redis_server_protocol: improve user comment

src/brpc/policy/redis_protocol.cpp

@@ -115,9 +115,13 @@ ParseResult ParseRedisMessage(butil::IOBuf* source, Socket* socket,
     }
     const Server* server = static_cast<const Server*>(arg);
     if (server) {
+        RedisService* rs = server->options().redis_service;
+        if (!rs) {
+            return MakeParseError(PARSE_ERROR_TRY_OTHERS);
+        }
         ServerContext* ctx = static_cast<ServerContext*>(socket->parsing_context());
         if (ctx == NULL) {
-            RedisConnection* conn = server->options().redis_service->NewConnection();
+            RedisConnection* conn = rs->NewConnection();
             if (!conn) {
                 LOG(ERROR) << "Fail to new redis connection from redis service";
                 return MakeParseError(PARSE_ERROR_TRY_OTHERS);

src/brpc/redis.h

@@ -208,7 +208,22 @@ private:
 
 std::ostream& operator<<(std::ostream& os, const RedisRequest&);
 std::ostream& operator<<(std::ostream& os, const RedisResponse&);
+class RedisConnection;
 
+// Implement this class and assign an instance to ServerOption.redis_service
+// to enable redis support. The return type of NewConnection(), which is
+// RedisConnection, should also be implemented by users.
+class RedisService {
+public:
+    virtual ~RedisService() {}
+    virtual RedisConnection* NewConnection() = 0;
+};
+
+// Implement this class and make RedisServiceImpl::NewConnection return the
+// implemented class. Notice that one TCP connection corresponds to one RedisConnection
+// instance, and for the same TCP connection, OnRedisMessage is called sequentially.
+// But OnRedisMessage are called concurrently between different TCP connections.
+// Read src/brpc/redis_message.h to get the idea how to read and write RedisMessage.
 class RedisConnection {
 public:
     virtual ~RedisConnection() {}
@@ -216,11 +231,6 @@ public:
             RedisMessage* output, butil::Arena* arena) = 0;
 };
 
-class RedisService {
-public:
-    virtual ~RedisService() {}
-    virtual RedisConnection* NewConnection() = 0;
-};
 
 } // namespace brpc
 

src/brpc/server.h

@@ -237,7 +237,7 @@ struct ServerOptions {
     // Customize parameters of HTTP2, defined in http2.h
     H2Settings h2_settings;
 
-    // For processing Redis conneections.
+    // For processing Redis connections. Read src/brpc/redis.h for details.
     // Default: NULL (disabled)
     RedisService* redis_service;