messi 0.6.0

Reliable message passing in distributed systems.

⚽ Messi

Reliable message passing for distributed systems.

Architecture

A Messi system consists of servers and clients. Once a client has successfully connected to a server, messages defined in their protocol specification can be sent between them.

Below is a sequence diagram showing typical communication between a server and a client. The messages FooReq, FooRsp and BarInd are defined in the protocol my_protocol, which is defined later in this document.

+--------+                               +--------+
| client |                               | server |
+--------+                               +--------+
    |             1. TCP connect              |
    |========================================>|
    |                2. ping                  |
    |---------------------------------------->|
    |                   pong                  |
    |<----------------------------------------|
    |               3. FooReq                 |
    |========================================>|
    |                  FooRsp                 |
    |<========================================|
    |               4. BarInd                 |
    |========================================>|
    |                  BarInd                 |
    |========================================>|
    .                                         .
    .                                         .
    .                                         .
    |                5. ping                  |
    |---------------------------------------->|
    .                                         .
    .                                         .
    .                                         .

Legend:

  ---: Background communication. No user interaction needed.
  ===: User initiated communication.

1. The client connects to the server.

2. The client sends a ping message to the server, which responds with a pong message. This is done in the background. No user interaction needed.

3. The client sends FooReq to the server, which responds with FooRsp.

4. The client sends BarInd twice to the server. No response is defined.

5. The client sends another ping message. This time the server does not respond, and the client application is notified that it has been disconnected from the server.

Messi protocol specification

All messages sent on the wire consists of a type, a size and optional payload. This enables both streaming and packet based transport protocols. The transport protocol must be reliable (guaranteed in order delivery).

Type and size are in network byte order (big endian).

+---------+---------+-----------------+
| 4b type | 4b size | <size>b payload |
+---------+---------+-----------------+

TYPE  SIZE  DESCRIPTION
------------------------------------------------------------------
   1     n  User message (server <-> client).

            Encoded "message ClientToServer" for client to server
            messages and "message ServerToClient" for server to
            client messages.

   2     0  Ping message (client --> server).
   3     0  Pong message (server --> client).

User messages

User messages are defined in Googles Protocol Buffers language version 3.

Here is an example defining a protocol called my_protocol. The two messages ClientToServer and ServerToClient must be present in every protocol specification. ClientToServer contains all messages sent from clients to servers, and ServerToClient contains all messages sent from servers to clients.

syntax = "proto3";

// The protocol name.
package my_protocol;

// Messages sent from client to server.
message ClientToServer {
    oneof messages {
        FooReq foo_req = 1;
        BarInd bar_ind = 2;
        FieRsp fie_rsp = 3;
    }
}

// Messages sent from server to client.
message ServerToClient {
    oneof messages {
        FooRsp foo_rsp = 1;
        FieReq fie_req = 2;
    }
}

// Message definitions.
message FooReq {
}

message FooRsp {
}

message BarInd {
}

message FieReq {
}

message FieRsp {
}

Ping and pong messages

A client pings its server periodically. A client will close the connection and report an error if the server does not answer with pong within given time. Likewise, the server will close the connection and report an error if it does not receive ping within given time.

The ping-pong mechanism is only used if the transport layer does not provide equivalent functionality.

Error handling

Messi aims to minimize the amount of error handling code in the user application. Almost all functions always succeeds from the caller point of view. For example, PROTO_client_send() returns void. If an error occurs, likely a connection issue, the disconnect callback is called to notify the user that the connection was dropped.

C source code

Generate server and client side C source code.

$ messi generate_c_source examples/chat/chat.proto

Use -p/--platform to select which platform to generate code for. Planned platforms are Linux (using epoll and timerfd) and async.

The generated code is not thread safe.

Client side

Per client.

void PROTO_client_init();   // Initialize given client.
void PROTO_client_start();  // Connect to the server. The connected callback is
                            // called once connected. Automatic reconnect if
                            // disconnected.
void PROTO_client_stop();   // Disconnect from the server. Call start to connect
                            // again.
void PROTO_client_send();   // Send prepared message to the server.

Per Linux client.

void PROTO_client_process();  // Process all pending events on given file
                              // descriptor (if it belongs to given client).

Per message.

void PROTO_client_init_MESSAGE(); // Prepare given message. Call send or reply to
                                  // send it.

Server side

Per server.

void PROTO_server_init();        // Initialize given server.
void PROTO_server_start();       // Start accepting clients.
void PROTO_server_stop();        // Disconnect any clients and stop accepting new
                                 // clients.
void PROTO_server_send();        // Send prepared message to given client.
void PROTO_server_reply();       // Send prepared message to current client.
void PROTO_server_broadcast();   // Send prepared message to all clients.

Per Linux server.

void PROTO_server_process();  // Process all pending events on given file
                              // descriptor (if it belongs to given server).

Per message.

void PROTO_server_init_MESSAGE(); // Prepare given message. Call send, reply or
                                  // broadcast to send it.