Actors, Observers and Data Pipelines.
The most important configuration setting is the mode, which governs how background tasks are handled.
By default it is :inline, so every command or query is handled synchronously. This is the ruby behaviour you know and love (although see the section on await below).
:async mode handles tasks using fibers (via the Async gem). Your code should include the "async" gem in its bundle, as Plumbing does not load it by default.
:threaded mode handles tasks using a thread pool via Concurrent Ruby). Your code should include the "concurrent-ruby" gem in its bundle, as Plumbing does not load it by default.
However, :threaded mode is not safe for Ruby on Rails applications. In this case, use :threaded_rails mode, which is identical to :threaded, except it wraps the tasks in the Rails executor. This ensures your actors do not interfere with the Rails framework. Note that the Concurrent Ruby's default :io scheduler will create extra threads at times of high demand, which may put pressure on the ActiveRecord database connection pool. A future version of plumbing will allow the thread pool to be adjusted with a maximum number of threads, preventing contention with the connection pool.
The timeout setting is used when performing queries - it defaults to 30s.
require "plumbing"
puts Plumbing.config.mode
# => :inline
Plumbing.configure mode: :async, timeout: 10
puts Plumbing.config.mode
# => :asyncIf you are running a test suite, you can temporarily update the configuration by passing a block.
require "plumbing"
puts Plumbing.config.mode
# => :inline
Plumbing.configure mode: :async do
puts Plumbing.config.mode
# => :async
first_test
second_test
end
puts Plumbing.config.mode
# => :inlineDefine a sequence of operations that proceed in order, passing their output from one operation as the input to another. Unix pipes in Ruby.
Use perform to define a step that takes some input and returns a different output.
Specify using to re-use an existing Plumbing::Pipeline as a step within this pipeline.
Use execute to define a step that takes some input, performs an action but passes the input, unchanged, to the next step.
If you have dry-validation installed, you can validate your input using a Dry::Validation::Contract. Alternatively, you can define a pre_condition to test that the inputs are valid.
You can also verify that the output generated is as expected by defining a post_condition.
Building an array using multiple steps with a pre-condition and post-condition
require "plumbing"
class BuildArray < Plumbing::Pipeline
perform :add_first
perform :add_second
perform :add_third
pre_condition :must_be_an_array do |input|
input.is_a? Array
end
post_condition :must_have_three_elements do |output|
output.length == 3
end
private
def add_first(input) = input << "first"
def add_second(input) = input << "second"
def add_third(input) = input << "third"
end
BuildArray.new.call []
# => ["first", "second", "third"]
BuildArray.new.call 1
# => Plumbing::PreconditionError("must_be_an_array")
BuildArray.new.call ["extra element"]
# => Plumbing::PostconditionError("must_have_three_elements")Validating input parameters with a contract
require "plumbing"
require "dry/validation"
class SayHello < Plumbing::Pipeline
validate_with "SayHello::Input"
perform :say_hello
private
def say_hello input
"Hello #{input[:name]} - I will now send a load of annoying marketing messages to #{input[:email]}"
end
class Input < Dry::Validation::Contract
params do
required(:name).filled(:string)
required(:email).filled(:string)
end
rule :email do
key.failure("must be a valid email") unless /\A[\w+\-.]+@[a-z\d\-]+(\.[a-z\d\-]+)*\.[a-z]+\z/i.match? value
end
end
end
SayHello.new.call(name: "Alice", email: "alice@example.com")
# => Hello Alice - I will now send a load of annoying marketing messages to alice@example.com
SayHello.new.call(some: "other data")
# => Plumbing::PreConditionErrorBuilding a pipeline through composition
require "plumbing"
class ExternalStep < Plumbing::Pipeline
perform :add_item_to_array
private
def add_item_to_array(input) = input << "external"
end
class BuildSequenceWithExternalStep < Plumbing::Pipeline
perform :add_first
perform :add_second, using: "ExternalStep"
perform :add_third
private
def add_first(input) = input << "first"
def add_third(input) = input << "third"
end
BuildSequenceWithExternalStep.new.call([])
# => ["first", "external", "third"]An actor defines the messages an object can receive, similar to a regular object. However, in traditional object-orientated programming, a thread of execution moves from one object to another. If there are multiple threads, then each object may be accessed concurrently, leading to race conditions or data-integrity problems - and very hard to track bugs.
Actors are different. Conceptually, each actor has it's own thread of execution, isolated from every other actor in the system. When one actor sends a message to another actor, the receiver does not execute its method in the caller's thread. Instead, it places the message on a queue and waits until its own thread is free to process the work. If the caller would like to access the return value from the method, then it must wait until the receiver has finished processing.
This means each actor is only ever accessed by a single thread and the vast majority of concurrency issues are eliminated.
Plumbing::Actor allows you to define the async public interface to your objects. Calling .start builds a proxy to the actual instance of your object and ensures that any messages sent are handled in a manner appropriate to the current mode - immediately for inline mode, using fibers for async mode and using threads for threaded and threaded_rails mode.
When sending messages to an actor, this just works.
