libmc

libmc is a memcached client library for Python without any other dependencies in runtime. It's mainly written in C++ and Cython. libmc can be considered as a drop in replacement for libmemcached and python-libmemcached.

libmc is developing and maintaining by Douban Inc. Currently, It is working in production environment, powering all web traffics in douban.com. Realtime benchmark result is available on travis.

Build and Installation

For users:

pip install libmc

Usage:

import libmc

mc = libmc.Client(['localhost:11211', 'localhost:11212'])
mc.set('foo', 'bar')
assert mc.get('foo') == 'bar'

Under the hood

Under the hood, libmc consists of 2 parts: an internal fully-functional memcached client implementation in C++ and a Cython wrapper around that implementation. Dynamic memory allocation and memory-copy are slow, so we tried our best to avoid them. The set_multi command is not natively supported by the memcached protocol. Some techniques are applied to make set_multi command extremely fast in libmc (compared to some other similiar libraries).

Configuration

import libmc
from libmc import (
    MC_HASH_MD5, MC_POLL_TIMEOUT, MC_CONNECT_TIMEOUT, MC_RETRY_TIMEOUT
)

mc = libmc.Client(
    [
    'localhost:11211',
    'localhost:11212',
    'remote_host',
    'remote_host mc.mike',
    'remote_host:11213 mc.oscar'
    ],
    do_split=True,
    comp_threshold=0,
    noreply=False,
    prefix=None,
    hash_fn=MC_HASH_MD5,
    failover=False
)

mc.config(MC_POLL_TIMEOUT, 100)  # 100 ms
mc.config(MC_CONNECT_TIMEOUT, 300)  # 300 ms
mc.config(MC_RETRY_TIMEOUT, 5)  # 5 s

• servers: is a list of memcached server addresses. Each address can be in format of hostname[:port] [alias]. port and alias are optional. If port is not given, default port 11211 will be used. alias will be used to compute server hash if given, otherwise server hash will be computed based on host and port (i.e.: If port is not given or it is equal to 11211, host will be used to compute server hash. If port is not equal to 11211, host:port will be used).

• do_split: Memcached server will refuse to store value if size >= 1MB, if do_split is enabled, large value (< 10 MB) will be splitted into several blocks. If the value is too large (>= 10 MB), it will not be stored. default: True

• comp_threshold: All kinds of values will be encoded into string buffer. If buffer length > comp_threshold > 0, it will be compressed using zlib. If comp_threshold = 0, string buffer will never be compressed using zlib. default: 0

• noreply: Whether to enable memcached's noreply behaviour. default: False

• prefix: The key prefix. default: ''

• hash_fn: hashing function for keys. possible values:

  • MC_HASH_MD5
  • MC_HASH_FNV1_32
  • MC_HASH_FNV1A_32
  • MC_HASH_CRC_32

  default: MC_HASH_MD5

  NOTE: fnv1_32, fnv1a_32, crc_32 implementations in libmc are per each spec, but they're not compatible with corresponding implementions in libmemcached.

• failover: Whether to failover to next server when current server is not available. default: False

• MC_POLL_TIMEOUT Timeout parameter used during set/get procedure. (default: 300 ms)

• MC_CONNECT_TIMEOUT Timeout parameter used when connecting to memcached server on initial phase. (default: 100 ms)

• MC_RETRY_TIMEOUT When a server is not available dur to server-end error. libmc will try to establish the broken connection in every MC_RETRY_TIMEOUT s until the connection is back to live.(default: 5 s)

NOTE: The hashing algorithm for host mapping on continuum is always md5.

Contributing to libmc

Feel free to send a Pull Request. For feature requests or any questions, please open an Issue.

For SECURITY DISCLOSURE, please disclose the information responsibly by sending an email to security@douban.com directly instead of creating a GitHub issue.

FAQ

Does libmc support PHP?

No. But if you like, you can write a wrapper for PHP based on the C++ implementation.

Is Memcached binary protocol supported ?

No. Only Memcached ASCII protocol is supported currently.

Why reinventing the wheel?

Before libmc, we're using python-libmemcached, which is a python extention for libmemcached. libmemcached is quite weird and buggy. After nearly one decade, there're still some unsolved bugs.

Is libmc thread-safe ?

libmc is a single-threaded memcached client. If you initialize a libmc client in one thread but reuse that in another thread, a Python Exception ThreadUnsafe will raise in Python.

Is libmc compatible with gevent?

Yes, with the help of greenify, libmc is friendly to gevent. Read tests/shabby/gevent_issue.py for details.

Acknowledgments

• Thanks to @fahrenheit2539 and the llvm project for the standalone. SmallVector implementation.
• Thanks to @miloyip for the high performance i64toa implementation.
• Thanks to Ivan Novikov for the research in THE NEW PAGE OF INJECTIONS BOOK: MEMCACHED INJECTIONS.
• Thanks to the PolarSSL project for the md5 implementation.
• Thanks to @lericson for the benchmark script in pylibmc.
• Thanks to the libmemcached project and some other projects possibly not mentioned here.

Contributors

• @mckelvin
• @zzl0
• @windreamer
• @lembacon
• @seansay

Documentation

https://github.com/douban/libmc/wiki

LICENSE

Copyright (c) 2014-2015, Douban Inc. All rights reserved.

Licensed under a BSD license: https://github.com/douban/libmc/blob/master/LICENSE.txt