Time and Watermarks in Confluent Cloud for Apache Flink

Timely stream processing is an extension of stateful stream processing that incorporates time into the computation. It’s commonly used for time series analysis, aggregations based on windows, and event processing where the time of occurrence is important.

If you’re working with timely Apache Flink® applications on Confluent Cloud, it’s important to consider certain factors to ensure optimal performance. Learn more about these considerations in the following sections.

Notions of time: Event Time and Processing Time

When referring to time in a streaming program, like when you define windows, different notions of time may apply.

Processing time

Processing time refers to the system time of the machine that’s executing the operation.

When a streaming program runs on processing time, all time-based operations, like time windows, use the system clock of the machines that run the operator.

An hourly processing time window includes all records that arrived at a specific operator between the times when the system clock indicated the full hour.

For example, if an application begins running at 9:15 AM, the first hourly processing time window includes events processed between 9:15 AM and 10:00 AM, the next window includes events processed between 10:00 AM and 11:00 AM, and so on.

Processing time is the simplest notion of time and requires no coordination between streams and machines. It provides the best performance and the lowest latency. But in distributed and asynchronous environments, processing time doesn’t provide determinism, because it’s susceptible to the speed at which records arrive in the system, like from a message queue, to the speed at which records flow between operators inside the system, and to outages (scheduled, or otherwise).

Event time

Event time is the time that each individual event occurred on its producing device. This time is typically embedded within the records before they enter Flink, and this event timestamp can be extracted from each record.

In event time, the progress of time depends on the data, not on any wall clocks. Event-time programs must specify how to generate event-time watermarks, which is the mechanism that signals progress in event time. This watermarking mechanism is described in the Event Time and Watermarks section.

In a perfect world, event-time processing would yield completely consistent and deterministic results, regardless of when events arrive, or their ordering. But unless the events are known to arrive in-order (by timestamp), event-time processing incurs some latency while waiting for out-of-order events. Because it’s only possible to wait for a finite period of time, this places a limit on how deterministic event-time applications can be.

Assuming all of the data has arrived, event-time operations behave as expected, and produce correct and consistent results even when working with out-of-order or late events, or when reprocessing historic data.

For example, an hourly event-time window contains all records that carry an event timestamp that falls into that hour, regardless of the order in which they arrive, or when they’re processed. For more information, see Lateness.

Sometimes when an event-time program is processing live data in real-time, it uses some processing time operations in order to guarantee that they are progressing in a timely fashion.

Event Time and Processing Time

Event Time and Processing Time

Event Time and Watermarks

Event time

A stream processor that supports event time needs a way to measure the progress of event time. For example, a window operator that builds hourly windows needs to be notified when event time has passed beyond the end of an hour, so that the operator can close the window in progress.

Event time can progress independently of processing time, as measured by wall clocks. For example, in one program, the current event time of an operator may trail slightly behind the processing time, accounting for a delay in receiving the events, while both proceed at the same speed. But another streaming program might progress through weeks of event time with only a few seconds of processing, by fast-forwarding through some historic data already buffered in an Apache Kafka® topic.


The mechanism in Flink to measure progress in event time is watermarks. Watermarks determine when to make progress during processing or wait for more records.

Certain SQL operations, like windows, interval joins, time-versioned joins, and MATCH_RECOGNIZE require watermarks. Without watermarks, they don’t produce output.

By default, every table has a watermark strategy applied.

A watermark means, “I have seen all records until this point in time”. It’s a long value that usually represents epoch milliseconds. The watermark of an operator is the minimum of received watermarks over all partitions of all inputs. It triggers the execution of time-based operations within this operator before sending the watermark downstream.

Watermarks can be emitted for every record, or they can be computed and emitted on a wall-clock interval. By default, Flink emits them every 200 ms.

The built-in function, CURRENT_WATERMARK, enables printing the current watermark for the executing operator.

Providing a timestamp is a prerequisite for providing a default watermark. Without providing some timestamp, neither a watermark nor a time attribute is possible.

In Flink SQL, only time attributes can be used for time-based operations.

A time attribute must be of type TIMESTAMP(p) or TIMESTAMP_LTZ(p), with 0 <= p <= 3.

Defining a watermark over a timestamp makes it a time attribute. This is shown as a ROWTIME in a DESCRIBE statement.

Watermarks and timestamps

Every Kafka record has a message timestamp which is part of the message format, and not in the payload or headers.

Timestamp semantics can be CreateTime (default) or LogAppendTime.

The timestamp is overwritten by the broker only if LogAppendTime is configured. Otherwise, it depends on the producer, which means that the timestamp can be user-defined, or it is set using the client’s clock if not defined by the user.

In most cases, a Kafka record’s timestamp is expressed in epoch milliseconds in UTC.

Watermarks flow as part of the data stream and carry a timestamp t. A Watermark(t) declares that event time has reached time t in that stream, meaning that there should be no more elements from the stream with a timestamp t’ <= t, that is, events with timestamps older or equal to the watermark.

The following diagram shows a stream of events with logical timestamps and watermarks flowing inline. In this example, the events are in order with respect to their timestamps, meaning that the watermarks are simply periodic markers in the stream.

A data stream with in-order events and watermarks

A data stream with in-order events and watermarks

Watermarks are crucial for out-of-order streams, as shown in the following diagram, where the events are not ordered by their timestamps. In general, a watermark declares that by this point in the stream, all events up to a certain timestamp should have arrived. Once a watermark reaches an operator, the operator can advance its internal event time clock to the value of the watermark.

