Elasticsearch's domain status stuck at processing

EBS volume change triggers a blue/green deployment process which means:

• AWS will provision a new set of nodes
• Data is migrated from old nodes to the new ones.
• Once this is complete, old nodes are terminated

While executing this kind of change, it is recommended to ensure :

• The workload in the cluster is comparatively low.
• There is only one configuration change at a time.
• The cluster is in a healthy and active state when the configuration change is triggered.
• There are dedicated master nodes ( 3 masters )

Now this may get stuck due to -

• Lack of exact instances type in the AZ
• Shard migration to new nodes is stuck
• Other things

Hence, to figure out what is exactly happening in your situation - you can raise a support ticket and let the support team do further investigation