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Events

Overview

Events are messages that you send and receive to communicate state change of a component or a system.
Anything can be an event - passing a message in a JavaScript application is an event and so is passing a message via Kafka or via a UDP socket.
The most common use case for events is to pass them through a message system such as RabbitMQ, Kafka or SQS to facilitate asynchronous work.
While any piece of data can be an event, you should strive to have well-composed events. A well-composed event should:
  • Use a common event envelope​
  • Indicate the event type
  • Not use ambiguous field names
  • Not share field names
  • Be easy to parse by a computer
  • Be type safe
NOTE: Batch uses "events" and "messages" terminology interchangeably throughout the platform - they are one and the same.

Message Passing Patterns

There are many ways to facilitate message passing.
The most common way to pass messages would be to utilize a messaging system such as:
More advanced use-cases may utilize technologies such as ZeroMQ, Nanomsg, Elixir's built-in RPC or pure gRPC.

Why utilize message passing?

There are many benefits to using a message passing mechanism but the primary reason is to gain additional reliability in your distributed system.
The quickest way to demonstrate this is through an example.

Example

Traditionally, if you (application A) want another application (B) to do some work, you would have A call B via a REST API. This is easy and it works but soon you may find out that:
  • Networks are unreliable and 1% of your requests don't complete
  • Application B gets deployed way more often than you thought
  • Application B gets restarted every night due to a memory leak
  • Application B is rate-limited to 5 requests per second (while application A often hits 100+ requests per second)
This all translates to poor service reliability.

A better approach

A way to improve service reliability would be to utilize an asynchronous approach that involves message passing.
  • Application A produces a message to a message queue
  • The message contains ALL of the necessary information for another application to be able to process the request and produce a result somewhere
  • Application B eventually picks up the message, processes it and produces a result by emitting another event that application A picks up

Takeaway

By utilizing an asynchronous approach, you are:
  • Alleviating service back pressure on application B
  • Increasing successful request completion for application A
  • Changing the relationship between A and B:
    • B is no longer a hard dependency for A - it is now a soft dependency, meaning, application A can survive if B goes away
  • And.. *drumroll* .. you are beginning to use event-driven architecture​