Coming Soon: Master Asynchronous Workflows in Ruby with EventMachine Deferrables

Understanding the Power of Deferrables

Modern software development often involves handling tasks that can run concurrently. EventMachine (EM), a well-known library in the Ruby community, provides powerful features for managing such tasks. Specifically, EM introduces two concurrency patterns to Ruby programmers: spawned processes and deferrables. In this article, we will delve into how to adopt deferrables for efficient and effective asynchronous workflows.

What are Deferrables?

EM's Deferrable module is heavily inspired by the 'deferred' concept in Python's Twisted framework. Essentially, Deferrables in Ruby allow you to execute blocks of code only when certain conditions are met, at a future point in time. This mechanism decouples the timing of an operation's initiation from its processing, facilitating a more flexible and responsive application design.

An Introductory Example

Consider the following Ruby code, which uses EM's Deferrable module:

 require 'eventmachine' class MyClass include EM::Deferrable def print_value(x) puts "MyClass instance received #{x}" end end EM.run { df = MyClass.new df.callback {|x| df.print_value(x) EM.stop } EM::Timer.new(2) { df.set_deferred_status :succeeded, 100 } } 

This code will wait for two seconds before printing "MyClass instance received 100" and exiting. While this might seem like a simplistic use-case, the same pattern is instrumental in handling more complex and asynchronous operations without blocking the main thread.

Practical Applications of Deferrables

Deferrables are incredibly useful in scenarios such as networking, where I/O operations do not need to block execution. For example, consider an HTTP server that needs to make requests to other services while still being responsive to new incoming connections. By employing deferrables, you can set up callbacks that get triggered upon the completion of these outbound requests, without halting the server's main event loop.

Manipulating Deferrable Execution

Deferrables allow you to attach any number of callbacks and errbacks that execute in response to success or failure status changes. This design pattern offers a separation of concerns, where the logic that initiates an operation is isolated from the logic that executes once the operation is complete or has failed. It not only enhances code clarity but also mitigates some common pitfalls associated with parallel programming such as race conditions.

Going Deeper with Callbacks and Errbacks

When you set the deferred status of an event using EM's Deferrable, all attached callbacks or errbacks are immediately executed in the order they were added. This promotes a predictable flow, which is often not the case with threaded applications. Moreover, if the deferred status is already determined, any new callbacks or errbacks added will run without delay.

Fine-Tuning Deferred Operations

If required, you can modify deferred status within callbacks or errbacks themselves, enabling dynamic control over subsequent operations. This facility for altering execution paths can be very powerful, accommodating complex scenarios where the data or the status obtained from an operation needs to be transformed before being used by another part of the program.

Extended Example with HTTP Request

Let's observe deferrables in action within the context of an HTTP request:

 require 'eventmachine' EM.run { df = EM::Protocols::HttpClient.request( :host=>"www.example.com", :request=>"/index.html" ) df.callback {|response| puts "Succeeded: #{response[:content]}" EM.stop } df.errback {|response| puts "ERROR: #{response[:status]}" EM.stop } } 

This code demonstrates how an immediate deferrable object is returned and is later updated with a success or failure, thus triggering the appropriate callback or errback. This pattern exemplifies asynchronous programming, where the request to 'www.example.com' is processed in the background, enabling your application to perform other tasks concurrently.

Conclusion and Further Resources

The Deferrable pattern, as implemented in Ruby's EventMachine, offers a robust mechanism for efficiently managing concurrency in your applications. By leveraging this pattern, software developers can write clean, maintainable, and non-blocking code that readily adapts to complex workflows across a variety of use cases. Interested readers should examine EM's unit tests and other auxiliary methods that offer syntactic conveniences around common Deferrable usage patterns.