pull-write

pull-write

base class for creating generic pull-sinks that write to some device via an async call.

Write(asyncWrite, reduce, max, cb)

asyncWrite(ary, cb)

async function called with an array of items to output. This function will only ever be called once at a time (per instance), and while it is working pull-write will buffer any subsequent writes, until the buffer has the length of at most max, or asyncWrite has called back.

reduce (queue, item)

queue is the current backlog of data the pull-write is getting ready to write. item is the next incoming item. reduce must add item into queue in whatever way is appropiate. If queue is empty, then it will be null. Your reduce function must handle that case and set an initial value.

by default, reduce will be a function that initializes a buffer, and then pushes the new items onto that buffer, this means max will be compared to the number of items in that buffer.

max

A number, when the .length property of the queue returned by reduce gets this big pull-write will stop reading more, until asyncWrite calls back.

example

Suppose we want a to take a stream of values from one leveldb, and write it to another. If we have the timestamp they where written to the first, we can track that in the second, then it's easy to keep them both up to date. We just need to always output latest ts separately.

var Write = require('pull-write')

var LevelWrite = function (db, cb) {
  var max = 100
  return Write(function (ary, cb) {
    db.batch(ary, cb)
  }, function (queue, data) {
    if(!queue)
      queue = [{key: '~meta~ts', value: 0, type: 'put'}]
    queue.push({key:data.key, value: data.value, type: 'put'})
    //the record of the current sequence is always the first value
    //in the batch, so we can update it easily.
    queue[0].value = data.ts
    return queue
  }, max, cb)
}

License

MIT