Note that the write*() methods on DataStream are mainly intended for debugging purposes. They are not participating in Flink’s checkpointing, this means these functions usually have at-least-once semantics. The data flushing to the target system depends on the implementation of the OutputFormat. This means that not all elements send to the OutputFormat are immediately showing up in the target system. Also, in failure cases, those records might be lost. For reliable, exactly-once delivery of a stream into a file system, use the FileSink. Also, custom implementations through the .addSink(...) method can participate in Flink’s checkpointing for exactly-once semantics.