[jira] [Comment Edited] (RATIS-498) Notify Follower to Install Snapshot through state machine

["Hanisha Koneru (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/RATIS-498?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16803556#comment-16803556 ] 

Hanisha Koneru edited comment on RATIS-498 at 3/28/19 3:21 AM:
---------------------------------------------------------------

Thank you [~szetszwo] for the review.
{quote}When a server is installing snapshot, how should it resply appendEntries requests? It seems the server should reject any appendEntries requests until installSnapshot completes.
For the example after #8, when N2 sends appendEntries, N3 should ignore the request and reply (AppendEntriesReplyProto) the nextIndex 100 (since it has installed snapshot 100). Then, it would work.
{quote}
Agree. Updated the patch to check if snapshot installation is going on in AppendEntries. Also checking that the first append entry index is greater than the latest snapshot index.

{quote}Also, when a leader requests notifyInstallSnapshot at index n, the follower could install any snapshots with index >= n. The follower should install the latest available snapshot. After the installation has completed, the follower reply (either notifyInstallSnapshot or appendEntries) with the new index.
{quote}
Yes, we do return the installed snapshot index and not the index n which was notified to the follower. This is returned as "snapshotIndex" in InstallSnapshotReplyProto.


was (Author: hanishakoneru):
{quote}When a server is installing snapshot, how should it resply appendEntries requests? It seems the server should reject any appendEntries requests until installSnapshot completes.
For the example after #8, when N2 sends appendEntries, N3 should ignore the request and reply (AppendEntriesReplyProto) the nextIndex 100 (since it has installed snapshot 100). Then, it would work.
{quote}
Agree. Updated the patch to check if snapshot installation is going on in AppendEntries. Also checking that the first append entry index is greater than the latest snapshot index.

{quote}Also, when a leader requests notifyInstallSnapshot at index n, the follower could install any snapshots with index >= n. The follower should install the latest available snapshot. After the installation has completed, the follower reply (either notifyInstallSnapshot or appendEntries) with the new index.
{quote}
Yes, we do return the installed snapshot index and not the index n which was notified to the follower. This is returned as "snapshotIndex" in InstallSnapshotReplyProto.

> Notify Follower to Install Snapshot through state machine
> ---------------------------------------------------------
>
>                 Key: RATIS-498
>                 URL: https://issues.apache.org/jira/browse/RATIS-498
>             Project: Ratis
>          Issue Type: New Feature
>          Components: server
>            Reporter: Hanisha Koneru
>            Assignee: Hanisha Koneru
>            Priority: Major
>         Attachments: RATIS-498.000.patch, RATIS-498.001.patch, RATIS-498.002.patch, RATIS-498.003.patch, RATIS-498.004.patch, RATIS-498.005.patch, RATIS-498.006.patch
>
>
> When a lagging Follower wants to catch up with the Leader, and the Leader only has logs with start index greater than the Followers's last log index, then the leader sends an InstallSnapshotRequest to the the Follower. 
> The aim of this Jira is to allow State Machine to decouple snapshot installation from the Ratis server. When Leader does not have the logs to get the Follower up to speed, it should notify the Follower to install a snapshot (if install snapshot through Log Appender is disabled). The Follower in turn notifies its state machine that a snapshot is required to catch up with the leader.



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