Apache Kudu concepts and architecture

You can familiarize yourself with some basic concepts related to Kudu and its architecture in this topic.

Columnar datastore
Kudu is a columnar datastore. A columnar datastore stores data in strongly-typed columns. With a proper design, a columnar store can be superior for analytical or data warehousing workloads for the following reasons:
Raft consensus algorithm
The Raft consensus algorithm provides a way to elect a leader for a distributed cluster from a pool of potential leaders. If a follower cannot reach the current leader, it transitions itself to become a candidate. Given a quorum of voters, one candidate is elected to be the new leader, and the others transition back to being followers.
Table
A table is where your data is stored in Kudu. A table has a schema and a totally ordered primary key. A table is split into segments called tablets, by primary key.
Tablet
A tablet is a contiguous segment of a table, similar to a partition in other data storage engines or relational databases. A given tablet is replicated on multiple tablet servers, and at any given point in time, one of these replicas is considered the leader tablet. Any replica can service reads. Writes require consensus among the set of tablet servers serving the tablet.
Tablet server
A tablet server stores and serves tablets to clients. For a given tablet, one tablet server acts as a leader and the others serve follower replicas of that tablet. Only leaders service write requests, while leaders or followers each service read requests. Leaders are elected using Raft consensus. One tablet server can serve multiple tablets, and one tablet can be served by multiple tablet servers.
Master
The master keeps track of all the tablets, tablet servers, the catalog table, and other metadata related to the cluster. At a given point in time, there can only be one acting master (the leader). If the current leader disappears, a new master is elected using Raft consensus.
Catalog table
The catalog table is the central location for metadata of Kudu. It stores information about tables and tablets. The catalog table is accessible to clients through the master, using the client API. The catalog table cannot be read or written directly. Instead, it is accessible only through metadata operations exposed in the client API.
Logical replication
Kudu replicates operations, not on-disk data. This is referred to as logical replication, as opposed to physical replication.
Architectural overview
The diagram in this topic shows a Kudu cluster with three masters and multiple tablet servers, each serving multiple tablets. It illustrates how Raft consensus is used to allow for both leaders and followers for both the masters and tablet servers. In addition, a tablet server can be a leader for some tablets and a follower for others. Leaders are shown in gold, while followers are shown in grey.

Parent topic: Apache Kudu overview