Behaviors¶

libmemcached is a lot more flexible than python-memcached, and has provisions for configuring so-called behaviors. pylibmc wraps these in a Python interface.

Not all of the available behaviors make sense for Python, or are hard to make use of, and as such some behaviors have been intentionally hidden or exposed in some other way (UDP and the binary protocol are examples of this.)

Generally, a behavior’s value should be an integer value. The exceptions are hashing and distribution, which pylibmc translates with the C constants’ string equivalents, for readability.

Other than that, the behaviors are more or less one to one mappings of libmemcached behavior constants.

"hash": Specifies the default hashing algorithm for keys. See Hashing for more information and possible values.

"distribution": Specifies different means of distributing values to servers. See Distribution for more information and possible values.

"ketama": Setting this behavior to True is a shortcut for setting "hash" to "md5" and "distribution" to "consistent ketama".

"ketama_weighted": Exactly like the "ketama" behavior, but also enables the weighting support.

"ketama_hash": Sets the hashing algorithm for host mapping on continuum. Possible values include those for the "hash" behavior.

"buffer_requests": Enabling buffered I/O causes commands to “buffer” instead of being sent. Any action that gets data causes this buffer to be be sent to the remote connection. Quiting the connection or closing down the connection will also cause the buffered data to be pushed to the remote connection.

"no_block": Enables asychronous I/O. This is the fastest transport available for storage functions.

"tcp_nodelay": Setting this behavior will enable the TCP_NODELAY socket option, which disables Nagle’s algorithm. This obviously only makes sense for TCP connections.

"cas": Enables support for CAS operations.

"verify_keys": Setting this behavior will test if the keys for validity before sending to memcached.

"connect_timeout": In non-blocking mode, this specifies the timeout of socket connection in milliseconds.

"receive_timeout": “This sets the microsecond behavior of the socket against the SO_RCVTIMEO flag. In cases where you cannot use non-blocking IO this will allow you to still have timeouts on the reading of data.”

"send_timeout": “This sets the microsecond behavior of the socket against the SO_SNDTIMEO flag. In cases where you cannot use non-blocking IO this will allow you to still have timeouts on the sending of data.”

"num_replicas"

Poor man’s high-availability solution. Specifies numbers of replicas that should be made for a given item, on different servers.

“[Replication] does not dedicate certain memcached servers to store the replicas in, but instead it will store the replicas together with all of the other objects (on the ‘n’ next servers specified in your server list).”

"dead_timeout": Once a server has been marked dead, wait this amount of time (in seconds) before checking to see if the server is alive again.

"remove_failed": If set, a server will be removed from the server list after this many operations on it in a row have failed. See the section on Failover.

"failure_limit"deprecated

Use "remove_failed" if at all possible, which has the same meaning but uses newer behaviour.

If set, a server will be removed from the server list after this many operations on it in a row have failed.

"auto_eject"deprecated

Use "remove_failed" if at all possible.

With this behavior set, hosts which have been disabled will be removed from the list of servers after "failure_limit".

Hashing¶

Basically, the hasher decides how a key is mapped to a specific memcached server.

The available hashers are:

"default" - libmemcached’s home-grown hasher
"md5" - MD5
"crc" - CRC32
"fnv1_64" - 64-bit FNV-1
"fnv1a_64" - 64-bit FNV-1a
"fnv1_32" - 32-bit FNV-1
"fnv1a_32" - 32-bit FNV-1a
"murmur" - MurmurHash

If pylibmc was built against a libmemcached using --enable-hash_hsieh, you can also use "hsieh".

Hashing and python-memcached¶

python-memcached up until version 1.45 used a CRC32-based hashing algorithm not reproducible by libmemcached. You can change the hasher for python-memcached using the cmemcache_hash module, which will make it not only compatible with cmemcache, but also the "crc" hasher in libmemcached.

python-memcached 1.45 and later incorporated cmemcache_hash as its default hasher, and so will interoperate with libmemcached provided the libmemcached clients are told to use the CRC32-style hasher. This can be done in pylibmc as follows:

>>> mc.behaviors["hash"] = "crc"

Distribution¶

When using multiple servers, there are a few takes on how to choose a server from the set of specified servers.

The default method is "modula", which is what most implementations use. You can enable consistent hashing by setting distribution to "consistent".

Modula-based distribution is very simple. It works by taking the hash value, modulo the length of the server list. For example, consider the key "foo" under the "crc" hasher:

>>> servers = ["a", "b", "c"]
>>> crc32_hash(key)
3187
>>> 3187 % len(servers)
1
>>> servers[1]
'b'

However, if one was to add a server or remove a server, every key would be displaced by one - in effect, changing your server list would more or less reset the cache.

Consistent hashing solves this at the price of a more costly key-to-server lookup function, last.fm’s RJ explains how it works.

Failover¶

Most people desire the classical “I don’t really care” type of failover support: if a server goes down, just use another one. This case is supported, but not by default. As explained above, the default distribution mechanism is not very smart, and libmemcached doesn’t support any meaningful failover for it. If a server goes down, it stays down, and all of its alloted keys will simply fail. The recommended failover behaviors is for that reason:

mc.behaviors['ketama'] = True
mc.behaviors['remove_failed'] = 1
mc.behaviors['retry_timeout'] = 1
mc.behaviors['dead_timeout'] = 60

This will enable ketama hashing, and remove failed servers from rotation on their first failure, and retry them once every minute. It is the most robust configuration.

To fully understand the failover state machine, peruse the following graph:

While it might seem daunting at first, a closer examination will bring clarity to this picture. When a server connection fails, the server is marked as temporarily failed. This state is exited either by retry_timeout expiring, in which case the connection is retried, or, if remove_failed connection attempts have been made.

When a server runs out of retries, it is marked dead. This removes it from rotation. However, only the ketama distribution actually removes servers.

Note

There used to be two behaviors called failure_limit and auto_eject; these still exist, but their interaction with the state machine is unclear, and should be avoided. remove_failed acts as a combination of the two.