>.
Uniqueness can't be maintained unless shard key is a prefix"
To maintain more than one unique indexes use a proxy collection.
Client generated _id is unique by design if you are using custom _id you must preserve uniqueness
from the application tier.

Shard key Limitations
48
Field(s) must exist on every document
If you try to shard a collection with null on shard key an exception will be produced:
"found missing value in key { : null } for doc: { _id: <value>}"
On compound shard keys none of fields is allowed to have null values.
A handy script to identify NULL values is the following. You need to execute it for each of the shard
key fields:
db.<collection_name>.find({<shard_key_element>:{$exists:false}})
A potential solution is to replace NULL with a dummy value that your application will read as NULL.
Be careful because “dummy NULL” might create a hotspot.

49
Updates and FindAndModify must use the shard key
Updates and FindAndmodify must use the shard key on the query predicates
If an update of FAM executed on a field different than the shard key or _id the following error is been
produced
"A single update on a sharded collection must contain an exact match on _id (and have the collection default collation) or
contain the shard key (and have the simple collation). Update request: { q: { <field1> }, u: { $set: { <field2> } }, multi: false,
upsert: false }, shard key pattern: { <shard_key>: 1 }”
For update operations the workaround is to use the {multi:true} flag.
For FindAndModify {multi:true} flag doesn’t exist.
For upserts _id instead of <shard key> is not applicable.

50
Sharding Existing Collection Data Size
You can’t shard collections that their size violate the maxCollectionSize as defined below:
maxSplits = 16777216 (bytes) / <average size of shard key values in bytes>
maxCollectionSize (MB) = maxSplits * (chunkSize / 2)
Maximum Number of Documents Per Chunk to Migrate
MongoDB cannot move a chunk if the number of documents in the chunk exceeds:
• 250000 documents or 1.3 times the result of dividing the configured chunk size by the
avgObjSize
For example, with an avgObjSize of 512 bytes and chunk size of 64MB a chunk is considered Jumbo
with 170394 documents.

51
Operation not allowed on a sharded collection
• group collection method Deprecated since version 3.4 - Use mapReduce or
aggregate instead
• db.eval() Deprecated since version 3.0
• $where does not permit references to the db object from the $where function. This
is uncommon in un-sharded collections.
• $isolated update modifier
• $snapshot queries
• The geoSearch command

Chunks
52
The purpose of shard key is to create chunks.
The logic partition your collection is divided into and how data is distributed across the cluster.
• Maximum size is defined in config.settings
• Default 64MB
• Hardcoded maximum document count of 250,000
• Chunk map is stored in config.chunks
• Continuous range from MinKey to MaxKey
• Chunk map is cached at both the mongos and mongod
• Query Routing
• Sharding Filter
• Chunks distributed by the Balancer
• Using moveChunk
• Up to maxSize

Chunk
53
{
"_id" : "mydb.mycoll-uuid_"00005cf6-1217-4414-935b-cf1bde09cc77"",
"lastmod" : Timestamp(1, 5),
"lastmodEpoch" : ObjectId("570733145f2bf94777a62155"),
"ns" : "mydb.mycoll",
"min" : {
"uuid" : "00005cf6-1217-4414-935b-cf1bde09cc77"
},
"max" : {
"uuid" : "7fe55637-74c0-4e51-8eed-ab6b411d2b6e"
},
"shard" : ”s1"
}
config.chunks

Additional Sharding Metadata
54
Collections
• Historical Sharding and Balancing Activity
• config.changelog
• config.actionlog
• Additional Balancing Settings
• config.settings
• All databases sharded (and non-sharded)
• config.databases
• Balancer Activity
• config.locks
• Shards
• config.shards

Shard Key Selection
● Profiling
● Cardinality
● Throughput
○ Targeted Operations
○ Broadcast Operations
● Anti-Patterns
● Data Distribution
55

Shard key selection
56
Famous quotes about sharding:
“Sharding is an art”
“There is no such thing as the perfect shard key”
“Friends don't let friends use sharding”

Shard key selection
57
Profiling
Identify Patterns
Constraints/Best
practices
Code changes
Apply shard
key(s)

Shard key selection
58
Profiling
Identify Patterns
Constraints/Best
practices
Code changes
Apply shard
key(s)

Shard key selection–Profiling
59
Profiling will help you identify your workload.
Enable statement profiling on level 2 (collects profiling data for all database operations)
db.getSiblingDB(<database>).setProfilingLevel(2)
To collect a representative sample you might have to increase profiler size
db.getSiblingDB(<database>).system.profile.drop()
db.getSiblingDB(<database>).createCollection( "system.profile", { capped: true, size: <size>} )
*size is in bytes

60
If you don’t have enough space you may either:
- Periodically dump the profiler and restore to a different instance
- Use a tailable cursor and save the output on a different instance
Profiler adds overhead to the instance due to extra inserts
When analyzing the output is recommended to:
- Dump the profiler to another instance using a non-capped collection
- Keep only the useful dataset
- Create the necessary indexes

61
{op:"query" , ns:<db.col>} , reveals find operations
Example: { "op" : "query", "ns" : "foo.foo", "query" : { "find" : "foo", "filter" : { "x" : 1 } ...
{op:"insert" , ns:<db.col>} , reveals insert operations
Example: { "op" : "insert", "ns" : "foo.foo", "query" : { "insert" : "foo", "documents" : [ { "_id" :
ObjectId("58dce9730fe5025baa0e7dcd"), "x" : 1} ] ...
{op:"remove" , ns:<db.col>} , reveals remove operations
Example: {"op" : "remove", "ns" : "foo.foo", "query" : { "x" : 1} ...
{op:"update" , ns:<db.col>} , reveals update operations
Example: { "op" : "update", "ns" : "foo.foo", "query" : { "x" : 1 }, "updateobj" : { "$set" : { "y" : 2 } } ...

