Suppose we have a huge collection of over a billion documents and we get a requirement whereby we need to create a separate index with a particular set of documents, or we want to break our index into two parts on the basis of certain criteria. Migration of documents to another collection makes this possible. Effectively, we can specify a source and a destination collection in SolrCloud. On the basis of the routing criteria, certain documents will be copied from the source to the destination collection. We can specify the migration time as the forward.timeout parameter during which all write requests will be forwarded to the target collection. The target collection must not receive any writes while the migrate command is running. Otherwise, some writes may be lost.

Let us look at a practical scenario.

We currently have two collections—catcollection and mycollection. Now catcollection contains documents belonging to the categories books, currency, and...