Add iterator(options) and return [Symbol.asyncIterator]

[MeirionHughes]

I'd like to propose adding a method to levelup that would allow direct iteration of the underlying store.iterator. This would negate the need for consumers to convert the ReadStream to an Iterable.

from a usage perspective, something along the lines of:

async function main(){
  let db = levelup(encode(leveldown()));

  await db.put("foo", "hello");
  await db.put("bar", "world");

  let search = db.read( { leq: "bar", limit: 10 });

  for await (let data of search){
    let key = data.key;
    let value = data.value;
  }
}

not a huge change to implement this:

LevelUP.prototype.read= function (options) {
  options = extend({ keys: true, values: true }, options)
  if (typeof options.limit !== 'number') { options.limit = -1 }
  return new LevelAsyncIterator(this.db.iterator(options), options)
}

where LevelAsyncIterator can be the stream iterator butchered to remove the Stream aspects and simply implement [Symbol.asyncIterator] instead - along with ability to end (clean) the underlying store.iterator

If people are happy with this proposal I can make the PRs and a new level-iterator repo, based off level-stream-iterator?

I'll do it as a plugin to levelup due to Symbol.asyncIterator being still stage-3.

[MeirionHughes]

admittedly, you could probably do this without the change:

async function main(){
  let store = encode(leveldown('/path/to/db')))
  let db = levelup(store);

  let search = new LevelIterator(store, { keys: true, values: true, leq: "bar", limit: 10 });

[ralphtheninja]

Would it be possible/feasible to implement something on top of a ReadableStream? E.g.

async function main(){
  const db = levelup(encode(leveldown()))

  await db.put("foo", "hello")
  await db.put("bar", "world")

  const search = new ReadableStreamIterator(db.createReadStream({ leq: "bar", limit: 10 }))

  for (let data of search) {
    let key = data.key
    let value = data.value
  }
}

This goes against your suggestion but also makes it more generic if it can operate on top of any ReadableStream.

[vweevers]

See also: support of Symbol.asyncIterator in Node.js streams (nodejs/readable-stream#254).

@mcollina @calvinmetcalf any thoughts? Until Symbol.asyncIterator lands in Node, do you think levelup should support it on its own? Not through streams, but a lower-level interface. Could be an interesting experiment, also re: performance. Easier to optimize I reckon, without a stream in the middle.

[MeirionHughes]

You can convert a stream to an asyncIterator, but you potentially have to buffer the stream if it produces faster than the consumer and its in push-mode. I don't like this solution, because the stream is redundent to using the underlying iterator directly and exposing it via Symbol.asyncIterator;

Problem with Symbol.asyncIterator, bar the fact its still stage-3, is that I doubt you'll see it in the wild without polyfill for a good few years; I'm on node 8.4 and even with --harmony the Symbol.asyncIterator global needs to be polyfilled. Even then, without v8 inlining Promises asyncIterator has major performance cost. So I don't recommend adding it to levelup, yet.

In the mean time. I'm just writing a plugin to augment the levelup prototype and give you direct access the iterator, via asyncIterator. Those that want it can just import and use it. Then at a later stage it can be merged into levelup.

I can update the levelup typings to allow merging definitions with third-party plugins to levelup, so the read() method will show up for typescript users.

[vweevers]

Problem with Symbol.asyncIterator bar the fact its still stage-3, is that I doubt you'll see it in the wild without polyfill for a good few years.

We can dream :)

SO.. I'm just writing a plugin to augment the levelup prototype and give you direct access the iterator, via asyncIterator. Those that want it can just import and use it. Then at a later stage it can be merged into levelup.

👍

Instead of read(), why not iterator()? In the future, levelup can perhaps expose iterator(options) which simply returns this.db.iterator(options), and then we implement Symbol.asyncIterator in abstract-leveldown. If that makes sense.

And instead of key-value objects, yield arrays, e.g.:

const db = levelup(leveldown('./db'))

for await (let [key, value] of db.iterator()) {
    console.log(key, value);
}

Just spitballing, say we add Iterator.prototype.seek, could still work:

const db = levelup(leveldown('./db'))
const iter = db.iterator()

for await (let [key, value] of iter) {
  if (key == 'something to skip') {
      iter.seek('yonder')
  }
}

[MeirionHughes]

Yes, ultimately you'd want this in the abstract iterator. But, again, I wouldn't recommend you do this yet.

Sounds good, I'll change what I've got and get some tests going.

[ralphtheninja]

@vweevers I've never seen the for await construct before. Where can I read more about it?

[MeirionHughes]

https://github.com/tc39/proposal-async-iteration

this might help if you want to try it: https://stackoverflow.com/questions/43694281/ts2318-cannot-find-global-type-asynciterableiterator-async-generator

[vweevers]

It is also available in node 7+ with the --harmony-async-iteration flag.

[vweevers]

@ralphtheninja a contrived example:

'use strict'

// Run with: node --harmony-async-iteration example.js

async function example() {
  for await(let [key, value] of new AbstractIterator()) {
    console.log('%s: %s', key, value)
  }

  console.log('done!')
}

class AbstractIterator {
  [Symbol.asyncIterator]() {
    const pairs = [
      ['key1', 'foo'],
      ['key2', 'bar']
    ]

    return {
      next() {
        return new Promise((resolve, reject) => {
          setTimeout(() => {
            const kv = pairs.shift()
            resolve({ value: kv, done: !kv })
          }, 500)
        })
      }
    }
  }
}

example()

[ralphtheninja]

this might help if you want to try it: https://stackoverflow.com/questions/43694281/ts2318-cannot-find-global-type-asynciterableiterator-async-generator

Upvoted! 😉

[ralphtheninja]

@vweevers What about the syntax for the AbstractIterator?

class AbstractIterator {
  [Symbol.asyncIterator]() { // <-- this one!
    return {
      next() {
        // ..
      }
    }
  }
}

[vweevers]

http://2ality.com/2015/02/es6-classes-final.html#computed-method-names. Sans classes you'd write:

AbstractIterator.prototype[Symbol.asyncIterator] = function() {
  return //..
}