MongoDB 2.4 and spring data

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MongoDB 2.4 and Spring
Data
June 10h, 2013

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Who Am I?
 Solutions Architect with ICF Ironworks
 Part-time Adjunct Professor
 Started with HTML and Lotus Notes in 1992
• In the interim there was C, C++, VB, Lotus Script, PERL, LabVIEW,
Oracle, MS SQL Server, etc.
 Not so much an Early Adopter as much as a Fast Follower of Java
Technologies
 Alphabet Soup (MCSE, ICAAD, ICASA, SCJP, SCJD, PMP, CSM)
 LinkedIn: http://www.linkedin.com/in/iamjimmyray
 Blog: http://jimmyraywv.blogspot.com/ Avoiding Tech-sand

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Tonight’s Agenda
 Quick introduction to NoSQL and MongoDB
• Configuration
• MongoView
 Introduction to Spring Data and MongoDB support
• Spring Data and MongoDB configuration
• Templates
• Repositories
• Query Method Conventions
• Custom Finders
• Customizing Repositories
• Metadata Mapping (including nested docs and DBRef)
• Aggregation Functions
• GridFS File Storage
• Indexes

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What is NoSQL?
 Official: Not Only SQL
• In reality, it may or may not use SQL*, at least in its truest form
• Varies from the traditional RDBMS approach of the last few decades
• Not necessarily a replacement for RDBMS; more of a solution for more
specific needs where is RDBMS is not a great fit
• Content Management (including CDNs), document storage, object storage,
graph, etc.
 It means different things to different folks.
• It really comes down to a different way to view our data domains for
more effective storage, retrieval, and analysis…albeit with tradeoffs
that effect our design decisions.

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From NoSQL-Database.org
“NoSQL DEFINITION: Next Generation Databases mostly
addressing some of the points: being non-relational, distributed,
open-source and horizontally scalable. The original intention has
been modern web-scale databases. The movement began early
2009 and is growing rapidly. Often more characteristics apply such
as: schema-free, easy replication support, simple API, eventually
consistent / BASE (not ACID), a huge amount of data and more.”

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Some NoSQL Flavors
 Document Centric
• MongoDB
• Couchbase
 Wide Column/Column
Families
• Cassandra
• Hadoop Hbase
 XML (JSON, etc.)
• MarkLogic
 Graph
• Neo4J
 Key/Value Stores
• Redis
 Object
• DB4O
 Other
• LotusNotes/Domino

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Why MongoDB
 Open Source
 Multiple platforms (Linux, Win, Solaris, Apple) and Language Drivers
 Explicitly de-normalized
 Document-centric and Schema-less (for the most part)
 Fast (low latency)
• Fast access to data
• Low CPU overhead
 Ease of scalability (replica sets), auto-sharding
 Manages complex and polymorphic data
 Great for CDN and document-based SOA solutions
 Great for location-based and geospatial data solutions

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Why MongoDB (more)
 Because of schema-less approach is more flexible, MongoDB is
intrinsically ready for iterative (Agile) projects.
 Eliminates “impedance-mismatching” with typical RDBMS solutions
 “How do I model my object/document based application in 3NF?”
 If you are already familiar with JavaScript and JSON, MongoDB storage
and document representation is easier to understand.
 Near-real-time data aggregation support
 10gen has been responsive to the MongoDB community

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What is schema-less?
 A.K.A. schema-free, 10gen says “flexible-schema”
 It means that MongoDB does not enforce a column data type on
the fields within your document, nor does it confine your document
to specific columns defined in a table definition.
 The schema “can be” actually controlled via the application API
layers and is implied by the “shape” (content) of your documents.
 This means that different documents in the same collection can
have different fields.
• So the schema is flexible in that way
• Only the _id field is mandatory in all documents.
 Requires more rigor on the application side.

