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Commit fcd303ac authored by Jeremy Andrews's avatar Jeremy Andrews
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Issue #1974088 by Jeremy, erikwebb: document memcache lock implementation and... 

Issue #1974088 by Jeremy, erikwebb: document memcache lock implementation and stampede protection; documentation cleanup.
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README.txt
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......@@ -92,16 +92,60 @@ The bin/cluster/server model can be described as follows:
- The default cluster is 'default'.
## LOCKING ##
The memcache-lock.inc file included with this module can be used as a drop-in
replacement for the database-mediated locking mechanism provided by Drupal
core. To enable, define the following in your settings.php:
$conf['lock_inc'] = 'sites/all/modules/memcache/memcache-lock.inc';
Locks are written in the 'semaphore' table, which will map to the 'default'
memcache cluster unless you explicitly configure a 'semaphore' cluster.
## STAMPEDE PROTECTION ##
Memcache includes stampede protection for rebuilding expired and invalid cache
items. To enable stampede protection, define the following in settings.php:
$conf['memcache_stampede_protection'] = TRUE;
To avoid lock stampedes, it is important that you enable the memacache lock
implementation when enabling stampede protection -- enabling stampede protection
without enabling the Memache lock implementation can cause worse performance.
Only change the following values if you're sure you know what you're doing,
which requires reading the memcachie.inc code.
The value passed to lock_acquire, defaults to '15':
$conf['memcache_stampede_semaphore'] = 15;
The value passed to lock_wait, defaults to 5:
$conf['memcache_stampede_wait_time'] = 5;
The maximum number of calls to lock_wait() due to stampede protection during a
single request, defaults to 3:
$conf['memcache_stampede_wait_limit'] = 3;
When adjusting these variables, be aware that:
- there is unlikely to be a good use case for setting wait_time higher
than stampede_semaphore;
- wait_time * wait_limit is designed to default to a number less than
standard web server timeouts (i.e. 15 seconds vs. apache's default of
30 seconds).
## EXAMPLES ##
Example 1:
First, the most basic configuration which consists of one memcached instance
running on localhost port 11211 and all caches except for cache_form being
stored in memcache. This requires minimal configuration as it is the default
behavior:
stored in memcache. We also enable stampede protection, and the memcache
locking mechanism.
$conf['cache_backends'][] = 'sites/all/modules/memcache/memcache.inc';
$conf['lock_inc'] = 'sites/all/modules/memcache/memcache-lock.inc';
$conf['memcache_stampede_protection'] = TRUE;
$conf['cache_default_class'] = 'MemCacheDrupal';
// The 'cache_form' bin must be assigned to non-volatile storage.
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
......@@ -115,21 +159,23 @@ configuration which is used in this case is equivalant to setting:
Example 2:
In this example we define three memcached instances, two accessed over the
network on localhost, and one on a Unix socket -- please note this is not a
recommended configuration and it's highly unlikely you'd want to configure
memcache to use both sockets and network addresses like this, instead you'd
consistently use one or the other.
The instance on localhost port 11211 belongs to the 'default' cluster where
everything gets cached that isn't otherwise defined. (We refer to it as a
"cluster", but in our example our "clusters" involve only one instance.) The
instance on port 11212 belongs to the 'pages' cluster, with the 'cache_page'
table mapped to it -- so the Drupal page cache is stored in this cluster.
Finally, the instance listening on a socket is part of the 'blocks' cluster,
with the 'cache_block' table mapped to it -- so the Drupal block cache is
stored here. Note that sockets do not have ports.
network, and one on a Unix socket -- please note this is only an illustration of
what is possible, and is not a recommended configuration as it's highly unlikely
you'd want to configure memcache to use both sockets and network addresses like
this, instead you'd consistently use one or the other.
