Github user tzulitai commented on a diff in the pull request:
https://github.com/apache/flink/pull/5922#discussion_r185452174
--- Diff: docs/dev/stream/state/broadcast_state.md ---
@@ -0,0 +1,281 @@
+---
+title: "The Broadcast State Pattern"
+nav-parent_id: streaming_state
+nav-pos: 2
+---
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+
+* ToC
+{:toc}
+
+[Working with State](state.html) described operator state which is either
**evenly** distributed among the parallel
+tasks of an operator, or state which **upon restore**, its partial (task)
states are **unioned** and the whole state is
+used to initialize the restored parallel tasks.
+
+A third type of supported *operator state* is the *Broadcast State*.
Broadcast state was introduced to support use-cases
+where some data coming from one stream is required to be broadcasted to
all downstream tasks, where it is stored locally
+and is used to process all incoming elements on the other stream. As an
example where broadcast state can emerge as a
+natural fit, one can imagine a low-throughput stream containing a set of
rules which we want to evaluate against all
+elements coming from another stream. Having the above type of use-cases in
mind, broadcast state differs from the rest
+of operator states in that:
+ 1. it has a map format,
+ 2. it is only available to streams whose elements are *broadcasted*,
--- End diff --
How about,
"it is only available to operators which have a broadcasted input stream"?
This might only be a matter of personal preference, so please take this as
a grain of salt.
It's just that I somehow find it easier to understand when thinking in
terms of states and operators.
---
