Thanks for the link. This paper provides an alternative, but similar implementation to that in Zookeeper. The key difference seems to be that the former supports membership reconfiguration.
Kafka replication is simpler because it separates the leader election part from log replication. Such separation has a few benefits: (1) the leader election part is easier to implement by leveraging a consensus system (e.g. Zookeeper); (2) the log format is simpler since the log itself is not used for leader election; (3) the replication factor for the log is decoupled from the number of parties required for leader election (e.g., in Kafka we can choose a replication factor of 2 for the log while using an ensemble of 5 for a Zookeeper cluster).
Both Rafe and Zookeeper are solving a harder problem than Kafka replication because they have no consensus service to rely upon for their own leader election since they are implementing a consensus service.
Jun On Tue, Apr 9, 2013 at 10:34 PM, Jay Kreps <[EMAIL PROTECTED]> wrote:
More notable differences from Kafka as far as log replication protocol is concerned -
- Raft considers log entries as committed as soon as it is acknowledged by a majority of the servers in a cluster. Compare this to Kafka where we have the notion of "in-sync followers" that are required to ack every batch of log entries in order for the leader to commit those.
- Raft uses the election voting mechanism to select a new leader whose log is as “up-to-date” as possible. Compare this to Kafka where we can pick ANY of the "in-sync followers" as the next leader, we typically pick the first one in the list. We do not try to pick the "in-sync follower" with the largest log for simplicity and fewer RPCs.
- In Raft, when the follower's log diverts from the leader's (in the presence of multiple failures), the leader-follower RPC truncates the follower's log up to the diversion point and then replicate the rest of the leader's log. This ensures that follower's log is identical to that of the leader's in such situations. Compare this to Kafka, where we allow the logs to divert and don't reconcile perfectly.
On Sun, Apr 14, 2013 at 9:42 PM, Jun Rao <[EMAIL PROTECTED]> wrote:
On the last point, in general, Kafka logs are identical among replicas. The only case that they may not be identical is when an unclean leader election happens, i.e., a leader has to be elected from a replica not in in-sync replica set). Unclean leader election should be rare since this requires multiple broker failures around the same time.
Jun On Tue, Apr 16, 2013 at 11:14 AM, Neha Narkhede <[EMAIL PROTECTED]>wrote:
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