class Concurrent::Agent
‘Agent` is inspired by Clojure’s [agent](clojure.org/agents) function. An agent is a shared, mutable variable providing independent, uncoordinated, asynchronous change of individual values. Best used when the value will undergo frequent, complex updates. Suitable when the result of an update does not need to be known immediately. ‘Agent` is (mostly) functionally equivalent to Clojure’s agent, except where the runtime prevents parity.
Agents are reactive, not autonomous - there is no imperative message loop and no blocking receive. The state of an Agent
should be itself immutable and the ‘#value` of an Agent
is always immediately available for reading by any thread without any messages, i.e. observation does not require cooperation or coordination.
Agent
action dispatches are made using the various ‘#send` methods. These methods always return immediately. At some point later, in another thread, the following will happen:
-
The given ‘action` will be applied to the state of the
Agent
and the `args`, if any were supplied. -
The return value of ‘action` will be passed to the validator lambda, if one has been set on the
Agent
. -
If the validator succeeds or if no validator was given, the return value of the given ‘action` will become the new `#value` of the
Agent
. See `#initialize` for details. -
If any observers were added to the
Agent
, they will be notified. See ‘#add_observer` for details. -
If during the ‘action` execution any other dispatches are made (directly or indirectly), they will be held until after the `#value` of the
Agent
has been changed.
If any exceptions are thrown by an action function, no nested dispatches will occur, and the exception will be cached in the Agent
itself. When an Agent
has errors cached, any subsequent interactions will immediately throw an exception, until the agent’s errors are cleared. Agent
errors can be examined with ‘#error` and the agent restarted with `#restart`.
The actions of all Agents get interleaved amongst threads in a thread pool. At any point in time, at most one action for each Agent
is being executed. Actions dispatched to an agent from another single agent or thread will occur in the order they were sent, potentially interleaved with actions dispatched to the same agent from other sources. The ‘#send` method should be used for actions that are CPU limited, while the `#send_off` method is appropriate for actions that may block on IO.
Unlike in Clojure, ‘Agent` cannot participate in `Concurrent::TVar` transactions.
## Example
“‘ def next_fibonacci(set = nil)
return [0, 1] if set.nil? set + [set[-2..-1].reduce{|sum,x| sum + x }]
end
# create an agent with an initial value agent = Concurrent::Agent.new(next_fibonacci)
# send a few update requests 5.times do
agent.send{|set| next_fibonacci(set) }
end
# wait for them to complete agent.await
# get the current value agent.value #=> [0, 1, 1, 2, 3, 5, 8] “‘
## Observation
Agents support observers through the {Concurrent::Observable} mixin module. Notification of observers occurs every time an action dispatch returns and the new value is successfully validated. Observation will not occur if the action raises an exception, if validation fails, or when a {#restart} occurs.
When notified the observer will receive three arguments: ‘time`, `old_value`, and `new_value`. The `time` argument is the time at which the value change occurred. The `old_value` is the value of the Agent
when the action began processing. The `new_value` is the value to which the Agent
was set when the action completed. Note that `old_value` and `new_value` may be the same. This is not an error. It simply means that the action returned the same value.
## Nested Actions
It is possible for an Agent
action to post further actions back to itself. The nested actions will be enqueued normally then processed after the outer action completes, in the order they were sent, possibly interleaved with action dispatches from other threads. Nested actions never deadlock with one another and a failure in a nested action will never affect the outer action.
Nested actions can be called using the Agent
reference from the enclosing scope or by passing the reference in as a “send” argument. Nested actions cannot be post using ‘self` from within the action block/proc/lambda; `self` in this context will not reference the Agent
. The preferred method for dispatching nested actions is to pass the Agent
as an argument. This allows Ruby to more effectively manage the closing scope.
Prefer this:
“‘ agent = Concurrent::Agent.new(0)
agent.send(agent) do |value, this|
this.send {|v| v + 42 } 3.14
end agent.value #=> 45.14 “‘
Over this:
“‘ agent = Concurrent::Agent.new(0)
agent.send do |value|
agent.send {|v| v + 42 } 3.14
end “‘
@!macro agent_await_warning
**NOTE** Never, *under any circumstances*, call any of the "await" methods ({#await}, {#await_for}, {#await_for!}, and {#wait}) from within an action block/proc/lambda. The call will block the Agent and will always fail. Calling either {#await} or {#wait} (with a timeout of `nil`) will hopelessly deadlock the Agent with no possibility of recovery.