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Is MongoDB really schema-less?
 Technically no.
 There is the System Catalog of system collections
• <database>.system.namespaces
• <database>.system.indexes
• <database>.system.profile
• <database>.system.users
 And…because of the nature of how docs are stored in collections
(JSON/BSON), field labels are store in every doc*

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Schema tips
 MongoDB has ObjectID, can be placed in _id
• If you have a natural unique ID, use that instead
 De-normalize when needed (you must know MongoDB restrictions)
• For example: Compound indexes cannot contain parallel arrays
 Create indexes that cover queries
• Mongo only uses one index at a time for a query
• Watch out for sorts
• What out for field sequence in compound indexes.
 Reduce size of collections (watch out for label sizes)

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MongoDB Data Modeling and Node Setups
 Schema Design is still important
 Understand your concerns
• Do you have read-intensive or write-intensive data
• Document embedding (fastest and atomic) vs. references (normalized)
• Atomicity – Document Level Only
• Can use 2-Phase Commit Pattern
• Data Durability
• Not “truly” available in a single-server setup
• Requires write concern tuning
• Need sharding and/or replicas
 10gen offers patterns and documentation:
• http://docs.mongodb.org/manual/core/data-modeling/

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Why Not MongoDB
 High speed and deterministic transactions:
• Banking and accounting
• See MongoDB Global Write Locking
– Improved by better yielding in 2.0
 Where SQL is absolutely required
• Where true Joins are needed*
 Traditional non-real-time data warehousing ops*
 If your organization lacks the controls and rigor to place schema
and document definition at the application level without
compromising data integrity**

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MongoDB
 Was designed to overcome some of the performance
shortcomings of RDBMS
 Some Features
• Memory Mapped IO (32bit vs. 64bit)
• Fast Querying (atomic operations, embedded data)
• In place updates (physical writes lag in-memory changes)
• Depends on Write Concern settings
• Full Index support (including compound indexes, text, spherical)
• Replication/High Availability (see CAP Theorem)
• Auto Sharding (range-based portioning, based on shard key) for
scalability
• Aggregation, MapReduce, geo-spatial
• GridFS

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MongoDB – In Place Updates
 No need to get document from the server, just send update
 Physical disk writes lag in-memory changes.
• Lag depends on Write-Concerns (Write-through)
• Multiple writes in memory can occur before the object is updated on
disk
 MongoDB uses an adaptive allocation algorithm for storing its
objects.
• If an object changes and fits in it’s current location, it stays there.
• However, if it is now larger, it is moved to a new location. This moving
is expensive for index updates
• MongoDB looks at collections and based on how many times items
grow within a collection, MongoDB calculates a padding factor that trys
to account for object growth
• This minimizes object relocation

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MongoDB – A Word About Sharding…
 Need to choose the right key
• Easily divisible (“splittable”– see cardinality) so that Mongo can
distribute data among shards
• “all documents that have the same value in the state field must reside on the
same shard” – 10Gen
• Enable distributed write operations between cluster nodes
• Prevents single-shard bottle-necking
• Make it possible for “Mongos” return most query operations from
multiple shards (or single shard if you can guarantee contiguous
storage in that shard**)
• Distribute write evenly among mongos
• Minimize disk seeks per mongos
• “users will generally have a unique value for this field (Phone)
– MongoDB will be able to split as many chunks as needed” – 10Gen
 Watch out for the need to perform range queries.

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MongoDB – Cardinality…
 In most cases, when sharding for performance, you want higher
cardinality to allow chunks of data to be split among shards
• Example: Address data components
• State – Low Cardinality
• ZipCode – Potentially low or high, depending population
• Phone Number – High Cardinality
 High cardinality is a good start for sharding, but..
• …it does not guarantee query isolation
• …it does not guarantee write scaling
• Consider computed keys (Hashed , MD5, etc.)

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CAP Theorem
 Consistency – all nodes see the same data at the same time
 Availability – all requests receive responses, guaranteed
 Partition Tolerance (network partition tolerance)
 The theorem states that you can never have all three, so you plan
for two and make the best of the third.
• For example: Perhaps “eventual consistency” is OK for a CDN
application.
• For large scalability, you would need partitioning. That leaves C & A to
choose from
• Would you ever choose consistency over availability?
 How does CLOUD implementations change this?