A data stream with out-of-order events and watermarks

A data stream with out-of-order events and watermarks

Event time is inherited by a freshly created stream element (or elements) from either the event that produced them or from the watermark that triggered creation of these elements.

Watermarks in parallel streams

Watermarks are generated at, or directly after, source functions. Each parallel subtask of a source function usually generates its watermarks independently. These watermarks define the event time at that particular parallel source.

As the watermarks flow through the streaming program, they advance the event time at the operators where they arrive. Whenever an operator advances its event time, it generates a new watermark downstream for its successor operators.

Some operators consume multiple input streams. For example, a union, or operators following a keyBy(…) or partition(…) function consume multiple input streams. Such an operator’s current event time is the minimum of its input streams’ event times. As its input streams update their event times, so does the operator.

The following diagram shows an example of events and watermarks flowing through parallel streams, and operators tracking event time.

Parallel data streams and operators with events and watermarks

Parallel data streams and operators with events and watermarks


It’s possible that certain elements violate the watermark condition, meaning that even after the Watermark(t) has occurred, more elements with timestamp t’ <= t occur.

In many real-world systems, certain elements can be delayed for arbitrary lengths of time, making it impossible to specify a time by which all elements of a certain event timestamp will have occurred. Furthermore, even if the lateness can be bounded, delaying the watermarks by too much is often not desirable, because it causes too much delay in the evaluation of event-time windows.

For this reason, streaming programs may explicitly expect some late elements. Late elements are elements that arrive after the system’s event time clock, as signaled by the watermarks, has already passed the time of the late element’s timestamp.

Currently, Flink does not support late events or allowed lateness.


Aggregating events, for example in counts and sums, works differently with streams than in batch processing. For example, it’s impossible to count all elements in a stream, because streams are, in general, infinite (unbounded). Instead, aggregates on streams, like counts and sums, are scoped by windows, like as “count over the last 5 minutes”, or “sum of the last 100 elements”.

Time windows and count windows on a data stream

Time windows and count windows on a data stream

Windows can be time driven, for example, “every 30 seconds”, or data driven, for example, “every 100 elements”.

There are different types of windows, for example:

  • Tumbling windows: no overlap
  • Sliding windows: with overlap
  • Session windows: punctuated by a gap of inactivity (currently, Flink SQL does not support session windows)

For more information, see:

Watermarks and windows

In the following example, the source is a Kafka topic with 4 partitions.

The Flink job is running with a parallelism of 2, and each instance of the Kafka source reads from 2 partitions.

Each event has a key, shown as a letter from A to D, and a timestamp.

The events shown in bold text have already been read. The events in gray, to the left of the read position, will be read next.

The events that have already been read are shuffled by key into the window operators, where the events are counted by key for each hour.

Example Flink job graph with windows and watermarks.

Example Flink job graph with windows and watermarks.

Because the hour from 1 to 2 o’clock hasn’t been finalized yet, the windows keep track of the counters for that hour. There have been two events for key A for that hour, one event for key B, and so on.

Because events for the following hour have already begun to appear, these windows also maintain counters for the hour from 2 o’clock to 3 o’clock.

These windows wait for watermarks to trigger them to produce their results. The watermarks come from the watermark generators in the Kafka source operators.

For each Kafka partition, the watermark generator keeps track of the largest timestamp seen so far, and subtracts from that an estimate of the expected out-of-orderness.

For example, for Partition 1, the largest timestamp is 1:30. Assuming that the events are at most 1 minute out of order, then the watermark for Partition 1 is 1:29.

A similar computation for Partition 3 yields a watermark of 1:30, and so on for the remaining partitions.

Each of the two Kafka source instances take as its watermark the minimum of these per-partition watermarks

From the point of view of the uppermost Kafka source operator, the watermark it produces should include a timestamp that reflects how complete the stream is that it is producing.

This stream from Kafka Source 1 includes events from both Partition 1 and Partition 3, so it can be no more complete than the furthest behind of these two partitions, which is Partition 1.

Although Partition 1 has seen an event with a timestamp as late as 1:30, it reports its watermark as 1:29, because it allowing for its events to be up to one minute out-of-order.

This same reasoning is applied as the watermarks flow downstream through the job graph. Each instance of the window operator has received watermarks from the two Kafka source instances.

The current watermark at both of the window operators is 1:17, because this is the furthest behind of the watermarks coming into the windows from the Kafka sources.

The furthest behind of all four Kafka partitions determines the overall progress of the windows.

Time Attributes

Confluent Cloud for Apache Flink can process data based on different notions of time.

  • Processing time refers to the machine’s system time that’s executing the operation. Processing time is also known as “epoch time”, for example, Java’s System.currentTimeMillis().
  • Event time refers to stream processing based on timestamps that are attached to each row. The timestamps can encode when an event happened.

Time attributes can be part of every table schema. They are defined when creating a table from a CREATE TABLE DDL statement.

Once a time attribute is defined, it can be referenced as a field and used in time-based operations. As long as a time attribute is not modified and is simply forwarded from one part of a query to another, it remains a valid time attribute.

Time attributes behave like regular timestamps, and are accessible for calculations. When used in calculations, time attributes are materialized and act as standard timestamps, but ordinary timestamps can’t be used in place of, or converted to, time attributes.