org.apache.flink.streaming.api

Enum CheckpointingMode

java.lang.Object

java.lang.Enum<CheckpointingMode>

org.apache.flink.streaming.api.CheckpointingMode

All Implemented Interfaces:

Serializable, Comparable<CheckpointingMode>

public enum CheckpointingMode
extends Enum<CheckpointingMode>

The checkpointing mode defines what consistency guarantees the system gives in the presence of failures.

When checkpointing is activated, the data streams are replayed such that lost parts of the processing are repeated. For stateful operations and functions, the checkpointing mode defines whether the system draws checkpoints such that a recovery behaves as if the operators/functions see each record "exactly once" (EXACTLY_ONCE), or whether the checkpoints are drawn in a simpler fashion that typically encounteres some duplicates upon recovery (AT_LEAST_ONCE)

Enum Constant Summary

Enum Constants

Enum Constant and Description
AT_LEAST_ONCE Sets the checkpointing mode to "at least once".
EXACTLY_ONCE Sets the checkpointing mode to "exactly once".

Method Summary

Methods

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

Methods inherited from class java.lang.Enum

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

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Enum Constant Detail

EXACTLY_ONCE

public static final CheckpointingMode EXACTLY_ONCE

Sets the checkpointing mode to "exactly once". This mode means that the system will checkpoint the operator and user function state in such a way that, upon recovery, every record will be reflected exactly once in the operator state.

For example, if a user function counts the number of elements in a stream, this number will consistently be equal to the number of actual elements in the stream, regardless of failures and recovery.

Note that this does not mean that each record flows through the streaming data flow only once. It means that upon recovery, the state of operators/functions is restored such that the resumed data streams pick up exactly at after the last modification to the state.

Note that this mode does not guarantee exactly-once behavior in the interaction with external systems (only state in Flink's operators and user functions). The reason for that is that a certain level of "collaboration" is required between two systems to achieve exactly-once guarantees. However, for certain systems, connectors can be written that facilitate this collaboration.

This mode sustains high throughput. Depending on the data flow graph and operations, this mode may increase the record latency, because operators need to align their input streams, in order to create a consistent snapshot point. The latency increase for simple dataflows (no repartitioning) is negligible. For simple dataflows with repartitioning, the average latency remains small, but the slowest records typically have an increased latency.

AT_LEAST_ONCE

public static final CheckpointingMode AT_LEAST_ONCE

Sets the checkpointing mode to "at least once". This mode means that the system will checkpoint the operator and user function state in a simpler way. Upon failure and recovery, some records may be reflected multiple times in the operator state.

For example, if a user function counts the number of elements in a stream, this number will equal to, or larger, than the actual number of elements in the stream, in the presence of failure and recovery.

This mode has minimal impact on latency and may be preferable in very-low latency scenarios, where a sustained very-low latency (such as few milliseconds) is needed, and where occasional duplicate messages (on recovery) do not matter.

Method Detail

values

public static CheckpointingMode[] 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 (CheckpointingMode c : CheckpointingMode.values())
    System.out.println(c);

Returns:

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

valueOf

public static CheckpointingMode 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