62
{ "op" : "command", "ns" : <db.col>, "command.findAndModify" : <col>} reveals FAMs
Example: { "op" : "command", "ns" : "foo.foo", "command" : { "findAndModify" : "foo", "query" : { "x" : "1" },
"sort" : { "y" : 1 }, "update" : { "$inc" : { "z" : 1 } } }, "updateobj" : { "$inc" : { "z" : 1 } } ...
{"op" : "command", "ns" : <db.col>, "command.aggregate" : <col>}: reveals aggregations
Example: { "op" : "command", "ns" : "foo.foo", "command" : { "aggregate" : "foo", "pipeline" : [ { "$match" : {
"x" : 1} } ] ...
{ "op" : "command", "ns" : <db.col>, "command.count" : <col>} : reveals counts
Example: { "op" : "command", "ns" : "foo.foo", "command" : { "count" : "foo", "query" : { "x" : 1} } ...
More commands: mapreduce, distinct, geoNear are following the same pattern
Be aware that profiler format may be different from major version to major version

Shard key selection
63
Profiling
Identify Patterns
Constraints/Best
practices
Code changes
Apply shard
key(s)

Shard key selection–Patterns
64
Identify the workload nature (type of statements)
db.system.profile.aggregate([{$match:{ns:<db.col>}},{$group: {_id:"$op",number : {$sum:1}}},{$sort:{number:-1}}])
var cmdArray = ["aggregate", "count", "distinct", "group", "mapReduce", "geoNear", "geoSearch", "find", "insert", "update", "delete",
"findAndModify", "getMore", "eval"];
cmdArray.forEach(function(cmd) {
var c = "<col>";
var y = "command." + cmd;
var z = '{"' + y + '": "' + c + '"}';
var obj = JSON.parse(z);
var x = db.system.profile.find(obj).count();
if (x>0) {
printjson(obj);
print("Number of occurrences: "+x);}
});

65
Identify statement patterns
var tSummary = {}
db.system.profile.find( { op:"query",ns : {$in : ['<ns>']}},{ns:1,"query.filter":1}).forEach(
function(doc){
tKeys=[];
if ( doc.query.filter === undefined) {
for (key in doc.query.filter){
tKeys.push(key)
}
}
else{
for (key in doc.query.filter){
tKeys.push(key)
}
}
sKeys= tKeys.join(',')
if ( tSummary[sKeys] === undefined){
tSummary[sKeys] = 1
print("Found new pattern of : "+sKeys)
print(tSummary[sKeys])
}else{
tSummary[sKeys] +=1
print("Incremented "+sKeys)
print(tSummary[sKeys])
}
print(sKeys)
tSummary=tSummary
}
)

66
At this stage you will be able to create a report similar to:
Collection <Collection Name> - Profiling period <Start time> , <End time> - Total statements: <num>
Number of Inserts: <num>
Number of Queries: <num>
Query Patterns: {pattern1}: <num> , {pattern2}: <num>, {pattern3}: <num>
Number of Updates: <num>
Update patterns: {pattern1}: <num> , {pattern2}: <num>, {pattern3}: <num>
Number of Removes: <num>
Remove patterns: {pattern1}: <num> , {pattern2}: <num>, {pattern3}: <num>
Number of FindAndModify: <num>
FindandModify patterns: {pattern1}: <num> , {pattern2}: <num>, {pattern3}: <num>

67
Using the statement’s patterns you may identify shard key candidates
Shard key candidates may be a single field ({_id:1}) or a group of fields ({name:1, age:1}).
Note down the amount and percentage of operations that each shard key candidate can “scale”
Note down the important operations on your cluster
Order the shard key candidates by descending <importance>,<scale%>
Identify exceptions and constraints as described on shard key limitations
If a shard key candidate doesn’t satisfy a constraint you may either:
- Remove it from the list, OR
- Make the necessary changes on your schema

Shard key selection
68
Profiling
Identify Patterns
Constraints/Best
practices
Code changes
Apply shard
key(s)

Shard key selection- Constraints
69
Shard key must not have NULL values:
db.<collection>.find({<shard_key>:{$exists:false}}) , in the case of a compound key each element must
be checked for NULL
Shard key is immutable:
Check if collected updates or FindAndModify have any of the shard key components on “updateobj”:
db.system.profile.find({"ns" : <col>,"op" : "update" },{"updateobj":1}).forEach(function(op) { if
(op.updateobj.$set.<shard_key> != undefined ) printjson(op.updateobj);})
db.system.profile.find({"ns" : <col>,"op" : "update" },{"updateobj":1}).forEach(function(op) { if
(op.updateobj.<shard_key> != undefined ) printjson(op.updateobj);})
Assuming you are running a replica set the oplog may also proven useful

70
Updates must use the shard key or _id on the query predicates:
We can identify the query predicates using the exported patterns
Shard key must have good cardinality
db.<col>.distinct(<shard_key>).length OR
db.<col>.aggregate([{$group: { _id:"$<shard_key>" ,n : {$sum:1}}},{$count:"n"}])
We define cardinality as <total docs> / <distinct values>
The closer to 1 the better, for example a unique constraint has cardinality of 1
Cardinality is important for splits. Fields with cardinality close to 0 may create indivisible jumbo chunks

71
Check for data hotspots
db.<col>.aggregate([{$group: { _id:"$<shard_key>" ,number : {$sum:1}},{$sort:{number:-
1}},{$limit:100}},{allowDiskUse:true}])
We don’t want a single range to exceed the 250K document limit
We have to try and predict the future based on the above values
Check for operational hotspots
db.system.profile.aggregate([{$group: { _id:"$query.filter.<shard_key>" ,number :
{$sum:1}},{$sort:{number:-1}},{$limit:100}},{allowDiskUse:true}])
We want uniformity as much as possible. Hotspoted ranges may affect scaling

72
Check for monotonically increased fields:
Monotonically increased fields may affect scaling as all new inserts are directed to maxKey chunk
Typical examples: _id , timestamp fields , client-side generated auto-increment numbers
db.<col>.find({},{<shard_key>:1}).sort({$natural:1})
Workarounds: Hashed shard key or use application level hashing or compound shard keys

73
Throughput:
Targeted operations
- Operations that using the shard key and will access only one shard
- Adding shards on targeted operations will increase throughput linearly (or almost linearly)
- Ideal type of operation on a sharded cluster
Scatter-gather operations
- Operations that aren’t using the shard key or using a part of the shard key
- Will access more than one shard - sometimes all shards
- Adding shards won’t increase throughput linearly
- Mongos opens a connection to all shards / higher connection count
- Mongos fetches a larger amount of data on each iteration
- A merge phase is required
- Unpredictable statement execution behavior
- Queries, Aggregations, Updates, Removes (Inserts and FAMs are always targeted)

Shard key selection-Constraints
74
Throughput:
Scatter-gather operations are not always “evil’’
- Sacrifice reads to scale writes on a write intensive workload
- Certain read workloads like Geo or Text searches are scatter-gather anyway
- Maybe is faster to divide an operation into N pieces and execute them in parallel
- Smaller indexes & dataset
- Other datastores like Elastic are using scatter-gather for queries

Shard key selection-Constraints
75
Data distribution
Factors that may cause an imbalance:
- Poor cardinality
- Good cardinality with data hotspots
- TTL indexes
- Data pruning
- Orphans
Always plan ahead
- Try to simulate the collection dataset in 3,6 and 12 months
- Consider your deletion strategy before capacity becomes an issue

Shard key selection-Anti-Patterns
76
- Monotonically increased fields
- _id or timestamps
- Use hashed or compound keys instead
- Poor cardinality field(s)
- country or city
- Eventually leads to jumbos
- Use Compound keys instead
- Operational hotspots
- client_id or user_id
- Affect scaling
- Data hotspots
- client_id or user_id
- Affect data distribution may lead to jumbo