@!macro thread_safe_variable_comparison
@see clojure.org/Agents Clojure Agents @see clojure.org/state Values and Change - Clojure’s approach to Identity and State
Constants
- AWAIT_ACTION
- AWAIT_FLAG
- DEFAULT_ERROR_HANDLER
- DEFAULT_VALIDATOR
- ERROR_MODES
- Job
Attributes
The error mode this Agent
is operating in. See {#initialize} for details.
Public Class Methods
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 448 def await(*agents) agents.each { |agent| agent.await } true end
Blocks the current thread (indefinitely!) until all actions dispatched thus far to all the given Agents, from this thread or nested by the given Agents, have occurred. Will block when any of the agents are failed. Will never return if a failed Agent
is restart with ‘:clear_actions` true.
@param [Array<Concurrent::Agent>] agents the Agents on which to wait @return [Boolean] true
@!macro agent_await_warning
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 462 def await_for(timeout, *agents) end_at = Concurrent.monotonic_time + timeout.to_f ok = agents.length.times do |i| break false if (delay = end_at - Concurrent.monotonic_time) < 0 break false unless agents[i].await_for(delay) end !!ok end
Blocks the current thread until all actions dispatched thus far to all the given Agents, from this thread or nested by the given Agents, have occurred, or the timeout (in seconds) has elapsed.
@param [Float] timeout the maximum number of seconds to wait @param [Array<Concurrent::Agent>] agents the Agents on which to wait @return [Boolean] true if all actions complete before timeout else false
@!macro agent_await_warning
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 481 def await_for!(timeout, *agents) raise Concurrent::TimeoutError unless await_for(timeout, *agents) true end
Blocks the current thread until all actions dispatched thus far to all the given Agents, from this thread or nested by the given Agents, have occurred, or the timeout (in seconds) has elapsed.
@param [Float] timeout the maximum number of seconds to wait @param [Array<Concurrent::Agent>] agents the Agents on which to wait @return [Boolean] true if all actions complete before timeout
@raise [Concurrent::TimeoutError] when timout is reached @!macro agent_await_warning
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 219 def initialize(initial, opts = {}) super() synchronize { ns_initialize(initial, opts) } end
Create a new ‘Agent` with the given initial value and options.
The ‘:validator` option must be `nil` or a side-effect free proc/lambda which takes one argument. On any intended value change the validator, if provided, will be called. If the new value is invalid the validator should return `false` or raise an error.
The ‘:error_handler` option must be `nil` or a proc/lambda which takes two arguments. When an action raises an error or validation fails, either by returning false or raising an error, the error handler will be called. The arguments to the error handler will be a reference to the agent itself and the error object which was raised.
The ‘:error_mode` may be either `:continue` (the default if an error handler is given) or `:fail` (the default if error handler nil or not given).
If an action being run by the agent throws an error or doesn’t pass validation the error handler, if present, will be called. After the handler executes if the error mode is ‘:continue` the Agent
will continue as if neither the action that caused the error nor the error itself ever happened.
If the mode is ‘:fail` the Agent
will become {#failed?} and will stop accepting new action dispatches. Any previously queued actions will be held until {#restart} is called. The {#value} method will still work, returning the value of the Agent
before the error.
@param [Object] initial the initial value @param [Hash] opts the configuration options
@option opts [Symbol] :error_mode either ‘:continue` or `:fail` @option opts [nil, Proc] :error_handler the (optional) error handler @option opts [nil, Proc] :validator the (optional) validation procedure
Public Instance Methods
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 330 def <<(action) send_off(&action) self end
Dispatches an action to the Agent
and returns immediately. Subsequently, in a thread from a thread pool, the {#value} will be set to the return value of the action. Appropriate for actions that may block on IO.
