level-ttl

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 (or Level) and sublevel to be installed separately.

var levelup  = require('level')
  , ttl      = require('level-ttl')
  , sublevel = require('level-sublevel')

levelup('/tmp/foo.db', function (err, db) {
  db = sublevel(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!).

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.

levelup('/tmp/foo.db', function (err, 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!

Shutting down

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!) but this can cause problems for processes that have a limited lifespan; particularly when testing. 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

Licence

Level TTL is Copyright (c) 2013 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.