Hi tison, > When a new node join an existing cluster in this case, how it exactly applies logs? Will it applies raft logs from the beginning one by one?
The leader takes a snapshot from time to time and then purges the old log entries. When a new server joins the group, the leader will first send a snapshot, if there is any, and then send the remaining log entries. Tsz-Wo On Sun, Jul 3, 2022 at 9:26 AM tison <[email protected]> wrote: > Hi Tsz-Wo, > > I have one question that may be a bit out of topic: > > When a new node join an existing cluster in this case, how it exactly > applies logs? Will it applies raft logs from the beginning one by one? TiKV > will sends a snapshot of state machine from leader to the new node in this > case to speed up the catch up progress and I'd like to know how Ratis > implements catch-up. > > Best, > tison. > > > Tsz Wo Sze <[email protected]> 于2022年7月2日周六 01:31写道： > >> > * It's possible to update the old configuration first by using >> client.admin().setConfiguration(), let's say set N=11 first, then start new >> nodes. However, since 5 < 11/2, the cluster won't be able to elect leader >> until at least 1 new node join. >> Yes, you are right. Also, even if one node has joined, the group has to >> wait for it to catch up with the previous log entries in order to obtain a >> majority for committing new entries. >> >> > * Or may be we should limit the count when scaling? From N=5 -> N=7 -> >> N=9 -> N=11. >> >> We may start the 6 new nodes as listeners first. Listeners receive log >> entries but they are not voting members and they won't be counted for >> majority. When the listeners catch up, we may change them to normal nodes >> so that they become voting members. >> >> Tsz-Wo >> >> >> On Fri, Jul 1, 2022 at 10:23 AM Tsz Wo Sze <[email protected]> wrote: >> >>> Hi Riguz, >>> >>> > Start 6 new nodes with new configuration N=11, while keeping the >>> previous nodes running >>> >>> This step probably won't work as expected since it will create a new >>> group but not adding nodes to the original group. We must use the >>> setConfiguration API to change configuration (add/remove nodes); see >>> https://github.com/apache/ratis/blob/bd83e7d7fd41540c8bda6bd92a52ac99ccec2076/ratis-client/src/main/java/org/apache/ratis/client/api/AdminApi.java#L35 >>> >>> Hope it helps. Thanks a lot for trying Ratis! >>> >>> Tsz-Wo >>> >>> On Fri, Jul 1, 2022 at 12:30 AM Riguz Lee <[email protected]> wrote: >>> >>>> >>>> >>>> Hi, >>>> >>>> >>>> I'm testing scaling up/down the raft cluster, but ratis is not working >>>> as expected in new cluster. My steps are: >>>> >>>> >>>> * Initialize a cluster with 5 nodes, the size and peers of the cluster >>>> is configured in a configuration file, let's say N=5. The cluster works >>>> perfectly, raft logs are synchronized across the cluster. >>>> >>>> * Start 6 new nodes with new configuration N=11, while keeping the >>>> previous nodes running >>>> >>>> * Recreate the previous nodes with N=11 one by one >>>> >>>> >>>> According the raft paper, raft should be able to handle configuration >>>> change by design, but after the above steps, what I've found is that: >>>> >>>> >>>> - New nodes not able to join the cluster >>>> >>>> - Old nodes still has a size of 5(by >>>> *client.getGroupManagementApi(peerId).info(groupId)*) >>>> >>>> >>>> So how should I scale the cluster correctly? A few thoughts of mine: >>>> >>>> >>>> * Definitely the old cluster should not be stopped while starting new >>>> nodes, otherwise new nodes might be able to elect new leader(eg. N=11 with >>>> 6 new nodes) and raft logs in old nodes will be overriden. >>>> >>>> * It's possible to update the old configuration first by using >>>> client.admin().setConfiguration(), let's say set N=11 first, then start new >>>> nodes. However, since 5 < 11/2, the cluster won't be able to elect leader >>>> until at least 1 new node join. >>>> >>>> * Or may be we should limit the count when scaling? From N=5 -> N=7 -> >>>> N=9 -> N=11. >>>> >>>> >>>> Thanks, >>>> >>>> Riguz Lee >>>> >>>> >>>> >>>>