Shard key selection
77
Profiling
Identify Patterns
Constraints/Best
practices
Code changes
Apply shard
key(s)

Shard key selection-Code Changes
78
Remove unique constraints
- Sharded collections can only support one unique index
- Use a satellite collection for multiple unique indexes
- Change your updates to use the unique index rather than the “_id”
- Careful of custom _id
Change statements
- Make sure FAM are using the shard key
- Make sure update are using the shard key or the {multi:true}
- Remove geoSearch, eval(), $isolate, $snapshot, group and $where
Schema changes
- Change null on shard key with “dummy” values
- Implement an Insert+Delete functionality for changing shard keys

Shard key selection
79
Profiling
Identify Patterns
Constraints/Best
practices
Code changes
Apply shard
key(s)

80
Lab 02
Lab 02. Navigate to /Percona2017/Lab02/exercises
Sharding Analysis

Balancing & Chunk
Management ● Sizing and Limits
● moveChunk
● Pre-Splitting
● Splitting
● Merging
81

Sizing and Limits
82
Under normal circumstances the default size is sufficient for most workloads.
• Maximum size is defined in config.settings
• Default 64MB
• Hardcoded maximum document count of 250,000
• Chunks that exceed either limit are referred to as Jumbo
• Can not be moved unless split into smaller chunks
• Ideal data size per chunk is (chunk_size/2)
• Chunks can have a zero size with zero documents
• Jumbo’s can grow unbounded
• Unique shard key guarantees no Jumbos (i.e. max document size is 16MB) post
splitting

dataSize
83
Estimated Size and Count (Recommended)
db.adminCommand({
dataSize: "mydb.mycoll",
keyPattern: { "uuid" : 1 },
min: { "uuid" : "7fe55637-74c0-4e51-8eed-ab6b411d2b6e" },
max: { "uuid" : "7fe55742-7879-44bf-9a00-462a0284c982" }, estimate=true });
Actual Size and Count
db.adminCommand({
dataSize: "mydb.mycoll",
keyPattern: { "uuid" : 1 },
min: { "uuid" : "7fe55637-74c0-4e51-8eed-ab6b411d2b6e" },
max: { "uuid" : "7fe55742-7879-44bf-9a00-462a0284c982" } });

Start moveChunk (>=3.4)
84
s1 s2
moveChunk

Build Indexes
85
s1 s2
createIndex
The destination shard builds all non-existing indexes for the collection.

Clone Collection
86
s1 s2
insert()
The destination shard requests documents from the source and inserts them into the collection.

Sync Collection
87
s1 s2
Synchronize _id’s
Operations executed on source shard after the start of the migration are logged for synchronizing.

Commit
88
s1 s2
Finalize the chunk migration by committing metadata changes.

Clean Up - RangeDeleter
89
This process is responsible for removing the chunk ranges that were moved by
the balancer (i.e. moveChunk).
● This thread only runs on the primary
● Is not persistent in the event of an election
● Can be blocked by open cursors
● Can have multiple ranges queued to delete(_waitForDelete: false)
● If a queue is present, the shard can not be the destination for a new chunk

Pre-splitting (Hashed)
90
Shard keys using MongoDB’s hashed index allow the use of numInitialChunks.
Hashing Mechanism
jdoe@example.com 694ea0904ceaf766c6738166ed89bafb NumberLong(“7588178963792066406”)
Value 64-bits of MD5 64-bit Integer
Estimation
Size = Collection size (in MB)/32
Count = Number of documents/125000
Limit = Number of shards*8192
numInitialChunks = Min(Max(Size, Count), Limit)
1,600 = 51,200/32
800 = 100,000,000/125,000
32,768 = 4*8192
1600 = Min(Max(1600, 800), 32768)
Command
db.runCommand( { shardCollection: "mydb.mycoll", key: { "uid": "hashed" }, numInitialChunks : 5510 } );

Pre-splitting - splitAt()
91
When the shard key is not hashed but throughput requires pre-splitting with sh.splitAt().
1. In a testing environment shard the collection to generate split points.
a) Alternative: Calculate split points manually by analyzing the shard key field(s)
db.runCommand( { shardCollection: "mydb.mycoll", key: { "u_id": 1, "c": 1 } } )
db.chunks.find({ns: "mydb.mycoll"}).sort({min: 1}).forEach(function(doc) {
print("sh.splitAt('" + doc.ns + "', { "u_id": ObjectId("" + doc.min.u_id + ""), "c": "" + doc.min.c + '" });');
});

splitAt() - cont.
92
2. Identify enough split points to create N chunks which is at least 2x the number of shards and build a
JavaScript file containing your split points
3. Shard the collection and then execute the splitAt() commands to create chunks
db.runCommand( { shardCollection: "mydb.mycoll", key: { "u_id": 1, "c": 1 } } )
sh.splitAt('mydb.mycoll', { "u_id": ObjectId("581939bd1b0e4d73dd3d72db"), "c": ISODate("2017-01-31T12:02:45.532Z") });
sh.splitAt('mydb.mycoll', { "u_id": ObjectId("5800ed895c5a687c99c2f2ae"), "c": ISODate("2017-01-31T11:02:46.522Z") });
….
sh.splitAt('mydb.mycoll', { "u_id": ObjectId("5823859e5c5a6829cc3b09d0"), "c": ISODate("2017-01-31T13:22:48.522Z") });
sh.splitAt('mydb.mycoll', { "u_id": ObjectId("582f53315c5a68669f569c21"), "c": ISODate("2017-01-31T13:12:49.532Z") });
….
sh.splitAt('mydb.mycoll', { "u_id": ObjectId("581939bd1b0e4d73dd3d72db"), "c": ISODate("2017-01-31T17:32:41.552Z") });
sh.splitAt('mydb.mycoll', { "u_id": ObjectId("58402d255c5a68436b40602e"), "c": ISODate("2017-01-31T19:32:42.562Z") });

splitAt() - cont.
93
4. Using moveChunk distribute the chunks evenly.
Optional: Now that the chunks are distributed evenly run more splits to prevent immediate splitting.
db.runCommand({
"moveChunk": "mydb.mycoll",
"bounds": [{
"u_id": ObjectId("52375761e697aecddc000026"), "c": ISODate("2017-01-31T00:00:00Z")
}, {
"u_id": ObjectId("533c83f6a25cf7b59900005a"), "c": ISODate("2017-01-31T00:00:00Z")
}
],
"to": "rs1",
"_secondaryThrottle": true
});

Shard
Management ● Adding Shards
● Removing Shards
● Replacing Shards
○ Virtual
○ Physical
94

Shard Management-Operations
95
Add shard(s):
- Increase capacity
- Increase throughput or Re-plan
Remove shard(s):
- Reduce capacity
- Re-plan
Replace shard(s):
- Change Hardware specs
- Hardware maintenance
- Move to different underlying platform