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Example MongoDB Isolated Setup

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Container Models: RDBMS vs. MongoDB
 RDBMS: Servers > Databases > Schemas > Tables > Rows
• Joins, Group By, ACID
 MongoDB: Servers > Databases > Collections > Documents
• No Joins**
• Instead: Db References (Linking) and Nested Documents (Embedding)

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MongoDB Collections
 Schema-less
 Can have up to 24000 (according to 10gen)
• Cheap to resource
 Contain documents (…of varying shapes)
• 100 nesting levels (version 2.2)
 Are namespaces, like indexes
 Can be “Capped”
• Limited in max size with rotating overwrites of oldest entries
• Logging anyone?
• Example: MongoDB oplog
 TTL Collections

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MongoDB Documents
 JSON (what you see)
• Actually BSON (Internal - Binary JSON - http://bsonspec.org/)
 Elements are name/value pairs
 16 MB maximum size
 What you see is what is stored
• No default fields (columns)

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JSON Syntax
 Curly braces are used for documents/objects – {…}
 Square brackets are used for arrays – […]
 Colons are used to link keys to values – key:value
 Commas are used to separate multiple objects or elements or
key/value pairs – {ke1:value1, key2:value2…}
 JavaScript has how many data types?
• 6 – Text, Number, Array, Object, null, Boolean

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JSON Syntax Example
{
“application”:”HR System”,
"users" : [{"name" : "bill",“age" : 60},
{"name" : "fred","age" : 29}]
}

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Why BSON?
 Adds data types that JSON did not support – (ISO Dates, ObjectId,
etc.)
 Optimized for performance
 Adds compression
 http://bsonspec.org/#/specification

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MongoDB Install
 Extract MongoDB
 Build config file, or use startup script
• Need dbpath configured
• Need REST configured for Web Admin tool
 Start Mongod (daemon) process
 Use Shell (mongo) to access your database
 Use MongoVUE (or other) for GUI access and to learn shell
commands

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Mongo Shell
 In Windows, mongo.exe
 Interactive JavaScript shell to mongod
 Command-line interface to MongoDB (sort of like SQL*Plus for
Oracle)
 JavaScript Interpreter, behaves like a read-eval-print loop
 Can be run without database connection (use –nodb)
 Uses a fluent API with lazy cursor evaluation
• db.locations.find({state:'MN'},{city:1,state:1,_id:0}).sort({city:-
1}).limit(5).toArray();

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MongoVUE
 GUI around MongoDB Shell
 Current version 1.61 (May 2013)
 Makes it easy to learn MongoDB Shell commands
• db.employee.find({ "lastName" : "Smith", "firstName" : "John"
}).limit(50);
• show collections
 Not sure if development is continuing, but very handy still.
 Demo…

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Mongo Explorer
 Silverlight GUI
 Current development has stopped – for now.
 http://mongoexplorer.com/
 Demo…

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Web Admin Interface
 Localhost:<mongod port + 1000>
 Quick stats viewer
 Run commands
 Demo
 There is also Sleepy Mongoose
• http://www.kchodorow.com/blog/2010/02/22/sleepy-mongoose-a-
mongodb-rest-interface/

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Other MongoDB Tools
 Edda – Log Visualizer
• http://blog.mongodb.org/post/28053108398/edda-a-log-visualizer-for-
mongodb
• Requires Python
 MongoDB Monitoring Service
• Free Cloud based service that monitors MongoDB instances via
configrued agents.
• Requires Python
• http://www.10gen.com/products/mongodb-monitoring-service
 Splunk
• www.splunk.com

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MongoImport
 Binary mongoimport
 Syntax: mongoimport --stopOnError --port 29009 --db geo --
collection geos --file
C:UserDataDocsJUGsTwinCitieszips.json
 Don’t use for backup or restore in production
• Use mongodump and mongorestore

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Spring Data
 Large Spring project with many subprojects
• Category: Document Stores, Subproject MongoDB
 “…aims to provide a familiar and consistent Spring-based
programming model…”
 Like other Spring projects, Data is POJO Oriented
 For MongoDB, provides high-level API and access to low-level API
for managing MongoDB documents.
 Provides annotation-driven meta-mapping
 Will allow you into bowels of API if you choose to hang out there

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Spring Data MongoDB Templates
 Implements MongoOperations (mongoOps) interface
• mongoOps defines the basic set of MongoDB operations for the Spring
Data API.
• Wraps the lower-level MongoDB API
 Provides access to the lower-level API
 Provides foundation for upper-level Repository API.
 Demo

