npm install level-hooksIntercept put/delete/batch operations on levelup.
hook(change, add)add(_change) to add a new item into the batch.Also, attaching hooks to leveldb is now simpler
`` js`
var Hooks = require('level-hooks')
Hooks(db) //previously: Hooks()(db)
` js
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!
If prefix is a string or object that defines the range the pre-hook triggers on.prefix' is a string, then the hook only triggers on keys that _start_ with that
If
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.
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.
MIT