The instance on port 11211 belongs to the 'default' cluster where everything
gets cached that isn't otherwise defined. (We refer to it as a "cluster", but in
this example our "clusters" involve only one instance.) The instance on port
11212 belongs to the 'pages' cluster, with the 'cache_page' table mapped to
it -- so the Drupal page cache is stored in this cluster. Finally, the instance
listening on a socket is part of the 'blocks' cluster, with the 'cache_block'
table mapped to it -- so the Drupal block cache is stored here. Note that
sockets do not have ports.
$conf['cache_backends'][] = 'sites/all/modules/memcache/memcache.inc';
$conf['lock_inc'] = 'sites/all/modules/memcache/memcache-lock.inc';
$conf['memcache_stampede_protection'] = TRUE;
$conf['cache_default_class'] = 'MemCacheDrupal';
// The 'cache_form' bin must be assigned no non-volatile storage.
......@@ -137,8 +183,8 @@ stored here. Note that sockets do not have ports.
// Important to define a default cluster in both the servers
// and in the bins. This links them together.
$conf['memcache_servers'] = array('localhost:11211' => 'default',
'localhost:11212' => 'pages',
$conf['memcache_servers'] = array('10.1.1.1:11211' => 'default',
'10.1.1.1:11212' => 'pages',
'unix:///path/to/socket' => 'blocks');
$conf['memcache_bins'] = array('cache' => 'default',
'cache_page' => 'pages',
......@@ -148,20 +194,22 @@ stored here. Note that sockets do not have ports.
Example 3:
Here is an example configuration that has two clusters, 'default' and
'cluster2'. Five memcached instances are divided up between the two
clusters. 'cache_filter' and 'cache_menu' bins go to 'cluster2'. All other
bins go to 'default'.
'cluster2'. Five memcached instances running on four different servers are
divided up between the two clusters. The 'cache_filter' and 'cache_menu' bins
go to 'cluster2'. All other bins go to 'default'.
$conf['cache_backends'][] = 'sites/all/modules/memcache/memcache.inc';
$conf['lock_inc'] = 'sites/all/modules/memcache/memcache-lock.inc';
$conf['memcache_stampede_protection'] = TRUE;
$conf['cache_default_class'] = 'MemCacheDrupal';
// The 'cache_form' bin must be assigned no non-volatile storage.
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
$conf['memcache_servers'] = array('123.45.67.89:11211' => 'default',
'123.45.67.89:11212' => 'default',
'123.45.67.90:11211' => 'default',
'123.45.67.91:11211' => 'cluster2',
'123.45.67.92:11211' => 'cluster2');
$conf['memcache_servers'] = array('10.1.1.6:11211' => 'default',
'10.1.1.6:11212' => 'default',
'10.1.1.7:11211' => 'default',
'10.1.1.8:11211' => 'cluster2',
'10.1.1.9:11211' => 'cluster2');
$conf['memcache_bins'] = array('cache' => 'default',
'cache_filter' => 'cluster2',
......@@ -200,7 +248,7 @@ $conf['memcache_options'] = array(
## SESSIONS ##
NOTE: Session.inc is not yet ported to Drupal 7 and is not recommended for use
in production..
in production.
Here is a sample config that uses memcache for sessions. Note you MUST have
a session and a users server set up for memcached sessions to work.
......@@ -213,12 +261,12 @@ $conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
$conf['session_inc'] = './sites/all/modules/memcache/memcache-session.inc';
$conf['memcache_servers'] = array(
'localhost:11211' => 'default',
'localhost:11212' => 'filter',
'localhost:11213' => 'menu',
'localhost:11214' => 'page',
'localhost:11215' => 'session',
'localhost:11216' => 'users',
'10.1.1.1:11211' => 'default',
'10.1.1.1:11212' => 'filter',
'10.1.1.1:11213' => 'menu',
'10.1.1.1:11214' => 'page',
'10.1.1.1:11215' => 'session',
'10.1.1.1:11216' => 'users',
);
$conf['memcache_bins'] = array(
'cache' => 'default',
......@@ -229,7 +277,6 @@ $conf['memcache_bins'] = array(
'users' => 'users',
);
## TROUBLESHOOTING ##
PROBLEM:
......
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