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  1. Cassandra
  2. CASSANDRA-9667

strongly consistent membership and ownership




      Currently, there is advice to users to "wait two minutes between adding new nodes" in order for new node tokens, et al, to propagate. Further, as there's no coordination amongst joining node wrt token selection, new nodes can end up selecting ranges that overlap with other joining nodes. This causes a lot of duplicate streaming from the existing source nodes as they shovel out the bootstrap data for those new nodes.

      This ticket proposes creating a mechanism that allows strongly consistent membership and ownership changes in cassandra such that changes are performed in a linearizable and safe manner. The basic idea is to use LWT operations over a global system table, and leverage the linearizability of LWT for ensuring the safety of cluster membership/ownership state changes. This work is inspired by Riak's claimant module.

      The existing workflows for node join, decommission, remove, replace, and range move (there may be others I'm not thinking of) will need to be modified to participate in this scheme, as well as changes to nodetool to enable them.

      Note: we distinguish between membership and ownership in the following ways: for membership we mean "a host in this cluster and it's state". For ownership, we mean "what tokens (or ranges) does each node own"; these nodes must already be a member to be assigned tokens.

      A rough draft sketch of how the 'add new node' workflow might look like is: new nodes would no longer create tokens themselves, but instead contact a member of a Paxos cohort (via a seed). The cohort member will generate the tokens and execute a LWT transaction, ensuring a linearizable change to the membership/ownership state. The updated state will then be disseminated via the existing gossip.

      As for joining specifically, I think we could support two modes: auto-mode and manual-mode. Auto-mode is for adding a single new node per LWT operation, and would require no operator intervention (much like today). In manual-mode, however, multiple new nodes could (somehow) signal their their intent to join to the cluster, but will wait until an operator executes a nodetool command that will trigger the token generation and LWT operation for all pending new nodes. This will allow us better range partitioning and will make the bootstrap streaming more efficient as we won't have overlapping range requests.


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              Unassigned Unassigned
              jasobrown Jason Brown
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