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ns_cache(3aolserver)					    AOLserver Built-In Commands 				      ns_cache(3aolserver)

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NAME

       ns_cache - Cache arbitrary data

SYNOPSIS

       ns_cache append cachename key string ?string ...?
       ns_cache create cachename ?-size maxsize? ?-timeout timeout? ?-thread thread?
       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
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DESCRIPTION

       AOLserver  implements a C API for caching arbitrary data. This module provides a Tcl API on top of the C API.  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.

       ns_cache create cachename ?-size maxsize? ?-timeout timeout? ?-thread thread?
	      This  command  creates a new cache named cachename. If -thread is given and is true, then it is a thread-private cache. Otherwise it
	      is a global cache. If maxsize is given, then it is a sized-based cache. If timeout is given, then it is a timeout-based cache.  Oth-
	      erwise, it is a timeout-based cache with an infinite timeout, meaning it will never be flushed.

	      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 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.

	      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 TYPES

       ns_cache supports three types of caches:

       Global Size-Limited Cache

       ns_cache create cachename -size maxsize

       Entries in a cache of this type are accessible to all threads. Each cache has its own mutex that protects access to 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 cache's size 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).

       Global Time-Limited Cache

       ns_cache create cachename -timeout timeout

       Entries in a cache of this type are accessible to all threads. Each cache has its own mutex that protects access to its entries.

       Cache values are stored as counted strings, as in a global size-limited cache.

       The  cache  has	a  maximum  entry  lifetime,  called its timeout, specified (in seconds) when the cache is created. Every timeout seconds,
       AOLserver flushes all cache entries that have not were not created or accessed in the last timeout seconds.

       Thread-Private Size-Limited Cache

       ns_cache create cachename -size maxsize -thread 1

       Each thread in AOLserver automatically gets its own private cache named cachename. Since a thread-private cache is  only  accessed  by  one
       thread, access to it does not require a mutex. Entries in one thread's cache are not visible to any other thread.

       Cache  values  are  stored as Tcl objects. When a value is stored in the cache, nscache computes its string form and uses the length of the
       string as the size of the value.

       The cache has a maximum size, like a global size-limited cache. However, because of the way the cache value sizes are computed, the  actual
       memory usage of the cache values may be several times larger than maxsize.

       Thread-private  caches  may  offer  higher  performance if the cached values are complex objects such as lists or scripts, but require more
       storage than global caches.

SEE ALSO

       Ns_Cache(3), nsv(n)

AOLserver								4.0						      ns_cache(3aolserver)