#' Create a disk cache object
#'
#' A disk cache object is a key-value store that saves the values as files in a
#' directory on disk. Objects can be stored and retrieved using the `get()`
#' and `set()` methods. Objects are automatically pruned from the cache
#' according to the parameters `max_size`, `max_age`, `max_n`,
#' and `evict`.
#'
#'
#' @section Missing Keys:
#'
#' The `missing` and `exec_missing` parameters controls what happens
#' when `get()` is called with a key that is not in the cache (a cache
#' miss). The default behavior is to return a [key_missing()]
#' object. This is a *sentinel value* that indicates that the key was not
#' present in the cache. You can test if the returned value represents a
#' missing key by using the [is.key_missing()] function. You can
#' also have `get()` return a different sentinel value, like `NULL`.
#' If you want to throw an error on a cache miss, you can do so by providing a
#' function for `missing` that takes one argument, the key, and also use
#' `exec_missing=TRUE`.
#'
#' When the cache is created, you can supply a value for `missing`, which
#' sets the default value to be returned for missing values. It can also be
#' overridden when `get()` is called, by supplying a `missing`
#' argument. For example, if you use `cache$get("mykey", missing =
#' NULL)`, it will return `NULL` if the key is not in the cache.
#'
#' If your cache is configured so that `get()` returns a sentinel value
#' to represent a cache miss, then `set` will also not allow you to store
#' the sentinel value in the cache. It will throw an error if you attempt to
#' do so.
#'
#' Instead of returning the same sentinel value each time there is cache miss,
#' the cache can execute a function each time `get()` encounters missing
#' key. If the function returns a value, then `get()` will in turn return
#' that value. However, a more common use is for the function to throw an
#' error. If an error is thrown, then `get()` will not return a value.
#'
#' To do this, pass a one-argument function to `missing`, and use
#' `exec_missing=TRUE`. For example, if you want to throw an error that
#' prints the missing key, you could do this:
#'
#' \preformatted{
#' diskCache(
#' missing = function(key) {
#' stop("Attempted to get missing key: ", key)
#' },
#' exec_missing = TRUE
#' )
#' }
#'
#' If you use this, the code that calls `get()` should be wrapped with
#' [tryCatch()] to gracefully handle missing keys.
#'
#' @section Cache pruning:
#'
#' Cache pruning occurs when `set()` is called, or it can be invoked
#' manually by calling `prune()`.
#'
#' The disk cache will throttle the pruning so that it does not happen on
#' every call to `set()`, because the filesystem operations for checking
#' the status of files can be slow. Instead, it will prune once in every 20
#' calls to `set()`, or if at least 5 seconds have elapsed since the last
#' prune occurred, whichever is first. These parameters are currently not
#' customizable, but may be in the future.
#'
#' When a pruning occurs, if there are any objects that are older than
#' `max_age`, they will be removed.
#'
#' The `max_size` and `max_n` parameters are applied to the cache as
#' a whole, in contrast to `max_age`, which is applied to each object
#' individually.
#'
#' If the number of objects in the cache exceeds `max_n`, then objects
#' will be removed from the cache according to the eviction policy, which is
#' set with the `evict` parameter. Objects will be removed so that the
#' number of items is `max_n`.
#'
#' If the size of the objects in the cache exceeds `max_size`, then
#' objects will be removed from the cache. Objects will be removed from the
#' cache so that the total size remains under `max_size`. Note that the
#' size is calculated using the size of the files, not the size of disk space
#' used by the files --- these two values can differ because of files are
#' stored in blocks on disk. For example, if the block size is 4096 bytes,
#' then a file that is one byte in size will take 4096 bytes on disk.
#'
#' Another time that objects can be removed from the cache is when
#' `get()` is called. If the target object is older than `max_age`,
#' it will be removed and the cache will report it as a missing value.
#'
#' @section Eviction policies:
#'
#' If `max_n` or `max_size` are used, then objects will be removed
#' from the cache according to an eviction policy. The available eviction
#' policies are:
#'
#' \describe{
#' \item{`"lru"`}{
#' Least Recently Used. The least recently used objects will be removed.
#' This uses the filesystem's mtime property. When "lru" is used, each
#' `get()` is called, it will update the file's mtime.
#' }
#' \item{`"fifo"`}{
#' First-in-first-out. The oldest objects will be removed.
#' }
#' }
#'
#' Both of these policies use files' mtime. Note that some filesystems (notably
#' FAT) have poor mtime resolution. (atime is not used because support for
#' atime is worse than mtime.)
#'
#'
#' @section Sharing among multiple processes:
#'
#' The directory for a DiskCache can be shared among multiple R processes. To
#' do this, each R process should have a DiskCache object that uses the same
#' directory. Each DiskCache will do pruning independently of the others, so if
#' they have different pruning parameters, then one DiskCache may remove cached
#' objects before another DiskCache would do so.
