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More multi-DC tweaks to MessageCache::load()

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* Use hash key volatility to propagate invalidations over DCs
  in addition to memcached->APC instances.
* Make use of stale APC cache due to hash mismatch instead
  of waiting around in some cases. This is similar to the
  TTL expiry and volatility cases.
* Renamed $hashExpired and added some comments for clarity.

Change-Id: I8890fb174cae72c4ce8872df64f52f5cbd55183b
This commit is contained in:
Aaron Schulz 2015-08-11 19:53:03 -07:00 committed by Ori.livneh
parent 95ebbff50f
commit db464b8a84
1 changed files with 31 additions and 24 deletions
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55
includes/cache/MessageCache.php vendored
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@ -260,29 +260,30 @@ class MessageCache {
# Loading code starts
$success = false; # Keep track of success
$staleCache = false; # a cache array with expired data, or false if none has been loaded
$hashExpired = false; # whether the cluster-local validation hash is stale
$where = array(); # Debug info, delayed to avoid spamming debug log too much
# Local cache
# Hash of the contents is stored in memcache, to detect if local cache goes
# out of date (e.g. due to replace() on some other server)
if ( $wgUseLocalMessageCache ) {
list( $hash, $hashExpired ) = $this->getValidationHash( $code );
if ( $hash ) {
$cache = $this->getLocalCache( $code );
if ( !$cache || !isset( $cache['HASH'] ) || $cache['HASH'] !== $hash ) {
$where[] = 'local cache is empty or has the wrong hash';
} elseif ( $this->isCacheExpired( $cache ) ) {
$where[] = 'local cache is expired';
$staleCache = $cache;
} elseif ( $hashExpired ) {
$where[] = 'local cache validation key is expired';
$staleCache = $cache;
} else {
$where[] = 'got from local cache';
$success = true;
$this->mCache[$code] = $cache;
}
# Hash of the contents is stored in memcache, to detect if data-center cache
# or local cache goes out of date (e.g. due to replace() on some other server)
list( $hash, $hashVolatile ) = $this->getValidationHash( $code );
if ( $wgUseLocalMessageCache && $hash ) {
# Try the local cache and check against the cluster hash key...
$cache = $this->getLocalCache( $code );
if ( !$cache ) {
$where[] = 'local cache is empty';
} elseif ( !isset( $cache['HASH'] ) || $cache['HASH'] !== $hash ) {
$where[] = 'local cache has the wrong hash';
$staleCache = $cache;
} elseif ( $this->isCacheExpired( $cache ) ) {
$where[] = 'local cache is expired';
$staleCache = $cache;
} elseif ( $hashVolatile ) {
$where[] = 'local cache validation key is expired/volatile';
$staleCache = $cache;
} else {
$where[] = 'got from local cache';
$success = true;
$this->mCache[$code] = $cache;
}
}
@ -292,7 +293,7 @@ class MessageCache {
# the lock can't be acquired, wait for the other thread to finish
# and then try the global cache a second time.
for ( $failedAttempts = 0; $failedAttempts < 2; $failedAttempts++ ) {
if ( $hashExpired && $staleCache ) {
if ( $hashVolatile && $staleCache ) {
# Do not bother fetching the whole cache blob to avoid I/O.
# Instead, just try to get the non-blocking $statusKey lock
# below, and use the local stale value if it was not acquired.
@ -304,6 +305,12 @@ class MessageCache {
} elseif ( $this->isCacheExpired( $cache ) ) {
$where[] = 'global cache is expired';
$staleCache = $cache;
} elseif ( $hashVolatile ) {
# DB results are slave lag prone until the holdoff TTL passes.
# By then, updates should be reflected in loadFromDBWithLock().
# One thread renerates the cache while others use old values.
$where[] = 'global cache is expired/volatile';
$staleCache = $cache;
} else {
$where[] = 'got from global cache';
$this->mCache[$code] = $cache;
@ -319,8 +326,8 @@ class MessageCache {
# We need to call loadFromDB. Limit the concurrency to one process.
# This prevents the site from going down when the cache expires.
# Note that the slam-protection lock here is non-blocking.
if ( $this->loadFromDBWithLock( $code, $where ) ) {
# Load from DB complete, no need to retry
$success = true;
break;
} elseif ( $staleCache ) {
@ -644,7 +651,7 @@ class MessageCache {
* Get the md5 used to validate the local disk cache
*
* @param string $code
* @return array (hash or false, bool expiry status)
* @return array (hash or false, bool expiry/volatility status)
*/
protected function getValidationHash( $code ) {
$curTTL = null;