However, as the caller, you do not have direct access to the return values of the messages that you send. Instead, you must call #value - or alternatively, wrap your call in await { ... }. The block form of await is added in to ruby's Kernel so it is available everywhere. It is also safe to use with non-actors (in which case it just returns the original value from the block).
@actor = MyActor.start name: "Alice"
@actor.name.value
# => "Alice"
await { @actor.name }
# => "Alice"
await { "Bob" }
# => "Bob"This then makes the caller's thread block until the receiver's thread has finished its work and returned a value. Or if the receiver raises an exception, that exception is then re-raised in the calling thread.
The actor model does not eliminate every possible concurrency issue. If you use value or await, it is possible to deadlock yourself.
Actor A, running in Thread 1, sends a message to Actor B and then awaits the result, meaning Thread 1 is blocked. Actor B, running in Thread 2, starts to work, but needs to ask Actor A a question. So it sends a message to Actor A and awaits the result. Thread 2 is now blocked, waiting for Actor A to respond. But Actor A, running in Thread 1, is blocked, waiting for Actor B to respond.
This potential deadlock only occurs if you use value or await and have actors that call back in to each other. If your objects are strictly layered, or you never use value or await (perhaps, instead using a Pipe to observe events), then this particular deadlock should not occur. However, just in case, every call to value has a timeout defaulting to 30s.
Even though inline mode is not asynchronous, you must still use value or await to access the results from another actor. However, as deadlocks are impossible in a single thread, there is no timeout.
Using async mode is probably the easiest way to add concurrency to your application. It uses fibers to allow for "concurrency but not parallelism" - that is execution will happen in the background but your objects or data will never be accessed by two things at the exact same time.
Using threaded (or threaded_rails) mode gives you concurrency and parallelism. If all your public objects are actors and you are careful about callbacks then the actor model will keep your code safe. But there are a couple of extra things to consider.
Firstly, when you pass parameters or return results between threads, those objects are "transported" across the boundaries.
Most objects are cloned. Hashes, keyword arguments and arrays have their contents recursively transported. And any object that uses GlobalID::Identification (for example, ActiveRecord models) are marshalled into a GlobalID, then unmarshalled back in to their original object. This is to prevent the same object from being amended in both the caller and receiver's threads.
Secondly, when you pass a block (or Proc parameter) to another actor, the block/proc will be executed in the receiver's thread. This means you must not access any variables that would normally be in scope for your block (whether local variables or instance variables of other objects - see note below) This is because you will be accessing them from a different thread to where they were defined, leading to potential race conditions. And, if you access any actors, you must not use value or await or you risk a deadlock. If you are within an actor and need to pass a block or proc parameter, you should use the safely method to ensure that your block is run within the context of the calling actor, not the receiving actor.
For example, when defining a custom filter, the filter adds itself as an observer to its source. The source triggers the received method on the filter, which will run in the context of the source. So the custom filter uses safely to move back into its own context and access its instance variables.
class EveryThirdEvent < Plumbing::CustomFilter
def initialize source:
super
@events = []
end
def received event
safely do
@events << event
if @events.count >= 3
@events.clear
self << event
end
end
end
end(Note: we break that rule in the specs for Pipe objects - we use a block observer that sets the value on a local variable. That's because it is a controlled situation where we know there are only two threads involved and we are explicitly waiting for the second thread to complete. For almost every app that uses actors, there will be multiple threads and it will be impossible to predict the access patterns).
Instead of constructing your object with .new, use .start. This builds a proxy object that wraps the target instance and dispatches messages through a safe mechanism. Only messages that have been defined as part of the actor are available in this proxy - so you don't have to worry about callers bypassing the actor's internal context.
If you're within a method inside your actor and you want to pass a reference to yourself, instead of using self, you should use proxy (which is also aliased as as_actor or async).
Also be aware that if you use actors in one place, you need to use them everywhere - especially if you're using threads. This is because as the actor sends messages to its collaborators, those calls will be made from within the actor's internal context. If the collaborators are also actors, the subsequent messages will be handled correctly, if not, data consistency bugs could occur. This does not mean that every class needs to be an actor, just your "public API" classes which may be accessed from multiple actors or other threads.
require "plumbing"
class Employee
include Plumbing::Actor
async :name, :job_title, :greet_slowly, :promote
attr_reader :name, :job_title
def initialize(name)
@name = name
@job_title = "Sales assistant"
end
private
def promote
sleep 0.5
@job_title = "Sales manager"
end
def greet_slowly
sleep 0.2
"H E L L O"
end
end
@person = Employee.start "Alice"
await { @person.name }
# => "Alice"
await { @person.job_title }
# => "Sales assistant"
# by using `await`, we will block until `greet_slowly` has returned a value
await { @person.greet_slowly }
# => "H E L L O"
# this time, we're not awaiting the result, so this will run in the background (unless we're using inline mode)
@person.greet_slowly
# this will run in the background
@person.promote
# this will block - it will not return until the previous calls, #greet_slowly, #promote, and this call to #job_title have completed
await { @person.job_title }
# => "Sales manager"Observers in Ruby are a pattern where objects (observers) register their interest in another object (the observable). This pattern is common throughout programming languages (event listeners in Javascript, the dependency protocol in Smalltalk).
