Reading on a stretched cluster

  • We have a test stand with stretched scylladb cluster:

    • and in dc1
    • in dc2
    • RF=3
    • CL=2
  • the attacking host placed in dc1

    • use gocql driver
    • use policy DCAwareRoundRobinPolicy in gocql driver (so assumed we must read from nearest hosts)
  • All nodes are same

    • scylla 4.5.3 in docker
    • OS - Ubuntu 22.03
    • 4 vcpu
    • 16GB RAM
    • SSD 160G
  • what is the reason that after the time expires, the application starts reading from host (which is obviously located further - in another data center)

  • And after some time, the situation changes exactly the opposite.

When you have a multi-DC setup, make sure the Consistency Level you use for you reads is a LOCAL_ one: LOCAL_ONE or LOCAL_QUORUM. Otherwise, the replicas will be selected from among all the nodes in the cluster, disregarding DC.
See Consistency Levels | ScyllaDB Docs.