I'm trying to write a logic (js script) to communicate with external system. As far as understand, logic will be executed on all endorsing peer. In this case, how can I avoid duplicate operation to external system ? For example, how to increment a value in external database ? If I write a logic to increment the value in js, I think the value will be incremented by all endorsing peer. I'll appreciate any comment.
How to communicate with external system
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Firstly, currently the only way you can interact with external systems is using the experimental
postAPI. This allows your Transaction Processor function to HTTP POST data to an external system and then to process the response.Documentation here: https://hyperledger.github.io/composer/integrating/call-out.html
You are correct in stating that if you have 4 peers, then the chain code container for each peer will run your logic, so you'd expect to see 4 calls to your HTTP service. This is required because each peer node is independent and Fabric must achieve consensus across the peers.
The external functions should therefore (ideally) be side-effect free "pure" functions (idempotent), meaning that for a given set of input parameters you always get the same set of output results.
Clearly a function that returns an incrementing integer doesn't fit this description! You probably need to rethink how you are structuring your problem to make it compatible with a decentralised blockchain-based approach.