mahan / majortomo

ZeroMQ MDP 0.2 (Majordomo) implementation in Python

Majortomo

Majortomo is a pure-Python ZeroMQ MDP 0.2 ("Majordomo") implementation. It provides a ready-to-use MDP Service Broker, as well as a Python 2.7 / 3.5+ library for implementing MDP clients and workers with just a few lines of code.

MDP / Majordomo is a protocol for implementing a highly scalable, lightweight service oriented messaging on top of ZeroMQ. It is very useful, for example, for facilitating communication between different micro-services in a scalable, robust and fault-tolerant manner.

Installation

The simplest way to install Majortomo is via pip:

$ pip install majortomo

If you just want to run the MDP broker, for example if you already have MDP workers and clients implemented in some other language / library, you can run simply run the Docker image without installing any Python packages:

# This doesn't actually work yet, but will at some point...
# $ docker run shoppimon/majortomo-broker:latest

Quick Start

Running the Broker

Exposing a Service using an MDP Worker

Consuming a Service using an MDP Client

Full Documentation

Project documentation is available here: https://majortomo.readthedocs.io/en/latest/

Usage

Running the Broker

In most cases the MDP broker can be used as-is, simply by running it with the right command line arguments. You can do this by building and running it using Docker:

# Build the Docker image
$ docker build -t shoppimon/mdp-broker -f mdp-broker/Dockerfile .

# Run the broker from Docker
$ docker run --rm -ti shoppimon/mdp-broker -b tcp://0.0.0.0:5555 --verbose

You can run the broker with --help for more command line options.

Of course, you can also run the broker directly using Python 3.5 and up:

$ python -m majortomo.broker --help

Note that this requires setting up a virtual environemnt with the project dependencies, which is described below.

Installing & Using the Client and Worker modules

TBD

Using the Client class

See majortomo.echo for a sample client implementation.

The Client class should normally be used directly (without subclassing) to send requests to the broker (and workers).

Opening and closing client connections

While a lower-level API is available (through the connect, is_connected, and close), managing the connection to the broker is easiest done through the context manager protocol:

with Client(broker_url='tcp://127.0.0.1:5555') as client:
    client.send(b'my-service', b'frame1', b'frame2')
    reply = client.recv_all_as_list(timeout=10.0)

The example above takes care of opening and closing the ZeroMQ socket as needed.

NOTE: ZeroMQ takes care of re-creating dropped TCP connections and waiting for not-yet-bound peers automatically.

Sending Requests & Receiving Replies

To send a request, use the send method:

client.send(service_name, frame1, frame2, frame3)

This method takes the service name (as bytes) as a first argument. All other arguments are sent as message frames - the MDP protocol supports sending requests with more than one frame to the broker. The contents of these frames is application dependent and is up to you.

Once a request has been sent, you must read the entire reply send back from the broker (or close the connection to the broker and reconnect if you wish to retry).

There are multiple methods for reading replies, depending on your needs:

recv_part(timeout: float=None) -> Optional[List[bytes]]

Receive one reply part from the broker (a reply part is a list of bytes, as it may contain multiple ZeroMQ frames).

If no more parts are available (i.e. the last part was a FINAL reply), will return None.

recv_all(timeout: float=None) -> Iterable[List[bytes]]

Returns an iterator that yields each message part as it is received, and exists after the FINAL reply has been received:

for part in client.recv_all(timeout=5.0): 
    do_something_with_reply_part(part)

Note: the timeout parameter in this case relates to the time between reply chunks, not to the time it would take to receive the entire reply until FINAL.

recv_all_as_list(timeout: float=None) -> List[bytes]

Returns a flat list of all message frames from all reply parts. Regardless of how many PARTIAL replies were sent by the worker until the FINAL reply, this method will always return a single one-dimentional list of bytes with message frames.

Timeouts & Retrying

All recv_* methods of the client receive a timeout argument which should specify the number of seconds to wait for a response (a float is expected so you can specify second fractions as well). If no timeout is specified, the function will wait forever.

Once recv_* times out, a majortomo.error.Timeout will be raised. It is sometimes useful to catch this exception and retry the operation after reconnecting to the broker:

while True:
    with Client(broker_url='tcp://127.0.0.1:5555') as client:
        try:
        
            client.send(b'my-service', b'frame1', b'frame2')
            reply = client.recv_all_as_list(timeout=10.0)
            break
        except majortomo.error.Timeout:
            logging.warning("Timed out waiting for a reply from broker, reconnecting")
            time.sleep(1.0)
            continue

Or, if you do not wish to rely on the context manager for reconnecting (e.g. if the context is managed in an outer scope):

# Here `client` is passed from an outer scope
while True:
    try:
        client.send(b'my-service', b'frame1', b'frame2')
        reply = client.recv_all_as_list(timeout=10.0)
        break
    except majortomo.error.Timeout:
        logging.warning("Timed out waiting for a reply from broker, reconnecting")
        time.sleep(1.0)
        client.connect(reconnect=True)
        continue

Even better, it is advisable to manage the number of retries and the sleep time between them using some kind of exponential backoff & retry library, for example backoff or redo

Implementing MDP Workers

See majortomo.echo for a sample worker implementation

More details TBD

Copyright & Credits

Majortomo was created and is maintained by the Shoppimon team, and is distributed under the terms of the Apache 2.0 License (see LICENSE).

Majortomo is (C) Copyright 2018 Shoppimon LTD.

ØMQ is Copyright (c) 2007-2014 iMatix Corporation and Contributors.