[GitHub] dgrove-oss commented on a change in pull request #16: Add parallel, map, and dynamic combinators

[GitBox <gi...@apache.org>]

dgrove-oss commented on a change in pull request #16: Add parallel, map, and dynamic combinators
URL: https://github.com/apache/incubator-openwhisk-composer/pull/16#discussion_r249178263
 
 

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 File path: README.md
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 @@ -143,6 +143,49 @@ Compositions are implemented by means of OpenWhisk conductor actions. The
 actions](https://github.com/apache/incubator-openwhisk/blob/master/docs/conductors.md)
 explains execution traces in greater details.
 
+While composer does not limit in principle the length of a composition,
+OpenWhisk deployments typically enforce a limit on the number of action
+invocations in a composition as well as an upper bound on the rate of
+invocation. These limits may result in compositions failing to execute to
+completion.
+
+## Parallel compositions with Redis
+
+Composer offers parallel combinators that make it possible to run actions or
+compositions in parallel, for example:
+```javascript
+composer.parallel('checkInventory', 'detectFraud')
+```
+
+The width of parallel compositions is not in principle limited by composer, but
+issuing many concurrent invocations may hit OpenWhisk limits leading to
+failures: failure to execute a branch of a parallel composition or failure to
+complete the parallel composition.
+
+These combinators require access to a Redis instance to hold intermediate
+results of parallel compositions. The Redis credentials may be specified at
+invocation time or earlier by means of default parameters or package bindings.
+The required parameter is named `$composer`. It is a dictionary with a `redis`
+field of type dictionary. The `redis` dictionary specifies the `uri` for the
+Redis instance and optionally a certificate as a base64-encoded string to enable
+tls connections. Hence, the input parameter object for our order-processing
+example should be:
+```json
+{
+    "$composer": {
+        "redis": {
+            "uri": "redis://...",
+            "ca": "optional base64 encoded tls certificate"
+        }
+    },
+    "order": { ... }
+}
+```
+
+The intend is to use Redis only temporarily as the parallel composition is
 
 Review comment:
   Typo: "The intend is " ==> "The intent is"

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