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Latest commit 08d4591 Oct 11, 2014

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)


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.

  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. (ch) {
    //{type: 'put'|'del', key: ..., value: ...}


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


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

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) op 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.

You can check what opperations are currently in the batch with the third argument. Do not modify the batch directly, instead use add

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

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

unsafe mode

normally, pre hooks prevent you from inserting into the hooked range when the hook is triggered. However, sometimes you do need to do this. In those cases, pass in a range with {start: START, end: END, safe: false} and level-hooks will not error. If you use this option, your hook must avoid triggering in a loop itself.

rm = (range?, hook)

Post hooks do not offer any chance to change the value. but do take a range option, just like pre returns a function that will remove the hook when called.