[2/6] flink git commit: [FLINK-5805] [docs] Improvements to docs for ProcessFunction

[se...@apache.org]

[FLINK-5805] [docs] Improvements to docs for ProcessFunction

This closes #3317


Project: http://git-wip-us.apache.org/repos/asf/flink/repo
Commit: http://git-wip-us.apache.org/repos/asf/flink/commit/8fd374c9
Tree: http://git-wip-us.apache.org/repos/asf/flink/tree/8fd374c9
Diff: http://git-wip-us.apache.org/repos/asf/flink/diff/8fd374c9

Branch: refs/heads/release-1.2
Commit: 8fd374c9ed5913e42298f71c43922f11e353987e
Parents: e9ada34
Author: David Anderson <da...@alpinegizmo.com>
Authored: Wed Feb 15 10:58:55 2017 +0100
Committer: Stephan Ewen <se...@apache.org>
Committed: Thu Feb 16 14:51:01 2017 +0100

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 docs/dev/stream/process_function.md | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)
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http://git-wip-us.apache.org/repos/asf/flink/blob/8fd374c9/docs/dev/stream/process_function.md
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diff --git a/docs/dev/stream/process_function.md b/docs/dev/stream/process_function.md
index a8da4a2..ce3c670 100644
--- a/docs/dev/stream/process_function.md
+++ b/docs/dev/stream/process_function.md
@@ -47,7 +47,7 @@ stream.keyBy("id").process(new MyProcessFunction())
 
 The timers allow applications to react to changes in processing time and in [event time](../event_time.html).
 Every call to the function `processElement(...)` gets a `Context` object with gives access to the element's
-event time timestamp, and the *TimerService*. The `TimerService` can be used to register callbacks for future
+event time timestamp, and to the *TimerService*. The `TimerService` can be used to register callbacks for future
 event-/processing- time instants. When a timer's particular time is reached, the `onTimer(...)` method is
 called. During that call, all states are again scoped to the key with which the timer was created, allowing
 timers to perform keyed state manipulation as well.
@@ -55,30 +55,35 @@ timers to perform keyed state manipulation as well.
 
 ## Low-level Joins
 
-To realize low-level operations on two inputs, applications can use the `CoProcessFunction`. It relates to the `ProcessFunction`
-in the same way as a `CoFlatMapFunction` relates to the `FlatMapFunction`: The function is typed to two different inputs and
+To realize low-level operations on two inputs, applications can use `CoProcessFunction`. It relates to `ProcessFunction`
+in the same way that `CoFlatMapFunction` relates to `FlatMapFunction`: the function is bound to two different inputs and
 gets individual calls to `processElement1(...)` and `processElement2(...)` for records from the two different inputs.
 
-Implementing a low level join follows typically the pattern:
+Implementing a low level join typically follows this pattern:
 
   - Create a state object for one input (or both)
   - Update the state upon receiving elements from its input
   - Upon receiving elements from the other input, probe the state and produce the joined result
 
+For example, you might be joining customer data to financial trades,
+while keeping state for the customer data. If you care about having
+complete and deterministic joins in the face of out-of-order events,
+you can use a timer to evaluate and emit the join for a trade when the
+watermark for the customer data stream has passed the time of that
+trade.
 
 ## Example
 
-The following example maintains counts per key, and emits the key/count pair if no update happened to the key for one minute
-(in event time):
+The following example maintains counts per key, and emits a key/count pair whenever a minute passes (in event time) without an update for that key:
 
   - The count, key, and last-modification-timestamp are stored in a `ValueState`, which is implicitly scoped by key.
   - For each record, the `ProcessFunction` increments the counter and sets the last-modification timestamp
   - The function also schedules a callback one minute into the future (in event time)
   - Upon each callback, it checks the callback's event time timestamp against the last-modification time of the stored count
-    and emits the key/count if the match (no further update happened in that minute)
+    and emits the key/count if they match (i.e., no further update occurred during that minute)
 
-*Note:* This simple example could also have been implemented on top of session windows, we simple use it to illustrate
-the basic pattern of how to use the `ProcessFunction`.
+*Note:* This simple example could have been implemented with session windows. We use `ProcessFunction` here to illustrate
+the basic pattern it provides.
 
 <div class="codetabs" markdown="1">
 <div data-lang="java" markdown="1">