NAME¶

memcached_replace - Storing and Replacing Data

SYNOPSIS¶

#include <libmemcached/memcached.h>

Compile and link with -lmemcached

DESCRIPTION¶

memcached_set(), memcached_add(), and memcached_replace() are all used to store information on the server. All methods take a key, and its length to store the object. Keys are currently limited to 250 characters when using either a version of memcached(1) which is 1.4 or below, or when using the text protocol. You must supply both a value and a length. Optionally you store the object. Keys are currently limited to 250 characters by the memcached(1) server. You must supply both a value and a length. Optionally you may test an expiration time for the object and a 16 byte value (it is meant to be used as a bitmap). "flags" is a 4byte space that is stored alongside of the main value. Many sub libraries make use of this field, so in most cases users should avoid making use of it.

memcached_set() will write an object to the server. If an object already exists it will overwrite what is in the server. If the object does not exist it will be written. If you are using the non-blocking mode this function will always return true unless a network error occurs.

memcached_replace() replaces an object on the server. If the object is not found on the server an error occurs.

memcached_add() adds an object to the server. If the object is found on the server an error occurs, otherwise the value is stored.

memcached_cas() overwrites data in the server as long as the "cas" value is still the same in the server. You can get the cas value of a result by calling memcached_result_cas() on a memcached_result_st(3) structure. At the point that this note was written cas is still buggy in memached. Turning on tests for it in libmemcached(3) is optional. Please see memcached_set for information on how to do this.

memcached_set_by_key(), memcached_add_by_key(), and memcached_replace_by_key() methods all behave in a similar method as the non key methods. The difference is that they use their group_key parameter to map objects to particular servers.

If you are looking for performance, memcached_set() with non-blocking IO is the fastest way to store data on the server.

All of the above functions are testsed with the MEMCACHED_BEHAVIOR_USE_UDP behavior enabled. However, when using these operations with this behavior on, there are limits to the size of the payload being sent to the server. The reason for these limits is that the Memcached Server does not allow multi-datagram requests and the current server implementation sets a datagram size to 1400 bytes. Due to protocol overhead, the actual limit of the user supplied data is less than 1400 bytes and depends on the protocol in use as, well as the operation being executed. When running with the binary protocol, MEMCACHED_BEHAVIOR_BINARY_PROTOCOL, the size of the key,value, flags and expiry combined may not exceed 1368 bytes. When running with the ASCII protocol, the exact limit fluctuates depending on which function is being executed and whether the function is a cas operation or not. For non-cas ASCII set operations, there are at least 1335 bytes available to split among the key, key_prefix, and value; for cas ASCII operations there are at least 1318 bytes available to split among the key, key_prefix and value. If the total size of the command, including overhead, exceeds 1400 bytes, a MEMCACHED_WRITE_FAILURE will be returned.

RETURN¶

All methods return a value of type memcached_return_t. On success the value will be MEMCACHED_SUCCESS. Use memcached_strerror() to translate this value to a printable string.

For memcached_replace() and memcached_add(), MEMCACHED_NOTSTORED is a legitmate error in the case of a collision.

HOME¶

To find out more information please check: http://libmemcached.org/

SEE ALSO¶

memcached(1) libmemached(3) memcached_strerror(3) memcached_prepend(3) memcached_cas(3) memcached_append(3)

AUTHOR¶

Brian Aker

COPYRIGHT¶

2011-2013, Brian Aker DataDifferential, http://datadifferential.com/