---
layout: documentation
title: Creating persistent workers
category: extending
---

# Creating Persistent Workers

[Persistent workers](persistent-workers.html) can make your build faster.
If you have repeated actions in your build that have a high startup cost or
would benefit from cross-action caching, you may want to implement your own
persistent worker to perform these actions.

The Bazel server communicates with the worker using `stdin`/`stdout`. It
supports the use of protocol buffers or JSON strings. Support for JSON is
experimental and thus subject to change. It is guarded behind the
`--experimental_worker_allow_json_protocol` flag.

The worker implementation has two parts:

* The [worker](#making-the-worker).
* The [rule that uses the worker](#making-the-rule-that-uses-the-worker).

## Making the worker

A persistent worker upholds a few requirements:

* It reads [WorkRequests](https://github.com/bazelbuild/bazel/blob/6d1b9725b1e201ca3f25d8ec2a730a20aab62c6e/src/main/protobuf/worker_protocol.proto#L35)
  from its `stdin`.
* It writes [WorkResponses](https://github.com/bazelbuild/bazel/blob/6d1b9725b1e201ca3f25d8ec2a730a20aab62c6e/src/main/protobuf/worker_protocol.proto#L49)
  (and only `WorkResponse`s) to its `stdout`.
* It accepts the `--persistent_worker` flag. The wrapper must recognize the
  `--persistent_worker` command-line flag and only make itself persistent if
  that flag is passed, otherwise it must do a one-shot compilation and exit.

If your program upholds these requirements, it can be used as a persistent worker!

### Work requests

A `WorkRequest` contains a list of arguments to the worker, a list of
path-digest pairs representing the inputs the worker can access (this isn’t
enforced, but you can use this info for caching), and a request id, which is 0
for singleplex workers.

```json
{
  "args" : ["--some_argument"],
  "inputs" : [
    { "/path/to/my/file/1" : "fdk3e2ml23d"},
    { "/path/to/my/file/2" : "1fwqd4qdd" }
  ],
  "request_id" : 12
}
```

The optional `verbosity` field can be used to request extra debugging output
from the worker. It is entirely up to the worker what and how to output. Higher
values indicate more verbose output. Passing the `--worker_verbose` flag to
Bazel sets the `verbosity` field to 10, but smaller or larger values can be
used manually for different amounts of output.

### Work responses

A `WorkResponse` contains a request id, a zero or nonzero exit
code, and an output string that describes any errors encountered in processing
or executing the request. The `output` field contains a short
description; complete logs may be written to the worker's `stderr`. Because
workers may only write `WorkResponses` to `stdout`, it's common for the worker
to redirect the `stdout` of any tools it uses to `stderr`.

```json
{
  "exit_code" : 1,
  "output" : "Action failed with the following message:\nCould not find input file \"/path/to/my/file/1\"",
  "request_id" : 12
}
```

As per the norm for protobufs, all fields are optional. However, Bazel requires
the `WorkRequest` and the corresponding `WorkResponse`, to have the same request
id, so the request id must be specified if it is nonzero. This is a valid
`WorkResponse`.

```json
{
  "request_id" : 12,
}
```

A `request_id` of 0 indicates a "singleplex" request, i.e. this request cannot
be processed in parallel with other requests. The server guarantees that a
given worker receives requests with either only `request_id` 0 or only
`request_id` greater than zero. Singleplex requests are sent in serial, i.e. the
server doesn't send another request until it has received a response (except
for cancel requests, see below).

**Notes**

* Each protocol buffer is preceded by its length in `varint` format (see
[`MessageLite.writeDelimitedTo()`](https://developers.google.com/protocol-buffers/docs/reference/java/com/google/protobuf/MessageLite.html#writeDelimitedTo-java.io.OutputStream-).
* JSON requests and responses are not preceded by a size indicator.
* JSON requests uphold the same structure as the protobuf, but use standard
 JSON.
* In order to maintain the same backward and forward compatibility
  properties as protobuf, JSON workers must tolerate unknown fields in
  these messages, and use the protobuf defaults for missing values.
* Bazel stores requests as protobufs and converts them to JSON using
[protobuf's JSON format](https://cs.opensource.google/protobuf/protobuf/+/master:java/util/src/main/java/com/google/protobuf/util/JsonFormat.java)

### Cancellation

Workers can optionally allow work requests to be cancelled before they finish.
This is particularly useful in connection with dynamic execution, where local
execution can regularly be interrupted by a faster remote execution. To allow
cancellation, add `supports-worker-cancellation: 1` to the
`execution-requirements` field (see below) and set the
`--experimental_worker_cancellation` flag.

A **cancel request** is a `WorkRequest` with the `cancel` field set (and
similarly a **cancel response** is a `WorkResponse` with the `was_cancelled`
field set). The only other field that must be in a cancel request or cancel
response is `request_id`, indicating which
request to cancel. The `request_id` field will be 0 for singleplex workers
or the non-0 `request_id` of a previously sent `WorkRequest` for multiplex
workers. The server may send cancel requests for requests that the worker has
already responded to, in which case the cancel request must be ignored.

