tbonfort at terriscope.fr
MapCache sometimes needs exclusive access to a given resource, and provides some mechanisms to ensure that no more than one MapCache instance can operate on that resource. This may happen when:
Sending a request to a source WMS server for a given metatile. Only a single MapCache instance should be sending that request; other MapCache instances processing a tile from that same metatile should wait for that request to finish rather than sending the same exact request again (in order not to overload the WMS server).
Writing a tile inside a TIFF cache. The TIFF library does not handle concurrent writes, so MapCache must ensure that only a single instance is accessing a given TIFF file for writing.
A locker is configured with:
<locker type="..."> <timeout>60</timeout> <retry>0.1</retry> ... </locker>
When a MapCache instance cannot acquire a lock because it has already been acquired by another instance, it will:
Check back every <retry> seconds to see if the lock has been released by the other instance.
Consider that after <timeout> seconds the other instance has failed, and delete the lock before acquiring it itself.
This locking mechanism places a file somewhere in the filesystem. The filesystem can be a network share in order to synchronize multiple MapCache instances.
<locker type="disk"> <directory>/path/to/lockdir</directory> </locker>
The configured directory should be read/write accessible by the MapCache instance.
This locking mechanism uses a third-party memcache instance to keep track of the locks.
<locker type="memcache"> <server> <host>localhost</host> <port>11211</port> </server> <server> <host>memcache-host</host> <port>11212</port> </server> </locker>
This "meta" locker will fall back to its second configured locker if the first one fails (typically used with a memcache instance if the memcache instance goes down).
<locker type="fallback" <locker type="memcache"> <server> <host>localhost</host> <port>11211</port> </server> <server> <host>memcache-host</host> <port>11212</port> </server> </locker> <locker type="disk"> <directory>/path/to/lockdir</directory> </locker> </locker>