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Spring Data MongoDB Templates - Configuration
 See mongo-config.xml

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 Or…see the config class

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Spring Data Repositories
 Convenience for data access
• Spring does ALL the work (unless you customize)
 Convention over configuration
• Uses a method-naming convention that Spring interprets during
implementation
 Hides complexities of Spring Data templates and underlying API
 Builds implementation for you based on interface design
• Implementation is built during Spring container load.
 Is typed (parameterized via generics) to the model objects you want to
store.
• When extending MongoRepository
• Otherwise uses @RepositoryDefinition annotation
 Demo

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Spring Data Bulk Inserts
 All things being equal, bulk inserts in MongoDB can be faster than
inserting one record at a time, if you have batch inserts to perform.
 As of MongoDB 1.8, the max BSON size of a batch insert was
increased from 4MB to 16MB
• You can check this with the shell command: db.isMaster() or
mongo.getMaxBsonObjectSize() in the Java API
 Batch sizes can be tuned for performance
 Demo

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Transformers
 Does the “heavy lifting” by preparing MongoDB objects for
insertion
 Transforms Java domain objects into MongoDB DBObjects.
 Demo

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Converters
 For read and write, overrides default mapping of Java objects to
MongoDB documents
 Implements the Spring…Converter interface
 Registered with MongoDB configuration in Spring context
 Handy when integrating MongoDB to existing application.
 Can be used to remove “_class” field

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Spring Data Meta Mapping
 Annotation-driven mapping of model object fields to Spring Data
elements in specific database dialect. – Demo

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MongoDB DBRef
 Optional
 Instead of nesting documents
 Have to save the “referenced” document first, so that DBRef exists
before adding it to the “parent” document

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MongoDB Custom Spring Data Repositories
 Hooks into Spring Data bean type hierarchy that allows you to add
functionality to repositories
 Important: You must write the implementation for part of this
custom repository
 And…your Spring Data repository interface must extend this
custom interface, along with the appropriate Spring Data repository
 Demo

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Creating a Custom Repository
 Write an interface for the custom methods
 Write the implementation for that interface
 Write the traditional Spring Data Repository application interface,
extending the appropriate Spring Data interface and the (above)
custom interface
 When Spring starts, it will implement the Spring Data Repository
normally, and include the custom implementation as well.

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MongoDB Queries
 In mongos using JS: db.collection.find( <query>, <projection> )
• Use the projection to limit fields returned, and therefore network traffic
 Example: db["employees"].find({"title":"Senior Engineer"})
 Or: db.employees.find({"title":"Senior Engineer"},{"_id":0})
 Or: db.employees.find({"title":"Senior
Engineer"},{"_id":0,"title":1})
 In Java use DBObject or Spring Data Query for mapping queries.
 You can include and exclude fields in the projection argument.
• You either include (1) or exclude (0)
• You can not include and exclude in the same projection, except for the
“_id” field.

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DBObject and BasicDBObject
 For the Mongo Java driver, DBObject is the Interface,
BasicDBObject is the class
 This is essentially a map with additional Mongo functionality
• See partial objects when up-serting
 DBObject is used to build commands, queries, projections, and
documents
 DBObjects are used to build out the JS queries that would normally
run in the shell. Each {…} is a potential DBObject.

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MongoDB Queries – And & Or
 Comma denotes “and”, and you can use $and
• db.employees.find({"title":"Senior
Engineer","lastName":"Bashian"},{"_id":0,"title":1})
 For Or, you must use the $or operator
• db.employees.find({$or:[{"lastName":"Bashian"},{"lastName":"Baik"}]},{"_id":0,
"title":1,"lastName":1})
 In Java, use DBObjects and ArrayLists…
• Nest or/and ArrayLists for compound queries
 Or use the Spring Data Query and Criteria classes with or criteria
 Also see QueryBuilder class
 Demo

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MongoDB Array Queries
 db.misc.insert({users:["jimmy", "griffin"]})
 db.misc.find({users:"griffin"})
• { "_id" : ObjectId("518a5b7e18aa54b5cf8fc333"), "users" : [
"jimmy", "griffin" ]}
 db.misc.find({users:{$elemMatch:{name:"jimmy",gender:"mal
e"}}})
 { "_id" : ObjectId("518a599818aa54b5cf8fc332"), "users" : [ {
"name" : "jimmy", "gender" : "male" }, { "name" :
"griffin", "gender": "male" } ] }

