In the previous chapters, we discussed how to create index mappings and index the data. But what if you already have the mappings created, and data indexed, but you want to modify the structure of the index? Of course one could say that we could just create a new index with new mappings, but that is not always a possibility, especially in a production environment. This is possible to some extent. For example, by default, if we index a document with a new field, Elasticsearch will add that field to the index structure. Let's now look at how to modify the index structure manually.
Note
For situations where mapping changes are needed and they are not possible because of conflicts with the current index structure, it is very good to use aliases – both read and write ones. We will discuss aliasing in the Index aliasing section of Chapter 10, Administrating Your Cluster.