#'
#' Even though it is possible for multiple processes to share a DiskCache
#' directory, this should not be done on networked file systems, because of
#' slow performance of networked file systems can cause problems. If you need
#' a high-performance shared cache, you can use one built on a database like
#' Redis, SQLite, mySQL, or similar.
#'
#' When multiple processes share a cache directory, there are some potential
#' race conditions. For example, if your code calls `exists(key)` to check
#' if an object is in the cache, and then call `get(key)`, the object may
#' be removed from the cache in between those two calls, and `get(key)`
#' will throw an error. Instead of calling the two functions, it is better to
#' simply call `get(key)`, and use `tryCatch()` to handle the error
#' that is thrown if the object is not in the cache. This effectively tests for
#' existence and gets the object in one operation.
#'
#' It is also possible for one processes to prune objects at the same time that
#' another processes is trying to prune objects. If this happens, you may see
#' a warning from `file.remove()` failing to remove a file that has
#' already been deleted.
#'
#'
#' @section Methods:
#'
#' A disk cache object has the following methods:
#'
#' \describe{
#' \item{`get(key, missing, exec_missing)`}{
#' Returns the value associated with `key`. If the key is not in the
#' cache, then it returns the value specified by `missing` or,
#' `missing` is a function and `exec_missing=TRUE`, then
#' executes `missing`. The function can throw an error or return the
#' value. If either of these parameters are specified here, then they
#' will override the defaults that were set when the DiskCache object was
#' created. See section Missing Keys for more information.
#' }
#' \item{`set(key, value)`}{
#' Stores the `key`-`value` pair in the cache.
#' }
#' \item{`exists(key)`}{
#' Returns `TRUE` if the cache contains the key, otherwise
#' `FALSE`.
#' }
#' \item{`size()`}{
#' Returns the number of items currently in the cache.
#' }
#' \item{`keys()`}{
#' Returns a character vector of all keys currently in the cache.
#' }
#' \item{`reset()`}{
#' Clears all objects from the cache.
#' }
#' \item{`destroy()`}{
#' Clears all objects in the cache, and removes the cache directory from
#' disk.
#' }
#' \item{`prune()`}{
#' Prunes the cache, using the parameters specified by `max_size`,
#' `max_age`, `max_n`, and `evict`.
#' }
#' }
#'
#' @param dir Directory to store files for the cache. If `NULL` (the
#' default) it will create and use a temporary directory.
#' @param max_age Maximum age of files in cache before they are evicted, in
#' seconds. Use `Inf` for no age limit.
#' @param max_size Maximum size of the cache, in bytes. If the cache exceeds
#' this size, cached objects will be removed according to the value of the
#' `evict`. Use `Inf` for no size limit.
#' @param max_n Maximum number of objects in the cache. If the number of objects
#' exceeds this value, then cached objects will be removed according to the
#' value of `evict`. Use `Inf` for no limit of number of items.
#' @param evict The eviction policy to use to decide which objects are removed
#' when a cache pruning occurs. Currently, `"lru"` and `"fifo"` are
#' supported.
#' @param destroy_on_finalize If `TRUE`, then when the DiskCache object is
#' garbage collected, the cache directory and all objects inside of it will be
#' deleted from disk. If `FALSE` (the default), it will do nothing when
#' finalized.
#' @param missing A value to return or a function to execute when
#' `get(key)` is called but the key is not present in the cache. The
#' default is a [key_missing()] object. If it is a function to
#' execute, the function must take one argument (the key), and you must also
#' use `exec_missing = TRUE`. If it is a function, it is useful in most
#' cases for it to throw an error, although another option is to return a
#' value. If a value is returned, that value will in turn be returned by
#' `get()`. See section Missing keys for more information.
#' @param exec_missing If `FALSE` (the default), then treat `missing`
#' as a value to return when `get()` results in a cache miss. If
#' `TRUE`, treat `missing` as a function to execute when
#' `get()` results in a cache miss.
#' @param logfile An optional filename or connection object to where logging
#' information will be written. To log to the console, use `stdout()`.
#'
#' @export
diskCache <- function(
dir = NULL,
max_size = 10 * 1024 ^ 2,
max_age = Inf,
max_n = Inf,
evict = c("lru", "fifo"),
destroy_on_finalize = FALSE,
missing = key_missing(),
exec_missing = FALSE,
logfile = NULL)
{
DiskCache$new(dir, max_size, max_age, max_n, evict, destroy_on_finalize,
missing, exec_missing, logfile)
}
DiskCache <- R6Class("DiskCache",
public = list(
initialize = function(
dir = NULL,
max_size = 10 * 1024 ^ 2,
max_age = Inf,
max_n = Inf,
evict = c("lru", "fifo"),
destroy_on_finalize = FALSE,
missing = key_missing(),
exec_missing = FALSE,
logfile = NULL)
{
if (exec_missing && (!is.function(missing) || length(formals(missing)) == 0)) {
stop("When `exec_missing` is true, `missing` must be a function that takes one argument.")
