January 2015
Intermediate to advanced
724 pages
13h 21m
English
Content preview from Elasticsearch: The Definitive Guide
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Chapter 4. Distributed Document Store
In the preceding chapter, we looked at all the ways to put data into your index and then retrieve it. But we glossed over many technical details surrounding how the data is distributed and fetched from the cluster. This separation is done on purpose; you don’t really need to know how data is distributed to work with Elasticsearch. It just works.
In this chapter, we dive into those internal, technical details to help you understand how your data is stored in a distributed system.
Routing a Document to a Shard
When you index a document, it is stored on a single primary shard. How does Elasticsearch know which shard a document belongs to? When we create a new document, how does it know whether it should store that document on shard 1 or shard 2?
The process can’t be random, since we may need to retrieve the document in the future. In fact, it is determined by a simple formula:
shard = hash(routing) % number_of_primary_shards
The routing value is an arbitrary string, which defaults to the document’s
_id but can also be set to a custom value. ...
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