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126 lines
4.3 KiB
R
126 lines
4.3 KiB
R
% Generated by roxygen2: do not edit by hand
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% Please edit documentation in R/cache.R
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\name{diskCache}
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\alias{diskCache}
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\title{Create a disk cache object}
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\usage{
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diskCache(dir = tempfile("DiskCache-"), max_size = 5 * 1024^2,
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max_age = Inf, max_n = Inf, evict = "fifo",
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destroy_on_finalize = TRUE)
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}
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\arguments{
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\item{dir}{Directory to store files for the cache. By default, it will use
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a temporary directory.}
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\item{max_size}{Maximum size of the cache, in bytes. If the cache exceeds
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this size, cached objects will be removed according to the value of the
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\code{evict}.}
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\item{max_age}{Maximum age of files in cache before they are evicted, in
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seconds.}
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\item{max_n}{Maximum number of objects in the cache. If the number of objects
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exceeds this value, then cached objects will be removed according to the
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value of \code{evict}.}
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\item{evict}{The eviction policy to use to decide which objects are removed
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when a cache pruning occurs. Currently, \code{"lru"} and \code{"fifo"} are
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supported.}
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\item{destroy_on_finalize}{If \code{TRUE}, then when the DiskCache object is
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garbage collected, the cache directory and all objects inside of it will be
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deleted from disk.}
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}
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\description{
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A disk cache object is a key-value store that saves the values as files in a
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directory on disk. Objects can be stored and retrieved using the \code{get()}
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and \code{set()} methods. Objects are automatically pruned from the cache
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according to the parameters \code{max_size}, \code{max_age}, \code{max_n},
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and \code{evict}.
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}
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\section{Cache pruning}{
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Cache pruning occurs each time \code{get()} and \code{set()} are called, or
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it can be invoked manually by calling \code{prune()}.
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If there are any objects that are older than \code{max_age}, they will be
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removed when a pruning occurs.
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The \code{max_size} and \code{max_n} parameters are applied to the cache as
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a whole, in contrast to \code{max_age}, which is applied to each object
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individually.
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If the number of objects in the cache exceeds \code{max_n}, then objects
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will be removed from the cache according to the eviction policy, which is
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set with the \code{evict} parameter. Objects will be removed so that the
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number of items is \code{max_n}.
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If the size of the objects in the cache exceeds \code{max_size}, then
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objects will be removed from the cache. Objects will be removed from
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the cache so that the total size remains under \code{max_size}. Note that
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the size is calculated using the size of the files, not the size of disk
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space used by the files -- these two values can differ because of files
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are stored in blocks on disk. For example, if the block size is 4096 bytes,
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then a file that is one byte in size will take 4096 bytes on disk.
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}
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\section{Eviction policies}{
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If \code{max_n} or \code{max_size} are used, then objects will be removed
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from the cache according to an eviction policy. The available eviction
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policies are:
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\describe{
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\item{\code{"lru"}}{
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Least Recently Used. The least recently used objects will be removed.
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This uses the filesystem's atime property. Some filesystems do not
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support atime, or have a very low atime resolution. The DiskCache will
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check for atime support, and if the filesystem does not support atime,
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a warning will be issued and the "fifo" policy will be used instead.
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}
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\item{\code{"fifo"}}{
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First-in-first-out. The oldest objects will be removed.
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}
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}
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}
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\section{Methods}{
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A disk cache object has the following methods:
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\describe{
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\item{\code{get(key)}}{
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Returns the value associated with \code{key}. If the key is not in the
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cache, this throws an error.
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}
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\item{\code{set(key, value)}}{
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Stores the \code{key}-\code{value} pair in the cache.
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}
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\item{\code{has(key)}}{
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Returns \code{TRUE} if the cache contains the key, otherwise
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\code{FALSE}.
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}
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\item{\code{size()}}{
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Returns the number of items currently in the cache.
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}
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\item{\code{keys()}}{
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Returns a character vector of all keys currently in the cache.
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}
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\item{\code{reset()}}{
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Clears all objects from the cache.
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}
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\item{\code{destroy()}}{
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Clears all objects in the cache, and removes the cache directory from
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disk.
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}
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\item{\code{prune()}}{
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Prunes the cache, using the parameters specified by \code{max_size},
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\code{max_age}, \code{max_n}, and \code{evict}.
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}
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}
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}
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