mongodb sync index

When operating MongoDB at scale, I found mysef have to add/remove replica a lot. One thing that always bug me is the index. I used Mongoid as my ODM, so once the index is defined, creating it is easily with:

rake db:mongoid:create_indexes

However, given my experience, lots of time the index wasn’t created properly. It’s maybe because old version of gem that I used. But that’s in the point of this post.

In this article, I want to show you how we can manually sync index from primary. When we add a new replica to replica set, the single index is created automatically but the compound index weren’t.

The mothod that I used is very simple. It go over the source database, read the index, and attempt to re-create them in the secondary. Here is the procedure.

Remove the secondary from replicaset

This is necessary because we cannot write to a secondary.

Create the index

Add the node back to replica set

Finally now the indexes are in-sync with master and any new index will be replicated to secondary as well