Introduction to segment merging

In the Full text searching section of Chapter 1, Getting Started with Elasticsearch Cluster, we mentioned segments and their immutability. We wrote that the Lucene library, and thus Elasticsearch, writes data to certain structures that are written once and never change. This allows for some simplification, but also introduces the need for additional work. One such example is deletion. Because segment, cannot be altered, information about deletions must be stored alongside and dynamically applied during search. This is done by filtering deleted documents from the returned result set. The other example is the inability to modify the documents (however, some modifications are possible, such as modifying numeric doc values). Of course, one can say that Elasticsearch supports document updates (refer to the Manipulating data with the REST API section of Chapter 1, Getting Started with Elasticsearch Cluster). However, under the hood, the old document is marked as...