io.atomix.copycat

Interface Command<T>

Type Parameters:

T - command result type

All Superinterfaces:

Operation<T>, Serializable

All Known Implementing Classes:

NoOpCommand

public interface Command<T>
extends Operation<T>

Base interface for operations that modify system state.

Commands are submitted by clients to a Raft server and used to modify Raft cluster-wide state. When a command is submitted to the cluster, if the command is received by a follower, the Raft protocol dictates that it must be forwarded to the cluster leader. Once the leader receives a command, it logs and replicates the command to a majority of the cluster before applying it to its state machine and responding with the result.

Compaction modes

Determinism is the number one rule of state machines!

When commands are submitted to the Raft cluster, they're written to a commit log on disk or in memory (based on the storage configuration) and replicated to a majority of the cluster. As disk usage grows over time, servers compact their logs to remove commands that no longer contribute to the state machine's state. In order to ensure state machines remain deterministic, commands must provide compaction modes to aid servers in deciding when it's safe to remove a command from the log. The compaction mode allows state machines to safely handle the complexities of removing state from state machines while ensuring state machines remain deterministic, particularly in the event of a failure and replay of the commit log. See the Command.CompactionMode documentation for more info.

Serialization

Commands 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:

Command.CompactionMode

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static class	Command.CompactionMode Constants for specifying command compaction modes.

Method Summary

Modifier and Type	Method and Description
default Command.CompactionMode	compaction() Returns the command compaction mode.

Method Detail

compaction

default Command.CompactionMode compaction()

Returns the command compaction mode.

The compaction mode will dictate the circumstances under which the command can be safely removed from the Raft replicated log. Commands and the state machines to which they apply must coordinate the compaction process via this mechanism.

Returns:

The command compaction mode.

See Also:

Command.CompactionMode