Brief: Build a Recoverable Capability#

Goal. Serve a native capability from two providers, invoke it by service name, remove the first provider between calls, and verify that a later call selects the standby. See Submitted Is Not Completed for the distinction between a returned call and a verified workflow outcome.

Prerequisites#

• Rust toolchain; cargo add net-mesh-sdk tokio serde.
• No external mesh needed — this brief stands up its own two (or three) in-process Mesh nodes, following adapters/mcp/tests/serve_end_to_end.rs (invoke_fails_over_when_the_primary_provider_goes_down).

Steps#

1. Serve the capability from two providers. Build two Mesh nodes, register the same typed handler on each under one service name, and make the capability substitutable so the mesh treats them as interchangeable:

   ▸ rust⎘copy

   let _h1 = primary.serve_rpc_typed("summarize", handler.clone())?;
   let _h2 = standby.serve_rpc_typed("summarize", handler.clone())?;

2. Invoke by service name, not node id — this is what makes failover possible:

   ▸ rust⎘copy

   let resp: SummarizeResp = caller.call_service_typed("summarize", &req, opts).await?;

3. Kill the primary between two calls, then invoke again. Wrap the call in the retry helper so the transient failure during cutover is absorbed:

   ▸ rust⎘copy

   use net_sdk::mesh_rpc_resilience::RetryPolicy;
   let resp = caller.call_service_typed_with_retry("summarize", &req, opts, &RetryPolicy::default()).await?;

Expected output#

• The first call_service_typed returns a result from the primary.
• After the primary is dropped, the retried call_service_* returns a result from the standby — one successful response, no caller-visible error.

Verify (acceptance)#

• The pre-kill call and the post-kill call both return a valid SummarizeResp.
• The post-kill result demonstrably came from the standby (tag the two handlers' output so you can tell them apart).
• Calling by a pinned node id instead of the service name does not select another provider. Provider reselection applies to service-name discovery.

Pitfalls#

• Call by service name for failover. A call pinned to a dead node id cannot reroute — it just fails.
• Retry only retryable errors. RetryPolicy re-issues transient/backpressure failures, not application errors — retrying a bad request forever is not recovery.
• For latency-sensitive paths, prefer hedging (call_service_with_hedge) over retry: race a second provider instead of waiting for the first to fail.
• A CircuitBreaker around a repeatedly-failing target stops you from burning every deadline on a provider that's already down.

See Recover a Failed Workflow and the per-SDK errors pages (e.g. Rust).