Package Exports

level-queue

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (level-queue) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

level-queue

queue plugin for leveldb.

Deprecated

Use level-trigger instead.

Stability

Deprecated: Expect no more changes. Avoid using this module.

Example

This module can be used to add job queues to leveldb, and is especially useful if used with level-hooks

var levelup = require('levelup')

levelup(file, function(err, db) {

  //adds .queue to db.
  require('level-queue')()(db)

  //add a worker, for a given job name.
  db.queue.add('job', function (value, done) {

    setTimeout(function () {
      console.log(value)
      //call done() to delete this job from the database.
      done()
    }, Math.random() * 1000)

  })

  db.queue('job', 'todo - may be any string or buffer')
  
})

If the process crashes before the job is completed, it will be restarted when the process restarts.

The job must be idempotent.

Example with hooks

make a twitter like news feed, where messages are posted to friend's feeds.

var levelup = require('levelup')
var map     = require('map-stream')

levelup(file, function(err, db) {


  //adds .queue to db.
  require('level-hooks')()(db)
  require('level-queue')()(db)

  //add a worker, for a given job name.
  db.queue.add('job', function (value, done) {

    setTimeout(function () {
      console.log(value)
      //call done() to delete this job from the database.
      done()
    }, Math.random() * 1000)

  })

  /**

    calling db.queue(name, value, false)

    will not PUT the queue into the database, but will instead return the insert
    so that it can be included into an atomic batch operation.

  **/

  db.hooks.pre(function (batch) {
    //if any of the inserts is a user 
    
    for(var i in batch) {
      var row = batch[i]
      if(isUserMessage(row.key))
        batch.push(db.queue('postToFriends', row.value, false))
    }
  })

  db.queue.add('postToFriends', function (value, done) {
    var message = JSON.parse(''+value)

    //retrive all friends of a poster.
    db.readStream({
      start: 'friends:'+message.author, 
      end: 'friends:'+message.author + '~'
    })
    //attach the message to all follower's feeds.
    .pipe(map(function (val, next) {
      db.put('feed:'+val+':'
        +message.timestamp()+':'
        +message.author, value, next)
      //note: since we take the timestamp from the message,
      //if the job accidentially gets run twice,
      //it will just overwrite the same message.
      //the job is idempotent!
    })
    //mark the job DONE!
    .on('end', done)
  })
  
})

License

MIT