1GRPC Health Checking Protocol 2================================ 3 4Health checks are used to probe whether the server is able to handle rpcs. The 5client-to-server health checking can happen from point to point or via some 6control system. A server may choose to reply “unhealthy” because it 7is not ready to take requests, it is shutting down or some other reason. 8The client can act accordingly if the response is not received within some time 9window or the response says unhealthy in it. 10 11 12A GRPC service is used as the health checking mechanism for both simple 13client-to-server scenario and other control systems such as load-balancing. 14Being a high 15level service provides some benefits. Firstly, since it is a GRPC service 16itself, doing a health check is in the same format as a normal rpc. Secondly, 17it has rich semantics such as per-service health status. Thirdly, as a GRPC 18service, it is able reuse all the existing billing, quota infrastructure, etc, 19and thus the server has full control over the access of the health checking 20service. 21 22## Service Definition 23 24The server should export a service defined in the following proto: 25 26``` 27syntax = "proto3"; 28 29package grpc.health.v1; 30 31message HealthCheckRequest { 32 string service = 1; 33} 34 35message HealthCheckResponse { 36 enum ServingStatus { 37 UNKNOWN = 0; 38 SERVING = 1; 39 NOT_SERVING = 2; 40 } 41 ServingStatus status = 1; 42} 43 44service Health { 45 rpc Check(HealthCheckRequest) returns (HealthCheckResponse); 46} 47``` 48 49A client can query the server’s health status by calling the `Check` method, and 50a deadline should be set on the rpc. The client can optionally set the service 51name it wants to query for health status. The suggested format of service name 52is `package_names.ServiceName`, such as `grpc.health.v1.Health`. 53 54The server should register all the services manually and set 55the individual status, including an empty service name and its status. For each 56request received, if the service name can be found in the registry, 57a response must be sent back with an `OK` status and the status field should be 58set to `SERVING` or `NOT_SERVING` accordingly. If the service name is not 59registered, the server returns a `NOT_FOUND` GRPC status. 60 61The server should use an empty string as the key for server's 62overall health status, so that a client not interested in a specific service can 63query the server's status with an empty request. The server can just do exact 64matching of the service name without support of any kind of wildcard matching. 65However, the service owner has the freedom to implement more complicated 66matching semantics that both the client and server agree upon. 67 68A client can declare the server as unhealthy if the rpc is not finished after 69some amount of time. The client should be able to handle the case where server 70does not have the Health service. 71