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.
memoryCache(max_size = 10 * 1024^2, max_age = Inf, max_n = Inf,
evict = c("lru", "fifo"), missing = key_missing(),
exec_missing = FALSE, logfile = NULL)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.
Maximum age of files in cache before they are evicted, in
seconds. Use Inf for no age limit.
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.
The eviction policy to use to decide which objects are removed
when a cache pruning occurs. Currently, "lru" and "fifo" are
supported.
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.
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.
An optional filename or connection object to where logging
information will be written. To log to the console, use stdout().
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 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.
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.
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.
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.