14 Caching
| (require file/cache) | package: base |
The file/cache library provides
utilities for managing a local cache of files, such as downloaded
files. The cache is safe for concurrent use across processes, since
it uses filesystem locks, and it isolates clients from filesystem
failures.
procedure
(cache-file dest-file [ #:exists-ok? exists-ok?] key cache-dir fetch [ #:notify-cache-use notify-cache-use #:max-cache-files max-files #:max-cache-size max-size #:evict-before? evict-before? #:log-error-string log-error-string #:log-debug-string log-debug-string]) → void? dest-file : path-string? exists-ok? : any/c = #f key : (not/c #f) cache-dir : path-string? fetch : (-> any) notify-cache-use : (string? . -> . any) = void max-files : real? = 1024 max-size : real? = (* 64 1024 1024)
evict-before? : (hash? hash? . -> . boolean?) =
(lambda (a b) (< (hash-ref a 'modify-seconds) (hash-ref b 'modify-seconds)))
log-error-string : (string? . -> . any) = (lambda (s) (log-error s))
log-debug-string : (string? . -> . any) = (lambda (s) (log-debug s))
Looks for a file in cache-dir previously cached with
key, and copies it to dest-file (which must not
exist already, unless exists-ok? is true) if a cached file
is found. Otherwise, fetch is called; if dest-file
exists after calling fetch, it is copied to cache-dir
and recorded with key. When a cache entry is used,
notify-cache-use is called with the name of the cache file.
When a new file is cached, max-files (as a file count) and max-size (in bytes) determine whether any previously cached files should be evicted from the cache. If so, evict-before? determines an order on existing cache entries for eviction; each argument to evict-before? is a hash table with at least the following keys:
'modify-seconds —
the file’s modification date 'size —
the file’s size in bytes 'key —
the cache entry’s key 'name —
the cache file’s name
The log-error-string and log-debug-string functions are used to record errors and debugging information.
procedure
(cache-remove key cache-dir [ #:log-error-string log-error-string #:log-debug-string log-debug-string]) → void? key : any/c cache-dir : path-string?
log-error-string : (string? . -> . any) = (lambda (s) (log-error s))
log-debug-string : (string? . -> . any) = (lambda (s) (log-debug s))
Removes the cache entry matching key (if any) from the cache
in cache-dir, or removes all cached files if key is
#f.
The log-error-string and log-debug-string functions are used to record errors and debugging information.