R: Create a disk cache object

diskCache {shiny} | R Documentation

## Create a disk cache object

### 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 `get()` and `set()` methods. Objects are automatically pruned from the cache according to the parameters `max_size`, `max_age`, `max_n`, and `evict`.

### 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

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

### 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:

      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.

### 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.

### 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 `get()` 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 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.

### Methods

A disk cache object has the following methods:

`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.

`set(key, value)`

  Stores the `key`-`value` pair in the cache.

`exists(key)`

  Returns `TRUE` if the cache contains the key, otherwise `FALSE`.

`size()`

  Returns the number of items currently in the cache.

`keys()`

  Returns a character vector of all keys currently in the cache.

`reset()`

  Clears all objects from the cache.

`destroy()`

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

`prune()`

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

[Package shiny version 1.2.0 Index]