Message Queue

Event bus, Event loop

This pattern is used to send a message from one component to one or more other components, asynchronously.

How does it work?

The sending component places a message on the message queue. The distributor reads the oldest message from the queue an passes it to the recipient component.

Message queue architecture diagram


The pattern can be used within a program, but also between programs, using a specialized queue application.


  • Email: send email to one or more recipients, or a group
  • The JavaScript event loop handles messages in a single thread
  • Message queue software

When should you use it?

Use it when it takes too long to wait for a response.


  • There is an element of uncertainty introduced when messages are handled asynchronously. You can't be sure if and when they are handled.
  • If the modules share large amounts of data, it may not be a good idea to pump these over the bus all the time. If you choose to share the data between the modules, make sure no synchronization issues occur.

Common implementation techniques

  • Modules may run in the same process or in different processes (as is the case with Microservices)
  • The event bus may be part of the same process or it may be another process. It may use the internet as its substrate.