Build Event Protocol

The Build Event Protocol allows third party programs to gain insight into a Bazel invocation. For example, you could use the Build Event Protocol to gather information for an IDE plugin or a dashboard that displays build results.

The protocol is a set of protocol buffer messages with some semantics defined on top of it. It includes information about build and test results, build progress, the build configuration and much more. The Build Event Protocol is intended to be consumed programmatically and makes parsing Bazel’s command line output a thing of the past.


Build Event Protocol Overview

The Build Event Protocol represents information about a build as events. A build event is a protocol buffer message consisting of a build event identifier, a set of child event identifiers, and a payload.

  • Build Event Identifier: Depending on the kind of build event, it might be an opaque string or structured information revealing more about the build event. A build event identifier is unique within a build.

  • Children: A build event may announce other build events, by including their build event identifiers in its children field. For example, the PatternExpanded build event announces the targets it expands to as children. The protocol guarantees that all events, except for the first event, are announced by a previous event.

  • Payload: The payload contains structured information about a build event, encoded as a protocol buffer message specific to that event. Note, that the payload might not be the expected type, but could be an Aborted message e.g. if the build aborted prematurely.

Build Event Graph

All build events form a directed acyclic graph through their parent and child relationship. Every build event except for the initial build event has one or more parent events. Please note that not all parent events of a child event must necessarily be posted before it. When a build is complete (succeeded or failed) all announced events will have been posted. In case of a Bazel crash or a failed network transport, some announced build events may never be posted.

The Build Event Protocol by Example

The full specification of the Build Event Protocol can be found in its protocol buffer definition and describing it here is beyond the scope of this document. However, it might be helpful to build up some intuition before looking at the specification.

Consider a simple Bazel workspace that consists of two empty shell scripts and and the following BUILD file:

    name = "foo",
    srcs = [""],

    name = "foo_lib",
    data = [":foo"],

    name = "foo_test",
    srcs = [""],
    deps = [":foo_lib"],

When running bazel test ... on this project the build graph of the generated build events will resemble the graph below. The arrows indicate the aforementioned parent and child relationship. Note that some build events and most fields have been omitted for brevity.


Initially, a BuildStarted event is published. The event informs us that the build was invoked through the bazel test command and it also announces five child events: OptionsParsed, WorkspaceStatus, CommandLine, PatternExpanded and Progress. The first three events provide information about how Bazel was invoked. The PatternExpanded build event provides insight into which specific targets the ... pattern expanded to: //:foo, //:foo_lib and //:foo_test. It does so by declaring three TargetConfigured events as children.

Note that the TargetConfigured event declares the Configuration event as a child event, even though Configuration has been posted before the TargetConfigured event.

Besides the parent and child relationship, events may also refer to each other using their build event identifiers. For example, in the above graph the TargetComplete event refers to the NamedSetOfFiles event in its fileSets field.

Build events that refer to files (i.e. outputs) usually don’t embed the file names and paths in the event. Instead, they contain the build event identifier of a NamedSetOfFiles event, which will then contain the actual file names and paths. The NamedSetOfFiles event allows a set of files to be reported once and referred to by many targets. This structure is necessary because otherwise in some cases the Build Event Protocol output size would grow quadratically with the number of files. A NamedSetOfFiles event may also not have all its files embedded, but instead refer to other NamedSetOfFiles events through their build event identifiers.

Below is an instance of the TargetComplete event for the //:foo_lib target from the above graph, printed in protocol buffer’s JSON representation. The build event identifier contains the target as an opaque string and refers to the Configuration event using its build event identifier. The event does not announce any child events. The payload contains information about whether the target was built successfully, the set of output files, and the kind of target built.

  "id": {
    "targetCompleted": {
      "label": "//:foo_lib",
      "configuration": {
        "id": "544e39a7f0abdb3efdd29d675a48bc6a"
  "completed": {
    "success": true,
    "outputGroup": [{
      "name": "default",
      "fileSets": [{
        "id": "0"
    "targetKind": "sh_library rule"

Consuming the Build Event Protocol

Consume in a Binary Format

To consume the Build Event Protocol in a binary format:

  1. Have Bazel serialize the protocol buffer messages to a file by specifying the option --build_event_binary_file=/path/to/file. The file will contain serialized protocol buffer messages with each message being length delimited. Each message is prefixed with its length encoded as a variable length integer. This format can be read using the protocol buffer library’s parseDelimitedFrom(InputStream) method.

  2. Then, write a program that extracts the relevant information from the serialized protocol buffer message.

Consume in Text Formats

The following Bazel command line flags will output the Build Event Protocol in a human-readable formats:


The Build Event Service

The Build Event Service Protocol is a generic gRPC service for transmitting build events. The Build Event Service protocol is independent of the Build Event Protocol and treats the Build Event Protocol events as opaque bytes. Bazel ships with a gRPC client implementation of the Build Event Service protocol that transmits Build Event Protocol events. One can specify the endpoint to send the events to using the --bes_backend=HOST:PORT flag. Bazel’s implementation also supports TLS which can be enabled by specifying the --tls_enabled flag.

There is currently no open source server-side Build Event Service implementation that we know of.

Build Event Service Flags

Bazel has several flags related to the Build Event Service protocol:

  • --bes_backend
  • --[no]bes_best_effort
  • --[no]bes_lifecycle_events
  • --bes_timeout
  • --project_id

For a description of each of these flags, see the Command-Line Reference.

Authentication and Security

Bazel’s Build Event Service implementation also supports authentication and TLS. These settings can be controlled using the below flags. Please note that these flags are also used for Bazel’s Remote Execution. This implies that the Build Event Service and Remote Execution Endpoints need to share the same authentication and TLS infrastructure.

  • --auth_credentials
  • --[no]auth_enabled
  • --auth_scope
  • --tls_certificate
  • --[no]tls_enabled

For a description of each of these flags, see the Command-Line Reference.

Build Event Service and Remote Caching

The BEP typically contains many references to log files (test.log, test.xml, etc. ) stored on the machine where Bazel is running. A remote BES server typically can't access these files as they are on different machines. A way to work around this issue is to use Bazel with [remote caching]. Bazel will upload all output files to the remote cache (including files referenced in the BEP) and the BES server can then fetch the referenced files from the cache.

See [GitHub issue 3689] for more details.

remote caching

GitHub issue 3689