Shard Mgmt – Add shard(s)
96
Command for adding shards sh.addShard(host)
Host is either a standalone or replica set instance
Alternatively db.getSiblingDB('admin').runCommand( { addShard: host} )
Optional parameters with runCommand:
maxSize (int) : The maximum size in megabytes of the shard
Name (string): A unique name for the shard
Localhost and hidden members can’t be used on host variable

Shard Mgmt-Remove shard(s)
97
Command for removing shards:
db.getSiblingDB('admin').runCommand( { removeShard: host } )
host’s data (sharded and unsharded) MUST migrated to other shards in the cluster
1) Ensure that the balancer is running
2) Execute db.getSiblingDB('admin').runCommand( { removeShard: <shard_name> } )
MongoDB will print:
"msg" : "draining started successfully",
"state" : "started",
"shard" : "<shard>",
"note" : "you need to drop or movePrimary these databases",
"dbsToMove" : [ ],
"ok" : 1

98
3) Balancer will now start move chunks from the <shard_name> to all other shards
4) Check the status using db.getSiblingDB('admin').runCommand( { removeShard: <shard_name> } ),
MongoDB will print:
{
"msg" : "draining ongoing",
"state" : "ongoing",
"remaining" : {
"chunks" : <num_of_chunks_remaining>,
"dbs" : 1
},
"ok" : 1
}

99
5) Run the db.getSiblingDB('admin').runCommand( { removeShard: host }) for one last time
MongoDB will print:
{
"msg" : "removeshard completed successfully",
"state" : "completed",
"shard" : "<shard_name>",
"ok" : 1
}
If the shard is the primary shard for a database(s) it may or may not host unsharded
collections
Removal will only completed when moving unsharded data to a different shard

100
6) Check the status using db.getSiblingDB('admin').runCommand( { removeShard: <shard_name> } ),
MongoDB will print:
{
"msg" : "draining ongoing",
"state" : "ongoing",
"remaining" : {
"chunks" : NumberLong(0),
"dbs" : NumberLong(1) },
"note" : "you need to drop or movePrimary these databases",
"dbsToMove" : ["<database name>"],
"ok" : 1
}

101
7) Use the following command to movePrimary:
db.getSiblingDB('admin').runCommand( { movePrimary: <db name>, to: "<shard name>" })
MongoDB will print: {"primary" : "<shard_name>:<host>","ok" : 1}
8) After you move all databases you will be able to remove the shard:
db.getSiblingDB('admin').runCommand( { removeShard: <shard_name> } )
MongoDB will print:
{"msg" : "removeshard completed successfully",
"state" : "completed",
"shard" : "<shard_name>",
"ok" : 1}

102
movePrimary:
- requires Write Downtime for the unsharded collections
- Without Write Downtime you may encounter dataloss
- May impact performance (massive writes &foreground index builds)
Write downtime:
• Application layer
- Database layer
- - Stop all mongos
- - Drop databasesusers (restart the mongos)
flushrouterconfig is mandatory when you movePrimary - db.adminCommand({flushRouterConfig: 1})

103
Calculate average chunk moves time:
db.getSiblingDB(”config").changelog.aggregate([{$match:{"what" : "moveChunk.from"}},
{$project:{"time1":"$details.step 1 of 7", "time2":"$details.step 2 of 7","time3":"$details.step 3 of 7","time4":"$details.step
4 of 7","time5":"$details.step 5 of 7","time6":"$details.step 6 of 7","time7":"$details.step 7 of 7", ns:1}},
{$group:{_id: "$ns","avgtime": { $avg: {$sum:["$time",
"$time1","$time2","$time3","$time4","$time5","$time6","$time7"]}}}},
{$sort:{avgtime:-1}},
{ $project:{collection:"$_id", "avgt":{$divide:["$avgtime",1000]}}}])
Calculate the number of chunks to be moved:
db.getSiblingDB(”config").chunks.aggregate({$match:{"shard" : <shard>}},{$group: {_id:"$ns",number : {$sum:1}}})

104
Calculate non-sharded collection size:
function FindCostToMovePrimary(shard){
moveCostMB = 0; DBmoveCostMB = 0;
db.getSiblingDB('config').databases.find({primary:shard,}).forEach(function(d){
db.getSiblingDB(d._id).getCollectionNames().forEach(function(c){
if ( db.getSiblingDB('config').collections.find({_id : d._id+"."+c, key: {$exists : true} }).count() < 1){
x=db.getSiblingDB(d._id).getCollection(c).stats();
collectionSize = Math.round((x.size+x.totalIndexSize)/1024/1024*100)/100;
moveCostMB += collectionSize;
DBmoveCostMB += collectionSize;
}
else if (! /system/.test(c)) { }
})
print(d._id);
print("Cost to move database :t"+ DBmoveCostMB+"M");
DBmoveCostMB = 0;
});
print("Cost to move:t"+ moveCostMB+"M");};

105
s1 s2
1) {removeShard: ‘s2’}
Chunks
2) Drain chunks
3) Move Primary
5) {removeShard: ‘s2’}
4) flushRouterConfig
Database
? ? ?

Shard Mgmt-Replace shard(s)
106
Drain Method using javascript
1)Stop the balancer
2)Add the target shard <target>
3)Create and execute chunk moves from <source> to <target>
db.chunks.find({shard:"<shard>"}).forEach(function(chunk){print("db.adminCommand({moveChunk : '"+ chunk.ns +"' ,
bounds:[ "+ tojson(chunk.min) +" , "+ tojson(chunk.max) +"] , to:<target>'})");})
mongo <host:port> drain.js | tail -n +1 | sed 's/{ "$maxKey" : 1 }/MaxKey/' | sed 's/{ "$minKey" : 1 }/MinKey/' >
run_drain.js
4)Remove the <source> shard (movePrimary may required)

107
Drain Method using the Balancer
1) Stop the balancer
2) Add the target shard <target>
3) Run the remove command for <source> shard
4) Optional: Set the maxSize=1 for all shards but <source> and <target>
5) Start the balancer - it will move chunks from <source> to <target>
6) Finalize the <source> removal (movePrimary may required)

108
Replica-set extension
1) Stop the balancer
2) Add additional mongods on replica set <source>
3) Wait for the nodes to become fully sync
4) Perform a stepdown and promote one of the newly added nodes as the new Primary
5) Remove <source> nodes from the replica set
6) Start the balancer

Troubleshooting
● Hotspots
● Imbalanced Data
● Configuration Servers
● RangeDeleter
● Orphans
● Collection Resharding
109

Hotspotting
110
s1 s2
80% 20%
Possible Causes:
• Shard Key
• Unbalanced Collections
• Unsharded Collections
• Data Imbalance
• Randomness

