Crate ump_ng

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Micro Message Pass: Next Generation (ump-ng) is a library for passing messages between thread/tasks. It is similar to the ump library, but with an added uni-directional message passing primitive.

The primary purpose of ump(-ng) is to create simple RPC like designs, but between threads/tasks within a process rather than between processes over networks.

High-level usage overview

An application calls channel to create a linked pair of a Server and a Client.

The server calls Server::wait()/Server::async_wait(), which blocks and waits for an incoming message from a client.

A client, in a separate thread or task, calls Client::post() to send a unidirectional message to the server, or Client::req()/ Client::areq() to send a message to the server and wait for a reply.

The server’s wait call returns either a post message or a request message that consist a pair of a message and a ReplyContext that is used to send a reply back to the client.

After processing its application-defined message, the server must call the ReplyContext::reply() on the returned reply context object to return a reply message to the client.

Typically the server calls wait again to wait for next message from a client.

The client receives the reply from the server and processes it.


use std::thread;

use ump_ng::{channel, MsgType};

let (server, client) = channel::<String, String, String, ()>();

let server_thread = thread::spawn(move || {
  // Wait for data to arrive from a client
  loop {
    println!("Server waiting for message ..");
    match server.wait().unwrap() {
      MsgType::Put(data) => {
        println!("Server received Put: '{}'", data);
      MsgType::Request(data, rctx) => {
        println!("Server received Request: '{}'", data);

        // Process data from client

        // Reply to client
        let reply = format!("Hello, {}!", data);
        println!("Server replying '{}'", reply);

  println!("Server done");

let msg = String::from("Client");
println!("Client putting '{}'", msg);
let reply =;

let msg = String::from("Client");
println!("Client requesting '{}'", msg);
let reply = client.req(msg).unwrap();
println!("Client received reply '{}'", reply);

println!("Client done");


In practice the send/reply types will probably be enums used to indicate command/return type with associated data. The third type argument to channel is an error type that can be used to explicitly pass errors back to the sender.


There are some potentially useful semantic quirks that can be good to know about, but some of them should be used with caution. This section will describe some semantics that you can rely on, and others that you should be careful about relying on.

Stable invariants

These are behaviors which should not change in future versions.

  • The reply contexts are independent of the Server context. This has some useful implications for server threads that spawn separate threads to process messages and return replies: The server can safely terminate while there are clients waiting for replies (implied: the server can safely terminate while there are reply contexts in-flight).
  • A cloned client is paired with the same server as its origin, but in all other respects the clone and its origin are independent of each other.
  • A client can be moved to a new thread.
  • Any permutation of sync/async server/clients can be combined. async code must use the async method variants when available.

Unstable invariants

These are invariants you can trust will work in the current version, but they exist merely as a side-effect of the current implementation. Avoid relying on these if possible.

  • A single client can be used from two different threads. If a Client object in placed in an Arc, is cloned and passed to another thread/task then both the clone and the original can be used simultaneously. In the future this may not be allowed. It is recommended that a new clone of the client be created instead.
  • Put/Request messages arrive in the same order they were added to the queue. In future versions one type may be prioritized over the other.


  • Representation of a clonable client object.
  • Object used to reply to requests passed to the server.
  • Representation of a server object.
  • Context used to wait for a server to reply to a request.


  • Module-specific error codes.
  • Mesage operation types.