Add Java workers blog post

(Written by Philipp Wollermann.)

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+layout: posts
+title: Persistent Worker Processes for Bazel
+---
+
+Bazel runs most build actions as a separate process. Many build actions invoke a compiler.  However, starting a compiler is often slow: they have to perform some initialization when they start up, read the standard library, header files, low-level libraries, and so on. That’s why some compilers and tools have a persistent mode, e.g. [sjavac](http://openjdk.java.net/jeps/199), [Nailgun](http://martiansoftware.com/nailgun/) and [gcc server](http://per.bothner.com/papers/GccSummit03/gcc-server.pdf). Keeping a single process for longer and passing multiple individual requests to the same server can significantly reduce the amount of duplicate work and cut down on compile times.
+
+In Bazel, we have recently added experimental support for delegating work to [persistent worker processes](https://github.com/bazelbuild/bazel/tree/master/src/main/java/com/google/devtools/build/lib/worker) that run as child processes of and are managed by Bazel. Our Javac wrapper (called JavaBuilder) is the first compiler that supports running as a worker.
+
+We’ve tried the persistent JavaBuilder for a variety of builds and are seeing a ~4x improvement in Java build times, as Javac can now benefit from JIT optimizations over multiple runs and we no longer have to start a new JVM for every compile action. For Bazel itself, we saw a reduction in build time for a clean build from ~58s to ~16s (on repeated builds).
+
+<img src="/assets/fullbuild.png" alt="Full build" class="img-responsive">
+<img src="/assets/incbuild.png" alt="Incremental build" class="img-responsive">
+
+If you often build Java code, we’d like you to give it a try. Just pass `--strategy=Javac=worker` to enable it or add `build --strategy=Javac=worker` to the .bazelrc in your home directory or in your workspace. Check the WorkerOptions class for [flags to further tune the workers’ behavior](https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/worker/WorkerOptions.java) or run “bazel help” and look for the “Strategy options” category. Let us know how it works for you.
+
+We’re currently using a simple [protobuf-based protocol](https://github.com/bazelbuild/bazel/blob/master/src/main/protobuf/worker_protocol.proto) to communicate with the worker process. Let us know if you want to add support for more compilers; in many cases, you can do that without any Bazel changes. However, the protocol is still subject to change based on your feedback.