Here is some information to keep in mind when you implement clustering:
- When setting up a cluster with multiple nodes, we recommend that you do so with a minimum of three machines. You can set up with a cluster with just two nodes. However, if one of those two nodes go down, the controller will no longer be operational.
- Every device that belongs to a cluster needs to have an identifier. For this purpose, OpenDaylight uses the node’s role. After you define the first node’s role as member-1 in the akka.conf file, OpenDaylight uses member-1 to identify that node.
- Data shards are used to house all or a certain segment of a module’s data. For example, one shard can contain all of a module’s inventory data while another shard contains all of it’s topology data. If you do not specify a module in the modules.conf file and do not specify a shard in module-shards.conf, then (by default) all the data is places onto the default shard (which must also be defined in module-shards.conf file). Each shard has replicas configured, and the module-shards.conf file is where you can specify where these replicas reside.
- Say you have a three node cluster on which HA is enabled. A replica of every defined data shard must be running on all three cluster nodes. This is because OpenDaylight’s clustering implementation requires a majority of the defined shard replicas to be running in order to function. If you only define data shard replicas on two of the cluster nodes and one of those nodes goes down, the corresponding data shards will not function.
- If you have a three node cluster and have defined replicas for a data shard on each of those nodes, that shard will still function even if only two of the cluster nodes are running. Note that if one of those two nodes go down, your controller will no longer be operational.
What considerations need to be made when setting the seed nodes for each member? Why are we referring to multiple seed nodes when you set only one IP address? Can you set multiple seed nodes for functional testing?
We recommend that you have multiple seed nodes configured. After a cluster member is started, it sends a message to all of its seed nodes. The cluster member then sends a join command to the first seed node that responds. If none of its seed nodes reply, the cluster member repeats this process until it successfully establishes a connection or it is shutdown.
What happens after one node becomes unreachable? Do the other two nodes function normally? When the first node reconnects, does it automatically synchronize with the other nodes?
After a node becomes unreachable, it remains down for configurable period of time (10 seconds, by default). Once a node goes down, you need to restart it so that it can rejoin the cluster. Once a restarted node joins a cluster, it will synchronize with the lead node automatically.
Can you run a two node cluster for functional testing?
For functional testing, yes. For HA testing, you need to run all three nodes.