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MongoDB Array Updates
db.misc.insert({"users":[{"name":"jimmy","gender":"male"},{"n
ame":"griffin","gender":"male"}]})
db.misc.update({"_id":ObjectId("518276054e094734807395b6"),
"users.name":"jimmy"}, {$set:{"users.$.name":"george"}})
db.employees.update({products:"Softball"},
{$pull:{products:"Softball" }},false,true)
db.employees.find({products:"Softball"}).count()
0

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Does Field Exist
 $exists
 db.locations.find({user:{$exists:false}})
 Type “it” for more – iterates over documents - paging

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MongoDB Advanced Queries
 http://www.mongodb.org/display/DOCS/Advanced+Queries#Advan
cedQueries-%24all
 May use Mongo Java driver and BasicDBObjectBuilder
 Spring Data fluent API is much easier
 Demo - $in, $nin, $gt ($gte), $lt ($lte), $all, ranges

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MongoDB RegEx Queries
 In JS:
db.employees.find({ "title" : { "$regex" : "seNior EngIneer" ,
"$options" : "i"}})
 In Java use java.util.regex.Pattern

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Optimizing Queries
 Use $hint or hint() in JS to tell MongoDB to use specific index
 Use hint() in Java API with fluent API
 Use $explain or explain() to see MongoDB query explain plan
• Number of scanned objects should be close to the number of returned
objects

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MongoDB Aggregation Functions
 Aggregation Framework
 Map/Reduce - Demo
 Distinct - Demo
 Group - Demo
• Similar to SQL Group By function
 Count
 Demo #7

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More Aggregation
 $unwind
• Useful command to convert arrays of objects, within documents, into
sub-documents that are then searchable by query.
db.depts.aggregate({"$project":{"employees":"$employees"}},{"$un
wind":"$employees"},{"$match":{"employees.lname":"Vural"}});
 Demo

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More Aggregation
 $unwind
• Useful command to convert arrays of objects, within documents, into
sub-documents that are then searchable by query.
db.depts.aggregate({"$project":{"employees":"$employees"}},{"$un
wind":"$employees"},{"$match":{"employees.lname":"Vural"}});
 Demo

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MongoDB GridFS
 “…specification for storing large files in MongoDB.”
 As the name implies, “Grid” allows the storage of very large files
divided across multiple MongoDB documents.
• Uses native BSON binary formats
 16MB per document
• Will be higher in future
 Large files added to GridFS get chunked and spread across
multiple documents.

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MongoDB GridFS
multiple documents.

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MongoDB GridFS
multiple documents.

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MongoDB Indexes
 Similar to RDBMS Indexes, Btree (support range queries)
 Can have many
 Can be compound
• Including indexes of array fields in document
 Makes searches, aggregates, and group functions faster
 Makes writes slower
 Sparse = true
• Only include documents in this index that actually contain a value in the
indexed field.

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Text Indexes
 Currently in BETA, as of 2.4, not recommended for
production…yet
 Requires enabled in mongod
• --setParameter textSearchEnabled=true
 In mongo (shelll)
• db["employees"].ensureIndex({"title":"text"})
• Index “title” field with text index

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Text Indexes
 Currently in BETA, as of 2.4, not recommended for
production…yet
 Requires enabled in mongod
• --setParameter textSearchEnabled=true
 In mongo (shelll)
• db["employees"].ensureIndex({"title":"text"})
• Index “title” field with text index

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GEO Spatial Operations
 One of MongoDB’s sweet spots
 Used to store, index, search on geo-spatial data for GIS
operations.
 Requires special indexes, 2d and 2dsphere (new with 2.4)
 Requires Longitude and Latitude (in that order) coordinates
contained in double precision array within documents.
 Demo

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GEO Spatial Operations
 One of MongoDB’s sweet spots
 Used to store, index, search on geo-spatial data for GIS
operations.
 Requires special indexes, 2d and 2dsphere (new with 2.4)
 Requires Longitude and Latitude (in that order) coordinates
contained in double precision array within documents.
 Demo

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Query Pagination
 Use Spring Data and QueryDSL - http://www.querydsl.com/
 Modify Spring Data repo extend QueryDslPredicateExecutor
 Add appropriate Maven POM entries for QueryDSL
 Use Page and PageRequest objects to page through result sets
 QueryDSL will create Q<MODEL> Java classes
• Precludes developers from righting pagination code

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Save vs. Update
 Java driver save() saves entire document.
 Use “update” to save time and bandwidth, and possibly indexing.
• Spring Data is slightly slower than lower level mongo Java driver
• Spring data fluent API is very helpful.

