Backup and Messaging-only Nodes

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Note
This feature is only available in Axon Server Enterprise versions 4.3 and higher.
When you have larger clusters and specific requirements, you may want to have nodes acting in a different way within a replication group. For this reason, in Axon Server Enterprise 4.3, we have introduced a number of new roles that can be assigned to a node.
These are:
Primary nodes -> This is the role for nodes prior to 4.3. Primary nodes handle client connections and messages, store events and may act as the leader for a replication group.
Backup nodes -> These nodes maintain a copy of the event store, but will never become the leader of a replication group. There are two types of Backup nodes - Active Backup and Passive Backup​
Messaging-only nodes ->  They handle client connections and all types of messages (Commands/Queries/Events) but they do not have an event store.
Secondary nodes ->  Support for multi-tier storage. If a replication group contains secondary nodes the primary nodes will only retain recent data.
Each replication group needs to have at least one Primary node. This node is capable of becoming a leader and coordinating transactions across the replication group between the different member nodes. Even if you are not using Axon Server as an event store, you still need a leader as transactions also include making changes to the replication group configuration and access control lists. It is also possible to change the role that a particular node plays within a context.
Backup nodes
You can use backup nodes for instance when you want to ensure that you have a copy of your event store in another data center. As clients will never connect to a backup node and the backup node will never become the leader for a replication group it reduces the risk of high latency, compared to having a normal (Primary) node in another data center. Just to reiterate, clients will never connect directly to a backup node.
Backup nodes come in two flavors
Active Backup node
Active backup nodes maintain a real time copy of the Event Store by being active participants in transactions. To expand this, suppose a replication group within an Axon Server EE cluster has Active Backup nodes assigned to it (in addition to the Primary nodes). When an event is raised within a context the transaction to commit it in the Event Store is only confirmed if it receives a successful acknowledgement from at-least a certain number of those Active Backup nodes.
It is possible to customize the number of active backup nodes involved in the transactions by changing the value of the property axoniq.axonserver.replication.min-active-backups . The default value of this parameter is "1" which means that if you have Active Backup nodes, at least one of them needs to be up at any time. The higher the value set for this property, the higher the number of Active backup member nodes that need to be available for a successful transaction, so this property affects the availability of the cluster and hence needs to be carefully managed.
There are three possible ways to assign the ACTIVE_BACKUP role to a node within a replication group:
A) The Axon Server EE UI Console. Navigate to the Replication Groups icon on the navigation menu of the console which will open up the replication group maintenance screen. The nodes can be added as an ACTIVE_BACKUP role within a replication group.
B) The add-node-to-replication-group command with the role option as ACTIVE_BACKUP
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$ java -jar axonserver-cli.jar add-node-to-replication-group  -S http://[node]:[port] -n [node name] -g [replication-group-name] -r PASSIVE_BACKUP
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Mandatory parameters
-g refers to an existing replication group
-n refers to the node name that should be a member of this replication group
-r as PASSIVE_BACKUP refers to the role of this node within the replication group 
Optional parameters
-S if not supplied connects by default to http://localhost:8024. If supplied, it should be any node serving the _admin context 
-t  refers to the access token to authenticate at server
C) Axon Server EE provided REST API (http:[server]:[port]/swagger-ui.html) which offers the replication-group-rest-controller to help perform role maintenance operations
Passive Backup node
Passive Backup nodes follow the primary nodes with on a best effort base. If they are disconnected for some time, it will not impact the overall availability of the replication group. Once the Passive backup nodes are connected again, they will update their event stores with the events that were added while they were gone. If you don't require the backup node to be fully up to date at any moment, you can configure one passive backup node.‌
There are three possible ways to assign the PASSIVE_BACKUP role to a node within a replication group:
A) The Axon Server EE UI Console. Navigate to the Replication Groups icon on the navigation menu of the console which will open up the replication group maintenance screen. The nodes can be added as a PASSIVE_BACKUP role within a replication group.
B) The add-node-to-replication-group command with the role option as PASSIVE_BACKUP
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$ java -jar axonserver-cli.jar add-node-to-replication-group  -S http://[node]:[port] -n [node name] -g [replication-group-name] -r PASSIVE_BACKUP
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Mandatory parameters
-g refers to an existing replication group
-n refers to the node name that should be a member of this replication group
-r as PASSIVE_BACKUP refers to the role of this node within the replication group 
Optional parameters
-S if not supplied connects by default to http://localhost:8024. If supplied, it should be any node serving the _admin context 
-t  refers to the access token to authenticate at server
C) Axon Server EE provided REST API (http:[server]:[port]/swagger-ui.html) which offers the replication-group-rest-controller to help perform role maintenance operations
Messaging-only nodes
You can add nodes as messaging-only nodes to a replication group, if you don't want to use Axon Server as an event store, or if you want to have a large number of Axon Server nodes for a single replication group, without storing the events on each node. As the name already suggests, messaging-only nodes only route messages, they do not store events themselves. They do not participate in transactions and will clearly never become the leader for a replication-group.
