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In the current implementation, ACM supports multi-participant with same supported element Type but different participantId, so they need different properties file.

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Participant replicas can be a kubernetes StatefulSets that consume different properties file with unique consumer groups and unique UUIDs/replica (participants with same UUIDs have different replica number).

The StatefulSet uses the SPRING_CONFIG_NAME environment variable pointing to the spring application properties file unique to each of the participant replica.

Each of the properties file with the names pod-0.yaml, pod-1.yaml is mounted to the volumes. And the SPRING_CONFIG_NAME variable can be set to /path/to/$HOSTNAME.yaml to use the corresponding

properties file. 

By this approach the participant can have multiple replicas to work with a shared data.

env:
- name: HOSTNAME
  valueFrom:
    fieldRef:
      fieldPath: metadata.name

- name: SPRING_CONFIG_NAME
  value: /path/to/${HOSTNAME}.yaml

For example considering the http participant replica, ${HOSTNAME} will be "policy-http-ppnt-0" and "policy-http-ppnt-1" and their 
corresponding properties files with the names "http-ppnt-0.yaml" and "http-ppnt-1.yaml" is volume mounted. 

Note: In a scenario of two participants in replicas (we are calling "policy-http-ppnt-0" and "policy-http-ppnt-1"), ACM-Runtime will assignee as before any composition definition in prime time to specific participant based of supported element definition type. All participants will receive the same messages and store same data, so all participants are synchronized with ACM-R. Into all messages from ACM-R to participants will be present the replica number to indicate what participant will do the job (like prime or deploy). Example ACM-R send deploy message with replica 0, so "policy-http-ppnt-0" save the new instance and deploy and "policy-http-ppnt-1" just save that instance. When "policy-http-ppnt-0" send outProperties, then ACM-R and "policy-http-ppnt-1" receive the message and save that properties. When "policy-http-ppnt-0" has completed the deploy, the send the message and then then ACM-R and "policy-http-ppnt-1" receive the message and save the result. No issue if ACM-R send an undeploy message with replica 1, because all applications are synchronized.

Changes in Participant:

• Register, Status and Unregister message have to contain the replica number
• store data from messages from ACM-R if the participantId is matching and enable the actions only if the participantId and replica number are matching with the message
• store data (outProperties, and action completed) from messages from participants with same participantId and different replica
• implement time-out to stop the process if is running out of time

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Solution 5: True Participant Replicas

This solution uses a similar approach to that used by Kubernetes when using etcd to implement CRDs and CRs.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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