#Oplog Transform Free your oplog

##Why? Often people want a duplicate of their data in another data store. Doing imports on an interval works but need to be scheduled. MongoDB replica sets have a tailable oplog that provides push like updates about changes happening to the records in the database. By tapping into this we can publish changes to other datastores too, keeping them up to date in real time.

oplog-transform is simply a module that provides an interface for insert, update, and remove hooks for you to fill in with how to modify the target datastore. You can then pass in oplog records and oplog-transform will use the hooks accordingly.

##Usage

Here is an example of initializing oplog-transform using an in-memory JSON object as our datastore.

var store = {}
  , Transform = require('oplog-transform')
  , transform = new Transform({
    insert: function(_id, doc, cb) {
      store[_id] = doc
      cb()
    },
    update: function(_id, done){
      done(store[_id], function(err, updatedDoc, cb) {
        store[_id] = updatedDoc
        cb()
      })
    },
    remove: function(_id, cb) {
      delete store[_id]
      cb()
    }
  })

You can write to the transform using the transform function

transform.transform(oplogDoc)

Or you can use the streaming interface to pipe oplog docs

tailedOplog.pipe(transform)

//or

transform.write(oplogDoc)

##Hooks ###insert

function(_id, doc, cb) {
  /* insert doc */
  cb()
}

The insert hook must be a function that accepts 3 parameters, the doc _id, the doc, and a callback to fire once you've inserted the document

###update

function(_id, done){
  var retrievedDoc = /* fetch doc from your secondary store */
  done(retrievedDoc, function(err, updatedDoc, cb) {
    /* store updated doc */
    cb()
  })
}

The update hook must be a function that accepts 2 parameters, the doc _id, a done callback. Once you've fetched the document from your target datastore call the done callback with the fetched document and a callback to receive an error, the updated document, and a final callback to fire once you've updated the record in your target datastore

###remove

function(_id, cb) {
  /* delete doc */
  cb()
}

The remove hook must be a function that accepts 2 parameters, the doc _id and a callback to fire once you've removed the doc.

Made with ⚡️ by @taterbase