io.atomix.resource

Enum ReadConsistency

java.lang.Object

java.lang.Enum<ReadConsistency>

io.atomix.resource.ReadConsistency

All Implemented Interfaces:

Serializable, Comparable<ReadConsistency>

public enum ReadConsistency
extends Enum<ReadConsistency>

Constants for controlling read consistency constraints on a per-request basis.

Read consistency levels dictate how queries should be evaluated in the Atomix cluster. Resources generally have significantly greater flexibility in controlling the consistency level of reads because Raft allows reads from leaders or followers. Consistency levels can be applied on a per-command basis or to all operations for a given resource.

Author:

Enum Constant Summary

Enum Constants

Enum Constant and Description
ATOMIC Guarantees atomicity (linearizability) for read operations.
ATOMIC_LEASE Provides linearizability under a leader lease.
SEQUENTIAL Guarantees sequential consistency for read operations.

Method Summary

Modifier and Type	Method and Description
Query.ConsistencyLevel	level()
static ReadConsistency	valueOf(String name) Returns the enum constant of this type with the specified name.
static ReadConsistency[]	values() Returns an array containing the constants of this enum type, in the order they are declared.

Methods inherited from class java.lang.Enum

compareTo, equals, getDeclaringClass, hashCode, name, ordinal, toString, valueOf

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Enum Constant Detail

ATOMIC

public static final ReadConsistency ATOMIC

Guarantees atomicity (linearizability) for read operations.

The atomic consistency level guarantees linearizability by contacting a majority of the cluster on every read. When a Query is submitted to the cluster with linearizable consistency, it must be forwarded to the current cluster leader. Once received by the leader, the leader will contact a majority of the cluster before applying the query to its state machine and returning the result. Note that if the leader is already in the process of contacting a majority of the cluster, it will queue the Query to be processed on the next round trip. This allows the leader to batch expensive quorum based reads for efficiency.

ATOMIC_LEASE

public static final ReadConsistency ATOMIC_LEASE

Provides linearizability under a leader lease.

Atomic lease consistency is a special implementation of linearizable reads that relies on the semantics of Raft's election timers to determine whether it is safe to immediately apply a query to the Raft state machine. When a linearizable Query is submitted to the Raft cluster with linearizable consistency, it must be forwarded to the current cluster leader. For lease-based linearizability, the leader will determine whether it's safe to apply the query to its state machine based on the last time it successfully contacted a majority of the cluster. If the leader contacted a majority of the cluster within the last election timeout, it assumes that no other member could have since become the leader and immediately applies the query to its state machine. Alternatively, if it hasn't contacted a majority of the cluster within an election timeout, the leader will handle the query as if it were submitted with ATOMIC consistency.

SEQUENTIAL

public static final ReadConsistency SEQUENTIAL

Guarantees sequential consistency for read operations.

Sequential read consistency requires that clients always see state progress in monotonically increasing order. Note that this constraint allows reads from followers. When a sequential Query is submitted to the cluster, the first server that receives the query will handle it. However, in order to ensure that state does not go back in time, the client must submit its last known index with the query as well. If the server that receives the query has not advanced past the provided client index, it will queue the query and await more entries from the leader.

Method Detail

values

public static ReadConsistency[] values()

Returns an array containing the constants of this enum type, in the order they are declared. This method may be used to iterate over the constants as follows:

for (ReadConsistency c : ReadConsistency.values())
    System.out.println(c);

Returns:

an array containing the constants of this enum type, in the order they are declared

valueOf

public static ReadConsistency valueOf(String name)

Returns the enum constant of this type with the specified name. The string must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.)

Parameters:

name - the name of the enum constant to be returned.

Returns:

the enum constant with the specified name

Throws:

IllegalArgumentException - if this enum type has no constant with the specified name

NullPointerException - if the argument is null

level

public Query.ConsistencyLevel level()