Level TTL

Add a 'ttl' (time-to-live) option to LevelUP for put() and batch()



Augment LevelUP to handle a new 'ttl' option on put() and batch() that specifies the number of milliseconds an entry should remain in the data store. After the TTL, the entry will be automatically cleared for you.

Requires LevelUP, Level or level-hyper to be installed separately.

Note 1: Version 1.0.0 data stores are not backward compatible with previous versions. If you have unexpired entries in a data store managed by pre-1.0.0, don't expect them to expire if you upgrade to 1.0.0+. This is due to a level-sublevel change. It is also recommended that you only use level-sublevel 6.0.0+ with level-ttl.

Note 2: level-ttl has partial support for level-spaces. It should work fine as long as you don't use the 'defaultTTL' feature, see below. This is being worked on so we can have full support for level-spaces as well.

``js
var levelup = require('level')
, ttl = require('level-ttl')

var db = levelup('/tmp/foo.db')
db = ttl(db)

// --------------------------- put() --------------------------- //
// this entry will only stay in the data store for 1 hour
db.put('foo', 'bar', { ttl: 1000 60 60 }, function (err) { / .. / })

// -------------------------- batch() -------------------------- //
// the two 'put' entries will only stay in the data store for 1 hour
db.batch([
{ type: 'put', key: 'foo', value: 'bar' }
, { type: 'put', key: 'bam', value: 'boom' }
, { type: 'del', key: 'w00t' }
], { ttl: 1000 60 60 }, function (err) { / .. / })
`

If you put the same entry twice, you refresh the TTL to the last put operation. In this way you can build utilities like session managers for your web application where the user's session is refreshed with each visit but expires after a set period of time since their last visit.

Alternatively, for a lower write-footprint you can use the ttl() method that is added to your LevelUP instance which can serve to insert or update a ttl for any given key in the database (even if that key doesn't exist but may in the future! Crazy!).

`js
db.put('foo', 'bar', function (err) { / .. / })
db.ttl('foo', 1000 60 60, function (err) { / .. / })
`

Level TTL uses an internal scan every 10 seconds by default, this limits the available resolution of your TTL values, possibly delaying a delete for up to 10 seconds. The resolution can be tuned by passing the 'checkFrequency' option to the ttl() initialiser.

`js
var db = levelup('/tmp/foo.db')
// scan for deletables every second
db = ttl(db, { checkFrequency: 1000 })

/ .. /
`

Of course, a scan takes some resources, particularly on a data store that makes heavy use of TTLs. If you don't require high accuracy for actual deletions then you can increase the 'checkFrequency'. Note though that a scan only involves invoking a LevelUP ReadStream that returns only the entries due to expire, so it doesn't have to manually check through all entries with a TTL. As usual, it's best to not do too much tuning until you have you have something worth tuning!

$3

You can set a default ttl value for all your keys by passing the 'defaultTTL' option to the ttl() initialiser. This can be overridden by explicitly setting the ttl value.

In the following examle 'foo' will expire in 15 minutes while 'beep' will expire in one minute.

`js
var db = levelup('/tmp/foo.db')
db = ttl(db, { defaultTTL: 15 60 1000 })
db.put('foo', 'bar', function (err) { / .. / })
db.put('beep', 'boop', { ttl: 60 1000 }, function (err) { / .. */ })
`

$3

You can provide a custom storage for the meta data by using the opts.sub property. If it's set, that storage will contain all the ttl meta data. A use case for this would be to avoid mixing data and meta data in the same keyspace, since if it's not set, all data will be sharing the same keyspace.

A db for the data and a separate to store the meta data:

`js
var level = require('level')
, ttl = require('level-ttl')
, meta = level('./meta')
, db = ttl(level('./db'), { sub: meta })
, batch = [
{ type: 'put', key: 'foo', value: 'foovalue' }
, { type: 'put', key: 'bar', value: 'barvalue' }
]

db.batch(batch, { ttl: 100 }, function (err) {
db.createReadStream()
.on('data', function (data) {
console.log('data', data)
})
.on('end', function () {
meta.createReadStream()
.on('data', function (data) {
console.log('meta', data)
})
})
})
`

For more examples on this please check the tests involving level-sublevel.

$3

Level TTL uses a timer to regularly check for expiring entries (don't worry, the whole data store isn't scanned, it's very efficient!). The db.close() method is automatically wired to stop the timer but there is also a more explicit db.stop() method that will stop the timer and not pass on to a close()` underlying LevelUP instance.

Contributors

Level TTL is powered by the following hackers:

* Rod Vagg
* Matteo Collina
* Josh Duff
* Erik Kristensen
* Lars-Magnus Skog

Licence

Level TTL is Copyright (c) 2013-2015 Rod Vagg @rvagg and licensed under the MIT licence. All rights not explicitly granted in the MIT license are reserved. See the included LICENSE file for more details.