A Python Petting Zoo

A Python Petting Zoo
Python and ZooKeeper
Devon Jones
Senior Software Engineer

knewt.ly/pettingzoo-slides

Knewton is an education technology company with a goal of
bringing adaptive education to the masses. Knewton makes it
possible to break courses into tiny parts that are delivered to
each student as personalized, real-time recommendations.

Knewton recommends the best work for an individual learner by
calculating data on what we know about a student, similar
students, the learning objective, and the content itself at a given
point in time.

The Knewton platform today has tens of thousands of students,
will have over 600k students starting Sept. 2012, and will soon
have millions of students.

What is this about?
● What is ZooKeeper, how is it useful
● State of ZooKeeper on python
● The release of PettingZoo, Knewton's
ZooKeeper recipes for managing a
distributed machine learning cluster

What is ZooKeeper?
According to ZooKeeper's Apache Site:

ZooKeeper is a centralized service for maintaining configuration information,
naming, providing distributed synchronization, and providing group services. All
of these kinds of services are used in some form or another by distributed
applications. Each time they are implemented there is a lot of work that goes
into fixing the bugs and race conditions that are inevitable. Because of the
difficulty of implementing these kinds of services, applications initially usually
skimp on them, which make them brittle in the presence of change and difficult
to manage. Even when done correctly, different implementations of these
services lead to management complexity when the applications are deployed.

Right, but what is it?

What is ZooKeeper?
ZooKeeper is a distributed filesystem based on
the PAXOS algorithim with a few valuable
features.
● No single point of failure
● Strictly ordered, observable state
● Events
● Sequential and ephemeral primitives
● ACLs

PAXOS
http://groups.csail.mit.edu/tds/papers/DePrisco/WDAG97.
pdf
● Distributed Finite State Machine
● Pairwise connections among hosts
● Rounds of Consensus Propositions
conducted by leader
● Time bound contracts on interactions
● Useful to have internal concurrency
contracts

What is ZooKeeper for?
ZooKeeper is a platform for creating protocols
for synchronization of distributed systems.
Uses include:
● Distributed configuration
● Queues
● Implementation of distributed concurrency
primitives such as locks, barriers, latches,
counters, etc.
In short, it's a system for managing shared
state between distributed systems.

Consistency Guarantees
ZooKeeper makes a number of promises:
● Sequential Consistency
● Atomicity
● Single System Image
● Reliability
● Timeliness

Watches
● Can be set by any read operation
● Will fire an event to the client who set them
once and only once
● Can be set on nodes or on their data.
● Will always be sent to you in a fixed order

Sequential Nodes
● Appends a monotonically increasing number
to the end of the znode (file)
● Can be used directly for leader election
● Provides communication of server's view of
ordering

Ephemeral Nodes
● Only exist for the life of a connection
● If your connection does not respond to a
keepalive request, will disappear
● Used to ensure reliability against service
disruption for most recipes
● Used to trigger events, such as
reconfiguration if a service goes down that
published discovery configs

Recipes
ZooKeeper's documentation explains how to
implement a number of recipes:
● Barriers
● Locks
● Queues
● Counters
● Two Phase Commit
● Leader Election

Recipes
As a community, we need well tested versions
of these recipes as well as other valuable
protocols built on ZooKeeper

State of ZooKeeper
● Very low level
● Coding for it is very complex
● Lots of edge cases
● ZooKeeper needs high level, well tested
libraries
● Very few complete, high level solutions exist

ZooKeeper Libraries
The first high level library with significant recipe
implementations has emerged from Netflix. It's
name is Curator.

Unfortunately for us, it's in Java, not Python.

Curator
https://github.com/Netfix/curator
● State
○ High level api
○ Non-Resilient Client
○ Documentation
○ Tests, Embeddable ZooKeeper for testing
● Recipes
○ Leader Latch, Leader Election
○ Multiple Locks and Semaphores
○ Multiple Queues
○ Barrier, Double Barrier
○ Shared Counter/Distributed Atomic Long

State of ZooKeeper on Python
● Current state is in a lot of flux
● Went from only low level bindings to a
number of incomplete bindings in first half of
2012
● So far nothing like Curator has emerged (but
it appears to be brewing)

One of the top ranked results for 'Python ZooKeeper'

Summary of Python ZooKeeper
bindings
● There are about 10 presently
● Many suffer from not handling known edge
cases in ZooKeeper
● Some suffer problems with resilient
connections
● The following is derived from the python
ZooKeeper binding census of Ben Bangert

Official Bindings
● State
○ Complete access to the ZooKeeper C bindings
○ Full of sharp edges
○ Not a resilient client
○ No recipes
○ Threads communication with ZooKeeper in a C
thread
○ Foundation for most other libraries
○ Very low level