Hotspotting - Profiling
111
Knowing the workload this is how you can identify your top u_id’s.
db.system.profile.aggregate([
{$match: { $and: [ {op:"update"}, {ns : "mydb.mycoll"} ] }},
{$group: { "_id":"$query.u_id", count:{$sum:1}}},
{$sort: {"count": -1}}, {$limit : 5 } ]);
{
"result" : [
{
"_id" : ObjectId('53fcc281a25cf72b59000013',
"count" : 28672
},
..........
], "ok" : 1 }

Hotspotting - SplitAt()
112
{
"_id" : "mydb.mycoll-u_id_ObjectId('533974d30daac135a9000049')",
"lastmod" : Timestamp(1, 5),
"lastmodEpoch" : ObjectId("570733145f2bf94777a62155"),
"ns" : "mydb.mycoll",
"min" : {
"u_id" : ObjectId('533974d30daac135a9000049')
},
"max" : {
"u_id" : ObjectId('545d29e74d32da4e290000cf'
},
"shard" : ”s1"
} { “u_id” : ObjectId('53fcc281a25cf72b59000013'), …
{ “u_id” : ObjectId('54062bd2a25cf7e051000017') , …
{ “u_id” : ObjectId('5432ada44d32dadcc400007b') , …
{ “u_id” : ObjectId('5433f7254d32daa1e3000198') , …
sh.splitAt()
Unbalanced Operations

Data Imbalance – Disk Usage
113
s1 s2
s1 s2
Unbalanced Resources

Data Imbalance
114
Common Causes
● Balancing has been disabled or window to small
○ sh.getBalancerState();
○ db.getSiblingDB("config").settings.find({"_id" : "balancer"});
● maxSize has been reached or misconfigured across the shards
○ db.getSiblingDB("config").shards.find({},{maxSize:1});
● Configuration servers in an inconsistent state
○ db.getSiblingDB("config").runCommand( {dbHash: 1} );
● Jumbo Chunks
○ Previously covered, chunks that exceed chunksize or 250,000 documents
● Empty Chunks
○ Chunks that have no size and contain no documents
● Unsharded Collections
○ Data isolated to primary shard for the database
● Orphaned Documents

Data Imbalance
115
s1 s2
80% 20%
s1 s2
Unbalanced Operations
Unbalanced Resources
Empty Chunks
{ "uuid" : "00005cf6-....." } -->> { "uuid" : "7fe55637-....." } on :
s1
100K Docs
@ 45MB
{ "uuid" : "7fe55637-....." } -->> { "uuid" : "7fe55742-....." } on :
s2
0 Docs
@ 0MB

Data Imbalance - Empty Chunks
116
Unevenly distributed remove or TTL operations
insert()
& split
insert()
& split
insert() -> split
->
remove()
insert() -> split
->
remove()
s1 s2 s3 s4
moveChunk

Data Imbalance - Merging
117
Using JavaScript the following process can resolve the imbalance.
1. Check balancer state, set to false if not already.
2. Identify empty chunks and their current shard.
a. dataSize command covered earlier in the presentation, record this output
3. Locate adjacent chunk and it's current shard, this is done by comparing max and min
a. The chunk map is continuous, there are no gaps between max and min
4. If shard is not the same move empty chunks to the shard containing the adjacent
chunk
a. Moving the empty chunk is cheap but will cause frequent metadata changes
5. Merge empty chunks into their adjacent chunks
a. Merging chunks is also non-blocking but will cause frequent meta changes
Consideration, how frequent is this type of maintenance required?

Configuration Servers - SCCC
118
Sync Cluster Connection Configuration
At times a configuration server can be corrupted, the following methods can be used to
fixed both configurations. This is applicable to versions <= 3.2 MMAP and 3.2 WiredTiger.
1. As pre-caution back up all three config servers via mongodump
2. If only one of the three servers is not functioning or out of sync
a. Stop one of the available configuration servers
b. Rsync the data directory to the broken configuration server and start mongod
3. If only the first configuration server is healthy or all three are out of sync
a. Open a Mongo and SSH session to the first configuration server
b. From the Mongo session use db.fsyncLock() to lock the mongod
c. From the SSH session make a copy of the data directory
d. In the existing Mongo session use db.fsyncUnlock() to unlock the mongod
e. Rsync the copy to the broken configuration servers and start mongod

Configuration Servers - CSRS
119
Config Servers as Replica Sets
With CSRS an initial sync from a another member of the cluster is the preferred method to
repair the health of the replica set. If for any reason the majority is lost the cluster metadata
will still be available but read-only.
CSRS became available in version 3.2 and mandatory in version 3.4.

RangeDeleter
120
This process is responsible for removing the chunk ranges that were moved by the
balancer (i.e. moveChunk).
● This thread only runs on the primary
● Is not persistent in the event of an election
● Can be blocked by open cursors
● Can have multiple ranges queued to delete
● If a queue is present, the shard can not be the destination for a new chunk
A queue being blocked by open cursors can create two potential problems:
● Duplicate results for secondary and secondaryPreferred read preferences
● Permanent Orphans

Range Deleter - Open Cursors
121
Log Line:
[RangeDeleter] waiting for open cursors before removing range [{ _id: -869707922059464413 }, {
_id: -869408809113996381 }) in mydb.mycoll, elapsed secs: 16747, cursor ids: [74167011554]
MongoDB does not provide a server side command to close cursors, in this example
74167011554 must be closed to allow RangeDeleter to proceed.
Solution using Python:
from pymongo import MongoClient
c = MongoClient('<host>:<port>')
c.the_database.authenticate('<user>','<pass>',source='admin')
c.kill_cursors([74167011554])

Orphans
122
moveChunk started and documents are being inserted into s2 from s1.
s1 s2
Commit
Range: [{ u_id: 100 }, { u_id: 200 }]

Orphans
123
s1 s2
s2 synchronizes the changes documents and s1 commits the meta data change.
Commit
Range: [{ u_id: 100 }, { u_id: 200 }]

Orphans
124
s1 s2
The delete process is asynchronous, at this time both shards contain the documents. Primary
read preference filters them but not secondary and secondaryPreferred.
RangeDeleter Waiting
On Open Cursor

Orphans
125
s1 s2
An unplanned election or or step down occurs on s1.
RangeDeleter Waiting
On Open Cursor

Orphans
126
s1 s2
The documents are now orphaned on s1.
Range: [{ u_id: 100 }, { u_id: 200 }]Range: [{ u_id: 100 }, { u_id: 200 }]

Orphans
127
moveChunk reads and writes to and from primary members. Primary members cache a
copy of the chunk map via the ChunkManager process.
To resolve the issue:
1. Set the balancer state to false.
2. Add a new shard to the cluster.
3. Using moveChunk move all chunks from s1 to sN.
4. When all chunks have been moved and RangeDeleter are complete, connect to
primary:
a. Run a remove() operation to removed remaining “orphaned” documents
b. Or: Drop and recreate empty collection with all indexes
Alternatively Use:
https://docs.mongodb.com/manual/reference/command/cleanupOrphaned/

