Memcache::addServer

(PECL memcache >= 2.0.0)

Memcache::addServer — Add a memcached server to connection pool

Description

Memcache::addServer(
    string $host,
    int $port = 11211,
    bool $persistent = ?,
    int $weight = ?,
    int $timeout = ?,
    int $retry_interval = ?,
    bool $status = ?,
    callable $failure_callback = ?,
    int $timeoutms = ?
): bool

Memcache::addServer() adds a server to the connection pool. You can also use the memcache_add_server() function.

When using this method (as opposed to Memcache::connect() and Memcache::pconnect()) the network connection is not established until actually needed. Thus there is no overhead in adding a large number of servers to the pool, even though they might not all be used.

Failover may occur at any stage in any of the methods, as long as other servers are available the request the user won't notice. Any kind of socket or Memcached server level errors (except out-of-memory) may trigger the failover. Normal client errors such as adding an existing key will not trigger a failover.

Note:
This function has been added to Memcache version 2.0.0.

Parameters

host

Point to the host where memcached is listening for connections. This parameter may also specify other transports like unix:///path/to/memcached.sock to use UNIX domain sockets, in this case port must also be set to 0.

port

Point to the port where memcached is listening for connections. Set this parameter to 0 when using UNIX domain sockets.

Please note: port defaults to memcache.default_port if not specified. For this reason it is wise to specify the port explicitly in this method call.

persistent

Controls the use of a persistent connection. Default to true.

weight

Number of buckets to create for this server which in turn control its probability of it being selected. The probability is relative to the total weight of all servers.

timeout

Value in seconds which will be used for connecting to the daemon. Think twice before changing the default value of 1 second - you can lose all the advantages of caching if your connection is too slow.

retry_interval

Controls how often a failed server will be retried, the default value is 15 seconds. Setting this parameter to -1 disables automatic retry. Neither this nor the persistent parameter has any effect when the extension is loaded dynamically via dl().

Each failed connection struct has its own timeout and before it has expired the struct will be skipped when selecting backends to serve a request. Once expired the connection will be successfully reconnected or marked as failed for another retry_interval seconds. The typical effect is that each web server child will retry the connection about every retry_interval seconds when serving a page.

status

Controls if the server should be flagged as online. Setting this parameter to false and retry_interval to -1 allows a failed server to be kept in the pool so as not to affect the key distribution algorithm. Requests for this server will then failover or fail immediately depending on the memcache.allow_failover setting. Default to true, meaning the server should be considered online.

failure_callback

Allows the user to specify a callback function to run upon encountering an error. The callback is run before failover is attempted. The function takes two parameters, the hostname and port of the failed server.

timeoutms

Return Values

Returns true on success or false on failure.

Examples

Example #1 Memcache::addServer() example

<?php

/* OO API */

$memcache = new Memcache;
$memcache->addServer('memcache_host', 11211);
$memcache->addServer('memcache_host2', 11211);

/* procedural API */

$memcache_obj = memcache_connect('memcache_host', 11211);
memcache_add_server($memcache_obj, 'memcache_host2', 11211);

?>

Notes

Warning

When the port is unspecified, this method defaults to the value set of the PHP ini directive memcache.default_port If this value was changed elsewhere in your application it might lead to unexpected results: for this reason it is wise to always specify the port explicitly in this method call.

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User Contributed Notes 6 notes

down

rstaveley at seseit dot com ¶

13 years ago

The Memcache client library is responsible for picking the right server to set/get data. That's why addServer is what you want to use rather than connect, when you have more than one Memcache server. A subsequent set/get will then connect on demand to the appropriate instance as needs be. Disconnection to whatever servers were connected to happening when you close or your script terminates.

Memcache instances added to your Memcache object via addServer should be added in the same order in your application to ensure that the same server is picked for use with the same key. 

A client library may be implemented to run a CRC on the key and do a modulus over the number of instances in the list to select an instance from the list for the set/get. This ensures that data is spread nicely across the nodes. 

That all works nicely behind the scenes for you in your PHP code, as long as you add your list of Memcache instances in a consistent manner with addServer.

down

enno dot rehling at gmail dot com ¶

13 years ago

The $timeoutms argument can be used to specify the timeout in milliseconds, but isn't available in all versions. For example, it exists in php_memcache 2.2.6, but not in 3.0.4. In 2.2.6, if you specify it, then it overrides $timeout.

Caveat emptor: If $timeoutms is not specified, it defaults to the value of memcache.default_timeout_ms in php.ini, which defaults to 1000 if not set. This also overrides $timeout, which has the curious effect that $timeout is always ignored in php_memcache 2.2.6 (either in favor of $timeoutms, memcache.default_timeout_ms or the value 1000, in that order of priority).

down

joewynn dot nz+phpnet at gmail dot com ¶

11 years ago

Note that this method will always return TRUE because a connection is not actually made at call time. See this bug report for more information: https://bugs.php.net/bug.php?id=58193

down

eu at serbannistor dot ro ¶

13 years ago

Actually if you have two memcached servers from which one of them is on localhost, and the other is on a remote machine you can communicate with both even if you specify the loopback address for the local one.

<?php
$memcache_obj = memcache_connect("127.0.0.1", 11211);
memcache_add_server($memcache_obj, "memcache_remote_host");
$memcache_obj->set('var_key', time());
?>

This WILL communicate with both hosts but however there are two aspects that must be taken into account:
1. the communication will be done through different network interfaces with the two hosts. It will use the loopback interface for the "127.0.0.1" host (lo in my case on Linux) and the external interface for the "memcache_remote_host" (eth0 in my case). Only if you want to use the same network interface to communicate with both hosts you must use the external IPs of both machines (and all communication will go out through the eth0 interface).
2. the connection with the two hosts will be established differently because of how memcache_connect() and memcache_add_server() work. Therefore the memcache_connect() will initiate the connection to localhost through the loopback interface when it is called, while memcache_add_server() will just add the second server to the pool, but it will not send any package through the network until it's absolutely needed (for example when a memcache_set() command is issued).

down

iwind dot liu at gmail dot com ¶

14 years ago

The weight of the server must be greater than 0.



If there is no memcached server to use, and you try to set/add variables, the apache will be crashed, with the error message "[notice] child pid 18725 exit signal Segmentation fault (11)" in error_log file.

down

-6

Jean-Baptiste Quenot ¶

16 years ago

The default value for the "weight" argument is 1

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Memcache::addServer

Description

Parameters

Return Values

Examples

Notes

See Also

Found A Problem?

User Contributed Notes 6 notes