Bindings
Twisted:
● txzookeeper

gevent:
● gevent-zookeeper
● kazoo

Bindings (Cont)
High level clients:
● zc.zk
● kazoo
● twitter zookeeper

Others:
● zkpython (low level)
● pykeeper (not resiliant)
● zoop (not resiliant)

Bindings (Cont)
Recipes only:
● zktools
● PettingZoo

State of ZooKeeper on Python:
Kind of a Mess
A project exists to merge some of the high level
bindings in an attempt to create a python
equivalent of Curator: https://github.
com/python-zk

Started by Ben Bangert to merge Kazoo & zc.
zk with an attempt to implement all Curator
recipes.

PettingZoo
https://github.com/Knewton/pettingzoo-python
● State
○ Relies on zc.zk
○ Documented, doc strings
○ Tests (mock ZooKeeper)
○ All recipes implemented in a Java version as well
● Recipes
○ Distributed Config
○ Distributed Bag
○ Leader Queue
○ Role Match

PettingZoo
● In heavy development
● Distributed Discovery, Distributed Bag are
well tested and used in production
● Leader Queue and Role Match are tested,
but undeployed
● PettingZoo will be ported to or merged with
the kazoo effort when it is ready

Our Problem
Need to be able to do stream processing of
observations of student interactions with course
material. This involves multiple models that
have interdependent parameters. This requires:
● Sharding along different axes dependent
upon the models
● Subscriptions between models for
parameters
● Dynamic reconfiguration of the environment
to deal with current load

Distributed Discovery
Allows services in a dynamic, distributed
environment to be able to be quickly alerted of
service address changes.
● Most service discovery recipes only contain
host:port, Distributed Discovery can share
arbitrary data as well (using yaml)
● Can handle load balancing through random
selection of config
● Handles rebalancing on pool change

How does this help us scale?
● Makes discovery of dependencies simple
● Adds to reliability of system by quickly
removing dead resources
● Makes dynamic reconfiguration simple as
additional resources become available

Distributed Discovery Example:
Write
from pettingzoo.discovery import write_distributed_config
from pettingzoo.utils import connect_to_zk
conn = connect_to_zk( 'localhost:2181' )
config = {
'header': {
'service_class' : 'kestrel', 'metadata' : {
'protocol' : 'memcached' , 'version': 1.0
}
},
'host': 'localhost' ,
'port': 22133
}
write_distributed_config(conn, 'kestrel', 'platform' , config)

Distributed Discovery Example:
Read
from pettingzoo.discovery import DistributedMultiDiscovery
dmd = DistributedMultiDiscovery(conn)
conf = None
def update_configs (path, updated_configs):
conf.update(updated_configs)
conf = dmd.load_config( 'kestrel', 'platform' , callback=update_configs)

Distributed Bag
Recipe for a distributed bag (dbag) that allows
processes to share a collection. Any
participant can post or remove data, alerting all
others.
● Used as a part of Role Match

● Useful for any case where

processes need to share
configuration determined at
runtime

● Can quickly alert processes as to who is
subscribing to them
● Reduces load by quickly yanking dead
subscriptions
● Provides event based subscriptions, making
implementation simpler

Distributed Bag
● Sequential items
Item
contain the actual data
Items
1Item
2Item ● Can be ephemeral
3 ● Clients set delete watch
on discrete items
<bag> ● Token is set to id of
highest item
● Clients set a child
Tokens
Token watch on the "Tokens"
3
node
● Can determine exact
adds and deletes with a
constant number of
messages per delta

Distributed Bag Example
import yaml
from pettingzoo.dbag import DistributedBag

...
dbag = DistributedBag(conn, '/some/data' )
docs = {}
def acb(cb_dbag, cb_id):
docs[cb_id] = cb_dbag.get(cb_id)
def rcb(cb_dbag, cb_id):
docs.remove(cb.id)

dbag.add_listeners(add_callback=acb, remove_callback=rcb)
add_id = dbag.add(yaml.load(document), ephemeral=True)
docs[add_id] = document

Leader Queue
Recipe is similar to Leader Election, but makes
it easy to monitor your spare capacity.
● Used in Role Match
● As services are ready to do work, they
create an ephemeral, sequential node in the
queue.
● Any member always knows if either they are
in the queue or at the front
● Watch lets leader know when it is elected

● Gives a convenient method of assigning
work
● Makes monitoring current excess capacity
easy

