C - Even the default profile can be single-cluster or multi-cluster "The settings in an instance's default app profile depend on the number of clusters the instance had when you first created it: If you created the instance with 1 cluster, the default app profile uses single-cluster routing, and it enables single-row transactions. This ensures that adding additional clusters later doesn't change the behavior of your existing applications."

C. To ensure you still get connected when a Bigtable instance has 2 or more clusters, you must use multi-cluster routing. The question states that a single cluster existed first, then a second cluster was added. Google’s documentation states, “if you created the instance with one cluster, the default app profile uses single-cluster routing. This ensures that adding additional clusters later does not change the behavior of your existing applications”. Simply adding a second cluster does not change the default profile from single-cluster routing to multi-cluster routing. Since you need multi-cluster routing, you’re going to need a custom app profile. So C is correct. https://cloud.google.com/bigtable/docs/app-profiles#default-app-profile

https://cloud.google.com/bigtable/docs/replication-settings#high-availability To configure your instance for a high availability (HA) use case, create a new app profile that uses multi-cluster routing, or update the default app profile to use multi-cluster routing. This configuration provides eventual consistency. You won't be able to enable single-row transactions because single-row transactions can cause data conflicts when you use multi-cluster routing

I think C is correct. To make B correct it should say "Update default app profile to use multi-cluster routing and then use it". But it says just "Use default app profile". So B is not complete in this case.

https://cloud.google.com/bigtable/docs/replication-settings#high-availability said that To configure your instance for a high availability (HA) use case, create a new app profile that uses multi-cluster routing, or update the default app profile to use multi-cluster routing.

The default app profile does not change when you add or remove clusters. You must manually update the default app profile to change its settings. However, as a best practice you should create and use a new app profile instead of changing the default app profile. Once the new default app with multi-cluster in place, the rest follows multi-cluster path.

Since you created your cluster with only one cluster, the default app profile will have single-cluster routing. You want multi-cluster routing so that if one of the clusters is unavailable, requests are routed automatically to the other cluster. Because of this you need a custom app profile and multi-cluster routing.

Default profile can be updated to use multi-cluster routing https://cloud.google.com/bigtable/docs/replication-settings#high-availability:~:text=To%20configure%20your%20instance%20for%20a%20high%20availability%20(HA)%20use%20case%2C%20create%20a%20new%20app%20profile%20that%20uses%20multi%2Dcluster%20routing%2C%20or%20update%20the%20default%20app%20profile%20to%20use%20multi%2Dcluster%20routing.

C From https://cloud.google.com/bigtable/docs/app-profiles#default-app-profile The default app profile does not change when you add or remove clusters. You must manually update the default app profile to change its settings. However, as a best practice you should create and use a new app profile instead of changing the default app profile.

C. Create a custom app profile with multi-cluster routing. Default with multi-cluster won't work as the BigTable was created with a single cluster initially. Hence Custom with multi-cluster option.

Check the documentation: The default app profile does not change when you add or remove clusters. You must manually update the default app profile to change its settings. However, as a best practice you should create and use a new app profile instead of changing the default app profile. https://cloud.google.com/bigtable/docs/app-profiles#how-they-work Best practice must be the C, a custom app profile.

might be better option

C If you created the instance with 1 cluster, the default app profile uses single-cluster routing, and it enables single-row transactions. This ensures that adding additional clusters later doesn't change the behavior of your existing applications.

C https://cloud.google.com/bigtable/docs/app-profiles#:~:text=Multi%2Dcluster%20routing%20automatically%20routes,they%20are%20in%20different%20zones.

A The default app profile does not change when you add or remove clusters. You must manually update the default app profile to change its settings. However, as a best practice you should create and use a new app profile instead of changing the default app profile.

Default profile can be single-cluster or multi-cluster https://cloud.google.com/bigtable/docs/app-profiles#:~:text=Multi%2Dcluster%20routing%20automatically%20routes,they%20are%20in%20different%20zones.

B: Use the default app profile with multi-cluster *** routing. What is Bigtable cluster? A cluster represents the Bigtable service in a specific location. Each cluster belongs to a single Bigtable instance, and an instance can have clusters in up to 8 regions. When your application sends requests to a Bigtable instance, those requests are handled by one of the clusters in the instance.