Oracle 5.0, Справочное руководство

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Using
memcached
1346
15.6.2.4.
memcached
Hashing/Distribution Types
The
memcached
client interface supports a number of different distribution algorithms that are used in
multi-server configurations to determine which host should be used when setting or getting data from
a given
memcached
instance. When you get or set a value, a hash is constructed from the supplied
key and then used to select a host from the list of configured servers. Because the hashing mechanism
uses the supplied key as the basis for the hash, the same server is selected during both set and get
operations.
You can think of this process as follows. Given an array of servers (a, b, and c), the client uses a
hashing algorithm that returns an integer based on the key being stored or retrieved. The resulting
value is then used to select a server from the list of servers configured in the client. Most standard
client hashing within
memcache
clients uses a simple modulus calculation on the value against the
number of configured
memcached
servers. You can summarize the process in pseudocode as:
@memcservers = ['a.memc','b.memc','c.memc'];
$value = hash($key);
$chosen = $value % length(@memcservers);
Replacing the above with values:
@memcservers = ['a.memc','b.memc','c.memc'];
$value = hash('myid');
$chosen = 7009 % 3;
In the above example, the client hashing algorithm chooses the server at index 1 (
7009 % 3 = 1
),
and store or retrieve the key and value with that server.
Note
This selection and hashing process is handled automatically by the
memcached
client you are using; you need only provide the list of
memcached
servers to
use.
You can see a graphical representation of this below in Figure 15.5, “
memcached
Hash Selection” .
Figure 15.5.
memcached
Hash Selection
The same hashing and selection process takes place during any operation on the specified key within
the
memcached
client.
Using this method provides a number of advantages:
• The hashing and selection of the server to contact is handled entirely within the client. This
eliminates the need to perform network communication to determine the right machine to contact.
• Because the determination of the
memcached
server occurs entirely within the client, the server can
be selected automatically regardless of the operation being executed (set, get, increment, etc.).