Collection Resharding
128
Overtime query patterns and writes can make a shard key not optimal or the wrong shard key
was implemented.
Dump And Restore
● Typically most time consuming
● Requires write outage if changing collection names
● Requires read and write outage if keeping the same collection name
Forked Writes
● Fork all writes to separate collection, database, or cluster
● Reduces or eliminates downtime completely using code deploys
● Increases complexity from a development perspective
● Allows for rollback and read testing

Collection Resharding
129
Incremental Dump and Restores (Append Only)
● For insert only collections begin incremental dump and restores
● Requires a different name space but reduces the cut-over downtime
● Requires read and write outage if keeping the same collection name
Mongo to Mongo Connector
● Connectors tail the oplog for namespace changes
● These changes can be filtered and applied to another namespace or cluster
● Similar to the forked write approach but handled outside of the application
● Allows for a rollback and read testing

130
Lab 03
Lab 04
Resharding and
Chunk Management

Miscellaneous
● User Management
● Zones
● Read Operations
● Connecting
131

User Management
● Users and Roles
○ Application Users
○ Admin Users
○ Monitoring
132
● Replica Set Access

Users And Roles
133
● Role based authentication framework with collection level granularity
○ Starting in version 3.4 views allow for finer granularity beyond roles
● To enable authentication for all components
○ Set security.authorization to enabled
○ Set security.keyFile to a file path containing a 6 - 1024 character random string
○ RBAC documents stored in admin.system.users, admin.system.roles, and
$external.users
● Once enabled the localhost exception can be used to create the initial admin user
○ For replica sets the admin database is stored and replicated to each member
○ For sharded clusters the admin data is stored on the configuration replica set
● Alternatively the user(s) can be created prior to enabling authentication
● External authentication is also available in the Enterprise* and Percona* distributions
○ SASL / LDAP*
○ Kerberos*

Cluster Authentication
134
s1 s2
Client

Replica Authentication
135
s1 s2
Client

Two Tier Authentication
136
s1 s2
Client
Discovery

Creating Users
137
use mydb
db.createUser({
user: "<user>",
pwd: "<secret>",
roles: [{ role: "readWrite", db: "mydb" }]
});
use admin
db.createUser({
user: "<user>",
pwd: "<secret>",
roles: [
{ role: "readWrite", db: "mydb1" },
{ role: "readWrite", db: "mydb2" },
{ role: "clusterMonitor", db: "admin" }
]
Application User Engineer
● Read/Write access to single database ● Read/Write access to multiple database
● Authentication scoped to admin
● Read-only cluster role

Built-In Roles
138
● Database
○ read
○ readWrite
● Database Administration
○ dbAdmin
○ dbOwner
○ userAdmin
● Backup And Restore
○ backup
○ restore
● Cluster Administration
○ clusterAdmin
○ clusterManager
○ clusterMonitor
○ hostManager
● All Databases
○ readAnyDatabase
○ readWriteAnyDatabase
○ userAdminAnyDatabase
○ dbAdminAnyDatabase
● Superuser
○ root

Creating Custom Roles
139
use admin
db.runCommand({ createRole: "<role>",
privileges: [
{ resource: { db: "local", collection: "oplog.rs" }, actions: [ "find" ] },
{ resource: { cluster: true }, actions: [ "listDatabases" ] }
],
roles: [ { role: "read", db: "mydb" } ]
});
Role Creation
• find() privilege on local.oplog.rs
• List all databases on the replica set
• Read only access on mydb
Granting Access
use admin
db.createUser( { user: "<user>", pwd: "<secret>",
roles: [ "<role>" ]
} );
Or
use admin
db.grantRolesToUser(
"<user>",
[ { role: "<role>", db: "admin" }]
);

Additional Methods
140
• db.updateUser()
• db.changeUserPassword()
• db.dropUser()
• db.grantRolesToUser()
• db.revokeRolesFromUser()
• db.getUser()
• db.getUsers()
User Management
• db.updateRole()
• db.dropRole()
• db.grantPrivilegesToRole()
• db.revokePrivilegesFromRole()
• db.grantRolesToRole()
• db.revokeRolesFromRole()
• db.getRole()
• db.getRoles()
Role Management

Zones
● Geographic Distribution
● Mixed Engine Topology
● Tiered Performance
141

Zones
142
A Zone is a range based on the shard key prefix, pre-3.4 was called Tag Aware
Sharding
A zone can be associated with one or more shards
A shard can be associated with any number of non-conflicting zones
Chunks covered by a zone will be migrated to associated shards.
We use zones for:
- Data isolation
- Geographically distributed clusters (keep data close to application)
- Heterogeneous hardware clusters

Zones
143
Zone : [“A”] Zone : [“B”] Zone : []
Zones:
[A] KEY: 1-10
[B] KEY: 10-20
{min , 10}
{10, 21}
{21, max}

Zones
144
Zone : [“A”] Zone : [“A”,“B”] Zone : [“C”]
Zones:
[A] KEY: 1-10
[B] KEY: 10-20
[C] KEY: 50-60
{min, 7}
{7, 30}
{31,max}

Zones
145
Define a zone:
sh.addTagRange(<collection>, { <shardkey>: min }, { <shardkey>: max }, <zone_name>)
Zone ranges are always inclusive of the lower boundary and exclusive of the upper boundary
The range ({ <shardkey>: min },{ <shardkey>: max }) must be a prefix of the shard key
Hashed keys are also supported - using the hash key ranges
Remove a zone:
There is no helper available yet - Manually remove from config.tags
db.getSiblingDB('config').tags.remove({_id:{ns:<collection>, min:{<shardkey>:min}},
tag:<zone_name>})

Zones
146
Add shards to a zone:
sh.addShardTag(<shard_name>, <zone_name>)
Remove zones from a shard:
sh.removeShardTag(<shard_name>, <zone_name>)
Manipulate zones may or may not cause chunk moves
Manipulate zones may be a long running operation

Zones
147
View existing zones/shards associations:
sh.status() list the tags next to the shards ,or
Query the config database db.getSiblingDB('config').shards.find({tags:<zone>})
View existing zones:
Query the config database db.getSiblingDB('config').tags.find({tags:<zone>})

Zones- Balancing
148
Balancer aims for an even chunk distribution
During a chunk migration:
- Balancer checks possible destination based on configured zones
- If the chunk range falls into a zone, it migrates the chunk to a shard within the zone
- Chunks that do not fall into a zone may exists on any shard
During balancing rounds balancer moves chunks that violate configured zones

