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The Action Cache (AC) service stores and retrieves action results, enabling incremental builds by reusing previously computed outputs.

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Overview

The Action Cache maps action digests to their execution results, including:
  • Exit code
  • stdout/stderr
  • Output file digests
  • Execution metadata
Key features:
  • Action result caching
  • Read-only mode support
  • Automatic result validation

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Configuration

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instance_name
string
required
The instance name identifying this AC endpoint
​
ac_store
string
required
Reference to the store configured in the stores section
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read_only
boolean
default:false
If true, only GetActionResult is allowed (no updates)
{
  services: {
    ac: [
      {
        instance_name: "main",
        ac_store: "AC_STORE",
        read_only: false
      }
    ]
  }
}

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gRPC Methods

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GetActionResult

Retrieve a cached action result. Request:
​
instance_name
string
required
The instance name
​
action_digest
Digest
required
Digest of the Action message
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inline_stdout
boolean
default:false
Include stdout inline if small enough
​
inline_stderr
boolean
default:false
Include stderr inline if small enough
​
inline_output_files
string[]
Paths of output files to include inline
Response:
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exit_code
int32
Process exit code
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stdout_digest
Digest
Digest of stdout (if not inlined)
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stdout_raw
bytes
Inline stdout (if requested and small)
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stderr_digest
Digest
Digest of stderr (if not inlined)
​
stderr_raw
bytes
Inline stderr (if requested and small)
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output_files
OutputFile[]
Output files produced by the action
​
output_directories
OutputDirectory[]
Output directories produced by the action
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execution_metadata
ExecutedActionMetadata
Metadata about the execution (worker, timing, etc.)
Example with grpcurl: 
grpcurl -plaintext \
  -d '{
    "instance_name": "main",
    "action_digest": {
      "hash": "abc123...",
      "size_bytes": 256
    },
    "inline_stdout": true,
    "inline_stderr": true
  }' \
  localhost:50051 build.bazel.remote.execution.v2.ActionCache/GetActionResult

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UpdateActionResult

Store an action result in the cache. Request:
​
instance_name
string
required
The instance name
​
action_digest
Digest
required
Digest of the Action message
​
action_result
ActionResult
required
The result to cache
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results_cache_policy
ResultsCachePolicy
Caching policy (priority, etc.)
Response:
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action_result
ActionResult
The stored action result (echoed back)
Updates fail with PERMISSION_DENIED if the AC is configured as read-only

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Cache Key Structure

The action digest serves as the cache key and is computed from:
  • Command line arguments
  • Environment variables
  • Input file digests
  • Platform properties
The action digest must be deterministic - same inputs always produce the same digest

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Implementation Details

From nativelink-service/src/ac_server.rs: 
pub struct AcServer {
    stores: HashMap<String, AcStoreInfo>,
}

pub struct AcStoreInfo {
    store: Store,
    read_only: bool,
}
Each instance can have independent read-only settings, allowing shared read-only caches with local read-write overlays.

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Error Codes

CodeDescription
NOT_FOUNDAction result not in cache
INVALID_ARGUMENTInvalid action digest or instance name
PERMISSION_DENIEDUpdate attempted on read-only AC
INTERNALStorage backend error
The AC service follows the Remote Execution API v2 specification

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