Leader Queue
● Candidates register with
sequential, ephemeral nodes
C_1
● Candidate sets delete watch
on predecessor
● Candidate is elected when it
is the smallest node
<queue> C_3 ● When elected, candidate
takes over its new role
● When ready, candidate
removes itself from the
queue
C_4
● Only one candidate needs to
call get_children upon any
node exiting

Leader Queue Usage Example
from pettingzoo.leader_queue import LeaderQueue, Candidate

class SomeCandidate (Candidate):
def on_elected (self):
<do something sexy>

leaderq = LeaderQueue(conn)
leaderq.add_candidate(SomeCandidate())

Role Match
Allows systems to expose needed, long lived
jobs, and for services to take over those jobs
until all are filled.
● Dbag used to expose jobs
● Leader queue used to hold applicants
● Records which jobs are presently held with
ephemeral node
● Lets a new process take over if a worker
dies
● We use it for sharding/segmentation to
dynamically adjust the shards as needed

● Core of our ability to dynamically adjust
shards
● Lets the controlling process adjust problem
spaces and have those tasks become
automatically filled
● Monitoring is easy to identify who is working
on what, when

Role Match

job
Distributed ● Leader monitors for
Bag
open jobs
● Job holder creates
an ephemeral
assignment
<match> applicant Leader Queue
● Assignment id
matches job id,
indicating that it is
Assgn
claimed
assignment
1
A_2

Future: Distributed Config
Next project is Distributed Config.
● Allows service config to be recorded and
changed with a yaml config
● Every process that connects creates a child
node of the appropriate service
● Any change in a child node's config
overrides the overall service config for that
process
● Any change of the parent or child fires a
watch to let the process know that it's config
has changed

Questions?
devon@knewton.com

knewt.ly/pettingzoo-slides

knewt.ly/pettingzoo

Appendix: Official Bindings
● State
○ Complete access to the ZooKeeper C bindings
○ Full of sharp edges
○ Not a resilient client
○ No recipes
○ Threads communication with ZooKeeper in a C
thread
○ Foundation for most other libraries
○ Very low level

Appendix: zkpython
https://github.com/duncf/zkpython/
improvements to a fork of the official bindings
● Status
○ Resilient Client
○ No docs beyond the official bindings
○ Good tests
● Recipes
○ Basic Lock (Using unique id rather than UUID)

Appendix: pykeeper
https://github.com/nkvoll/pykeeper
● State
○ Non-resilient Client (not resilient to errors)
○ High level client
○ Documented, No doc strings
○ Tests (Requires running ZooKeeper)

Appendix: zoop
https://github.com/davidmiller/zoop
● State
○ Doesn't handle node create edge-case
○ Doesn't handle retryable exceptions
○ Tests (Requires running ZooKeeper)
● Recipes
○ Revocable Lock (Doesn't handle create node edge-
case, uses a permanent node instead of ephemeral)

Appendix: txzookeeper
https://launchpad.net/txzookeeper
● State
○ Doc strings, no additional documentation
○ Supports twisted (only)
○ Well tested
● Recipes
○ Basic Lock (Not using UUID)
○ Queue
○ ReliableQueue
○ SerializedQueue

Appendix: twitter zookeeper
https://github.com/twitter/commons/tree/master/src/python/twitter/common/zookeeper

● State
○ Handles node create edge-case
○ Some documentation
○ Well Tested
○ Tied to a lot of twitter commons code, difficult to
extract
● Recipes
○ Service Registration/Discovery

Appendix: gevent-zookeeper
https://github.com/jrydberg/gevent-zookeeper/
● State
○ Supports gevent
○ No documentation
○ No tests

Appendix: Kazoo
https://github.com/nimbusproject/kazoo
● Status
○ Doc strings, no additional documentation
○ Supports gevent
○ Minimal tests
● Recipes
○ Basic Lock (Uses UUID properly)

Appendix: zc.zk
https://github.com/python-zk/zc.zk
● State
○ High level client
○ Higher level automatic watch functionality
○ Higher level restoration of ephemeral nodes
○ Tests (Mock ZooKeeper)
○ Documentation, no doc strings
● Recipes
○ Service Registration/Discovery

Appendix: zktools
https://github.com/mozilla-services/zktools
● State
○ Relies on zc.zk
○ Tests (Requires running ZooKeepe)
● Recipes
○ Shared Read/Write Locks
○ AsyncLock
○ Revocable Locks

Appendix: PettingZoo
https://github.com/Knewton/pettingzoo-python
● State
○ Relies on zc.zk
○ Tests (mock ZooKeeper)
○ All Recipes implemented in a java version as well
● Recipes
○ Distributed Config
○ Distributed Bag
○ Leader Queue
○ Role Match

A Python Petting Zoo

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