io.atomix.copycat.server.storage.snapshot

Class SnapshotStore

java.lang.Object

io.atomix.copycat.server.storage.snapshot.SnapshotStore

All Implemented Interfaces:

AutoCloseable

public class SnapshotStore
extends Object
implements AutoCloseable

Persists server snapshots via the Storage module.

The server snapshot store is responsible for persisting periodic state machine snapshots according to the configured storage level. Each server with a snapshottable state machine persists the state machine state to allow commands to be removed from disk.

When a snapshot store is created, the store will load any existing snapshots from disk and make them available for reading. Only snapshots that have been written and completed will be read from disk. Incomplete snapshots are automatically deleted from disk when the snapshot store is opened.

   
   SnapshotStore snapshots = storage.openSnapshotStore("test");
   Snapshot snapshot = snapshots.snapshot(1);
   
 

To create a new Snapshot, use the createSnapshot(long) method. Each snapshot must be created with a unique index which represents the index of the server state machine at the point at which the snapshot was taken. Snapshot indices are used to sort snapshots loaded from disk and apply them at the correct point in the state machine.

   
   Snapshot snapshot = snapshots.create(10);
   try (SnapshotWriter writer = snapshot.writer()) {
     ...
   }
   snapshot.complete();
   
 

Snapshots don't necessarily represent the beginning of the log. Typical Raft implementations take a snapshot of the state machine state and then clear their logs up to that point. However, in Copycat a snapshot may actually only represent a subset of the state machine's state. Indeed, internal Copycat server state machines use log cleaning and store no state in snapshots. When a snapshot is taken of the state machine state, only prior entries that contributed to the state stored in the snapshot - commands marked with the SNAPSHOT compaction mode - are removed from the log prior to the snapshot.

Author:

Constructor Summary

Constructors

Constructor and Description
SnapshotStore(String name, Storage storage, Serializer serializer)

Method Summary

Modifier and Type	Method and Description
void	close()
Snapshot	createSnapshot(long index) Creates a new snapshot.
Snapshot	currentSnapshot() Returns the most recent completed snapshot.
Serializer	serializer() Returns the snapshot store serializer.
Snapshot	snapshot(long index) Returns a snapshot by index.
Collection<Snapshot>	snapshots() Returns a collection of all snapshots stored on disk.
String	toString()

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

SnapshotStore

public SnapshotStore(String name,
                     Storage storage,
                     Serializer serializer)

Method Detail

serializer

public Serializer serializer()

Returns the snapshot store serializer.

Returns:

The snapshot store serializer.

currentSnapshot

public Snapshot currentSnapshot()

Returns the most recent completed snapshot.

The current snapshot is the last Snapshot successfully written and completed. It represents the most recent snapshot successfully written to disk and from which the server state can be restored.

Returns:

The most recent completed snapshot.

snapshots

public Collection<Snapshot> snapshots()

Returns a collection of all snapshots stored on disk.

Snapshots will be loaded from the underlying Storage when the snapshot store is created. The returned collection of Snapshots will include any stored complete snapshots. Both stored and new incomplete snapshots will be excluded from this list.

Returns:

A collection of all snapshots.

snapshot

public Snapshot snapshot(long index)

Returns a snapshot by index.

Parameters:

index - The snapshot index.

Returns:

The snapshot.

createSnapshot

public Snapshot createSnapshot(long index)

Creates a new snapshot.

Parameters:

index - The snapshot index.

Returns:

The snapshot.

close

public void close()

Specified by:

close in interface AutoCloseable

toString

public String toString()

Overrides:

toString in class Object