With load balancer, each RPC pick a different backend based on the load balancing policy. Stickiness policies try to preserve peers for the duration of a session, so that RPCs with the same stickiness key will be directed to the same server.

Note that there's only “soft” stickiness now, which means RPCs with the same stickienss key could still be sent to different servers. If stickiness is critical for the system, server side application level handling is still necessary.

Stickiness Key

A stickiness key works as the session id. RPCs with the same stickiness key will be assigned to the same backend.

Stickiness key is set as part of the custom metadata.

Enable stickiness

Stickiness can be enabled by setting stickinessKey field in service config.

  "stickinessKey": "sessionid"

The value sesseionid will be used as the key of the metadata entry that defines the stickiness key for each RPC.

Send RPC with stickiness

To set the stickiness key for an RPC, set the corresponding metadata. The following RPC will be sent with stickiness key session1.

// "sessionid" is the metadata key specified by service config, "session1" is
// the stickiness key for this RPC.
md := metadata.Paris("sessionid", "session1")

ctx := metadata.NewOutgoingContext(context.Background(), md)
resp, err := client.SomeRPC(ctx, req)