Read Operations
● Covering Queries
● Sharding Filter
● Secondary Reads
149

Read Operations – Covered Queries
150
Covered query is a query that can be satisfied entirely using an index and does not have
to examine any documents
On unsharded collection a query on db.foo.find({x:1},{x:1,_id:0}) is covered by the {x:1}
index
On a sharded collection the query is covered if:
- {x:1} is the shard key, or
- compound index on {x:1,<shard_key>} exists

Read Operations – Covered Queries
151
Reading from primary db.foo.find({x:1},{x:1,_id:0}) using {x:1} index explain() reports:
"totalKeysExamined" : N
"totalDocsExamined" : N
Reading from a secondary db.foo.find({x:1},{x:1,_id:0}) is covered by the {x:1} index.
explain() reports:
"totalKeysExamined" : N,
"totalDocsExamined" : 0,

Read Operations – Sharding filter
152
Reading from Primaries:
- Queries must fetch the shard key to filter the orphans
- The cost of sharding filter reports on explain() plan "stage" : "SHARDING_FILTER”
- For operation that return a high number of results, sharding filter can be expensive
Secondaries don’t apply the sharding filter
But… on a sharded collection may return orphans or “on move documents”
You can’t fetch orphans when:
- Read preference is set to primary
- Read preference is set to secondary and shard key exists on read predicates

153
Distinct and Count commands return orphans even with primary read preference
Always use aggregation framework instead of Distinct and Count on sharded collections
Reveal Orphans:
Compare "nReturned" of SHARDING_FILTER with "nReturned" of EXECUTION stage
When SHARDING_FILTER < EXECUTION orphans may exists
Compare count() with itcount()
When itcount() < count() orphans may exists

154
A handy JavaScript that reveals orphans
db.getMongo().getDBNames().forEach(function(database)
{var re = new RegExp('config|admin'); if (!database.match(re)) {print("Checking database:
"+database);print("");db.getSiblingDB(database).getCollectionNames().forEach(function(co
llection){print("Checking collection: "+collection);
db.getSiblingDB(database).getCollection(collection).find().explain(true).executionStats.exe
cutionStages.shards.forEach(function(orphans)
{if (orphans.executionStages.chunkSkips>0){print("Shard " +
orphans.shardName + " has " + orphans.executionStages.chunkSkips +"
orphans")}})
})}
})

Read Operations
155
Find: Mongos merges the results from each shard
Aggregate: Runs on each shard and a random shard perform the merge
In older versions the mongos or the primary shard only was responsible for the merge
Sorting: Each shard sorts its own results and mongos performs a merge sort
Limits: Each shard applies the limit and then mongos re-apply it on the results
Skips: Each shard returns the full result set (un-skipped) and mongos applies the skip
Explain() output contains individual shard execution plan and the merge phase
It's not uncommon each shard to follow different execution plan

Connecting
● URI parameters
● Load Balancing
● New parameters
156

Connecting – Server discovery
157
URI Format mongodb://[username:password@]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]
Server Discovery:
- The client measure the duration of an ismaster call (RTT)
- Variable "localThresholdMS" defines the acceptable RTT ,default is 15 millis.
- The driver load-balances randomly across mongos that fall within the localThresholdMS
serverSelectionTimeoutMS: how long to block for server selection before throwing an exception. The
default is 30 seconds.
heartbeatFrequencyMS: The interval between server checks, counted from the end of the previous
check until the beginning of the next one, default 10 seconds.
A client waits between checks at least minHeartbeatFrequencyMS. Default 500 millis.
Important: Each driver may use its own algorithm to measure average RTT

Connecting – Load Balancing
158
Driver:
- Increases heartbeat traffic/availability
Driver with virtual mongos pools:
- N number of mongos
- Each application server uses a portion , for example N/2
- Reduce heartbeat traffic/availability
Use a Load Balancer
- Hash based routing (avoid round-robin)
- Minimize heartbeat traffic
- Use more than one LB to avoid Single point of failure
- Be aware of hot app servers and client-side NAT

Connecting – Connection Pool
159
TaskExecutorPoolSize (SERVER-25027)
The number of connection pools to use for a given mongos. Defaults to 4 <=
NUM_CORES <= 64.
Decreasing the value will decrease the number of open connections
ShardingTaskExecutorPoolHostTimeoutMS (SERVER-25027)
How long is the wait before dropping all connections to a host that mongos haven’t
communicated with. Default 5 minutes
Increasing the value will smooth connection spikes

160
ShardingTaskExecutorPoolMaxSize (SERVER-25027)
The maximum number of connections to hold to a given host at any time. Default is
unlimited
Controls simultaneous operations from a mongos to a host (Poolsize*PoolMaxSize)
ShardingTaskExecutorPoolMinSize (SERVER-25027)
The minimum number of connections to hold to a given host at any time. Default is 1
Increasing the value will increase the number of open connections per mongod
(PoolMinSize*PoolSize*MONGOS)

161
ShardingTaskExecutorRefreshRequirement (SERVER-25027)
How long mongos will wait before attempting to heartbeat an idle connection in the pool.
Default 1 minute
Increasing the value will reduce the heartbeat traffic
Lower the value may decrease the connection failures
ShardingTaskExecutorRefreshTimeout (SERVER-25027)
How long mongos will wait before timing out a heartbeat. Default 20 seconds
Tuning this value up may help if heartbeats taking too long.

162
ShardingTaskExecutorPoolMaxConnecting (SERVER-29237)
Limit the rate at which mongosnodes add connectons to connection pools
Only N connections can be in the processing state at the same time (setup/refresh state)
Avoid overwhelming mongod nodes with connection requests
Per connection pool , C*N can be in setup/refresh
Our patch for SERVER-26654
setParameter:
taskExecutorPoolSize: 8
ShardingTaskExecutorPoolRefreshTimeoutMS: 600000
ShardingTaskExecutorPoolHostTimeoutMS: 86400000
ShardingTaskExecutorPoolMinSize: 100
ShardingTaskExecutorPoolMaxConnecting: 25

Backup & Disaster
Recovery ● Methods
● Topologies
163

mongodump
164
This utility is provided as part of the MongoDB binaries that creates a binary backup of
your database(s) or collection(s).
● Preserves data integrity when compared to mongoexport for all BSON types
● Recommended for stand-a-lone mongod and replica sets
○ It does work with sharded clusters but be cautious of cluster and data
consistency
● When dumping a database (--database) or collection (--collection) you have the
option of passing a query (--query) and a read preference (--readPreference) for
more granular control
● Because mongodump can be time consuming --oplog is recommended so the
operations for the duration of the dump are also captured
● To restore the output from mongodump you use mongorestore, not mongoimport

mongorestore
165
This utility is provided as part of the MongoDB binaries that restores a binary backup of
your database(s) or collection(s).
● Preserves data integrity when compared to mongoimport for all BSON types
● When restoring a database (--database) or collection (--collection) you have the
option of removing (--drop) collections contained in the backup and the option to
replay the oplog events (--oplogReplay) to a point in time
● Be mindful of destination of the restore, restoring (i.e. inserting) does add
additional work to the already existing workload in additional to the number of
collections being restored in parallel (--numParallelCollections)
● MongoDB will also restore indexes in the foreground (blocking), indexes can be
created in advance or skipped (--noIndexRestore) depending on the scenario

Data Directory Backup
166
While a mongod process is stopped a filesystem copy or snapshot can be performed to
make a consistent copy of the replica set member.
● Replica set required to prevent interruption
● Works for both WiredTiger and MMAPv1 engines
● Optionally you can use hidden secondary with no votes to perform this process to
prevent affectioning quorum
● For MMAPv1 this will copy fragmentation which increases the size of the backup
Alternatively fsyncLock() can be used to flush all pending writes and lock the mongod
process. At this time a file system copy can be performed, after the copy has completed
fsyncUnlock() can be used to return to normal operation.