There are three possible ways to assign the MESSAGING_ONLY role to a node within a replication group:
A) The Axon Server EE UI Console. Navigate to the Replication Groups icon on the navigation menu of the console which will open up the replication group maintenance screen. The nodes can be added as a MESSAGING_ONLY role within a replication group.
B) The add-node-to-replication-group command with the role option as MESSAGING_ONLY
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$ java -jar axonserver-cli.jar add-node-to-replication-group  -S http://[node]:[port] -n [node name] -g [replication-group-name] -r MESSAGING_ONLY
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Mandatory parameters
-c refers to an existing replication group
-n refers to the node name that should be a member of this replication group
-r as MESSAGING_ONLY refers to the role of this node within the replication group 
Optional parameters
-S if not supplied connects by default to http://localhost:8024. If supplied, it should be any node serving the _admin context 
-t  refers to the access token to authenticate at server
C) Axon Server EE provided REST API (http:[server]:[port]/swagger-ui.html) which offers the replication-group-rest-controller to help perform role maintenance operations
Secondary nodes
Axon Server version 4.4 and higher
You can add nodes as secondary nodes to a replication group, if you want to limit the amount of data stored on primary nodes. You could for example have fast (more expensive) storage on the primary nodes and less expensive storage on the secondary nodes. Most activity will take place on recent data, and only when you need to access older data you have to go to the secondary nodes.
Once you have defined secondary nodes for a replication group, this will apply for all contexts that are defined in the replication group. You can configure the retention time per context, so for some contexts you may have a longer retention time than for others.
As the old data still needs to be available at least one SECONDARY node needs to be up at all times. PRIMARY nodes will not delete information from their event stores until all SECONDARY nodes have this information.
There are three possible ways to assign the SECONDARY role to a node within a replication group:
A) The Axon Server EE UI Console. Navigate to the Replication Groups icon on the navigation menu of the console which will open up the replication group maintenance screen. The nodes can be added as a MESSAGING_ONLY role within a replication group.
B) The add-node-to-replication-group command with the role option as SECONDARY.
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$ java -jar axonserver-cli.jar add-node-to-replication-group  -S http://[node]:[port] -n [node name] -g [replication-group-name] -r SECONDARY
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Mandatory parameters
-c refers to an existing replication group
-n refers to the node name that should be a member of this replication group
-r as SECONDARY refers to the role of this node within the replication group 
Optional parameters
-S if not supplied connects by default to http://localhost:8024. If supplied, it should be any node serving the _admin context 
-t  refers to the access token to authenticate at server
C) Axon Server EE provided REST API (http:[server]:[port]/swagger-ui.html) which offers the replication-group-rest-controller to help perform role maintenance operations.
Changing node roles
Sometimes you may want to change the role a node has for a specific replication group. This may happen when you have a pre-existing cluster replication grou configuration (pre 4.3) and now you want to be able to start using the new roles. The way to do this is to remove a node from a replication group and then add it again in the new role.
When you remove the node from the replication group you have an option to preserve the event store. Preserving the event store is recommended when you want to change the role for a node from PRIMARY to BACKUP, or vice versa, as it would prevent a full replication of the event store when the node is added again with the new role.
There are three possible ways to change the role of a node within a replication group:
A) The Axon Server EE UI Console. Navigate to the Replication Groups icon on the navigation menu of the console which will open up the replication group maintenance screen. You can choose to delete the specific node from the context (using the delete icon). In case you would like to preserve the event store, click on the check-box in the pop-up.
B) The delete-node-from-replication-group command
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$ java -jar ./axonserver-cli.jar delete-node-from-replication-group -S http://[node]:[port] -g [replication-group-name] -n [node name]
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Mandatory parameters
-g refers to an existing replication group
-n refers to the node name that should be a member of this replication group
Optional parameters
-S if not supplied connects by default to http://localhost:8024. If supplied, it should be any node serving the _admin context 
-t  refers to the access token to authenticate at server
--preserve-event-store removes the node from the replication group but leaves the event store files on that node.
C) Axon Server EE provided REST API (http:[server]:[port]/swagger-ui.html) which offers the replication-group-rest-controller to help perform role maintenance operations
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Backups
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