diskCache.Rd

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/cache-disk.R
\name{diskCache}
\alias{diskCache}
\title{Create a disk cache object}
\usage{
diskCache(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)
}
\arguments{
\item{dir}{Directory to store files for the cache. If \code{NULL} (the
default) it will create and use a temporary directory.}

\item{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
\code{evict}. Use \code{Inf} for no size limit.}

\item{max_age}{Maximum age of files in cache before they are evicted, in
seconds. Use \code{Inf} for no age limit.}

\item{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 \code{evict}. Use \code{Inf} for no limit of number of items.}

\item{evict}{The eviction policy to use to decide which objects are removed
when a cache pruning occurs. Currently, \code{"lru"} and \code{"fifo"} are
supported.}

\item{destroy_on_finalize}{If \code{TRUE}, then when the DiskCache object is
garbage collected, the cache directory and all objects inside of it will be
deleted from disk. If \code{FALSE} (the default), it will do nothing when
finalized.}

\item{missing}{A value to return or a function to execute when
\code{get(key)} is called but the key is not present in the cache. The
default is a \code{\link[=key_missing]{key_missing()}} object. If it is a function to
execute, the function must take one argument (the key), and you must also
use \code{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
\code{get()}. See section Missing keys for more information.}

\item{exec_missing}{If \code{FALSE} (the default), then treat \code{missing}
as a value to return when \code{get()} results in a cache miss. If
\code{TRUE}, treat \code{missing} as a function to execute when
\code{get()} results in a cache miss.}

\item{logfile}{An optional filename or connection object to where logging
information will be written. To log to the console, use \code{stdout()}.}
}
\description{
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 \code{get()}
and \code{set()} methods. Objects are automatically pruned from the cache
according to the parameters \code{max_size}, \code{max_age}, \code{max_n},
and \code{evict}.
}
\section{Missing Keys}{


The \code{missing} and \code{exec_missing} parameters controls what happens
when \code{get()} is called with a key that is not in the cache (a cache
miss). The default behavior is to return a \code{\link[=key_missing]{key_missing()}}
object. This is a \emph{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 \code{\link[=is.key_missing]{is.key_missing()}} function. You can
also have \code{get()} return a different sentinel value, like \code{NULL}.
If you want to throw an error on a cache miss, you can do so by providing a
function for \code{missing} that takes one argument, the key, and also use
\code{exec_missing=TRUE}.

When the cache is created, you can supply a value for \code{missing}, which
sets the default value to be returned for missing values. It can also be
overridden when \code{get()} is called, by supplying a \code{missing}
argument. For example, if you use \code{cache$get("mykey", missing = NULL)}, it will return \code{NULL} if the key is not in the cache.

If your cache is configured so that \code{get()} returns a sentinel value
to represent a cache miss, then \code{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 \code{get()} encounters missing
key. If the function returns a value, then \code{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 \code{get()} will not return a value.

To do this, pass a one-argument function to \code{missing}, and use
\code{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 \code{get()} should be wrapped with
\code{\link[=tryCatch]{tryCatch()}} to gracefully handle missing keys.
}

\section{Cache pruning}{


Cache pruning occurs when \code{set()} is called, or it can be invoked
manually by calling \code{prune()}.

The disk cache will throttle the pruning so that it does not happen on
every call to \code{set()}, because the filesystem operations for checking
the status of files can be slow. Instead, it will prune once in every 20
calls to \code{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
\code{max_age}, they will be removed.

The \code{max_size} and \code{max_n} parameters are applied to the cache as
a whole, in contrast to \code{max_age}, which is applied to each object
individually.

If the number of objects in the cache exceeds \code{max_n}, then objects
will be removed from the cache according to the eviction policy, which is
set with the \code{evict} parameter. Objects will be removed so that the
number of items is \code{max_n}.

If the size of the objects in the cache exceeds \code{max_size}, then
objects will be removed from the cache. Objects will be removed from the
cache so that the total size remains under \code{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
\code{get()} is called. If the target object is older than \code{max_age},
it will be removed and the cache will report it as a missing value.
}

\section{Eviction policies}{


If \code{max_n} or \code{max_size} are used, then objects will be removed
from the cache according to an eviction policy. The available eviction
policies are:

\describe{
\item{\code{"lru"}}{
Least Recently Used. The least recently used objects will be removed.
This uses the filesystem's mtime property. When "lru" is used, each
\code{get()} is called, it will update the file's mtime.
}
\item{\code{"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 \code{exists(key)} to check
if an object is in the cache, and then call \code{get(key)}, the object may
be removed from the cache in between those two calls, and \code{get(key)}
will throw an error. Instead of calling the two functions, it is better to
simply call \code{get(key)}, and use \code{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 \code{file.remove()} failing to remove a file that has
already been deleted.
}

\section{Methods}{


A disk cache object has the following methods:

\describe{
\item{\code{get(key, missing, exec_missing)}}{
Returns the value associated with \code{key}. If the key is not in the
cache, then it returns the value specified by \code{missing} or,
\code{missing} is a function and \code{exec_missing=TRUE}, then
executes \code{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{\code{set(key, value)}}{
Stores the \code{key}-\code{value} pair in the cache.
}
\item{\code{exists(key)}}{
Returns \code{TRUE} if the cache contains the key, otherwise
\code{FALSE}.
}
\item{\code{size()}}{
Returns the number of items currently in the cache.
}
\item{\code{keys()}}{
Returns a character vector of all keys currently in the cache.
}
\item{\code{reset()}}{
Clears all objects from the cache.
}
\item{\code{destroy()}}{
Clears all objects in the cache, and removes the cache directory from
disk.
}
\item{\code{prune()}}{
Prunes the cache, using the parameters specified by \code{max_size},
\code{max_age}, \code{max_n}, and \code{evict}.
}
}
}