This examples shows how ClientConn can pick different load balancing policies.
Note: to show the effect of load balancers, an example resolver is installed in this example to get the backend addresses. It's suggested to read the name resolver example before this example.
go run server/main.go
go run client/main.go
Two echo servers are serving on “:50051” and “:50052”. They will include their serving address in the response. So the server on “:50051” will reply to the RPC with this is examples/load_balancing (from :50051).
Two clients are created, to connect to both of these servers (they get both server addresses from the name resolver).
Each client picks a different load balancer (using grpc.WithBalancerName): pick_first or round_robin. (These two policies are supported in gRPC by default. To add a custom balancing policy, implement the interfaces defined in https://godoc.org/google.golang.org/grpc/balancer).
Note that balancers can also be switched using service config, which allows service owners (instead of client owners) to pick the balancer to use. Service config doc is available at https://github.com/grpc/grpc/blob/master/doc/service_config.md.
The first client is configured to use pick_first. pick_first tries to connect to the first address, uses it for all RPCs if it connects, or try the next address if it fails (and keep doing that until one connection is successful). Because of this, all the RPCs will be sent to the same backend. The responses received all show the same backend address.
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The second client is configured to use round_robin. round_robin connects to all the addresses it sees, and sends an RPC to each backend one at a time in order. E.g. the first RPC will be sent to backend-1, the second RPC will be be sent to backend-2, and the third RPC will be be sent to backend-1 again.
this is examples/load_balancing (from :50051) this is examples/load_balancing (from :50051) this is examples/load_balancing (from :50052) this is examples/load_balancing (from :50051) this is examples/load_balancing (from :50052) this is examples/load_balancing (from :50051) this is examples/load_balancing (from :50052) this is examples/load_balancing (from :50051) this is examples/load_balancing (from :50052) this is examples/load_balancing (from :50051)
Note that it‘s possible to see two continues RPC sent to the same backend. That’s because round_robin only picks the connections ready for RPCs. So if one of the two connections is not ready for some reason, all RPCs will be sent to the ready connection.