@param [Proc] action the action dispatch to be enqueued @return [Concurrent::Agent] self @see send_off
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 349 def await wait(nil) self end
Blocks the current thread (indefinitely!) until all actions dispatched thus far, from this thread or nested by the Agent
, have occurred. Will block when {#failed?}. Will never return if a failed Agent
is {#restart} with ‘:clear_actions` true.
Returns a reference to ‘self` to support method chaining:
“‘ current_value = agent.await.value “`
@return [Boolean] self
@!macro agent_await_warning
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 362 def await_for(timeout) wait(timeout.to_f) end
Blocks the current thread until all actions dispatched thus far, from this thread or nested by the Agent
, have occurred, or the timeout (in seconds) has elapsed.
@param [Float] timeout the maximum number of seconds to wait @return [Boolean] true if all actions complete before timeout else false
@!macro agent_await_warning
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 376 def await_for!(timeout) raise Concurrent::TimeoutError unless wait(timeout.to_f) true end
Blocks the current thread until all actions dispatched thus far, from this thread or nested by the Agent
, have occurred, or the timeout (in seconds) has elapsed.
@param [Float] timeout the maximum number of seconds to wait @return [Boolean] true if all actions complete before timeout
@raise [Concurrent::TimeoutError] when timout is reached
@!macro agent_await_warning
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 239 def error @error.value end
When {#failed?} and {#error_mode} is ‘:fail`, returns the error object which caused the failure, else `nil`. When {#error_mode} is `:continue` will always return `nil`.
@return [nil, Error] the error which caused the failure when {#failed?}
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 401 def failed? !@error.value.nil? end
Is the Agent
in a failed state?
@see restart
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 423 def restart(new_value, opts = {}) clear_actions = opts.fetch(:clear_actions, false) synchronize do raise Error.new('agent is not failed') unless failed? raise ValidationError unless ns_validate(new_value) @current.value = new_value @error.value = nil @queue.clear if clear_actions ns_post_next_job unless @queue.empty? end true end
When an Agent
is {#failed?}, changes the Agent
{#value} to ‘new_value` then un-fails the Agent
so that action dispatches are allowed again. If the `:clear_actions` option is give and true, any actions queued on the Agent
that were being held while it was failed will be discarded, otherwise those held actions will proceed. The `new_value` must pass the validator if any, or `restart` will raise an exception and the Agent
will remain failed with its old {#value} and {#error}. Observers, if any, will not be notified of the new state.
@param [Object] new_value the new value for the Agent
once restarted @param [Hash] opts the configuration options @option opts [Symbol] :clear_actions true if all enqueued but unprocessed
actions should be discarded on restart, else false (default: false)
@return [Boolean] true
@raise [Concurrent:AgentError] when not failed
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 277 def send(*args, &action) enqueue_action_job(action, args, Concurrent.global_fast_executor) end
@!macro agent_send
Dispatches an action to the Agent and returns immediately. Subsequently, in a thread from a thread pool, the {#value} will be set to the return value of the action. Action dispatches are only allowed when the Agent is not {#failed?}. The action must be a block/proc/lambda which takes 1 or more arguments. The first argument is the current {#value} of the Agent. Any arguments passed to the send method via the `args` parameter will be passed to the action as the remaining arguments. The action must return the new value of the Agent. * {#send} and {#send!} should be used for actions that are CPU limited * {#send_off}, {#send_off!}, and {#<<} are appropriate for actions that may block on IO * {#send_via} and {#send_via!} are used when a specific executor is to be used for the action @param [Array<Object>] args zero or more arguments to be passed to the action @param [Proc] action the action dispatch to be enqueued @yield [agent, value, *args] process the old value and return the new @yieldparam [Object] value the current {#value} of the Agent @yieldparam [Array<Object>] args zero or more arguments to pass to the action @yieldreturn [Object] the new value of the Agent
@!macro send_return
@return [Boolean] true if the action is successfully enqueued, false if the Agent is {#failed?}
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 286 def send!(*args, &action) raise Error.new unless send(*args, &action) true end
@!macro agent_send
@!macro send_bang_return_and_raise
@return [Boolean] true if the action is successfully enqueued @raise [Concurrent::Agent::Error] if the Agent is {#failed?}
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 293 def send_off(*args, &action) enqueue_action_job(action, args, Concurrent.global_io_executor) end
@!macro agent_send @!macro send_return
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 301 def send_off!(*args, &action) raise Error.new unless send_off(*args, &action) true end
@!macro agent_send @!macro send_bang_return_and_raise
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 310 def send_via(executor, *args, &action) enqueue_action_job(action, args, executor) end
@!macro agent_send @!macro send_return @param [Concurrent::ExecutorService] executor the executor on which the
action is to be dispatched
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 318 def send_via!(executor, *args, &action) raise Error.new unless send_via(executor, *args, &action) true end
@!macro agent_send @!macro send_bang_return_and_raise @param [Concurrent::ExecutorService] executor the executor on which the
action is to be dispatched
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 228 def value @current.value # TODO (pitr 12-Sep-2015): broken unsafe read? end
The current value (state) of the Agent
, irrespective of any pending or in-progress actions. The value is always available and is non-blocking.
