| # ORCA Load Reporting | |
| ORCA is a protocol for reporting load between servers and clients. This | |
| example shows how to implement this from both the client and server side. For | |
| more details, please see [gRFC | |
| A51](https://github.com/grpc/proposal/blob/master/A51-custom-backend-metrics.md) | |
| ## Try it | |
| ``` | |
| go run server/main.go | |
| ``` | |
| ``` | |
| go run client/main.go | |
| ``` | |
| ## Explanation | |
| gRPC ORCA support provides two different ways to report load data to clients | |
| from servers: out-of-band and per-RPC. Out-of-band metrics are reported | |
| regularly at some interval on a stream, while per-RPC metrics are reported | |
| along with the trailers at the end of a call. Both of these mechanisms are | |
| optional and work independently. | |
| The full ORCA API documentation is available here: | |
| https://pkg.go.dev/google.golang.org/grpc/orca | |
| ### Out-of-band Metrics | |
| The server registers an ORCA service that is used for out-of-band metrics. It | |
| does this by using `orca.Register()` and then setting metrics on the returned | |
| `orca.Service` using its methods. | |
| The client receives out-of-band metrics via the LB policy. It receives | |
| callbacks to a listener by registering the listener on a `SubConn` via | |
| `orca.RegisterOOBListener`. | |
| ### Per-RPC Metrics | |
| The server is set up to report query cost metrics in its RPC handler. For | |
| per-RPC metrics to be reported, the gRPC server must be created with the | |
| `orca.CallMetricsServerOption()` option, and metrics are set by calling methods | |
| on the returned `orca.CallMetricRecorder` from | |
| `orca.CallMetricRecorderFromContext()`. | |
| The client performs one RPC per second. Per-RPC metrics are available for each | |
| call via the `Done()` callback returned from the LB policy's picker. |