Created Design: Runtime extension points (markdown)

Design:-Runtime-extension-points.md

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+(Original thread: https://groups.google.com/forum/#!topic/node-red/LFu_z85G3Fg)
+
+There are a number of places in the node-red runtime where it would be desirable to plug-in alternative implementations, or provide hooks to add functionality. There are a number of use cases this would enable.
+
+This page is an attempt to start organising these thoughts.
+
+## Use Cases
+
+### Running multiple instances of a flow
+
+This is where the flows are designed such that multiple instances can be run with some sort of load balancing in front of them. This is suited to HTTP-initiated flows, with any shared state maintained via an external service such as Redis.
+
+The requirement is to be able to trigger the deploy of flows across all instances.
+
+This could be done in the existing storage-plugin layer.
+
+### Running a flow across multiple instances
+
+This is where a flow is distributed across multiple runtimes. Two connected nodes may not be running on the same instance.
+
+The requirements are:
+
+ - co-ordinate the deploy of the flow across multiple machines
+ - be able to route messages between the instances are defined by the flow
+
+## Extension Points
+
+ - Storage layer - how/where things get stored - [already done](http://nodered.org/docs/api/storage/index.html).
+ - Message routing - how a message gets routed. Default implementation does the in-memory direct access to a node's `.receive` function. Alternative implementations could serialise the message to JSON and send over the network
+ - Flow handling - sits in the load/save path for flows. Need to refine how much is exposed to the API and where exactly it sits in the deploy process.