io.atomix.copycat

Interface Query<T>

All Superinterfaces:

Operation<T>, Serializable

public interface Query<T>
extends Operation<T>

Base interface for operations that read system state.

Queries are submitted by clients to a Raft server to read Raft cluster-wide state. In contrast to commands, queries allow for more flexible consistency levels that trade consistency for performance.

Consistency levels

All queries must specify a consistency() with which to execute the query. The provided consistency level dictates how queries are submitted to the Raft cluster. When a query is submitted to the cluster, the query is sent in a message to the server to which the client is currently connected. The server handles the query requests based on the configured Query.ConsistencyLevel. For Query.ConsistencyLevel.SEQUENTIAL consistency, followers are allowed to execute queries with certain constraints for faster reads. For higher consistency levels like Query.ConsistencyLevel.LINEARIZABLE and Query.ConsistencyLevel.LINEARIZABLE_LEASE, queries are forwarded to the cluster leader. See the Query.ConsistencyLevel documentation for more info.

By default, all queries should use the strongest consistency level, Query.ConsistencyLevel.LINEARIZABLE. It is essential that users understand the trade-offs in the various consistency levels before using them.

Serialization

Queries must be serializable both by the client and by all servers in the cluster. By default, all operations use Java serialization. However, default serialization in slow because it requires the full class name and does not allocate memory efficiently. For this reason, it's recommended that commands implement CatalystSerializable or register a custom TypeSerializer for better performance. Serializable types can be registered on the associated client/server Serializer instance.

Author:

Jordan Halterman

See Also:

Query.ConsistencyLevel

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static class	Query.ConsistencyLevel Constants for specifying Raft Query consistency levels.

Method Summary

Modifier and Type	Method and Description
default Query.ConsistencyLevel	consistency() Returns the query consistency level.

Method Detail

consistency

default Query.ConsistencyLevel consistency()

Returns the query consistency level.

The consistency will dictate how the query is executed on the server state. Stronger consistency levels can guarantee linearizability in all or most cases, while weaker consistency levels trade linearizability for more performant reads from followers. Consult the Query.ConsistencyLevel documentation for more information on the different consistency levels.

By default, this method enforces strong consistency with the Query.ConsistencyLevel.LINEARIZABLE consistency level.

Returns:

The query consistency level.