level-hooks

Pre/Post hooks for leveldb

Intercept put/delete/batch operations on levelup.

Warning - Breaking Changes

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)

Example

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

levelup(file, {createIfMissing: true}, function (err, db) {

  //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()+'-'+e.type, value: e.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!

API

rm = db.hooks.pre (range?, hook(change, add(change, prefix?)))

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.

rm = db.hooks.post (range?, hook)

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.

License

MIT