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AWS Open Sources Event Ruler


AWS recently announced that Event Ruler, the component managing the routing rules of Amazon EventBridge, is now open source. The project is a new option for developers in need to match lots of patterns, policies, or expressions against any amount of events in near real-time.

Written in Java, Event Ruler offers APIs for declaring pattern-matching rules, presenting data records (events) and finding out at scale which rules match each event, allowing developers to build applications that can match any number of rules against events at several hundred thousand events per second.

Events and rules are JSON objects, but rules additionally can be expressed through an inbuilt query language that helps describe custom matching patterns. For example, a JSON event of an image:


  "Image" : {
	"Width" :  800,
	"Height" : 600,
	"Title" :  "View from 15th Floor",
	"Thumbnail" : {
  	"Url" :	"",
  	"Height" : 125,
  	"Width" :  100
	"Animated" : false,
	"IDs" : [116, 943, 234, 38793]

can be filtered by a JSON rule that filters for static images only:


  "Image": {
    "Animated" : [ false ]


Rishi Baldawa, principal software engineer at AWS, explains:

This offers a novel solution for anyone in need to match lots of patterns, policies, or expressions against any amount of events without compromising on speed. Whether the events are in single digits or several hundred thousand, you can route, filter, or compare them in near real-time against any traffic. This speed is mostly independent of the number of rules or the patterns you define within these rules.

Event Ruler is in production in multiple Amazon services, including Amazon EventBridge, the serverless event bus service that helps developers and architects to connect applications with data from a variety of sources. The new open source project includes features that are not yet available on the AWS managed service. Nick Smit, principal product manager for Amazon EventBridge at AWS, tweets:

You’ll notice it has some features such as $or, suffix match, and Equals-ignore-case, which are not yet in EventBridge. We plan to include those in future. Excited to see the new ideas the community will bring!

Talking about use cases for the new project in a "Hello, Ruler" article, Tim Bray, formerly VP and distinguished engineer at AWS and one of the developer behind Event Ruler, writes:

The software is widely used inside AWS. Will it be useful outside the cloud-infrastructure world? My bet is yes, because more and more apps use loosely-coupled event-driven interconnections. For example, I think there are probably a lot of Kafka applications where the consumers could be made more efficient by the application of this sort of high-performance declarative filtering. Yes, I know there are already predicates, but still.

A year after announcing OpenSearch, there are different new AWS open source projects and initiatives, mostly covered in the AWS Open Source Blog and the AWS open source newsletter, with many believing that AWS is improving its open source reputation. Bray adds:

AWS has benefited hugely from the use of open-source. So it is nice to see them giving something back, something built from scratch, something that does not particularly advantage AWS.

Event Ruler is available on GitHub under the Apache 2.0 license.


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