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There are two choices for how you'd like your events delivered to you when using ZK, and you specify that choice using the :thread option to the callback, with the values :single and :per_callback.

The Short Short Version

  • The default is :single for backwards compatibility and stability.
  • :per_callback is probably the better choice for 1.9.x, jruby, and rubinius
  • If you're using 1.8.7, test :per_callback thoroughly before using it in production.

The Slightly Longer Explanation

Use :per_callback if:

  • If you're using ZK and doing a lot of asynchronous event processing, you would likely benefit from using the :per_callback option, which provides greater concurrency, and looser coupling between callbacks.
  • You're using 1.8.7 and your callbacks are fairly decoupled from one another.

Use :single if:

  • If you're using ZK mainly for Locker or for synchronous operations, the :per_callback option may not give an advantage, but it also should not cause isssues.
  • If you're using a framework like Celluloid that does a lot of thread management of its own, or your code is structured in such a way as you don't have to worry about callbacks blocking the event delivery thread.
  • If you're using 1.8.7 and are doing a lot of synchronization (i.e. using Monitor/Mutex) between threads, you may run into deadlocks or issues with your code when using :per_callback. I have tested extensively with 1.8.7, and the well known thread scheduler issues can cause problems (the Election codebase has always proven a bit much for 1.8.7). The deadlocks I'm talking about come from the increased number of threads in play, not from instability in the ZK code.

Hopefully, the following will make the tradeoffs clear.

Single Event Thread

This is the current default, and is how ZK has operated since before 0.8. Events are dispatched to callbacks using a single thread, in the order they come in, to the callbacks in the order they were registered.

require 'zk'

include ZookeeperConstants

logger = Logger.new($stderr).tap { |l| l.level = Logger::DEBUG }

zk = ZK.new

# a hack to inject events directly into to the event queue
def zk.deliver(path, type)
  event_handler.process(ZK::Event.new(:path => path, :type => type))
end

zk.register('/foo') do |event|
  logger.debug "FIRST! /foo callback #{event.event_name}"
end

zk.register('/foo') do |event|
  logger.debug "second /foo callback, #{event.event_name}"
end

zk.register('/bar') do |event|
  logger.debug "/bar callback, #{event.event_name}"
end

# simulates the event delivery thread
Thread.new do
  zk.deliver('/foo', ZOO_CREATED_EVENT)
  zk.deliver('/foo', ZOO_DELETED_EVENT)
  zk.deliver('/bar', ZOO_CREATED_EVENT)
  zk.deliver('/bar', ZOO_CHANGED_EVENT)
end

sleep(2)
produces the output:

DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT
DEBUG -- : second /foo callback, ZOO_CREATED_EVENT
DEBUG -- : FIRST! /foo callback ZOO_DELETED_EVENT
DEBUG -- : second /foo callback, ZOO_DELETED_EVENT
DEBUG -- : /bar callback, ZOO_CREATED_EVENT
DEBUG -- : /bar callback, ZOO_CHANGED_EVENT

This is how we would expect things to work, even in the case where there are pauses due to processing in one of the callbacks, only one callback is executed at a time, and events are delivered in order. This is fine for most simple applications and for initial development work, as it provides the most predictable execution.

Thread per Callback

In this model, each callback you register is given its own thread and queue. Events are delivered to the callback's queue, and the thread takes care of calling the callback with them one at a time. This model allows us to increase the parallelism of the app while also allowing us to guarantee:

  • Only one thread will be executing the callback at a time
  • Events will be delivered to the callback in the order they were received
  • A callback blocking will only affect the delivery of the next event to that callback.

The first two are also true for the single thread model, the last one is an advantage of the thread-per-callback model. The following difference is unique to the thread-per-callback-model, though:

  • The same event may be delivered at different times system-wide.

Which makes sense in a multi-threaded system.

A small example

Here, we're going to deliver 10 events each on '/foo' and '/bar' using the :per_callback option, and each callback block is going to add a small random sleep before returning (simulating some kind of "work performed"). You'll see in the output afterwards that while the events were delivered at different times to the different callbacks, the order to all callbacks was the same

require 'zk'

include ZookeeperConstants

logger = Logger.new($stderr).tap { |l| l.level = Logger::DEBUG }

zk = ZK.new('localhost:2181', :thread => :per_callback)

# a hack to inject events directly into to the event queue
def zk.deliver(path, type, context)
  event_handler.process(ZK::Event.new(:path => path, :type => type, :context => context))
end

zk.register('/foo') do |event|
  logger.debug "FIRST! /foo callback #{event.event_name}, #{event.context}"
  sleep(rand(10) * 0.001)
end

zk.register('/foo') do |event|
  logger.debug "second /foo callback, #{event.event_name}, #{event.context}"
  sleep(rand(10) * 0.001)
end

zk.register('/bar') do |event|
  logger.debug "/bar callback, #{event.event_name}, #{event.context}"
  sleep(rand(10) * 0.01)
end