Percona Hot Backup
167
For Percona Server running WiredTiger or RocksDB backups can be taken using an
administrative backup command.
● Can be executed on a mongod as a non-blocking operation
● Creates a backup of an the entire dbPath
○ Similar to data directory backup extra storage capacity required
> use admin
switched to db admin
> db.runCommand({createBackup: 1, backupDir: "/tmp/backup"})
{ "ok" : 1 }

Replicated DR
168
s1 s2
s1 s2
DC or AZ
Interconnect
Preferred Zone
Secondary Zone
● Primary zones replicas
configured with a higher
priority and more votes for
election purposes
● All secondary zones
replica members are
secondaries until
secondary zone is utilized
● An rs.reconfig() is required
to remove a preferred
zone member and level
votes to elect a primary in
a failover event

169
Lab 05
Lab 06
Zone Management and
Backup / Restore

Use Cases
● Social Application
● Aggregation
● Caching
● Retail
● Time Series
170

Use Cases - Caching
171
Use Case: A url/content cache
Schema example:
{
_id:ObjectId,
url:<string>
response_body:<string>
ts:<timestamp>
}
Operations:
- Insert urls into cache
- Invalidate cache (for example TTL index)
- Read from cache (Read the latest url entry based on ts)

Use Cases - Caching
172
Shard key Candidates:
{url:1}
- Advantages: Scale Reads Writes
- Disadvantages: Key length, may create empty chunks
{url:hashed}
- Advantages: Scale Reads Writes
- Disadvantages: CPU cycles to convert url to hashed, orderBy is not supported, may create
empty chunks
Hash the {url:1} on the application
- Advantages: Scale Reads/Writes, One index less
- Disadvantages: Num of initial chunks not supported, may create empty chunks

Use Cases – Social app
173
Use Case: A social app , a users “interact” with other users
Schema example:
{
_id:ObjectId,
toUser :<string> or <ObjectId> or <number>
fromUser :<string> or <ObjectId> or <number>
interaction:<string>
ts:<timestamp>
}
Operations:
- Insert interaction
- Remove interaction
- Read interaction (fromUser and toUser)

174
{toUser:1} or {fromUser:1}
- Advantages: Scale a portion of Reads
- Disadvantages: Write Hotspots, Jumbo Chunks, Empty Chunks
{toUser:1, fromUser:1}
- Advantages: Scale a portion of Reads, Improved Write distribution but not perfect
- Disadvantages: Frequent splits & moves, Empty Chunks
{fromUser:1, toUser:1}
- Advantages: Scale a portion of Reads, Improved Write distribution but not perfect
- Disadvantages: Frequent splits & moves, Empty Chunks

175
Shard key Candidates(cont.):
{toUser+fromUser:”hashed”}
- Advantages: Scale writes, Avoid Empty and Jumbo Chunks
- Disadvantages: All queries are Scatter-Gather but one query type
- Similar to _id:”hashed”
Perfect example that there is no perfect shard key:
- Application behavior will point us to the shard key
- Pre-define user_ids and split might not be effective

Use Cases – Email app
176
Use Case: Provide inbox access to users
Schema example:
{
_id:ObjectId,
recipient :<string> or <ObjectId> or <number>
sender :<string>
title:<string>
body:<string>
ts:<timestamp>
}
Operations:
- Read emails
- Insert emails
- Delete emails

Use Cases – Email app
177
{recipient:1}
- Advantages: All Reads will access one shard
{recipient:1, sender:1}
- Advantages: Some Reads will access one shard
- Disadvantages: Write Hotspots, Empty Chunks
Introduce buckets (an auto-increment number per user)
{recipient:1, bucket:1}
- Advantages: Reads will access one shard for every iteration
- Disadvantages: Write Hotspots, Empty Chunks

Use Cases – Time-series data
178
Use Case: A server-log collector
Schema example:
{
_id:ObjectId,
host :<string> or <ObjectId> or <number>
type:<string> or <number>
message :<string>
ts:<timestamp>
}
Operations:
- Insert logs
- Archive logs
- Read logs (host,type,<range date>)

179
{host:1}
- Advantages: Targets Reads
{host:hashed}
- Advantages: Targets Reads
{host:1,type:1,ts:1}
- Advantages: Decent cardinality, may Target Reads
- Disadvantages: Monotonically increased per host,type, Write Hotspots , Empty Chunks

180
Shard key Candidates (cont.):
{host:1,type:1, date_bucket:1}
- Advantages: Decent cardinality, Read targeting
- Disadvantages: Monotonically increased per host, Write Hotspots , Empty Chunks
Pre-create and distribute chunks easily
Merge host, type, date_bucket on the application layer and use hashed key - Queries {shard_key,
ts:<range>}

Use Cases – Retail app
181
Use Case: A product catalog
{
_id:ObjectId,
product :<string>
description:<string>
category:<string>
dimensions:<subdocument>
weight: <float>
price:<float>
rating:<float>
shipping :<string>
released_date:<ISODate>
related_products:<array>
}

182
Operations:
- Insert Products
- Update Products
- Delete Products
- Read from Products - various query patterns
{category:1}
- Advantages: Read targeting for most of the queries
- Disadvantages: Jumbo Chunks, Read hotspots

183
Shard key Candidates (cont.):
{category:1, price:1}
- Advantages: Read targeting for most of the queries
- Disadvantages: Jumbo Chunks, Read hotspots
Tag documents with price ranges 0-100,101-200 etc
Use a shard key of the type {category:1,price_range:1,release_date:1}
Try to involve as much of the query criteria on your shard key, but careful of Read hotspots
For example rating as users tend to search for 5-stars

Questions?
184
or…

185
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team?
https://www.objectrocket.com/careers
Reach out to us directly at careers@objectrocket.com

Thank you!
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Austin, TX 78701
Support:
1-800-961-4454
Sales:
1-888-440-3242
186

MongoDB – Sharded cluster tutorial - Percona Europe 2017

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