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The Build Event Protocol (BEP) allows third-party programs to gain insight into a Bazel invocation. For example, you could use the BEP 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 BEP is intended to be consumed programmatically and makes parsing Bazel’s command line output a thing of the past. 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 if the build aborted prematurely.

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Build event graph

All build events form a directed acyclic graph (DAG) through their parent and child relationships. Every build event except for the initial BuildStarted event is announced by one or more earlier parent events.

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Announcement contract

The protocol requires that every event in the stream (except BuildStarted) is announced in the children list of an earlier event:
  • Static parents: High-level lifecycle events announce their natural children. For example, BuildStarted announces PatternExpanded and BuildFinished, and PatternExpanded announces TargetConfigured for each matching target.
  • Dynamic chaining via Progress: Events discovered dynamically during evaluation that have no natural static parent—such as NamedSetOfFiles, Configuration, WorkspaceConfig, and Fetch—are announced on the fly by intervening Progress events before they are posted.
  • Completion invariant: A build is considered complete if and only if all announced children have been posted or accounted for with an Aborted event.

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Announcement vs. delivery ordering

Being announced in children advertises that an event will eventually occur, but it does not imply delivery sequence:
  • Bazel posts events concurrently as actions complete across worker threads.
  • Some events (such as TargetComplete, TestResult, and TestSummary) specify order constraints (postedAfter in the Bazel implementation). Bazel holds back delivering these events until their prerequisite events have been posted.
  • For test executions, TargetComplete announces initial test attempts (TestResult), while each retry attempt is announced by the attempt that immediately preceded it.

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Lifecycle stages

The event graph reflects the phases of a Bazel invocation:
  1. Invocation Start: The root BuildStarted event is published first. It announces command metadata (OptionsParsed, CommandLine, WorkspaceStatus), target pattern expansion (PatternExpanded), and build completion (BuildFinished).
  2. Analysis & Configuration: PatternExpanded announces TargetConfigured events for parsed targets. Intervening Progress events chain in build configurations (Configuration).
  3. Execution: Each TargetConfigured event announces its corresponding TargetComplete event. TargetComplete references output file sets (NamedSetOfFiles, which are dynamically announced via intervening Progress events) and announces target-level summaries (TargetSummary) as well as test events (TestResult, TestSummary) for test targets.
  4. Completion & Tool Logs: The BuildFinished event is published after command evaluation finishes, providing the authoritative exit code. It announces post-build events such as BuildMetrics, BuildToolLogs, and, for bazel run without --script_path, ExecRequestConstructed. These may arrive in any order; the final event of the stream has last_message set.

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Consuming Build Event Protocol

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Consume in binary format

To consume the BEP 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.

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Consume in text or JSON formats

The following Bazel command line flags will output the BEP in human-readable formats, such as text and JSON:

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Build Event Service

The Build Event Service Protocol is a generic gRPC service for publishing build events. The Build Event Service protocol is independent of the BEP and treats BEP events as opaque bytes. Bazel ships with a gRPC client implementation of the Build Event Service protocol that publishes Build Event Protocol events. One can specify the endpoint to send the events to using the --bes_backend=HOST:PORT flag. If your backend uses gRPC, you must prefix the address with the appropriate scheme: grpc:// for plaintext gRPC and grpcs:// for gRPC with TLS enabled.

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Build Event Service flags

Bazel has several flags related to the Build Event Service protocol, including:
  • --bes_backend
  • --[no]bes_lifecycle_events
  • --bes_results_url
  • --bes_timeout
  • --bes_instance_name
For a description of each of these flags, see the Command-Line Reference.

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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.
  • --[no]google_default_credentials
  • --google_credentials
  • --google_auth_scopes
  • --tls_certificate
  • --[no]tls_enabled
For a description of each of these flags, see the Command-Line Reference.

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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.