Multi-region deployment of Dgraph, role of follower Zeroes

[rajas (Rajas Vanjape)]

Hi @martinthenth,

Thanks for your post.

@martinthenth

I’m deploying a setup of:

• 2 Zeroes
• 2 Alphas

Quoting from Dgraph paper at Dgraph Whitepapers: In-Depth Insights and Analysis,
Dgraph consists of Zeros and Alphas, each representing a group that they are serving. Each group is a Raft Cluster of an odd number of members. Zeros store and propagate metadata about the cluster while Alphas store user data.
As mentioned in Consistent Replication section of Cluster Setup - Deploy, it is recommended to use odd number of zeros and alphas for maintaining consensus among the group.

@martinthenth

First, in the example at https://dgraph.io/docs/deploy/production-checklist/#ha-setup-6-node-cluster all Alphas belong to the leader Zero. Does that mean the Alpha’s only listen to the leader Zero? Or would, if I query a follower Zero, a local Alpha respond to the follower Zero?

Alphas listen to Zero group (the Raft cluster) as a whole. In a Raft cluster, every write is consistently replicated to the group. No matter which zero you query, you will get the same response.

@martinthenth

Second, if I post data to a follower Zero, will that be propagated to the leader Zero?

Quoting from paper, Zeros store and propagate metadata about the cluster while Alphas store user data.
In case data is posted to a follower Alpha, it will be propagated and replicated to leader Alpha.

@martinthenth

Third, what would happen if I set up 1 Alpha to listen to the leader Zero, and 1 Alpha to the follower Zero? For example:

The --peer parameter is used to point out the Zero cluster to the Alpha. In the given example, as both zeros belong to the same Zero group (Raft cluster), the result will be same as setting both Alphas to the leader Zero.