jpkessle commented on a change in pull request #3202: Support action
continuations in the controller
URL:
https://github.com/apache/incubator-openwhisk/pull/3202#discussion_r163054943
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File path: docs/compositions.md
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@@ -0,0 +1,144 @@
+# Action compositions
+
+Action compositions make it possible to dynamically build and invoke a series
of actions. Action compositions are similar to sequences. However, whereas the
components of a sequence must be specified before invoking the sequence,
components of a composition can be decided as the composition is running.
+
+## Example
+
+Suppose we define an _increment_ action:
+
+```
+$ cat > increment.js
+function main({ value }) { return { value: value + 1 } }
+^D
+
+$ wsk action create increment increment.js
+```
+
+We can use this _increment_ action in a composition as follows:
+
+```
+$ cat > composition.js
+function main(params) {
+ switch (params.$step || 0) {
+ case 0: delete params.$step; return { params, action: 'increment',
state: { $step: 1 } }
+ case 1: delete params.$step; return { params, action: 'increment',
state: { $step: 2 } }
+ case 2: delete params.$step; return { params }
+ }
+}
+^D
+
+$ wsk action create composition composition.js -a conductor true
+```
+
+The key to making this action a composition is the _conductor_ annotation,
which we discuss in the next section.
+
+This example composition executes two _increment_ actions in a sequence:
+
+```
+$ wsk action invoke composition -br -p value 3
+{
+ "value": 5
+}
+```
+
+In essence, this composition defines a program with three steps:
+
+- step 0: invoke increment on the input dictionary,
+- step 1: invoke increment on the output dictionary from step 1,
+- step 2: return the output dictionary from step 2.
+
+## Conductor Actions
+
+An action composition is driven by a _conductor action_. A _conductor action_,
or in short a _conductor_, is an action with a _conductor_ annotation. The
value of the annotation is irrelevant.
Review comment:
An Action composition is driven by a _conductor action_. A _conductor
action_, or in short a _conductor_, is an Action with a _conductor_
annotation. The value of the annotation is irrelevant.
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