Protocol and API surface

Who this is for: anyone writing or debugging a client, or checking whether a given REAPI feature is available. What you'll have at the end: the complete service list with implementation status. Time: use as a lookup.

NativeLink implements the Remote Execution API plus the adjacent protocols it depends on (ByteStream, long-running Operations, Remote Asset and the Build Event Protocol), and defines exactly one service of its own. Every vendored .proto under nativelink-proto/ is copied from upstream with only its licence header replaced; there are no NativeLink-specific fields or extensions on the standard messages.

Services

Each is registered only if the configuration asks for it. Registration order at startup is fixed: action cache, CAS, execution, Operations, fetch, push, ByteStream, capabilities, worker API, BEP.

WorkerApi: the only NativeLink-defined service

Defined in worker_api.proto. It has exactly one RPC:

rpc ConnectWorker(stream UpdateForScheduler) returns (stream UpdateForWorker);

One bidirectional stream carries a worker's entire relationship with its scheduler: registration, heartbeats, action assignment, progress and completion. No polling RPC and no second connection exist. See worker execution.

events.proto in the same tree declares message types only, no services.

Partially implemented RPCs

Three Operations RPCs are registered and return unimplemented:

A client that relies on cancellation to release cluster resources will not get it here. Actions are instead reaped by timeout; see client_action_timeout_s, which kills an action nobody is listening to.

What Capabilities advertises

Two of these need a caveat.

exec_enabled is hardcoded to true, with a TODO in the source acknowledging it. Whenever a capabilities entry names a remote_execution scheduler, execution is advertised as enabled; there is no way to advertise exec_enabled: false. A cache-only capabilities entry (no remote_execution) omits execution_capabilities altogether, which is the correct signal for a cache-only deployment.

The 64 KiB batch limit is advisory and small relative to what some clients assume. The CAS handlers do not enforce it themselves; the hard stop is the listener's max_decoding_message_size (4 MiB unless configured), and a batch request above that is rejected at the gRPC layer. Clients that honour the advertised limit send anything larger through ByteStream.

Wire-level details worth knowing

ByteStream resource names are parsed right to left. A resource name is {instance_name}/blobs/{hash}/{size}, and the instance name may itself contain slashes, so left-to-right parsing cannot find the boundary, while right-to-left can, because the trailing components are fixed. See ResourceInfo.

GetTree page tokens are "{hash}-{size}", constructed and parsed by splitting on -. They are neither opaque nor signed. The protocol asks clients to treat them as opaque, and clients that do will keep working if the format changes.

The Execute pre-flight input check is shallow and racy by design. It does not walk the full input tree, and a blob can be evicted between the check and the worker's fetch. It exists to turn the common "inputs were never uploaded" mistake into a fast error, not to provide a guarantee. See ExecutionServer.

ActionStage::CacheCheck is never emitted. The cache lookup happens in a decorator in front of the scheduler, so the state is skipped entirely. A client waiting to observe it will wait forever.

Final action failure arrives as success. An action that exhausts its retries returns ActionStage::Completed with ActionResult.error set: a successful RPC describing a failed action. This is what the REAPI specifies. Clients that only inspect the gRPC status will misread it.

server_logs is never populated. The field exists; the worker does not fill it.

Related

• Architecture deep dive: where each service sits in the process
• Scheduler internals: what happens between Execute and a worker
• Worker execution: the WorkerApi stream from the worker's side
• Configuration reference: which services a given config enables