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About Ketama
We wrote ketama to replace how our memcached clients mapped keys to servers.
Previously, clients mapped keys->servers like this:
server = serverlist[hash(key)%serverlist.length];
This meant that whenever we added or removed servers from the pool, everything
hashed to different servers, which effectively wiped the entire cache.
Ketama solves this problem in the following way:
* Take your list of servers (eg:,,
* Hash each server string to several (100-200) unsigned ints
* Conceptually, these numbers are placed on a circle called the continuum.
(imagine a clock face that goes from 0 to 2^32)
* Each number links to the server it was hashed from, so servers appear
at several points on the continuum, by each of the numbers they hashed to.
* To map a key->server, hash your key to a single unsigned int, and find the
next biggest number on the continuum. The server linked to that number is
the correct server for that key.
* If you hash your key to a value near 2^32 and there are no points on the
continuum greater than your hash, return the first server in the continuum.
If you then add or remove a server from the list, only a small proportion of
keys end up mapping to different servers.
The server file looks like this: 900 300 1500
ip:port and weighting, \t separated, \n line endings.
Just use the number of megs allocated to the server as the weight.
The weightings are realised by adding more or less points to the continuum.
Included in this tarball:
* libketama
* php_ketama
* java_ketama
* python_ketama
libketama is a general purpose C library that maps keys to a list of servers.
The server list is read from a file, and the continuum is created and stored
in shared memory for future access. If the file modification time changes,
the continuum is regenerated and shared memory is updated.
php_ketama is a PHP extenstion that wraps libketama. We use this in our PHP
memcached client library.
java_ketama implements the same logic in pure Java, and has been fitted into our
Java memcached client library.
python_ketama is a python module (with libketama dependencies) contributed by
* libketama (the general purpose C library)
$ cd libketama
$ make
$ make test
$ su -c "make install"
This will compile libketama and install it to the default prefix /usr/local.
You can change the prefix by editing the PREFIX variable in 'Makefile'.
* php_ketama (PHP extension that wraps libketama and therefore depends on it)
$ cd php-4.4.x/ext
$ ln -s /your/ketama/php_ketama ketama
$ cd ..
$ rm -Rf autom4te.cache
$ ./buildconf --force
$ ./configure --all_your_configure_options --with-ketama[=/your/ketama/prefix]
$ make
$ su -c "make install"
* python_ketama (python module that depends on libketama)
$ cd python_ketama
$ python build
$ sudo python install
$ python
Don't forget you might have to restart your httpd!