R: Create a memory cache object

memoryCache {shiny} | R Documentation

## Create a memory cache object

### Description

A memory cache object is a key-value store that saves the values in an environment. 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

    memoryCache(max_size = 10 * 1024^2, max_age = Inf, max_n = Inf,
      evict = c("lru", "fifo"), missing = key_missing(),
      exec_missing = FALSE, logfile = NULL)

### Arguments

`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.
`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()`.

### Details

In a `MemoryCache`, R objects are stored directly in the cache; they are not not serialized before being stored in the cache. This contrasts with other cache types, like `diskCache`, where objects are serialized, and the serialized object is cached. This can result in some differences of behavior. For example, as long as an object is stored in a MemoryCache, it will not be garbage collected.

### 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()`.

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 atime property. Some filesystems do not support atime, or have a very low atime resolution. The DiskCache will check for atime support, and if the filesystem does not support atime, a warning will be issued and the "fifo" policy will be used instead.

`"fifo"`

  First-in-first-out. The oldest objects will be removed.

### 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]