6.9.7 Summary: object caching ``flow chart''

To summarize and fill in missing details about the above discussion, the Cache works as follows:

• When the Cache starts, it writes its process identifier to the file cached.pid in the same directory as where cached.conf resides. This can be useful if you want to write programs that monitor the status of the cache.

• When a request is made for an object currently in the cache, it is returned to the client and noted in the cache log files, and noted in the cache's ``Least Recently Used'' (LRU) chain.

• When a request is made for an object not currently in the cache, the cache retrieves the object and simultaneously feeds bytes to the client and stores the object into both VM and disk. If the object turns out to be bigger than the parameterized cache_mem setting (from the cached.conf file) or if it's bigger than the maximum object size for that access type (also specified in the cached.conf file), the object is rejected, and an error is returned to the user (rather than returning the object). In a future version of Harvest we will allow proxying for objects that are too large, passing them to the client but removing them from VM and disk.

• The cache performs LRU replacement in both VM and disk, but with different size limits, as specified in cache_mem and cache_swap. Any object in VM will also be on disk, but not necessarily vice versa.

• When the amount of RAM used by the hot-object VM cache reaches cache_mem_high % of the specified cache_mem size, the cache starts throwing objects out of the VM cache, until they reach cache_mem_low % of the specified cache_mem size. (These values are set in the cached.conf file.) Objects evicted from the VM cache stay in the disk cache until their cache-assigned TTL expires, they are evicted by the disk cache replacement policy, or the user manually evicts them by clicking the browser's ``reload'' button.

  The high/low water mark mechanism is intended to let the Cache cut back on memory usage when it starts using too much memory, so it can avoid constantly bringing in objects and then having to evict them. There's no high/low water mark on disk.