level-ttl

Add a ttl (time-to-live) option to levelup for put() and batch().

Table of Contents

• Install
• Usage
• Contributing
• Donate
• License

Install

With npm do:

npm install level-ttl

Usage from TypeScript also requires npm install @types/readable-stream.

Usage

If you are upgrading: please see UPGRADING.md.

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 one of its variants like level-rocksdb to be installed separately.

import level from 'classic-level'
import ttl from 'level-ttl'

const db = ttl(level('./db'))

// This entry will only stay in the store for 1 hour
await db.put('foo', 'bar', { ttl: 1000 * 60 * 60 })

await db.batch([
  // Same for these two entries
  { type: 'put', key: 'foo', value: 'bar' },
  { type: 'put', key: 'bam', value: 'boom' },
  { type: 'del', key: 'w00t' }
], { ttl: 1000 * 60 * 5 })

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!

await db.put('foo', 'bar')
await db.ttl('foo', 1000 * 60 * 60)

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.

// Scan every second
const db = ttl(level('./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!

Default TTL

You can set a default ttl value for all your keys by passing the defaultTTL option to the ttl() initialiser. This can be overridden per operation. In the following example A will expire in 15 minutes while B will expire in one minute.

const db = ttl(level('./db'), {
  defaultTTL: 15 * 60 * 1000
})

await db.put('A', 'beep')
await db.put('B', 'boop', { ttl: 60 * 1000 })

opts.sub

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:

import level from 'classic-level'
import ttl from 'level-ttl'
import { EntryStream }  from 'level-read-stream'

const rootDb = level('./db')
const meta = rootDb.sublevel('meta')

const db = ttl(rootDb, { sub: meta })

const batch = [
  { type: 'put', key: 'foo', value: 'foo value' },
  { type: 'put', key: 'bar', value: 'bar value' }
]

await db.batch(batch, { ttl: 100 })

new EntryStream(db)
  .on('data', function (data) {
    console.log('data', data)
  })
  .on('end', function () {
    new EntryStream(meta)
      .on('data', function (data) {
        console.log('meta', data)
      })
  })
})

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!). 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 close the underlying levelup instance.

Contributing

Level/level-ttl is an OPEN Open Source Project. This means that:

Individuals making significant and valuable contributions are given commit-access to the project to contribute as they see fit. This project is more like an open wiki than a standard guarded open source project.

See the Contribution Guide for more details.

Donate

Support us with a monthly donation on Open Collective and help us continue our work.

License

MIT