Pre/Post hooks for leveldb

Intercept put/delete/batch operations on levelup.

The API for implementing pre hooks has changed. Instead of mutating an array at once, the prehook is called on each change hook(change, add) and may call add(_change) to add a new item into the batch.

Also, attaching hooks to leveldb is now simpler

var Hooks = require('level-hooks')
Hooks(db) //previously: Hooks()(db)

var levelup   = require('levelup')
var timestamp = require('monotonic-timestamp')
var hooks     = require('level-hooks')

var db = levelup(file)

//install hooks onto db.
hooks(db)

db.hooks.pre({start: '', end: '~'}, function (change, add) {
  //change is same pattern as the an element in the batch array.
  //add a log to record every put operation.
  add({type: 'put', key: '~log-'+timestamp()+'-'+change.type, value: change.key})
})

//add a hook that responds after an operation has completed.
db.hooks.post(function (ch) {
  //{type: 'put'|'del', key: ..., value: ...}
})

Used by map-reduce to make map-reduce durable across crashes!

var levelup   = require('levelup')
var timestamp = require('monotonic-timestamp')
var hooks     = require('level-hooks')

var db = levelup(file)

//install hooks onto db.
hooks(db)

db.hooks.pre('counter!', function (op, done) {
  db.get(op.key, function (err, val) {
    op.value = Number(op.value || 0) + Number(val || 0)
    cb()
  })
})

db.put('counter!foo', 1, function (err) {
  db.put('counter!foo', 2, function (err) {
    db.get('counter!foo', console.log) //3!
  })
})

If prefix is a string or object that defines the range the pre-hook triggers on. If prefix' is a string, then the hook only triggers on keys that _start_ with that string. If the hook is an object it must be of form {start: START, end: END}`

hook is a function, and will be called on each item in the batch (if it was a put or del, it will be called on the change) change is always of the form {key: key, value: value, type:'put' | 'del'}

Pass additional changes to add to add them to the batch. If add is passed a string as the second argument it will prepend that prefix to any keys you add.

To veto (remove) the current change call add(false).

db.hooks.pre returns a function that will remove the hook when called.

Post hooks do not offer any chance to change the value. but do take a range option, just like pre

db.hooks.post returns a function that will remove the hook when called.

Async hooks are another kind of prehook that allow IO to happen before the batch/put/del is processed.

Also, async hooked keys are processed strictly in series, subsequent calls being queued until the previous call has returned.

This may be an issue for write heavy applications!

MIT