bionic (3) ns_cache.3aolserver.gz
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. Otherwise, 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)