}
if (is.null(dir)) {
dir <- tempfile("DiskCache-")
}
if (!is.numeric(max_size)) stop("max_size must be a number. Use `Inf` for no limit.")
if (!is.numeric(max_age)) stop("max_age must be a number. Use `Inf` for no limit.")
if (!is.numeric(max_n)) stop("max_n must be a number. Use `Inf` for no limit.")
if (!dirExists(dir)) {
private$log(paste0("initialize: Creating ", dir))
dir.create(dir, recursive = TRUE)
}
private$dir <- normalizePath(dir)
private$max_size <- max_size
private$max_age <- max_age
private$max_n <- max_n
private$evict <- match.arg(evict)
private$destroy_on_finalize <- destroy_on_finalize
private$missing <- missing
private$exec_missing <- exec_missing
private$logfile <- logfile
private$prune_last_time <- as.numeric(Sys.time())
},
get = function(key, missing = private$missing, exec_missing = private$exec_missing) {
private$log(paste0('get: key "', key, '"'))
self$is_destroyed(throw = TRUE)
validate_key(key)
private$maybe_prune_single(key)
filename <- private$key_to_filename(key)
# Instead of calling exists() before fetching the value, just try to
# fetch the value. This reduces the risk of a race condition when
# multiple processes share a cache.
read_error <- FALSE
tryCatch(
{
value <- suppressWarnings(readRDS(filename))
if (private$evict == "lru"){
Sys.setFileTime(filename, Sys.time())
}
},
error = function(e) {
read_error <<- TRUE
}
)
if (read_error) {
private$log(paste0('get: key "', key, '" is missing'))
if (exec_missing) {
if (!is.function(missing) || length(formals(missing)) == 0) {
stop("When `exec_missing` is true, `missing` must be a function that takes one argument.")
}
return(missing(key))
} else {
return(missing)
}
}
private$log(paste0('get: key "', key, '" found'))
value
},
set = function(key, value) {
private$log(paste0('set: key "', key, '"'))
self$is_destroyed(throw = TRUE)
validate_key(key)
file <- private$key_to_filename(key)
temp_file <- paste0(file, "-temp-", createUniqueId(8))
save_error <- FALSE
ref_object <- FALSE
tryCatch(
{
saveRDS(value, file = temp_file,
refhook = function(x) {
ref_object <<- TRUE
NULL
}
)
file.rename(temp_file, file)
},
error = function(e) {
save_error <<- TRUE
# Unlike file.remove(), unlink() does not raise warning if file does
# not exist.
unlink(temp_file)
}
)
if (save_error) {
private$log(paste0('set: key "', key, '" error'))
stop('Error setting value for key "', key, '".')
}
if (ref_object) {
private$log(paste0('set: value is a reference object'))
warning("A reference object was cached in a serialized format. The restored object may not work as expected.")
}
private$prune_throttled()
invisible(self)
},
exists = function(key) {
self$is_destroyed(throw = TRUE)
validate_key(key)
file.exists(private$key_to_filename(key))
},
# Return all keys in the cache
keys = function() {
self$is_destroyed(throw = TRUE)
files <- dir(private$dir, "\\.rds$")
sub("\\.rds$", "", files)
},
remove = function(key) {
private$log(paste0('remove: key "', key, '"'))
self$is_destroyed(throw = TRUE)
validate_key(key)
file.remove(private$key_to_filename(key))
invisible(self)
},
reset = function() {
private$log(paste0('reset'))
self$is_destroyed(throw = TRUE)
file.remove(dir(private$dir, "\\.rds$", full.names = TRUE))
invisible(self)
},
prune = function() {
# TODO: It would be good to add parameters `n` and `size`, so that the
# cache can be pruned to `max_n - n` and `max_size - size` before adding
# an object. Right now we prune after adding the object, so the cache
# can temporarily grow past the limits. The reason we don't do this now
# is because it is expensive to find the size of the serialized object
# before adding it.
private$log(paste0('prune'))
self$is_destroyed(throw = TRUE)
current_time <- Sys.time()
filenames <- dir(private$dir, "\\.rds$", full.names = TRUE)
info <- file.info(filenames)
info <- info[info$isdir == FALSE, ]
info$name <- rownames(info)
rownames(info) <- NULL
# Files could be removed between the dir() and file.info() calls. The
# entire row for such files will have NA values. Remove those rows.
info <- info[!is.na(info$size), ]
