Fault Tolerance Guarantees of Data Sources and Sinks

Flink’s fault tolerance mechanism recovers programs in the presence of failures and continues to execute them. Such failures include machine hardware failures, network failures, transient program failures, etc.

Flink can guarantee exactly-once state updates to user-defined state only when the source participates in the snapshotting mechanism. The following table lists the state update guarantees of Flink coupled with the bundled connectors.

Please read the documentation of each connector to understand the details of the fault tolerance guarantees.

Source	Guarantees	Notes
Apache Kafka	exactly once	Use the appropriate Kafka connector for your version
AWS Kinesis Streams	exactly once
RabbitMQ	at most once (v 0.10) / exactly once (v 1.0)
Twitter Streaming API	at most once
Collections	exactly once
Files	exactly once
Sockets	at most once

To guarantee end-to-end exactly-once record delivery (in addition to exactly-once state semantics), the data sink needs to take part in the checkpointing mechanism. The following table lists the delivery guarantees (assuming exactly-once state updates) of Flink coupled with bundled sinks:

Sink	Guarantees	Notes
HDFS rolling sink	exactly once	Implementation depends on Hadoop version
Elasticsearch	at least once
Kafka producer	at least once
Cassandra sink	at least once / exactly once	exactly once only for idempotent updates
AWS Kinesis Streams	at least once
File sinks	at least once
Socket sinks	at least once
Standard output	at least once
Redis sink	at least once