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#31 in WebSocket

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35K SLoC


Condure is a service that manages network connections in order to allow controlling the connections from multiple processes. It can manage incoming connections as well as outgoing connections. Applications communicate with Condure over ZeroMQ.

Condure can only manage connections for protocols it knows about. Currently this is HTTP/1 and WebSockets. See Supported protocols.

The project was inspired by Mongrel2.

Use cases

  • Pass connection ownership from one process to another.
  • Restart an application without its connections getting disconnected.
  • Balance connection ownership among multiple processes.

Basic usage

Start the server:

$ condure --listen 8000 --zclient-stream ipc://client

Connect a handler to it, such as this simple Python program:

# this handler responds to every request with "hello world"

import os
import time
import tnetstring
import zmq

instance_id = 'basichandler.{}'.format(os.getpid()).encode()

ctx = zmq.Context()
in_sock = ctx.socket(zmq.PULL)
out_sock = ctx.socket(zmq.PUB)

# await subscription

while True:
    m_raw = in_sock.recv()
    req = tnetstring.loads(m_raw[1:])
    print('IN {}'.format(req))

    resp = {}
    resp[b'from'] = instance_id
    resp[b'id'] = req[b'id']
    resp[b'code'] = 200
    resp[b'reason'] = b'OK'
    resp[b'headers'] = [[b'Content-Type', b'text/plain']]
    resp[b'body'] = b'hello world\n'

    print('OUT {}'.format(resp))
    out_sock.send(req[b'from'] + b' T' + tnetstring.dumps(resp))

A client request:

$ curl -i http://localhost:8000
HTTP/1.1 200 OK
Content-Type: text/plain
Content-Length: 12

hello world

The process that receives the request doesn't need to be the same one that responds! For example, here's a program that outputs request IDs to stdout:

# this handler just outputs the request ID

import tnetstring
import zmq

ctx = zmq.Context()
sock = ctx.socket(zmq.PULL)

while True:
    m = sock.recv_multipart()
    req = tnetstring.loads(m[0][1:])
    print('{} {}'.format(req[b'from'].decode(), req[b'id'].decode()))

We can see request ID information when a client request is made:

$ python examples/printreq.py
condure 0-0-0

From another shell we can respond using a program like this:

# this program sends a response to a certain request ID

import sys
import time
import tnetstring
import zmq

body = sys.argv[1]
addr = sys.argv[2].encode()
rid = sys.argv[3].encode()

ctx = zmq.Context()
sock = ctx.socket(zmq.PUB)

# await subscription

resp = {}
resp[b'from'] = b'sendresp'
resp[b'id'] = rid
resp[b'code'] = 200
resp[b'reason'] = b'OK'
resp[b'headers'] = [[b'Content-Type', b'text/plain']]
resp[b'body'] = '{}\n'.format(body).encode()

m = [addr + b' T' + tnetstring.dumps(resp)]


For example:

$ python examples/sendresp.py "responding from another process" condure 0-0-0

The client sees:

$ curl -i http://localhost:8000
HTTP/1.1 200 OK
Content-Type: text/plain
Content-Length: 32

responding from another process

For easy testing, the programs can be piped together:

$ python -u examples/printreq.py | xargs -n 2 python examples/sendresp.py "responding from another process"

Suspending and resuming connections

When passing control of a connection from one process to another, it is important to suspend the connection first. This is done by sending a handoff-start message and waiting for a handoff-proceed message. At that point, the connection information can be given to another process, and the connection can be resumed by sending any message (such as keep-alive). See the ZHTTP spec.

REQ mode

In addition to the stream mode which uses PUSH/ROUTER/SUB sockets, there is a "REQ" mode available which uses a DEALER socket. To enable it, set req as the mode on a listen port. This mode can be handy for implementing simple request/response servers using ZeroMQ.

Supported protocols

Condure supports HTTP/1 and WebSockets.

Condure manages connections at layer 7 and only supports protocols it knows about. This is to simplify its usage. Handling arbitrary protocols would require applications to build protocol stacks capable of suspending/resuming sessions at arbitrary byte positions in TCP streams, making Condure usage prohibitive. Instead, Condure is protocol-aware, and provides parsed frames to applications, so that applications are only required to support suspending/resuming sessions at frame boundaries.


Condure was built for high performance. It uses numerous optimization techniques, including minimal heap allocations, ring buffers, vectored I/O, hierarchical timing wheels, and fast data structures (e.g. slabs). Over 1M concurrent connections have been tested on a single instance using just 2 workers (4 threads total). See https://blog.fanout.io/2020/08/11/rewriting-pushpins-connection-manager-in-rust/

Comparison to Mongrel2

  • Condure supports acting as a server and as a client.
  • Condure supports multiple cores.
  • Condure supports listening on multiple ports without requiring multiple processes.
  • Condure does not support multiple routes and is not intended to be a shared server. Each application that wants to keep connections in a separate process should spawn its own Condure instance.
  • Condure has no config file. Configuration is supplied using command line arguments.
  • Condure uses a different ZeroMQ-based protocol, ZHTTP, which is easier to use than Mongrel2's protocol and more reliable.

Future plans

  • HTTP/2
  • HTTP/3


~294K SLoC