74
MongoDB Security
 http://www.mongodb.org/display/DOCS/Security+and+Authenticati
on
 Default is trusted mode, no security
 --auth
 --keyfile
• Replica sets require this option
 New with 2.4:
• Kerberos Support

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MongoDB Auth Security
 Use –auth switch to enable
 Create users with roles
 Use db.authenticate in the code (if need be)

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MongoDB Auth Security with Spring
 May need to add credentials to Spring MongoDB config
 Do not authenticate twice
java.lang.IllegalStateException: can't call authenticate twice on
the same DBObject
at com.mongodb.DB.authenticate(DB.java:476)

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MongoDB Write Concerns
 Describes quality of writes (or write assurances) to MongoDB
 Application (MongoDB client) is concerned with this quality
 Write concerns describe the durability of a write, and can be tuned
based on application and data needs
 Adjusting write concerns can have an affect (maybe deleterious)
on write performance.

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MongoDB Encryption
 MongoDB does not support data encryption, per se
 Use application-level encryption and store encrypted data in BSON
fields
 Or…use TDE (Transparent Data Encryption) from Gazzang
• http://www.gazzang.com/encrypt-mongodb

79
MongoDB Licensing
 Database
• “Free Software Foundation's GNU AGPL v3.0.” – 10gen
• “Commercial licenses are also available from 10gen, including free
evaluation licenses.” – 10gen
 Drivers (API):
• “mongodb.org supported drivers: Apache License v2.0.” – 10gen
• “Third parties have created drivers too; licenses will vary there.” –
10gen

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MongoDB 2.2
 Drop-in replacement for 1.8 and 2.0.x
 Aggregation without Map Reduce
 TTL Collections (alternative to Capped Collections)
 Tag-aware Sharding
 http://docs.mongodb.org/manual/release-notes/2.2/

81
MongoDB 2.4
 Text Search
• Must be enabled, off by default
• Introduces considerable overhead for processing and storage
• Not recommended for PROD systems; it is a BETA feature.
 Hashed Index and sharding
 http://docs.mongodb.org/manual/release-notes/2.4/

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New JavaScript Engine – V8
 MongoDB 2.4 uses the Google V8 JavaScript Engine
• https://code.google.com/p/v8/
• Open source, written in C++,
• High performance, with improved concurrency for multiple JavaScript
operations in MongoDB at the same time.

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Some Useful Commands
 use <db> - connects to a DB
 use admin; db.runCommand({top:1})
• Returns info about collection activity
 db.currentOp() – returns info about operations currently running in mongo db
 db.serverStatus()
 db.hostInfo()
 db.isMaster()
 db.runCommand({"buildInfo":1})
 it
 db.runCommand({touch:"employees",data:true,index:true})
• { "ok" : 1 }

84
Helpful Links
 Spring Data MongoDB - Reference Documentation: http://static.springsource.org/spring-
data/data-mongodb/docs/1.0.2.RELEASE/reference/html/
 http://nosql-database.org/
 www.mongodb.org
 http://www.mongodb.org/display/DOCS/Java+Language+Center
 http://www.mongodb.org/display/DOCS/Books
 http://openmymind.net/2011/3/28/The-Little-MongoDB-Book/
 http://jimmyraywv.blogspot.com/2012/05/mongodb-and-spring-data.html
 http://jimmyraywv.blogspot.com/2012/04/mongodb-jongo-and-morphia.html
 https://www.10gen.com/presentations/webinar/online-conference-deep-dive-mongodb
 http://docs.mongodb.org/manual/faq/developers/#faq-developers-query-for-nulls

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