io.atomix.copycat.server.storage.snapshot

Class Snapshot

java.lang.Object

io.atomix.copycat.server.storage.snapshot.Snapshot

All Implemented Interfaces:

AutoCloseable

public abstract class Snapshot
extends Object
implements AutoCloseable

Manages reading and writing a single snapshot file.

User-provided state machines which implement the Snapshottable interface transparently write snapshots to and read snapshots from files on disk. Each time a snapshot is taken of the state machine state, the snapshot will be written to a single file represented by this interface. Snapshots are backed by a Buffer dictated by the parent StorageLevel configuration. Snapshots for file-based storage levels like DISK and MAPPED will be stored in a RandomAccessFile backed buffer, and MEMORY snapshots will be stored in an on-heap buffer.

Snapshots are read and written by a SnapshotReader and SnapshotWriter respectively. To create a reader or writer, use the reader() and writer() methods.

   
   Snapshot snapshot = snapshotStore.snapshot(1);
   try (SnapshotWriter writer = snapshot.writer()) {
     writer.writeString("Hello world!");
   }
   snapshot.complete();
   
 

A SnapshotReader is not allowed to be created until a SnapshotWriter has completed writing the snapshot file and the snapshot has been marked complete. This allows snapshots to effectively be written and closed but not completed until other conditions are met. Prior to the completion of a snapshot, a failure and recovery of the parent SnapshotStore will not recover an incomplete snapshot. Once a snapshot is complete, the snapshot becomes immutable, can be recovered after a failure, and can be read by multiple readers concurrently.

Author:

Method Summary

Modifier and Type	Method and Description
void	close() Closes the snapshot.
Snapshot	complete() Completes writing the snapshot to persist it and make it available for reads.
void	delete() Deletes the snapshot.
abstract long	index() Returns the snapshot index.
abstract SnapshotReader	reader() Returns a new snapshot reader.
abstract long	timestamp() Returns the snapshot timestamp.
abstract SnapshotWriter	writer() Returns a new snapshot writer.

Methods inherited from class java.lang.Object

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

Method Detail

index

public abstract long index()

Returns the snapshot index.

The snapshot index is the index of the state machine at the point at which the snapshot was written.

Returns:

The snapshot index.

timestamp

public abstract long timestamp()

Returns the snapshot timestamp.

The timestamp is the logical state machine time at the index at which the snapshot was written.

Returns:

The snapshot timestamp.

writer

public abstract SnapshotWriter writer()

Returns a new snapshot writer.

Only a single SnapshotWriter per Snapshot can be created. The single writer must write the snapshot in full and complete() the snapshot to persist it to disk and make it available for reads.

Returns:

A new snapshot writer.

Throws:

IllegalStateException - if a writer was already created or the snapshot is complete

reader

public abstract SnapshotReader reader()

Returns a new snapshot reader.

A SnapshotReader can only be created for a snapshot that has been fully written and completed. Multiple concurrent readers can be created for the same snapshot since completed snapshots are immutable.

Returns:

A new snapshot reader.

Throws:

IllegalStateException - if the snapshot is not complete

complete

public Snapshot complete()

Completes writing the snapshot to persist it and make it available for reads.

Snapshot writers must call this method to persist a snapshot to disk. Prior to completing a snapshot, failure and recovery of the parent SnapshotStore will not result in recovery of this snapshot. Additionally, no readers can be created until the snapshot has been completed.

Returns:

The completed snapshot.

close

public void close()

Closes the snapshot.

Specified by:

close in interface AutoCloseable

delete

public void delete()

Deletes the snapshot.