The Topology Service is a set of backend processes. This service is exposed to all Vitess components. It delivers a key/value service that is highly available and consistent, while being offset by having higher latency cost and very low throughput. The Topology Service is used for several things by Vitess:
- It enables tablets to coordinate among themselves as a cluster.
- It enables Vitess to discover tablets, so it knows where to route queries.
- It stores Vitess configuration provided by the database administrator which is required by the different components in the Vitess cluster and that must persist between server restarts.
The main functions the Topology Service provides are:
- It is both a repository for topology metadata and a distributed lock manager.
- It is used to store configuration data about the Vitess cluster. It stores small data structures (a few hundred bytes) per object.
- E.g. information about the Keyspaces, the Shards, the Tablets, the Replication Graph, and the Serving Graph.
- It supports a watch interface that signals a client when changes occur on an object. This is used, for instance, to know when the keyspace topology changes (e.g. for resharding).
- It supports master election.
- It supports quorum reads and writes.