com.hazelcast.cp.internal.raft.impl.persistence

Class NopRaftStateStore

java.lang.Object

com.hazelcast.cp.internal.raft.impl.persistence.NopRaftStateStore

All Implemented Interfaces:

RaftStateStore, Closeable, AutoCloseable

public final class NopRaftStateStore
extends Object
implements RaftStateStore

Used when a Raft node works transiently (its state is not persisted).

Field Summary

Fields

Modifier and Type	Field and Description
static RaftStateStore	INSTANCE Non-persisting RaftStateStore instance

Method Summary

Modifier and Type	Method and Description
void	close()
void	deleteEntriesFrom(long startIndexInclusive) Rolls back the log by deleting all entries starting with the given index.
void	flushLogs() Forces all buffered (in any layer) Raft log changes to be written to the storage layer and returns after those changes are written.
void	open() Initializes the store before starting to persist Raft state.
void	persistEntry(LogEntry entry) Persists the given log entry.
void	persistInitialMembers(RaftEndpoint localMember, Collection<RaftEndpoint> initialMembers) Persists the given local Raft endpoint and initial Raft group members.
void	persistSnapshot(SnapshotEntry entry) Persists the given snapshot entry.
void	persistTerm(int term, RaftEndpoint votedFor) Persists the term and the Raft endpoint that the local node voted for in the given term.

Methods inherited from class java.lang.Object

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

Field Detail

INSTANCE

public static final RaftStateStore INSTANCE

Non-persisting RaftStateStore instance

Method Detail

open

public void open()

Description copied from interface: RaftStateStore

Initializes the store before starting to persist Raft state. This method is called before any other method in this interface. After this method returns, the state store must be ready to accept all other method calls.

Specified by:

open in interface RaftStateStore

persistInitialMembers

public void persistInitialMembers(@Nonnull
                                  RaftEndpoint localMember,
                                  @Nonnull
                                  Collection<RaftEndpoint> initialMembers)

Description copied from interface: RaftStateStore

Persists the given local Raft endpoint and initial Raft group members. When this method returns, all the provided data has become durable.

Specified by:

persistInitialMembers in interface RaftStateStore

persistTerm

public void persistTerm(int term,
                        @Nonnull
                        RaftEndpoint votedFor)

Description copied from interface: RaftStateStore

Persists the term and the Raft endpoint that the local node voted for in the given term. When this method returns, all the provided data has become durable.

Specified by:

persistTerm in interface RaftStateStore

persistEntry

public void persistEntry(@Nonnull
                         LogEntry entry)

Description copied from interface: RaftStateStore

Persists the given log entry.

Log entries are appended to the Raft log with sequential log indices. The first log index is 1.

A block of consecutive log entries has no gaps in the indices, but a gap can appear between the snapshot entry and its preceding regular entry. This happens in an edge case where a follower has fallen so far behind that the missing entries are no longer available from the leader. In that case the leader will send its snapshot entry instead.

In a rare failure scenario Raft must delete a range of the newest entries, rolling back the index of the next persisted entry. Consider the following case where Raft persists three log entries and then deletes entries from index 2:

• persistEntry(1)
• persistEntry(2)
• persistEntry(3)
• deleteEntriesFrom(2)

After this call sequence log indices will remain sequential and the next persistEntry() call will be for index=2.

Specified by:

persistEntry in interface RaftStateStore

See Also:

RaftStateStore.flushLogs(), RaftStateStore.persistSnapshot(SnapshotEntry), RaftStateStore.deleteEntriesFrom(long), RaftAlgorithmConfig

persistSnapshot

public void persistSnapshot(@Nonnull
                            SnapshotEntry entry)

Description copied from interface: RaftStateStore

Persists the given snapshot entry.

After a snapshot is persisted at index=i and RaftStateStore.flushLogs() is called, the log entry at index=i and all the preceding entries are no longer needed and can be evicted from storage. Failing to evict stale entries will not cause a consistency problem, but it will increase the time to recover after a restart. Therefore eviction can be done in a background task.

Raft takes snapshots at a predetermined interval, controlled by commitIndexAdvanceCountToSnapshot. For instance, if it is 100, snapshots will occur at indices 100, 200, 300, and so on.

The snapshot index can lag behind the index of the newest log entry that was already persisted, but there is an upper bound to this difference, controlled by uncommittedEntryCountToRejectNewAppends. For instance, if uncommittedEntryCountToRejectNewAppends is 10, and a persistSnapshot() call is made with snapshotIndex=100, the index of the preceding persistEntry() call can be at most 110.

On the other hand, the snapshot index can also be ahead of the newest log entry. This can happen when a Raft follower has fallen so far behind the leader that the leader no longer holds the missing entries. In that case the follower receives a snapshot from the leader. There is no upper bound on the gap between the newest log entry and the index of the received snapshot.

Specified by:

persistSnapshot in interface RaftStateStore

See Also:

RaftStateStore.flushLogs(), RaftStateStore.persistEntry(LogEntry), RaftAlgorithmConfig

deleteEntriesFrom

public void deleteEntriesFrom(long startIndexInclusive)

Description copied from interface: RaftStateStore

Rolls back the log by deleting all entries starting with the given index. A deleted log entry is no longer valid and must not be restored (or at least must be ignored during the restore process).

There is a bound on the number of entries that can be deleted, specified by uncommittedEntryCountToRejectNewAppends. Say that it is 5 and the highest persisted log entry index is 20. At most 5 newest entries can be deleted, hence deletion can start at index=16 or higher.

Specified by:

deleteEntriesFrom in interface RaftStateStore

See Also:

RaftStateStore.flushLogs(), RaftStateStore.persistEntry(LogEntry), RaftAlgorithmConfig

flushLogs

public void flushLogs()

Description copied from interface: RaftStateStore

Forces all buffered (in any layer) Raft log changes to be written to the storage layer and returns after those changes are written.

Specified by:

flushLogs in interface RaftStateStore

See Also:

RaftStateStore.persistEntry(LogEntry), RaftStateStore.persistSnapshot(SnapshotEntry), RaftStateStore.deleteEntriesFrom(long)

close

public void close()

Specified by:

close in interface Closeable

Specified by:

close in interface AutoCloseable