Startup of Dgraph nodes

Once the Big Data Discovery cluster is started, it activates the Dgraph processes on a subset of the nodes that are hosting the Dgraph instances. This topic discusses the behavior of the Dgraph nodes at startup.

On startup of a Dgraph, the following actions take place:

A Dgraph starts up without any Dgraph databases mounted.
If a Dgraph gets a Web service request involving a database that it has not mounted, it tries to mount it. The Dgraph mounts the database as a follower node by default or as a leader node if it has already been appointed leader by the Dgraph Gateway.
Follower Dgraphs do not alter the Dgraph database in any way; they continue answering queries based on the version of the database to which they have access at startup, even if the leader Dgraph node is in the process of updating, merging, or deleting the database. Follower Dgraph nodes do not receive update requests; they acquire access to the new database once the updates complete.

Note that the Dgraph can start up without ZooKeeper being up. In this case, the Dgraph comes up in a running but not ready-for-requests state, which means that means that the Dgraph will not be able to service any requests involving accessing data. The Dgraph continues to wait and connects with ZooKeeper when ZooKeeper comes up.