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Solution 5: True Participant Replicas

The requirements are:

1. Participants can be replicated, each participant can have an arbitrary number of replicas
2. Composition definitions, instances, element instances and all their data including properties is identical in all participant replicas
3. When anything is changed in one replica, the change is propagated to all the replicas of a participant
4. An operation on a composition element can be sent to any replica of  a participant, which means that for a given element, the deploy could be on replica 1, the update could be on replica 2 and the delete could be on replica 3, as one would expect in any HA solution
5. A single REST operation called on ACM-R will select a participant replica (probably using round robin initially but we could support other algorithms in the future), and use that replica for that operation.
6. The ACM runtime will be made HA (more than one replica of ACM-R will be supported), it will run on a HA postgres.
7. The implementation of change propagation is transparent to participant API users
8. Replicas are "eventually consistent", with consistency typically occurring in 100s of milliseconds

This solution uses a similar approach to that used by Kubernetes when using etcd to implement CRDs and CRs. It implements replication in participants by introducing

1. The concept of participant replicas into ACM-R and the Participant Intermediary
2. A lightweight mechanism for replica update between replicas and ACM-R. Every time a replica changes its data, the change is reported to ACM-R and ACM-R updates all other replicas for that participant

In this solution:

1. Participant Design backward compatibility is preserved, there is no change to the participant intermediary interface for participant implementations
2. Participant Configuration Backward compatibility is preserved, apart from a new "replicas" parameter (optional, default is 1), there is no change to participant configuration
3. ACM-R introduces a new REST API for replica management
4. ACM-R is made HA so that it itself can scale

Optimal Solution:

After analysis, it is clear that the best solution to use is number 3.

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