The YB-Master service keeps the system metadata and records, such as tables and location of their tablets, users and roles with their associated permissions, and so on.

The YB-Master service is also responsible for coordinating background operations, such as load-balancing or initiating rereplication of under-replicated data, as well as performing a variety of administrative operations such as creating, altering, and dropping tables.

The YB-Master is highly available, as it forms a Raft group with its peers, and it is not in the critical path of I/O against user tables.

Functions of YB-Master

The YB-Master has a number of important functions within the system.

Coordination of universe-wide administrative operations

Examples of such operations include user-issued CREATE TABLE, ALTER TABLE, and DROP TABLE requests, as well as creating a backup of a table. The YB-Master performs these operations with a guarantee that the operation is propagated to all tablets irrespective of the state of the YB-TServers hosting these tablets. This is essential because a YB-TServer failure while one of these universe-wide operations is in progress cannot affect the outcome of the operation by failing to apply it on some tablets.

Storage of system metadata

Each YB-Master stores system metadata, including information about namespaces, tables, roles, permissions, and assignments of tablets to YB-TServers. These system records are replicated across the YB-Masters for redundancy using Raft as well. The system metadata is also stored as a DocDB table by the YB-Masters.

Authoritative source of tablet assignments to YB-TServers

The YB-Master stores all tablets and the corresponding YB-TServers that currently host them. This map of tablets to the hosting YB-TServers is queried by clients, such as, for example, the YugabyteDB query layer. Applications using the YugabyteDB smart clients for the YCQL and YSQL APIs are efficient in retrieving data. The smart clients query the YB-Master for the tablet to YB-TServer map and cache it. By doing so, the smart clients can communicate directly with the correct YB-TServer to serve various queries without incurring additional network hops.

Background operations

Some operations are performed throughout the lifetime of the universe, in the background, without impacting foreground read and write performance.

Data placement and load balancing

The YB-Master leader does the initial placement (at CREATE TABLE time) of tablets across YB-TServers to enforce any user-defined data placement constraints and ensure uniform load. In addition, during the lifetime of the universe, as nodes are added, fail or become decommissioned, it continues to balance the load and enforce data placement constraints automatically.

Leader balancing

Aside from ensuring that the number of tablets served by each YB-TServer is balanced across the universe, the YB-Masters also ensures that each node has a symmetric number of tablet-peer leaders across eligible nodes.

Rereplication of data on extended YB-TServer failure

The YB-Master receives heartbeats from all the YB-TServers, and tracks their liveness. It detects if any YB-TServers has failed and keeps track of the time interval for which the YB-TServer remains in a failed state. If the time duration of the failure extends beyond a threshold, it finds replacement YB-TServers to which the tablet data of the failed YB-TServer is rereplicated. Rereplication is initiated in a throttled fashion by the YB-Master leader so as to not impact the foreground operations of the universe.