ActionCache is a service that can be used to query whether a defined action has already been executed and, if so, download its result. The service API is defined in the Remote Execution API. ActionCache service would require ContentAddressableStorage service to store file data.

An Action encapsulates all the information required to execute an action. Such information includes the command, input tree containing subdirectory/file tree, environment variables, platform information. All the information will contribute to the digest computation of an Action so that execution of an Action multiple times will produce the same output. With this, hash of an Action can be used as a key to cached ActionResult, which store result and output of an Action after an Action is completed. ActionResults can be populated in ActionCache service after Actions get completed by an Execution service. They can also come from a local Bazel client that has executed the Actions and put the ActionResults into the cache by using the UpdateActionCache method. In other words, The ActionCache service can be used without using/implementing the Execution service.

By leveraging Action definition, ActionCache service is responsible for mapping Actions to the ActionResults.

Methods

GetActionResult

Essentially the “get” method, which is responsible for finding an ActionResult and retrieving it. Before invoking this method, Bazel client should compute the input tree and the Action message for the action needs to be done. Then Bazel can use this GetActionCache method to see if the action has been completed successfully and, if so, use bytestream.Read to download the outputs.

UpdateActionResult

As mentioned above, the ActionCache service doesn’t necessarily need an Execution Service. In this case, a “put” method is required so that an ActionResult can be directly put into the cache. This is what UpdateActionResult is designed for. With this method, Bazel clients running different machines can upload their build results into a cache pool, which can be available to other users through GetActionResult.

Buildfarm Implementations

Buildfarm provides implementations of both ActionCache and Execution. ActionResult can be populated through Execution service or uploaded by local Bazel clients through UpdateActionCache.

The ActionCache service is hosted on the server-side of Buildfarm. When a GetActionResultRequest received through GetActionResult call, by using the instance_name field of the request, the ActionCache service will find the Instance that is supposed to have ActionResult, and it will find the Instance and get the ActionResult asynchronously. Similarly, an UpdateActionResultRequest will be sent to the ActionCache service through UpdateActionResult rpc call. The service will find the Instance that is supposed to have the ActionResult through the instance_name field of the request and update the corresponding instance.