Ns cache

NAME

ns_cache − Cache arbitrary data

SYNOPSIS

ns_cache append cachename key string ?string ...?

ns_cache create cachename ?-size maxsize? ?-timeout timeout? ?-maxwait maxwait?

ns_cache eval cachename key script

ns_cache flush cachename key

ns_cache get cachename key ?varname?

ns_cache incr cachename key ?value?

ns_cache lappend cachename key string ?string ...?

ns_cache names cachename ?pattern?

ns_cache set cachename key string

DESCRIPTION

AOLserver implements a C API for caching arbitrary data. A cache, in this context, is simply a dictionary that maps keys to values. Keys are always stored as NUL-terminated strings. How values are stored depends on the type of cache. In past versions of AOLserver, an addon module called nscache provided a Tcl API on top of the C API to allow storing arbitrary strings. In AOLserver 4.5.1, ns_cache is now a builtin command which is syntax-compatible with the nscache module but is implemented differently and has several important behavioral differences:

• The module supported thread-specific, virtual-server-specific and serverwide (i.e. process-wide) caches, with virtual-server-specific as the default. AOLserver 4.5.1's builtin ns_cache allows but ignores -thread and -serverwide switches and always creates serverwide caches shared by all virtual servers.
• In AOLserver 4.5.1, all caches are size-limited whether -size is specified or not (if omitted, maxsize defaults to 1 Megabyte (1024*1000 bytes)) plus can optionally have a timeout.
• The new built-in ns_cache get does not wait for a concurrent ns_cache eval to execute its script, instead it behaves as if the cache entry does not exist (ns_cache set and incr throw an "entry busy: ..." error if there is a concurrent ns_cache eval for the same key).
• Instead it is now ns_cache eval that waits on another concurrent ns_cache eval for the same key but only up to the maxwait defined for the cache as a whole at creation time (defaults to 2 seconds) and then throws a "timeout waiting for update: ..." error. The default of 2 seconds can easily be exceeded by an expensive database operation the result of which you want to cache.
• append and lappend are new options (they also throw an "entry busy" error if a concurrent ns_cache eval is executing)

ns_cache create cachename ?-size maxsize? ?-timeout timeout? ?-maxwait maxwait?

This command creates a new global process-wide cache named cachename limited to 1 Megabyte by default or the number of bytes specified in maxsize. If timeout is given, then inactive (not accessed) entries will be flushed after the specified number of seconds. The -maxwait parameter is new to the AOLserver 4.5.1 implementation and provides a maximum wait in seconds that ns_cache eval will wait for another ns_cache eval for the same key to run its script and update the cache before throwing an error. The default maxwait is 2 seconds.

This command returns nothing if it is successful.

ns_cache eval cachename key script

This command atomically sets and gets a cache value. First, it looks up key in the cache named cachename. If it finds an unexpired entry, it returns the value of that entry. Otherwise, it executes script, stores the return value in the cache, and also returns that value.

Script may optionally use the return command to return its value. For example, this will store the value "2" in mycache, if mykey is not already present:

ns_cache eval mycache mykey { expr {1+1} }

This will also store the value "2" in mycache:

ns_cache eval mycache mykey { return [expr {1+1}] }

If script raises an error, or exits with break or continue, then ns_cache eval simply returns the same condition without modifying the cache.

ns_cache flush cachename key

This command removes the entry for key from the cache named cachename. If the cache has no entry for key, then nothing happens.

Note that ns_cache eval detects concurrent evals for the same key and For global caches, ns_cache flush interacts with ns_cache eval. Suppose thread 1 has called get_thing A and is executing the long operation to compute the value for A. Thread 2 calls get_thing A and starts waiting for thread 1 to finish. Thread 3 calls ns_cache flush thing_cache A. Thread 1 will continue executing the long operation, but thread 2 will also start the long operation. When thread 1 completes the long operation, ns_cache eval returns the (now stale) value it computed, but it does not store the value in the cache. When thread 2 completes the long operation, ns_cache eval stores the (fresh) value it computed in the cache and returns the fresh value.

ns_cache get cachename key ?varname?

This command looks up key in the specified cache. It operates differently depending on whether varname was given.

If varname absent and the key exists the value is returned and if the key is missing an error is raised. If varname is provided and the key exists the command sets varname to the value and returns 1, otherwise it returns 0.

ns_cache names cachename ?pattern?

This command returns a list of all keys currently in the specified cache. If pattern is specified, only matching entries are returned (match pattern syntax like in string match).

If the cache is thread‐private, then the list only includes keys that are in the thread’s private cache.

ns_cache set cachename key value

This command stores value for key in the specified cache.

CACHE DETAILS

Entries in the globacl cache are accessible to all threads. Each cache has its own SINGLE mutex that protects access to ALL its entries.

Cache values are stored as counted strings, so arbitrary binary data can be cached. A global cache stores strings instead of Tcl objects to prevent race conditions that could lead to heap corruption.

The cache has a maximum size specified when the cache is created. The size of the cache is the sum of the sizes of all the values in the cache; keys do not count toward a cache’s size. If inserting a value into the cache makes the size of the cache exceed its maximum, then cache entries are evicted starting with the least-recently used entry until the size is below the maximum size (or until only the new value remains in the cache).

If -timeout is specified when the cache is created, the cache has an inactive timeout for entries. Every timeout seconds, AOLserver flushes all cache entries that were not created or accessed in the last timeout seconds. Note that the purpose here is to purge from memory entries that aren't being used, not to provide a strict expiration such as exists in ns_adp_include -cache, for example.

SEE ALSO

nsv

Category Documentation - Category Core Tcl API