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From se...@apache.org
Subject flink git commit: [docs] Add description and illustration about Watermarks in streams
Date Wed, 09 Mar 2016 19:38:29 GMT
Repository: flink
Updated Branches:
  refs/heads/master cdc8f0a9b -> e0947cc3e


[docs] Add description and illustration about Watermarks in streams


Project: http://git-wip-us.apache.org/repos/asf/flink/repo
Commit: http://git-wip-us.apache.org/repos/asf/flink/commit/e0947cc3
Tree: http://git-wip-us.apache.org/repos/asf/flink/tree/e0947cc3
Diff: http://git-wip-us.apache.org/repos/asf/flink/diff/e0947cc3

Branch: refs/heads/master
Commit: e0947cc3e3e9f5aa6b8ab9b759a618e728fd0d5d
Parents: cdc8f0a
Author: Stephan Ewen <sewen@apache.org>
Authored: Wed Mar 9 20:31:52 2016 +0100
Committer: Stephan Ewen <sewen@apache.org>
Committed: Wed Mar 9 20:31:52 2016 +0100

----------------------------------------------------------------------
 docs/apis/streaming/event_time.md               |  62 ++-
 .../fig/parallel_streams_watermarks.svg         | 516 +++++++++++++++++++
 .../streaming/fig/stream_watermark_in_order.svg | 314 +++++++++++
 .../fig/stream_watermark_out_of_order.svg       | 314 +++++++++++
 4 files changed, 1205 insertions(+), 1 deletion(-)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/flink/blob/e0947cc3/docs/apis/streaming/event_time.md
----------------------------------------------------------------------
diff --git a/docs/apis/streaming/event_time.md b/docs/apis/streaming/event_time.md
index ad01f04..90eb67a 100644
--- a/docs/apis/streaming/event_time.md
+++ b/docs/apis/streaming/event_time.md
@@ -112,6 +112,7 @@ stream
 <div data-lang="scala" markdown="1">
 {% highlight scala %}
 val env = StreamExecutionEnvironment.getExecutionEnvironment
+
 env.setStreamTimeCharacteristic(TimeCharacteristic.ProcessingTime)
 
 // alternatively:
@@ -141,8 +142,67 @@ to use timestamp assignment and watermark generation in the Flink DataStream API
 
 # Event Time and Watermarks
 
-*Note: For a deep introduction to Event Time, please refer also to the paper on the [Dataflow Model](https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/43864.pdf)*
+*Note: Flink implements many techniques from the Dataflow Model. For a good introduction to Event Time and, have also a look at these articles*
+
+  - [Streaming 101](https://www.oreilly.com/ideas/the-world-beyond-batch-streaming-101) by Tyler Akidau
+  - The [Dataflow Model paper](https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/43864.pdf)
+
+
+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 reached the
+next full hour, such that the operator can close the next window.
+
+*Event Time* can progress independently of *Processing Time* (measures by wall clocks).
+For example, in one program, the current *event time* of an operator can trail slightly behind the processing time
+(accounting for a delay in receiving the latest elements) and both proceed at the same speed. In another streaming
+program, which reads fast-forward through some data already buffered in a Kafka topic (or another message queue), event time
+can progress by weeks in seconds.
+
+------
+
+The mechanism in Flink to measure progress in event time are **Watermarks**.
+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 all events with a timestamps *t' < t* have occurred.
+
+The figure below shows a stream of events with (logical) timestamps, and watermarks flowing inline. The events are in order
+(with respect to their timestamp), meaning that watermarks are simply periodic markers in the stream with an in-order timestamp.
+
+<img src="fig/stream_watermark_in_order.svg" alt="A data stream with events (in order) and watermarks" class="center" width="65%" />
+
+Watermarks are crucial for *out-of-order* streams, as shown in the figure below, where, events do not occur ordered by their timestamp.
+Watermarks establish points in the stream where all events up to a certain timestamp have occurred. Once these watermarks reach an
+operator, the operator can advance its internal *event time clock* to the value of the watermark.
+
+<img src="fig/stream_watermark_out_of_order.svg" alt="A data stream with events (out of order) and watermarks" class="center" width="65%" />
+
+
+## Watermarks in Parallel Streams
+
+Watermarks are generated at source functions, 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.
+
+Operators that consume multiple input streams (e.g., after a *keyBy(...)* or *partition(...)* function, or a union) track the event time
+on each of their input streams. The operator's current event time is the minimum of the input streams' event time. As the input streams
+update their event time, so does the operator.
+
+The figure below shows an example of events and watermarks flowing through parallel streams, and operators tracking event time.
+
+<img src="fig/parallel_streams_watermarks.svg" alt="Parallel data streams and operators with events and watermarks" class="center" width="80%" />
+
+
+## Late Elements
 
+It is possible that certain elements violate the watermark condition, meaning that even after the *Watermark(t)* has occurred,
+more elements with timestamp *t' < t* will occur. In fact, in many real world setups, certain elements can be arbitrarily
+delayed, which it is impossible to define a time when all elements of a certain event timestamp have occurred.
+Further more, even if the lateness can be bounded, delaying the watermarks by too much is often not desirable, because it delays
+the evaluation of the event time windows by too much.
 
+Due to that, some streaming programs will explicitly expect a number of *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.
 
 

http://git-wip-us.apache.org/repos/asf/flink/blob/e0947cc3/docs/apis/streaming/fig/parallel_streams_watermarks.svg
----------------------------------------------------------------------
diff --git a/docs/apis/streaming/fig/parallel_streams_watermarks.svg b/docs/apis/streaming/fig/parallel_streams_watermarks.svg
new file mode 100644
index 0000000..f6a4c4b
--- /dev/null
+++ b/docs/apis/streaming/fig/parallel_streams_watermarks.svg
@@ -0,0 +1,516 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
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http://git-wip-us.apache.org/repos/asf/flink/blob/e0947cc3/docs/apis/streaming/fig/stream_watermark_in_order.svg
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new file mode 100644
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http://git-wip-us.apache.org/repos/asf/flink/blob/e0947cc3/docs/apis/streaming/fig/stream_watermark_out_of_order.svg
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