# 1. Remove any files where the age exceeds max age.
if (is.finite(private$max_age)) {
timediff <- as.numeric(current_time - info$mtime, units = "secs")
rm_idx <- timediff > private$max_age
if (any(rm_idx)) {
private$log(paste0("prune max_age: Removing ", paste(info$name[rm_idx], collapse = ", ")))
file.remove(info$name[rm_idx])
info <- info[!rm_idx, ]
}
}
# Sort objects by priority. The sorting is done in a function which can be
# called multiple times but only does the work the first time.
info_is_sorted <- FALSE
ensure_info_is_sorted <- function() {
if (info_is_sorted) return()
info <<- info[order(info$mtime, decreasing = TRUE), ]
info_is_sorted <<- TRUE
}
# 2. Remove files if there are too many.
if (is.finite(private$max_n) && nrow(info) > private$max_n) {
ensure_info_is_sorted()
rm_idx <- seq_len(nrow(info)) > private$max_n
private$log(paste0("prune max_n: Removing ", paste(info$name[rm_idx], collapse = ", ")))
rm_success <- file.remove(info$name[rm_idx])
info <- info[!rm_success, ]
}
# 3. Remove files if cache is too large.
if (is.finite(private$max_size) && sum(info$size) > private$max_size) {
ensure_info_is_sorted()
cum_size <- cumsum(info$size)
rm_idx <- cum_size > private$max_size
private$log(paste0("prune max_size: Removing ", paste(info$name[rm_idx], collapse = ", ")))
rm_success <- file.remove(info$name[rm_idx])
info <- info[!rm_success, ]
}
private$prune_last_time <- as.numeric(current_time)
invisible(self)
},
size = function() {
self$is_destroyed(throw = TRUE)
length(dir(private$dir, "\\.rds$"))
},
destroy = function() {
if (self$is_destroyed()) {
return(invisible(self))
}
private$log(paste0("destroy: Removing ", private$dir))
# First create a sentinel file so that other processes sharing this
# cache know that the cache is to be destroyed. This is needed because
# the recursive unlink is not atomic: another process can add a file to
# the directory after unlink starts removing files but before it removes
# the directory, and when that happens, the directory removal will fail.
file.create(file.path(private$dir, "__destroyed__"))
# Remove all the .rds files. This will not remove the setinel file.
file.remove(dir(private$dir, "\\.rds$", full.names = TRUE))
# Next remove dir recursively, including sentinel file.
unlink(private$dir, recursive = TRUE)
private$destroyed <- TRUE
invisible(self)
},
is_destroyed = function(throw = FALSE) {
if (!dirExists(private$dir) ||
file.exists(file.path(private$dir, "__destroyed__")))
{
# It's possible for another process to destroy a shared cache directory
private$destroyed <- TRUE
}
if (throw) {
if (private$destroyed) {
stop("Attempted to use cache which has been destroyed:\n ", private$dir)
}
} else {
private$destroyed
}
},
finalize = function() {
if (private$destroy_on_finalize) {
self$destroy()
}
}
),
private = list(
dir = NULL,
max_age = NULL,
max_size = NULL,
max_n = NULL,
evict = NULL,
destroy_on_finalize = NULL,
destroyed = FALSE,
missing = NULL,
exec_missing = FALSE,
logfile = NULL,
prune_throttle_counter = 0,
prune_last_time = NULL,
key_to_filename = function(key) {
validate_key(key)
# Additional validation. This 80-char limit is arbitrary, and is
# intended to avoid hitting a filename length limit on Windows.
if (nchar(key) > 80) {
stop("Invalid key: key must have fewer than 80 characters.")
}
file.path(private$dir, paste0(key, ".rds"))
},
# A wrapper for prune() that throttles it, because prune() can be
# expensive due to filesystem operations. This function will prune only
# once every 20 times it is called, or if it has been more than 5 seconds
# since the last time the cache was actually pruned, whichever is first.
# In the future, the behavior may be customizable.
prune_throttled = function() {
# Count the number of times prune() has been called.
private$prune_throttle_counter <- private$prune_throttle_counter + 1
if (private$prune_throttle_counter > 20 ||
private$prune_last_time - as.numeric(Sys.time()) > 5)
{
self$prune()
private$prune_throttle_counter <- 0
}
},
# Prunes a single object if it exceeds max_age. If the object does not
# exceed max_age, or if the object doesn't exist, do nothing.
maybe_prune_single = function(key) {
obj <- private$cache[[key]]
if (is.null(obj)) return()
timediff <- as.numeric(Sys.time()) - obj$mtime
if (timediff > private$max_age) {
private$log(paste0("pruning single object exceeding max_age: Removing ", key))
rm(list = key, envir = private$cache)
}
},
log = function(text) {
if (is.null(private$logfile)) return()
text <- paste0(format(Sys.time(), "[%Y-%m-%d %H:%M:%OS3] DiskCache "), text)
writeLines(text, private$logfile)
}
)
)
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