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README.md

Erlang to CZMQ Bindings

Goals:

  • Provide a canonical CZMQ interface from Erlang
  • Safe: bugs, errors, assertion failures in CZMQ must not crash Erlang
  • Reasonably performant

Non Goals:

  • Value atop CZMQ - this is left to applications
  • Performance at the expense of safety

Approach

The "bindings" (this is a lose term given the approach here) are implemented as a C Port to ensure that crashes don't effect the Erlang VM.

The API mirrors that of CZMQ with all functions being available through the czmqmodule.

Building erlang-czmq

$ git clone https://github.com/gar1t/erlang-czmq.git
$ cd erlang-czmq
$ ./configure
$ make check

This will build the library and run the tests.

Please report and problems by opening an issue here:

https://github.com/gar1t/erlang-czmq/issues

Port to CZMQ Mapping

The port manages a single ZMQ context. All context managed state is associated with a port.

The port manages its state appropriately:

  • ZMQ context
  • Sockets
  • Auth object (limited to one per context)
  • Certs

We use dynamic arrays (vectors) to store references to ZMQ and CZMQ objects. Objects are referenced using their array index.

When an object is destroyed, it's associated element in the applicable array is set to NULL.

Sockets - Sending and Receiving

Erlang C ports use a synchronous request/response protocol over standard input output. This makes them unsuitable for handling the asynchronous events that are endemic to ZeroMQ. Received events must be routinely polled using non blocking operations.

Messages can be checked explicitly or routinely using czmq_poller. Messages received by czmq_poller can be delivered as Erlang messages to another process, effectively simulating asynchronous message delivery, albiet with some latency introduced by the polling sleep interval.

Using erlang-czmq

Benchmarks

While safety is the prime consideration for this binding, performance is important as well. erlang-czmq provides a simple framework for measuring message send/receive throughput using different bindings.

Benchmarks follow this approach:

  • A receiver binds to a local port and receives messages as quickly as it can, printing the number of received messages per second.
  • A sender connects to the receiver port and sends messages as quickly as it can for a period of time.

This scheme can be used to test different combinations of bindings for sending and receiving. Below are some preliminary results, which are useful as a rough gage for the relative performance differences of bindings.

C Receiver / C Sender

To test the native (i.e C) performance of CZMQ, use czmq-benchmark located in priv after compiling erlang-czmq. First, start the receiver:

$ cd erlang-czmq/priv
$ ./czmq-benchmark recv

The receiver will print the total number of messages it receives for each interval.

Next, in a separate shell, start the sender:

$ cd erlang-czmq/priv
$ ./czmq-benchmark send

You will see the number of messages the receiver received during the time the sender was sending. Discard the first and last observations as they reflect partial intervals.

C Receiver / erlang-czmq Sender

This test measures the throughput of using an erlang-czmq sender with a C receiver.

Start the receiver as with the C / C test above.

Next,

Benchark Summary - Lenovo X220 at 2.7 GHz

+---------------------------+-------------+---+
| Recv / Send               | Average MPS | N |
|---------------------------|-------------|---|
| C / C                     |     1190500 | 5 |
| C / erlzmq                |      128136 | 5 |
| C / erlang-czmq           |       35990 | 5 |
| erlzmq / C                |      152678 | 5 |
| erlang-czmq / C           |       10126 | 5 |
| erlzmq / erlzmq           |      134234 | 5 |
| erlang-czmq / erlang-czmq |        9614 | 5 |
+---------------------------+-------------+---+

Versioning Scheme

This project uses the traditional versioning scheme:

MAJOR "." MINOR "." REVISION

After major version "1", changes to Major represent potential API changes.

Minor version increments represent major milestones and will be documented.

Revision increments represent minor milestones including bug fixes.

Versions will be indicated using tags.

Changes are documented in CHANGES.md.

Releases

This project doesn't make formal releases by instead tags git commits using the versioning scheme above.

To increment a revision:

  1. Change the vsn in src/czmq.app.src
  2. Add an entry to CHANGES.md with reasonable detail to help users understand what changed in between the new documented release and the previous documented release