Awesome
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()+'-'+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!
API
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 = 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