@return [Object] the current value
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 392 def wait(timeout = nil) latch = Concurrent::CountDownLatch.new(1) enqueue_await_job(latch) latch.wait(timeout) end
Blocks the current thread until all actions dispatched thus far, from this thread or nested by the Agent
, have occurred, or the timeout (in seconds) has elapsed. Will block indefinitely when timeout is nil or not given.
Provided mainly for consistency with other classes in this library. Prefer the various ‘await` methods instead.
@param [Float] timeout the maximum number of seconds to wait @return [Boolean] true if all actions complete before timeout else false
@!macro agent_await_warning
Private Instance Methods
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 509 def enqueue_action_job(action, args, executor) raise ArgumentError.new('no action given') unless action job = Job.new(action, args, executor, @caller.value || Thread.current.object_id) synchronize { ns_enqueue_job(job) } end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 515 def enqueue_await_job(latch) synchronize do if (index = ns_find_last_job_for_thread) job = Job.new(AWAIT_ACTION, [latch], Concurrent.global_immediate_executor, Thread.current.object_id) ns_enqueue_job(job, index+1) else latch.count_down true end end end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 542 def execute_next_job job = synchronize { @queue.first } old_value = @current.value @caller.value = job.caller # for nested actions new_value = job.action.call(old_value, *job.args) @caller.value = nil return if new_value == AWAIT_FLAG if ns_validate(new_value) @current.value = new_value observers.notify_observers(Time.now, old_value, new_value) else handle_error(ValidationError.new) end rescue => error handle_error(error) ensure synchronize do @queue.shift unless failed? || @queue.empty? ns_post_next_job end end end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 575 def handle_error(error) # stop new jobs from posting @error.value = error if @error_mode == :fail @error_handler.call(self, error) rescue # do nothing end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 528 def ns_enqueue_job(job, index = nil) # a non-nil index means this is an await job return false if index.nil? && failed? index ||= @queue.length @queue.insert(index, job) # if this is the only job, post to executor ns_post_next_job if @queue.length == 1 true end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 583 def ns_find_last_job_for_thread @queue.rindex { |job| job.caller == Thread.current.object_id } end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 489 def ns_initialize(initial, opts) @error_mode = opts[:error_mode] @error_handler = opts[:error_handler] if @error_mode && !ERROR_MODES.include?(@error_mode) raise ArgumentError.new('unrecognized error mode') elsif @error_mode.nil? @error_mode = @error_handler ? :continue : :fail end @error_handler ||= DEFAULT_ERROR_HANDLER @validator = opts.fetch(:validator, DEFAULT_VALIDATOR) @current = Concurrent::AtomicReference.new(initial) @error = Concurrent::AtomicReference.new(nil) @caller = Concurrent::ThreadLocalVar.new(nil) @queue = [] self.observers = Collection::CopyOnNotifyObserverSet.new end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 538 def ns_post_next_job @queue.first.executor.post { execute_next_job } end
Source
# File lib/concurrent-ruby/concurrent/agent.rb, line 569 def ns_validate(value) @validator.call(value) rescue false end