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.

cache_mem(
  max_size = 512 * 1024^2,
  max_age = Inf,
  max_n = Inf,
  evict = c("lru", "fifo"),
  missing = key_missing(),
  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. The default is 1 gigabyte.

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 when get(key) is called but the key is not present in the cache. The default is a key_missing() object. It is actually an expression that is evaluated each time there is a cache miss. See section Missing keys for more information.

logfile

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

Value

A memory caching object, with class cache_mem.

Details

In a cache_mem, 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 cache_disk(), 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 cache_mem, it will not be garbage collected.

Missing keys

The missing parameter 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 an expression for missing, as in missing = stop("Missing key").

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.

The missing parameter is actually an expression which is evaluated each time there is a cache miss. A quosure (from the rlang package) can be used.

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().

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.

"fifo"

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

Methods

A disk cache object has the following methods:

get(key, missing)

Returns the value associated with key. If the key is not in the cache, then it evaluates the expression specified by missing and returns the value. If missing is specified here, then it will override the default that was set when the cache_mem 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.