The OPA gRPC server returned an error
Your pipeline stalled. Logs are silent. The policy engine sits there, waiting, but no decision comes.
Open Policy Agent (OPA) gRPC errors often hide behind generic failure messages. They may read like rpc error: code = Unknown desc = evaluation error or break under status codes like Unavailable, DeadlineExceeded, or Internal. These are not random—they point to specific points of failure in policy evaluation over gRPC.
The most common causes boil down to three clusters:
1. Connection and Transport Failures
When OPA runs as a sidecar or remote server, gRPC transport can fail if address bindings change, TLS certs expire, or ports are blocked. Always verify OPA’s --addr or --grpc-addr matches the client configuration. Confirm grpc.max_message_size fits the payload. Check firewall rules. In Kubernetes, inspect service endpoints and pod health.
2. Policy Compilation and Runtime Errors
OPA loads Rego policies before serving gRPC calls. If a policy has syntax errors, or if an import path is wrong, evaluation will throw InvalidArgument. Enable --decision-logs to capture the request and evaluation trace. Run opa eval locally against the same input to confirm it passes before sending over gRPC.
3. Resource and Timeout Issues
Large JSON inputs can push OPA over CPU or memory limits. When gRPC times out (DeadlineExceeded), it usually means OPA couldn’t complete evaluation in time. Reduce input size, pre-process data, or increase gRPC deadlines on the client side. Monitor OPA’s prometheus metrics for evaluation durations.
To debug fast, set OPA log level to debug for gRPC (opa run --log-level debug). Capture both sides of the wire. Use gRPC health checks to detect dead servers before requests are made. If you run in production, deploy readiness probes so failing OPA instances stop receiving traffic.
The fastest path from error to resolution is narrowing scope: verify connectivity, validate policy syntax, check resource metrics. OPA’s gRPC errors are clear once context is stripped away. Every fix begins with understanding the exact code and logging surrounding it.
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