Plumbing::Pipe makes observers "composable". Instead of simply just registering for notifications from a single observable, we can build sequences of pipes. These sequences can filter notifications and route them to different listeners, or merge multiple sources into a single stream of notifications.
Pipes are implemented as actors, meaning that event notifications can be dispatched asynchronously. The observer's callback will be triggered from within the pipe's internal context so you should immediately trigger a command on another actor to maintain safety.
require "plumbing"
@source = Plumbing::Pipe.start
@observer = @source.add_observer do |event|
puts event.type
end
@source.notify "something_happened", message: "But what was it?"
# => "something_happened" require "plumbing"
@source = Plumbing::Pipe.start
@filter = Plumbing::Filter.start source: @source do |event|
%w[important urgent].include? event.type
end
@observer = @filter.add_observer do |event|
puts event.type
end
@source.notify "important", message: "ALERT! ALERT!"
# => "important"
@source.notify "unimportant", message: "Nothing to see here"
# => <no output> require "plumbing"
class EveryThirdEvent < Plumbing::CustomFilter
def initialize source:
super source: source
@events = []
end
def received event
# #received is called in the context of the `source` actor
# in order to safely access the `EveryThirdEvent` instance variables
# we need to move into the context of our own actor
safely do
# store this event into our buffer
@events << event
# if this is the third event we've received then clear the buffer and broadcast the latest event
if @events.count >= 3
@events.clear
self << event
end
end
end
end
@source = Plumbing::Pipe.start
@filter = EveryThirdEvent.start(source: @source)
@observer = @filter.add_observer do |event|
puts event.type
end
1.upto 10 do |i|
@source.notify i.to_s
end
# => "3"
# => "6"
# => "9" require "plumbing"
@first_source = Plumbing::Pipe.start
@second_source = Plumbing::Pipe.start
@junction = Plumbing::Junction.start @first_source, @second_source
@observer = @junction.add_observer do |event|
puts event.type
end
@first_source.notify "one"
# => "one"
@second_source.notify "two"
# => "two"Define an interface or protocol specifying which messages you expect to be able to send.
Then cast an object into that type. This first tests that the object can respond to those messages and then builds a proxy that responds to those messages (and no others). However, if you take one of these proxies, you can safely re-cast it as another type (as long as the original target object responds to the correct messages).
Define your interface (Person in this example), then cast your objects (instances of PersonData and CarData).
Casting objects as duck-types:
require "plumbing"
Person = Plumbing::RubberDuck.define :first_name, :last_name, :email
LikesFood = Plumbing::RubberDuck.define :favourite_food
PersonData = Struct.new(:first_name, :last_name, :email, :favourite_food)
CarData = Struct.new(:make, :model, :colour)
@porsche_911 = CarData.new "Porsche", "911", "black"
@person = @porsche_911.as Person
# => Raises a TypeError as CarData does not respond_to #first_name, #last_name, #email
@alice = PersonData.new "Alice", "Aardvark", "alice@example.com", "Ice cream"
@person = @alice.as Person
@person.first_name
# => "Alice"
@person.email
# => "alice@example.com"
@person.favourite_food
# => NoMethodError - #favourite_food is not part of the Person rubber duck (even though it is part of the underlying PersonData struct)
# Cast our Person into a LikesFood rubber duck
@hungry = @person.as LikesFood
@hungry.favourite_food
# => "Ice cream"You can also use the same @object.as type to type-check instances against modules or classes. This creates a RubberDuck proxy based on the module or class you're casting into. So the cast will pass if the object responds to the correct messages, even if a strict .is_a? test would fail.
require "plumbing"
module Person
def first_name = @first_name
def last_name = @last_name
def email = @email
end
module LikesFood
def favourite_food = @favourite_food
end
PersonData = Struct.new(:first_name, :last_name, :email, :favourite_food)
CarData = Struct.new(:make, :model, :colour)
@porsche_911 = CarData.new "Porsche", "911", "black"
expect { @porsche_911.as Person }.to raise_error(TypeError)
@alice = PersonData.new "Alice", "Aardvark", "alice@example.com", "Ice cream"
@alics.is_a? Person
# => false - PersonData does not `include Person`
@person = @alice.as Person
# This cast is OK because PersonData responds to :first_name, :last_name and :email
expect(@person.first_name).to eq "Alice"
expect(@person.email).to eq "alice@example.com"
expect { @person.favourite_food }.to raise_error(NoMethodError)
@hungry = @person.as LikesFood
expect(@hungry.favourite_food).to eq "Ice cream"Note: this gem is licensed under the LGPL. This may or may not make it unsuitable for use by you or your company.
Install the gem and add to the application's Gemfile by executing:
bundle add standard-procedure-plumbingThen:
require 'plumbing'
# Set the mode for your Actors and Pipes
Plumbing.config mode: :asyncAfter checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.
To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.
Bug reports and pull requests are welcome on GitHub at https://github.com/standard_procedure/plumbing. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.
Everyone interacting in the Plumbing project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.