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pages/blog/2024-10-15-release_4_0.md

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+---
+title: 'Release 4.0: 🚄 Faster actor testing and modularized expressions'
+---
+
+Earlier this year, [Comunica version 3.0 was released](/blog/2024-03-19-release_3_0/),
+which introduced better querying planning across data sources and several convenience features.
+Since then, we had several minor releases introducing multiple additions and performance improvements.
+In [release 3.2](/blog/2024-07-05-release_3_2/), we introduced better techniques for discovering performance bottleneck.
+This allowed us identify some low-level bottlenecks in the internals of Comunica which limited performance.
+Fixing these issues requires breaking changes to the internal API of Comunica,
+which is the main reason for this major update.
+As a result of these changes, we see **performance improvements of 20% to 40%**.
+Breaking changes are limited to internal Comunica APIs.
+This means that developers that make use of Comunica can have their cake _and_ eat it 🎂;
+no breaking changes _and_ gaining a performance boost.
+
+<!-- excerpt-end -->
+
+## 🪢 New `Actor.test` contract for better performance
+
+To determine which actors can answer a certain action,
+all Comunica actors expose a `test` method, which used to look something like this:
+
+```typescript
+class MyActor extends Actor {
+  public async test(action: IAction): Promise<IActorTest> {
+    if (conditionNotMet(action)) {
+      throw new Error('This actor can not handle the action');
+    }
+    return true;
+  }
+}
+```
+
+The problem with the above is that JavaScript engines such as V8
+will eagerly build internal stacktraces when creating `Error` objects.
+Since Comunica has a large number of actors (240 at the time of writing),
+an average query execution can lead to a huge number of internal `Error` objects being created.
+According to our measurements, this produced a non-negligible performance overhead.
+
+As such, we refactored the contract of the `test` method to not rely on these `Error` objects anymore.
+Instead, `test` methods now make use of `TestResult` objects,
+which in practise look like this:
+
+```typescript
+class MyActor extends Actor {
+  public async test(action: IAction): Promise<TestResult<IActorTest>> {
+    if (conditionNotMet(action)) {
+      return failTest('This actor can not handle the action');
+    }
+    return passTestVoid();
+  }
+}
+```
+
+For various benchmarks on in-memory triple stores, this change makes queries up to 20% faster.
+
+## 🧩 Modularization of expressions logic
+
+Thanks to [Jitse De Smet](https://jitsedesmet.be/)'s monumental effort,
+all expressions-related logic in Comunica is now fully modularized.
+Previously, the handling of filters and aggregates were all delegated to the singular `sparqlee` package.
+While this package did a great job of handling filters and aggregates,
+it lacked the modularity that existed for all other parts of query execution.
+For example, it was not possible to easily plug in your own actor to evaluate the `SUM` aggregator in a different way.
+
+With this release, the `sparqlee` has been split up into multiple buses and actors,
+which are responsible for term comparators, function, and aggregators.
+For this, we avoided any kind of performance degradation.
+
+Learn more about [expressions evaluation in our documentation](https://comunica.dev/docs/modify/advanced/expression-evaluator/).
+
+## 🚄 Performance improvements
+
+Besides the changes mentioned above,
+there are a number of smaller changes that have a positive impact on performance that are worth mentioning:
+
+- [Optimize Bindings merge logic](https://github.com/comunica/comunica/commit/20daf1761b1ec4c82357909a45fa4f84e2754080)
+- [Fix internal cardinalities being wrong for SPARQL endpoints with VoID](https://github.com/comunica/comunica/commit/075f5dde03354f0516398235146d9a93883e8b66)
+- [Hash Joins now always use 32-bit numbers](https://github.com/comunica/comunica/commit/c1474e4167f1978e144bee9b7663fcb2bafc21bf), which speeds up operations in the V8 engine.
+- [Hash joins use the faster Murmur3 hash method](https://github.com/comunica/comunica/commit/331865c773882be7e061a690833fc841ece27339)
+- [Only consider overlapping vars when testing undef in join actors](https://github.com/comunica/comunica/commit/5ddb64bb1df4b1179529f3e2ea0c05fb321dfce9)
+- [Refactor HTTP fetch and retry logic](https://github.com/comunica/comunica/commit/b37d7e008b77067f6c6249aadfc901baf42a6914), which leads to more stable query execution over servers that make use of rate limits
+
+## 🤝 Contributors
+
+This release has been made possible thanks to the help of the following contributors (in no particular order):
+
+- [Jonni Hanski](https://github.com/surilindur)
+- [Jitse De Smet](https://github.com/jitsedesmet)
+- [Ruben Eschauzier](https://github.com/RubenEschauzier)
+- [Karel Klíma](https://github.com/karelklima)
+- [Ieben Smessaert](https://github.com/smessie)
+- [Maarten Vandenbrande](https://github.com/maartyman)
+- [Bryan-Elliott Tam](https://github.com/constraintAutomaton)
+- [Jesse Wright](https://github.com/jeswr)
+- [Ruben Taelman](https://github.com/rubensworks/)
+
+## Full changelog
+
+While this blog post explained the primary changes in Comunica 4.x,
+there are actually many more smaller changes internally that will make your lives easier.
+If you want to learn more about these changes, check out the [full changelog](https://github.com/comunica/comunica/blob/master/CHANGELOG.md#v401---2024-10-15).