ZooKeeper Reading 12-01-2012

Some recent Zoo Keeper reading.

Overview

  • There is a leader in the cluster.
  • Hierarchical model of znodes which act like directories and files. Uses “/” as the path separator.
  • All in memory, high throughput low latency.
  • “The ZooKeeper implementation puts a premium on high performance, highly available, strictly ordered access.”
  • Transaction logs and snapshots on disk.
  • Clients hold a TCP connection for duplex messaging.
  • All updates have a globally ordered TxID.
  • Works best in read heavy workloads, think 10:1 R:W ratios.
  • znodes may have data and children.
  • znodes have a version number for their local (?) state.
  • Reads and writes on a znode are atomic with respect to the version of data.
  • znodes can be protected by an ACL.
  • Ephemeral znodes are deleted when the session that created them ends.
  • Clients can set a watch on znode that is triggered when it changes. Is this for the local data or local data and children ?
  • Guarantees:
    • Sequential Consistency - Updates from a client will be applied in the order that they were sent.
    • Atomicity - Updates either succeed or fail. No partial results.
    • Single System Image - A client will see the same view of the service regardless of the server that it connects to.
    • Reliability - Once an update has been applied, it will persist from that time forward until a client overwrites the update.
    • Timeliness - The clients view of the system is guaranteed to be up-to-date within a certain time bound.
  • Write to WAL before apply to in memory DB.
  • Reads are serviced by the local DB on the node, writes are services by an agreement protocol. Guess this is why it’s tuned for read.
  • Writes go to the leader, are then distributed to the other (follower) nodes. Local DB’s should never diverge.
  • Performance 3 servers should give between 20k/sec and 80k/sec requests depending on the read/write mix.
  • Reliability less than 200ms to elect a new leader, failure of a follower reduces throughput.
  • What’s the recover model for a follower that is down for a while ? Does this affect performance ? Answer (from the internals) If too many Proposals are missing a snapshot is sent.

ZooKeeper Internals

  • “At the heart of ZooKeeper is an atomic messaging system that keeps all of the servers in sync.”
  • Guarantees:
    • Reliable Delivery
    • Total Order
    • Causal Order
  • The messaging layer is build around FIFO channels between nodes, and relies on the properties of TCP for this. Specifically:
    • Ordered delivery
    • No message after close
  • The protocol is composed of:
    • Packet: a sequence of bytes sent through a FIFO channel
    • Proposal: a unit of agreement. Proposals are agreed upon by exchanging packets with a quorum of ZooKeeper servers. Most proposals contain messages, however the NEW_LEADER proposal is an example of a proposal that does not correspond to a message.
    • Message: a sequence of bytes to be atomically broadcast to all ZooKeeper servers. A message put into a proposal and agreed upon before it is delivered.
  • QUORUM is (n/2) +1 by default.
  • QUORUM can be majority quorums, weights, or a hierarchy of groups.
  • Proposals are stamped with the zxid and sent to all servers, a server ack’s when it is on persistent store. Messages in the proposal are then delivered.
  • zxid has two parts: the epoch and a counter. Implemented as a 64 bit int, high 32 bits are the epoch, low 32 are the count.
  • “The epoch number represents a change in leadership. Each time a new leader comes into power it will have its own epoch number.”
  • Messaging consists of two phases, Leader Activation and Active Messaging.
  • Leader Activation may appear to have worked but later fail when checking the invariant that a QUORUM of followers follow the same leader. During the election it must only hold with a high probability.
  • In Active Messaging:
    • Leader sends PROPOSE to all followers for a new proposal.
    • Followers commit to non-volatile storage and then ACK
    • Leader sends COMMIT to all followers once a QUOURM have ACK‘d.

Getting Started

  • Grab the latest distro and start a single node with bin/zkServer.sh start-foreground conf/zoo_sample.cfg
  • Fire up the command line interface with bin/zkCli.sh -server 127.0.0.1:2181 and work through the examples in the doc.

Zope ZooKeeper client for Python

  • Requires zc-zookeeper-static
  • zc-zookeeper-static is a wrapper around the C libs, it’s pretty low level. e.g. you get an int handle and pass that into methods, not OO. zc.zk ads an OO wrapper and some other stuff I cannot work out.
  • A lot of methods on the zc.zk.ZooKepper object are pass through to the zc-zookeeper-static package and do not have any docs. Check to docs on zookeeper for the function help. For example zc.zk.ZooKeeper.get has no docs and a crap (*arg, **kwargs) param list, look at zookeeper.get.

Get a connection

import zc.zk
zk = zc.zk.ZooKeeper('127.0.0.1:2181')

Get the children of a znode

In [6]: zk.get_children("/")
Out[6]: ['consumers', 'brokers', 'zookeeper', 'zk_test']
# some stuff from kafka there.

Get the properties of a znode

# Get a zc.zk.Properties for the path
# **NOTE:** This is heavy weight, for single reads use get_properties()
In [55]: p = zk.properties("/zookeeper")

In [56]: p.data
Out[56]: {}

In [58]: p.values()
Out[58]: []

# simple get_properties()
In [64]: zk.get_properties("/zk_test")
Out[64]: {'string_value': 'foo'}

#zv.zk assume node data is json
In [49]: zk.set("/zk_test", "foo")
Out[49]: 0

In [51]: p = zk.properties("/zk_test")

In [53]: p.values()
Out[53]: ['foo']

In [54]: p.data
Out[54]: {'string_value': 'foo'}

Tree operations

In [59]: zk.print_tree("/zookeeper")
/zookeeper
  /quota

znode operations

# create an ephemeral node

# must have an ACL this is an open one 
In [78]: acl = [{"perms" : zookeeper.PERM_ALL, "scheme" : "world", "id" : "anyone"}]

# Parent path must exist
In [85]: zk.create( "/fake/ephemeral", "some data", acl, zookeeper.EPHEMERAL)
...
NoNodeException: no node

In [84]: zk.create( "/zk_test/ephemeral", "some data", acl, zookeeper.EPHEMERAL)
Out[84]: '/zk_test/ephemeral'

# node now listed (locally) on the connection 
In [86]: zk.ephemeral
Out[86]: 
{'/zk_test/ephemeral': {'acl': [{'id': 'anyone',
                                 'perms': 31,
                                 'scheme': 'world'}],
                        'data': 'some data',
                        'flags': 1}}
                        
# View from the cluster
In [88]: zk.get_properties("/zk_test/ephemeral")
Out[88]: {'string_value': 'some data'}

In [89]: p = zk.properties("/zk_test/ephemeral")
In [91]: p.meta_data
Out[91]: 
{'aversion': 0,
 'ctime': 1326337991257L,
 'cversion': 0,
 'czxid': 1950L,
 'dataLength': 9,
 'ephemeralOwner': 86922380708675587L,
 'mtime': 1326337991257L,
 'mzxid': 1950L,
 'numChildren': 0,
 'pzxid': 1950L,
 'version': 0}

Watch for changes

In [8]: children = zk.children("/zk_test")

In [9]: def my_callback(node):
   ...:   print "Called with node: ", str(node)
   ...: 

In [11]: children(my_callback)
Called with node:  zc.zk.Children(0, /zk_test)
Out[11]: zc.zk.Children(0, /zk_test)

In [14]: acl = [{"perms" : zookeeper.PERM_ALL, "scheme" : "world", "id" : "anyone"}]

In [15]: zk.create( "/zk_test/ephemeral", "some data", acl, zookeeper.EPHEMERAL)
Out[15]: '/zk_test/ephemeral'
Called with node:  zc.zk.Children(0, /zk_test)    
zookeeper
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