# make this repeatable
srand(1)

events = [ZOO_CREATED_EVENT, ZOO_CHANGED_EVENT, ZOO_CHILD_EVENT, ZOO_CHANGED_EVENT]

# simulates the event delivery thread
Thread.new do
  10.times do |n|
    zk.deliver('/foo', events.sample, n)
    zk.deliver('/bar', events.sample, n)
  end
end

sleep(2)

Produces the output:

[01:17:41.709086] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 0
[01:17:41.709221] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 0
[01:17:41.709293] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 0
[01:17:41.719466] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 1
[01:17:41.719633] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 1
[01:17:41.726058] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 2
[01:17:41.727401] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 3
[01:17:41.727517] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 4
[01:17:41.728783] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 5
[01:17:41.728884] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 2
[01:17:41.738058] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 6
[01:17:41.738199] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 3
[01:17:41.741572] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 7
[01:17:41.748336] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 4
[01:17:41.750703] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 8
[01:17:41.754165] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 9
[01:17:41.756349] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 5
[01:17:41.762182] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 6
[01:17:41.763585] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 7
[01:17:41.773710] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 8
[01:17:41.777252] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 9
[01:17:41.780594] DEBUG -- : /bar callback, ZOO_CHANGED_EVENT, 1
[01:17:41.861766] DEBUG -- : /bar callback, ZOO_CREATED_EVENT, 2
[01:17:41.872999] DEBUG -- : /bar callback, ZOO_CHANGED_EVENT, 3
[01:17:41.913864] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 4
[01:17:41.913972] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 5
[01:17:41.944432] DEBUG -- : /bar callback, ZOO_CHANGED_EVENT, 6
[01:17:42.035611] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 7
[01:17:42.055915] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 8
[01:17:42.056029] DEBUG -- : /bar callback, ZOO_CREATED_EVENT, 9

So the '/bar' callback has a sleep multiplier that is one order-of-magnitude larger than the '/foo' callbacks, and we can see that it processes the first event and sleeps, but doesn't block the delivery of events to other callbacks. You can also see that both the first and second '/foo' callbacks receive their events in the same order (denoted by the 'context' number at the EOL).

Running that same code with :thread => :single we see

D, [01:25:00.288553] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 0
D, [01:25:00.298346] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 0
D, [01:25:00.308591] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 0
D, [01:25:00.379793] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 1
D, [01:25:00.386812] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 1
D, [01:25:00.396146] DEBUG -- : /bar callback, ZOO_CHANGED_EVENT, 1
D, [01:25:00.407168] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 2
D, [01:25:00.407333] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 2
D, [01:25:00.408621] DEBUG -- : /bar callback, ZOO_CREATED_EVENT, 2
D, [01:25:00.489797] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 3
D, [01:25:00.498985] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 3
D, [01:25:00.502502] DEBUG -- : /bar callback, ZOO_CHANGED_EVENT, 3
D, [01:25:00.593444] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 4
D, [01:25:00.602622] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 4
D, [01:25:00.610623] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 4
D, [01:25:00.641788] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 5
D, [01:25:00.648270] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 5
D, [01:25:00.653985] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 5
D, [01:25:00.664595] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 6
D, [01:25:00.674749] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 6
D, [01:25:00.678275] DEBUG -- : /bar callback, ZOO_CHANGED_EVENT, 6
D, [01:25:00.719443] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 7
D, [01:25:00.728667] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 7
D, [01:25:00.729917] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 7
D, [01:25:00.770318] DEBUG -- : FIRST! /foo callback ZOO_CHANGED_EVENT, 8
D, [01:25:00.770449] DEBUG -- : second /foo callback, ZOO_CHANGED_EVENT, 8
D, [01:25:00.774027] DEBUG -- : /bar callback, ZOO_CHILD_EVENT, 8
D, [01:25:00.864362] DEBUG -- : FIRST! /foo callback ZOO_CREATED_EVENT, 9
D, [01:25:00.866898] DEBUG -- : second /foo callback, ZOO_CREATED_EVENT, 9
D, [01:25:00.867004] DEBUG -- : /bar callback, ZOO_CREATED_EVENT, 9

All progress is gated by the slow '/bar' callback blocking the (only) delivery thread.

How common is this?

To be honest, it is (in my experience) fairly uncommon to have multiple callbacks processing events related to a single znode path. Most of the ZooKeeper examples try to avoid this pattern, as it could be an indication of a "Thundering Herd." As a design principle, you want to have messages be as specific as possible (and wake as few threads as possible).

If different paths represent mostly independent processing tasks in your app, then you should't run into trouble with the :per_callback option.

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