gRPC supports two types of load balancing policies on the client side:
- PickFirst: The client will connect to the first server that it resolves.
- RoundRobin: The client will connect to the servers in a round robin fashion.
The default load balancing policy is PickFirst. To use the RoundRobin load balancing policy, we need to set the default service config. The default service config is a JSON string that contains the load balancing policy.
fooConn, err := grpc.Dial(service,
grpc.WithTransportCredentials(insecure.NewCredentials()),
grpc.WithDefaultServiceConfig(`{"loadBalancingPolicy":"round_robin"}`),
)With round robin, client connects to all addresses it sees and sends RPC to each backend address one at a time. If the connection is not ready for any reason, client may send RPCs to the same backend address multiple times.
-
The sample server takes a comma separated list of ports on the input and starts a gRPC server on each of the ports.
-
Similar client takes a comma separated list of addresses to connect to. Client then uses the sample resolver (FooResolver) added in the name_resolving example to resolve the sample service
foo:///resolver.foo.barto these backend addresses in a round robin fashion. -
Here is a sample output with debugging logs enabled on the client side. Can see that sometimes RPCs are sent to same server consecutively, depending on the connection readiness.
✗ go run server/main.go --ports 50505,50506,50507
2023/04/25 21:17:29 Listening on port 50505
2023/04/25 21:17:29 Listening on port 50506
2023/04/25 21:17:29 Listening on port 50507
2023/04/25 21:17:32 ServerPort:50507 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50507 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50505 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50506 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50507 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50505 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50506 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50507 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50505 UnaryEcho: foo:///resolver.foo.bar
2023/04/25 21:17:32 ServerPort:50506 UnaryEcho: foo:///resolver.foo.bar
✗ GRPC_GO_LOG_VERBOSITY_LEVEL=99 GRPC_GO_LOG_SEVERITY_LEVEL=info go run client/main.go --addr localhost:50507,localhost:50505,localhost:50506
2023/04/25 21:30:17 INFO: [core] [Channel #1] Channel created
2023/04/25 21:30:17 INFO: [core] [Channel #1] original dial target is: "foo:///resolver.foo.bar"
2023/04/25 21:30:17 INFO: [core] [Channel #1] parsed dial target is: {Scheme:foo Authority: URL:{Scheme:foo Opaque: User: Host: Path:/resolver.foo.bar RawPath:
OmitHost:false ForceQuery:false RawQuery: Fragment: RawFragment:}}
2023/04/25 21:30:17 INFO: [core] [Channel #1] Channel authority set to "resolver.foo.bar"
2023/04/25 21:30:17 INFO: [core] [Channel #1] Resolver state updated: {
"Addresses": [
{
"Addr": "localhost:50507",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
},
{
"Addr": "localhost:50505",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
},
{
"Addr": "localhost:50506",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}
],
"ServiceConfig": null,
"Attributes": null
} (resolver returned new addresses)
2023/04/25 21:30:17 INFO: [balancer] base.baseBalancer: got new ClientConn state: {{[{ [68/11852]
"Addr": "localhost:50507",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
} {
"Addr": "localhost:50505",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
} {
"Addr": "localhost:50506",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}] <nil> <nil>} <nil>}
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #2] Subchannel created
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #3] Subchannel created
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #4] Subchannel created
2023/04/25 21:30:17 INFO: [roundrobin] roundrobinPicker: Build called with info: {map[]}
2023/04/25 21:30:17 INFO: [core] [Channel #1] Channel Connectivity change to CONNECTING
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #2] Subchannel Connectivity change to CONNECTING
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #3] Subchannel Connectivity change to CONNECTING
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #2] Subchannel picks a new address "localhost:50507" to connect
2023/04/25 21:30:17 INFO: [balancer] base.baseBalancer: handle SubConn state change: 0x140000ae810, CONNECTING
2023/04/25 21:30:17 INFO: [balancer] base.baseBalancer: handle SubConn state change: 0x140000ae828, CONNECTING
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #3] Subchannel picks a new address "localhost:50505" to connect
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #4] Subchannel Connectivity change to CONNECTING
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #4] Subchannel picks a new address "localhost:50506" to connect
2023/04/25 21:30:17 INFO: [balancer] base.baseBalancer: handle SubConn state change: 0x140000ae840, CONNECTING
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #4] Subchannel Connectivity change to READY
2023/04/25 21:30:17 INFO: [balancer] base.baseBalancer: handle SubConn state change: 0x140000ae840, READY
2023/04/25 21:30:17 INFO: [roundrobin] roundrobinPicker: Build called with info: {map[0x140000ae840:{{
"Addr": "localhost:50506", [29/11852]
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}}]}
2023/04/25 21:30:17 INFO: [core] [Channel #1] Channel Connectivity change to READY
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #2] Subchannel Connectivity change to READY
2023/04/25 21:30:17 INFO: [balancer] base.baseBalancer: handle SubConn state change: 0x140000ae810, READY
2023/04/25 21:30:17 INFO: [roundrobin] roundrobinPicker: Build called with info: {map[0x140000ae810:{{
"Addr": "localhost:50507",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}} 0x140000ae840:{{
"Addr": "localhost:50506",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}}]}
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #3] Subchannel Connectivity change to READY
2023/04/25 21:30:17 INFO: [balancer] base.baseBalancer: handle SubConn state change: 0x140000ae828, READY
2023/04/25 21:30:17 INFO: [roundrobin] roundrobinPicker: Build called with info: {map[0x140000ae810:{{
"Addr": "localhost:50507",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}} 0x140000ae828:{{
"Addr": "localhost:50505",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}} 0x140000ae828:{{
"Addr": "localhost:50505",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}} 0x140000ae840:{{
"Addr": "localhost:50506",
"ServerName": "",
"Attributes": null,
"BalancerAttributes": null,
"Type": 0,
"Metadata": null
}}]}
2023/04/25 21:30:17 echo(0): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(1): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(2): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(3): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(4): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(5): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(6): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(7): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(8): foo:///resolver.foo.bar
2023/04/25 21:30:17 echo(9): foo:///resolver.foo.bar
2023/04/25 21:30:17 INFO: [core] [Channel #1] Channel Connectivity change to SHUTDOWN
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #2] Subchannel Connectivity change to SHUTDOWN
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #2] Subchannel deleted
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #3] Subchannel Connectivity change to SHUTDOWN
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #3] Subchannel deleted
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #4] Subchannel Connectivity change to SHUTDOWN
2023/04/25 21:30:17 INFO: [core] [Channel #1 SubChannel #4] Subchannel deleted
2023/04/25 21:30:17 INFO: [core] [Channel #1] Channel deleted