First you need to have Terracotta server or cluster setup.
You already asked how to do that and itβs captured here:
Then both IS server instances must point to the same external database, not Derby.
If IS instances were setup using CC composite template similar to is-template, then DB information, e.g. db.type, db.url, etc should have been identical for both IS instances.
Then COMMON-CLUSTER configuration must be created on both IS instances and point to the TC server/cluster.
Normally all of this is done by applying is-layer template with environment.type=cluster and passing is.tc.url and db.xxx parameters for the database, plus two or more hosts for the is.hosts= array, e.g.
This is example execution from the default is-layer template.yaml
cc exec templates composite apply is-layer environment.type=cluster 
is.hosts=[host1,host2,host3] db.type=oracle db.host=host0 db.name=db1 db.username=user db.password=pass 
is.tc.url=host4:9010 os.username=vmtest os.password=pass
If your IS instances are setup on the same host, in the same installation directory then you can execute the same template for each instance, e.g.
cc exec templates composite apply is-layer environment.type=cluster 
is.hosts=[host1] db.type=oracle db.host=host0 db.name=db1 db.username=user db.password=pass 
is.tc.url=host4:9010 os.username=vmtest os.password=pass 
is.instance.name=inst1 is.primary.port=5551 ....
cc exec templates composite apply is-layer environment.type=cluster 
is.hosts=[host1] db.type=oracle db.host=host0 db.name=db1 db.username=user db.password=pass 
is.tc.url=host4:9010 os.username=vmtest os.password=pass 
is.instance.name=inst2 is.primary.port=5552 ....
Notice is.instance.name and is.xxx.port parameters must be different for each instance, but DB and TC parameters are identical.
Thanks
Sergei
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