R: Create a file cache object

FileCache {sass} | R Documentation

## Create a file cache object

### Description

Create a file cache object

Create a file cache object

### Details

A file cache object is a key-file store that saves the values as files in a directory on disk. The objects are files on disk. They are stored and retrieved using the `get_file()`, `get_content()`, `set_file()`, and `set_content()` methods. Objects are automatically pruned from the cache according to the parameters `max_size`, `max_age`, `max_n`, and `evict`.

### Cache pruning

Cache pruning occurs when `set_file()` or `set_content()` 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_file()` or `set_content()`, because the filesystem operations for checking the status of files can be slow. Instead, it will prune once in every 20 calls to `set_file()` or `set_content()`, 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_file()` or `get_content()` 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.

### 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:

`"lru"`

  Least Recently Used. The least recently used objects will be removed. This uses the filesystem's mtime property. When "lru" is used, each time `get_file()` or `get_content()` is called, it will update the file's mtime.

`"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.)

### Sharing among multiple processes

The directory for a FileCache can be shared among multiple R processes. To do this, each R process should have a FileCache object that uses the same directory. Each FileCache will do pruning independently of the others, so if they have different pruning parameters, then one FileCache may remove cached objects before another FileCache would do so.

Even though it is possible for multiple processes to share a FileCache 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_file(key)`, the object may be removed from the cache in between those two calls, and `get_file(key)` will throw an error. Instead of calling the two functions, it is better to simply call `get_file(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.

### Methods

#### Public methods

- `FileCache$new()`

- `FileCache$get_file()`

- `FileCache$get_content()`

- `FileCache$set_file()`

- `FileCache$set_content()`

- `FileCache$exists()`

- `FileCache$keys()`

- `FileCache$remove()`

- `FileCache$reset()`

- `FileCache$dir()`

- `FileCache$prune()`

- `FileCache$size()`

- `FileCache$destroy()`

- `FileCache$is_destroyed()`

- `FileCache$finalize()`

- `FileCache$clone()`

#### Method `new()`

Create a FileCache object.

##### Usage

    FileCache$new(
      dir = NULL,
      max_size = 40 * 1024^2,
      max_age = Inf,
      max_n = Inf,
      evict = c("lru", "fifo"),
      destroy_on_finalize = FALSE,
      logfile = NULL
    )

##### Arguments

`dir`

  Directory to store files for the cache. If `NULL` (the default) it will create and use a temporary directory.

`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.

`max_age`

  Maximum age of files in cache before they are evicted, in seconds. Use `Inf` for no age limit.

`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.

`evict`

  The eviction policy to use to decide which objects are removed when a cache pruning occurs. Currently, `"lru"` and `"fifo"` are supported.

`destroy_on_finalize`

  If `TRUE`, then when the FileCache 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.

`logfile`

  An optional filename or connection object to where logging information will be written. To log to the console, use `stdout()`.

#### Method `get_file()`

Get the content associated with `key`, and save in a file named `outfile`.

##### Usage

    FileCache$get_file(key, outfile, overwrite = TRUE)

##### Arguments

`key`

  Key. Must be lowercase numbers and letters.

`outfile`

  Name of output file. If `NULL`, return the content as

`overwrite`

  If the output file already exists, should it be overwritten?

##### Returns

`TRUE` if the object is found in the cache and copying succeeds, `FALSE` otherwise.

#### Method `get_content()`

Get the content associated with `key`, and return as either string or a raw vector.

##### Usage

    FileCache$get_content(key, mode = c("text", "raw"))

##### Arguments

`key`

  Key. Must be lowercase numbers and letters.

`mode`

  If `"text"`, return the content as a UTF-8-encoded text string (a one element char vector). If `"raw"`, return the content as a raw vector.

##### Returns

A character or raw vector if the object is found in the cache, `NULL` otherwise.

#### Method `set_file()`

Sets content associated with `key`, from a file named `infile`.

##### Usage

    FileCache$set_file(key, infile)

##### Arguments

`key`

  Key. Must be lowercase numbers and letters.

`infile`

  Name of input file.

##### Returns

`TRUE` if copying the file into the cache succeeds, `FALSE` otherwise.

#### Method `set_content()`

Sets content associated with `key`, from a single-element vector.

##### Usage

    FileCache$set_content(key, content)

##### Arguments

`key`

  Key. Must be lowercase numbers and letters.

`content`

  A character or raw vector. If it is a character vector, it will be written with UTF-8 encoding, with with elements collapsed with `\\n` (consistent across platforms).

##### Returns

`TRUE` if setting the content in the cache succeeds, `FALSE` otherwise.

#### Method `exists()`

Check if content associated with `key` exists in cache

##### Usage

    FileCache$exists(key)

##### Arguments

`key`

  Key. Must be lowercase numbers and letters.

##### Returns

`TRUE` if the object is in the cache, `FALSE` otherwise.

#### Method `keys()`

Get all keys

##### Usage

    FileCache$keys()

##### Returns

A character vector of all keys currently in the cache.

#### Method `remove()`

Remove an object

##### Usage

    FileCache$remove(key)

##### Arguments

`key`

  Key. Must be lowercase numbers and letters.

##### Returns

`TRUE` if the object was found and successfully removed, `FALSE` otherwise.

#### Method `reset()`

Clear all objects from the cache.

##### Usage

    FileCache$reset()

#### Method `dir()`

Returns the directory used for the cache.

##### Usage

    FileCache$dir()

#### Method `prune()`

Prune the cache, using the parameters specified by `max_size`, `max_age`, `max_n`, and `evict`.

##### Usage

    FileCache$prune()

#### Method `size()`

Return the number of items currently in the cache.

##### Usage

    FileCache$size()

#### Method `destroy()`

Clears all objects in the cache, and removes the cache directory from disk.

##### Usage

    FileCache$destroy()

#### Method `is_destroyed()`

Reports whether the cache has been destroyed.

##### Usage

    FileCache$is_destroyed(throw = FALSE)

##### Arguments

`throw`

  Should this function throw an error if the cache has been destroyed?

#### Method `finalize()`

A finalizer for the cache.

##### Usage

    FileCache$finalize()

#### Method `clone()`

The objects of this class are cloneable with this method.

##### Usage

    FileCache$clone(deep = FALSE)

##### Arguments

`deep`

  Whether to make a deep clone.

[Package sass version 0.3.1 Index]