@isaacs/ttlcache
The time-based use-recency-unaware cousin of
lru-cache
Usage
Essentially, this is the same API as
lru-cache
, but it does not do LRU tracking,
and is bound primarily by time, rather than space. Since entries are not
purged based on recency of use, it can save a lot of extra work managing
linked lists, mapping keys to pointers, and so on.
TTLs are millisecond granularity.
If a capacity limit is set, then the soonest-expiring items are purged first, to bring it down to the size limit.
Iteration is in order from soonest expiring until latest expiring.
If multiple items are expiring in the same ms, then the soonest-added items are considered "older" for purposes of iterating and purging down to capacity.
A TTL must be set for every entry, which can be defaulted in the constructor.
Custom size calculation is not supported. Max capacity is simply the count of items in the cache.
const TTLCache = require('@isaacs/ttlcache')
const cache = new TTLCache({ max: 10000, ttl: 1000 })
// set some value
cache.set(1, 2)
// 999 ms later
cache.has(1) // returns true
cache.get(1) // returns 2
// 1000 ms later
cache.get(1) // returns undefined
cache.has(1) // returns false
Caveat Regarding Timers and Graceful Exits
On Node.js, this module uses the Timeout.unref()
method to
prevent its internal setTimeout
calls from keeping the process
running indefinitely. However, on other systems such as Deno,
where the setTimeout
method does not return an object with an
unref()
method, the process will stay open as long as any
unexpired entry exists in the cache.
You may call cache.cancelTimer()
to clear the timeout and
allow the process to exit normally. Be advised that canceling the
timer in this way will of course prevent anything from expiring.
API
const TTLCache = require('@isaacs/ttlcache')
or import TTLCache from '@isaacs/ttlcache'
Default export is the TTLCache
class.
new TTLCache({ ttl, max = Infinty, updateAgeOnGet = false, checkAgeOnGet = false, noUpdateTTL = false, noDisposeOnSet = false })
Create a new TTLCache
object.
-
max
The max number of items to keep in the cache. Must be positive integer orInfinity
, defaults toInfinity
(ie, limited only by TTL, not by item count). -
ttl
The max time in ms to store items. Overridable on theset()
method. Must be a positive integer orInfinity
(see note below about immortality hazards). Ifundefined
in constructor, then a TTL must be provided in eachset()
call. -
updateAgeOnGet
Should the age of an item be updated when it is retrieved? Defaults tofalse
. Overridable on theget()
method. -
checkAgeOnGet
Check the TTL whenever an item is retrieved withget()
. If the item is past its ttl, but the timer has not yet fired, then delete it and return undefined. By default, the cache will return a value if it has one, even if it is technically beyond its TTL. -
noUpdateTTL
Should setting a new value for an existing key leave the TTL unchanged? Defaults tofalse
. Overridable on theset()
method. (Note that TTL is always updated if the item is expired, since that is treated as a newset()
and the old item is no longer relevant.) -
dispose
Method called with(value, key, reason)
when an item is removed from the cache. Called once item is fully removed from cache. It is safe to re-add at this point, but note that adding whenreason
is'set'
can result in infinite recursion ifnoDisponseOnSet
is not specified.Disposal reasons:
-
'stale'
TTL expired. -
'set'
Overwritten with a new different value. -
'evict'
Removed from the cache to stay within capacity limit. -
'delete'
Explicitly deleted withcache.delete()
orcache.clear()
-
-
noDisposeOnSet
Do not calldispose()
method when overwriting a key with a new value. Defaults tofalse
. Overridable onset()
method.
When used as an iterator, like for (const [key, value] of cache)
or
[...cache]
, the cache yields the same results as the entries()
method.
cache.size
The number of items in the cache.
cache.set(key, value, { ttl, noUpdateTTL, noDisposeOnSet } = {})
Store a value in the cache for the specified time.
ttl
and noUpdateTTL
optionally override defaults on the constructor.
Returns the cache object.
cache.get(key, {updateAgeOnGet, checkAgeOnGet, ttl} = {})
Get an item stored in the cache. Returns undefined
if the item is not in
the cache (including if it has expired and been purged).
If updateAgeOnGet
is true
, then re-add the item into the
cache with the updated ttl
value. All options default to the
settings on the constructor.
If checkAgeOnGet
, then an item will be deleted if it is found
to be beyond its TTL, which can happen if the setTimeout timer
has not yet fired to trigger its expiration.
Note that using updateAgeOnGet
can effectively simulate a
"least-recently-used" type of algorithm, by repeatedly updating
the TTL of items as they are used. However, if you find yourself
doing this, consider using
lru-cache
, as it is much more
optimized for an LRU use case.
cache.getRemainingTTL(key)
Return the remaining time before an item expires. Returns 0
if the item
is not found in the cache or is already expired.
cache.has(key)
Return true if the item is in the cache.
cache.delete(key)
Remove an item from the cache.
cache.clear()
Delete all items from the cache.
cache.entries()
Return an iterator that walks through each [key, value]
from soonest
expiring to latest expiring. (Items expiring at the same time are walked
in insertion order.)
Default iteration method for the cache object.
cache.keys()
Return an iterator that walks through each key
from soonest expiring to
latest expiring.
cache.values()
Return an iterator that walks through each value
from soonest expiring to
latest expiring.
cache.cancelTimer()
Clear the internal timer, and stop automatically expiring items when their TTL expires.
This allows the process to exit normally on Deno and other
platforms that lack Node's Timer.unref()
method.
Internal Methods
You should not ever call these, they are managed automatically.
purgeStale
Internal
Removes items which have expired. Called automatically.
purgeToCapacity
Internal
Removes soonest-expiring items when the capacity limit is reached. Called automatically.
dispose
Internal
Called when an item is removed from the cache and should be disposed. Set this on the constructor options.
setTimer
Internal
Called when an with a ttl is added. This ensures that only one timer is setup at once. Called automatically.
Algorithm
The cache uses two Map
objects. The first maps item keys to their
expiration time, and the second maps item keys to their values. Then, a
null-prototype object uses the expiration time as keys, with the value
being an array of all the keys expiring at that time.
This leverages a few important features of modern JavaScript engines for fairly good performance:
-
Map
objects are highly optimized for referring to arbitrary values by arbitrary keys. - Objects with solely integer-numeric keys are iterated in sorted numeric order rather than insertion order, and insertions in the middle of the key ordering are still very fast. This is true of all modern JS engines tested at the time of this module's creation, but most particularly V8 (the engine in Node.js).
When it is time to prune, we can always walk the null-prototype object in iteration order, deleting items until we come to the first key greater than the current time.
Thus, the start
time doesn't need to be tracked, only the expiration
time. When an item age is updated (either explicitly on get()
, or by
setting to a new value), it is deleted and re-inserted.
Immortality Hazards
It is possible to set a TTL of Infinity
, in which case an item
will never expire. As it does not expire, its TTL is not
tracked, and getRemainingTTL()
will return Infinity
for that
key.
If you do this, then the item will never be purged. Create
enough immortal values, and the cache will grow to consume all
available memory. If find yourself doing this, it's probably
better to use a different data structure, such as a Map
or
plain old object to store values, as it will have better
performance and the hazards will be more obvious.