plugins v2 (sectioned db, with hooks)

Basically, instead of hooking new behaviour into the main database, you can create sub sections:

SubLevel(db)
var fooDb = db.sublevel('foo', '~')

fooDb is an object with the levelup api, except when you do an fooDb.put('bar') the key is prefixed with ~foo~ so that it's separated from the other keys in the main db. This is great, because if you want to build a section of the database with special behaviour in it, you can create a subsection, and extend the behaviour in anyway you like -- but it will only affect that section! So, you can monkeypatch it - whatever, you won't introduce bugs into other parts of the program.

Most of the plugins I built needed some sort of interception, where a value is inserted into one range, which triggers an action which inserts something into a different section. To get reliably consistent data this needs to be atomic.

so, you can tie set hooks on a subsection, that trigger an insert into another subsection.

when a key is inserted into the main db, index that write with a timestamp, saved into another subsection.

var SubLevel = require('level-sublevel'); SubLevel(db)
var sub = db.sublevel('SEQ')

db.pre(function (ch, add) {
  add({
    key: ''+Date.now(), 
    value: ch.key, 
    type: 'put'
  }, sub) //NOTE pass the destination db to add
          //and the value will end up in that subsection!
})

db.put('key', 'VALUE', function (err) {

  //read all the records inserted by the hook!
  sub.createReadStream()
    .on('data', console.log)
})

db.pre(function hook (ch, add) {...}) registers a function to be called whenever a key is inserted into that section. ch is a change, like a row argument to db.batch: {key: k, value: v, type: 'put' || 'del'}.

If the hook function calls add(ch) ch is added to the batch (a regular put/del is turned into a batch) but, if a subsection is passed in add(ch, sub) then that put will be added to that subsection.

Compare subsection code for queue/trigger https://github.com/dominictarr/level-sublevel/blob/e2d27cc8e8356cde6ecf4d50c980c2ba93d87b95/examples/queue.js
with the old code -
https://github.com/dominictarr/level-queue/blob/master/index.js
and https://github.com/dominictarr/level-trigger/blob/18d0a1daa21aab1cbc1d0f7ff3690b91c1e0291d/index.js

the new version is only ~ 60 lines, down from about ~ 200, also it's possible to use multiple different queue/trigger libs within the same db. And also, there is no tricky code that refers to ranges or prefixes in the subsection based code!

in summary,

• create subsections, add any features to your subsection.
• use pre(fun) to trigger atomic inserts into your subsection.

Start by comparing level-sublevel/examples/queue.js with level-queue/index.js and level-trigger/index.js, as linked in the issue. Treat the proposal as design work: completion would require subsections and pre hooks that support atomic inserts into other subsections, with behavior verified for the described LevelUP-style operations.