Each non-cancel `WorkRequest` message must be answered exactly once, whether
or not it was cancelled. Once the server has sent a cancel request, the worker
may respond with a `WorkResponse` with the `request_id` set
and the `was_cancelled` field set to true. Sending a regular `WorkResponse`
is also accepted, but the `output` and `exit_code` fields will be ignored.

Once a response has been sent for a `WorkRequest`, the worker must not touch
the files in its working directory. The server is free to clean up the files,
including temporary files.

## Making the rule that uses the worker

You'll also need to create a rule that generates actions to be performed by the
worker. Making a Starlark rule that uses a worker is just like [creating any other rule](https://github.com/bazelbuild/examples/tree/HEAD/rules).

In addition, the rule needs to contain a reference to the worker itself, and
there are some requirements for the actions it produces.

### Referring to the worker
The rule that uses the worker needs to contain a field that refers to the worker
itself, so you'll need to create an instance of a `\*\_binary` rule to define
your worker. If your worker is called `MyWorker.Java`, this might be the
associated rule:

```python
java_binary(
    name = "worker",
    srcs = ["MyWorker.Java"],
)
```

This creates the "worker" label, which refers to the worker binary. You'll then
define a rule that *uses* the worker. This rule should define an attribute that
refers to the worker binary.

If the worker binary you built is in a package named "work", which is at the top
level of the build, this might be the attribute definition:

```python
"worker": attr.label(
    default = Label("//work:worker"),
    executable = True,
    cfg = "host",
)
```

`cfg = "host"` indicates that the worker should be built to run on your host
platform.

### Work action requirements

The rule that uses the worker creates actions for the worker to perform. These
actions have a couple of requirements.

* The _“arguments”_ field. This takes a list of strings, all but the last
  of which are arguments passed to the worker upon startup. The last element in
  the “arguments” list is a `flag-file` (@-preceded) argument. Workers read
  the arguments from the specified flagfile on a per-WorkRequest basis. Your
  rule can write non-startup arguments for the worker to this flagfile.

* The _“execution-requirements”_ field, which takes a dictionary containing
  `"supports-workers" : "1"`, `"supports-multiplex-workers" : "1"`, or both.

  The "arguments" and "execution-requirements" fields are required for all
  actions sent to workers. Additionally, actions that should be executed by
  JSON workers need to include `"requires-worker-protocol" : "json"` in the
  execution requirements field. `"requires-worker-protocol" : "proto"` is also
  a valid execution requirement, though it’s not required for proto workers,
  since they are the default.

  You can also set a `worker-key-mnemonic` in the execution requirements. This
  may be useful if you're reusing the executable for multiple action types and
  want to distinguish actions by this worker.

* Temporary files generated in the course of the action should be saved to the
  worker's directory. This enables sandboxing.

**Note**: To pass an argument starting with a literal `@`, start the argument
with `@@` instead. If an argument is also an external repository label, it will
not be considered a flagfile argument.

Assuming a rule definition with "worker" attribute described above, in addition
to a "srcs" attribute representing the inputs, an "output" attribute
representing the outputs, and an "args" attribute representing the worker
startup args, the call to `ctx.actions.run` might be:

```python
ctx.actions.run(
  inputs=ctx.files.srcs,
  outputs=[ctx.outputs.output],
  executable=ctx.executable.worker,
  mnemonic="someMnemonic",
  execution_requirements={
    "supports-workers" : "1",
    "requires-worker-protocol" : "json",
  },
  arguments=ctx.attr.args + ["@flagfile"]
 )
```

For another example, see [Implementing persistent workers](persistent-workers.html#implementation).

## Examples

The Bazel code base uses [Java compiler workers](https://github.com/bazelbuild/bazel/blob/a4251eab6988d6cf4f5e35681fbe2c1b0abe48ef/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/BazelJavaBuilder.java),
in addition to an [example JSON worker](https://github.com/bazelbuild/bazel/blob/c65f768fec9889bbf1ee934c61d0dc061ea54ca2/src/test/java/com/google/devtools/build/lib/worker/ExampleWorker.java) that is used in our integration tests.

You can use their [scaffolding](https://github.com/bazelbuild/bazel/blob/a4251eab6988d6cf4f5e35681fbe2c1b0abe48ef/src/main/java/com/google/devtools/build/lib/worker/WorkRequestHandler.java) to make any Java-based tool into a worker by passing in the correct
callback.

For an example of a rule that uses a worker, take a look at Bazel's
[worker integration test](https://github.com/bazelbuild/bazel/blob/22b4dbcaf05756d506de346728db3846da56b775/src/test/shell/integration/bazel_worker_test.sh#L106).

External contributors have implemented workers in a variety of languages; take a
look at [Polyglot implementations of Bazel persistent workers](https://github.com/Ubehebe/bazel-worker-examples).
You can [find many more examples on GitHub](https://github.com/search?q=bazel+workrequest&type=Code)!