Introduce follower replication #249

Fullstop000 · 2019-07-01T08:31:23Z

#136

New features

Add some new fields in Message
New concepts Group and Delegate in the cluster topology
~~A message now could be sent to a proxy and the proxy redirects the message to the destination~~

TODOs

The leader can specify the Delegate of a Group
The Delegate can bcast_append or send_append to the other members in the Group
Stable group delegate
Scenarios based testing

Problems

~~The ConfChange protocol for raft groups might need to be discussed.~~ Solved as GroupConfig is now volatile.

The implementation has been ported to etcd: etcd-io/etcd#11455.

Fullstop000 · 2019-09-05T02:24:07Z

I've finished the main part of this PR. But I'm not sure the best way to implement the ConfChange for altering raft groups dynamically so I want a discussion with your guys.

PTAL @siddontang @hicqu @BusyJay @Hoverbear

Hoverbear · 2019-09-09T20:38:17Z

@Fullstop000 Can you please include links to any design docs you worked on?

Fullstop000 · 2019-09-10T06:22:57Z

@Hoverbear Can I make a WIP PR in tikv/rfc ？

Hoverbear · 2019-09-18T21:45:02Z

@Fullstop000 I think it's OK. :) I'd still really appreciate seeing an RFC (like https://docs.google.com/document/d/1Sp9Tnc_nk_i0feTOgLGPT3jZYVFjfP7C6pwA-wuVHko/edit?ts=5c8c5fe3 which you worked on)

Hoverbear · 2019-11-07T19:36:38Z

@hicqu PTAL

src/raft_log.rs

src/raft.rs

src/group/mod.rs

hicqu · 2019-12-03T12:29:37Z

src/group/mod.rs

+
+    /// The max number of members of a group of which contains leader and the leader never pick a delegate.
+    /// If the group size is larger than this, a delegate will be picked even the leader belongs to this group.
+    pub max_leader_group_no_delegate: usize,


Personally I prefer to remove this, because without explicit information about group_id, follower replication may be not helpful. For example 7 peers in one datacenter, follower replication can't save any network traffics.

src/group/mod.rs

hicqu · 2019-12-03T12:37:21Z

src/raft.rs

+    // Also, A peer with smaller 'match' is able to receive more un-compacted entries from leader
+    // and then send them to others in same group.
+    //
+    fn pick_delegate(&self, group: &[u64], prs: &ProgressSet) -> u64 {


Seems in the new implementation, choose the peer with most raft logs is better?

It seems more reasonable. Actually maybe we can choose any unpaused peer?

hicqu · 2019-12-04T11:43:24Z

src/raft.rs

@@ -130,6 +130,9 @@ pub struct Raft<T: Storage> {
    /// The list of messages.
    pub msgs: Vec<Message>,

+    /// The especial map for messages to be sent to group delegates.
+    pub delegated_msgs: HashMap<u64, Message>,


I prefer to put it with groups

hicqu · 2019-12-04T11:56:58Z

src/raft.rs

@@ -545,41 +546,159 @@ impl<T: Storage> Raft<T> {
        is_batched
    }

+    // Whether the given peer could use Follower Replication
+    fn use_delegate(&self, to: u64) -> bool {


I prefer

fn use_delegate(&self, to: u64) -> bool { self.is_leader() && self.groups.get_members(to).map_or(0, |members| members.len()) > 1 && !self.groups.in_same_group(self.id, to) }

And we need to make sure that peers with invalid group_id shouldn't exist in groups.

hicqu · 2019-12-04T11:58:49Z

src/raft.rs

+    /// of produced message should be set to the leader id.
+    pub fn send_append(&mut self, to: u64, prs: &mut ProgressSet) {
+        if self.use_delegate(to) {
+            if let Some(gid) = self.groups.get_group_id(to) {


Too many indents. Maybe we can define use_delegate(to) -> Option<group_id>.

hicqu · 2019-12-04T12:02:24Z

src/raft.rs

+                            None => self.pick_delegate(&members, prs),
+                        };
+                        if delegate != INVALID_ID {
+                            if let Some(pr) = prs.get_mut(delegate) {


I think calling unwrap is better, because the progress must exist.

hicqu · 2019-12-04T12:08:04Z

src/raft.rs

+                    }
+                    None => {
+                        let members = self.groups.get_members(to).cloned().unwrap(); // this is safe because of the checking in `self.use_delegate`
+                        let delegate = match self.groups.get_delegate(gid) {


I prefer let delegate = self.pick_delegate(prs), which already contains get_delegate logic. And, its valid only if the pr is not paused.

hicqu · 2019-12-04T12:17:14Z

src/raft.rs

        }
        *maybe_commit = true;
    }

+    fn handle_append_response_in_delegate(


Seems only difference between this and handle_append_response is the latter needs to handle leader transferring. That logic is compatible for delegates. So I think this function is unnecessary.

hicqu · 2019-12-04T12:29:54Z

src/raft.rs

+                self.set_prs(prs);
+            } else {
+                let members = self.groups.get_members(m.from_delegate).cloned();
+                self.bcast_append(members.as_ref());


bcast_append calls send_append, which checks members' delegate again. I believe it can be improved, maybe we can take a try?

hicqu · 2019-12-04T12:35:30Z

src/raft.rs

@@ -1811,14 +1993,63 @@ impl<T: Storage> Raft<T> {
                };
                self.read_states.push(rs);
            }
+            MessageType::MsgAppendResponse => {
+                if self.prs().get(m.from).is_none() {


It's a common case for receiving responses from removed peers. So I think the log is unnecessary. Without it the code could be cleaner.

hicqu · 2019-12-04T12:42:00Z

src/raft.rs

+    //
+    // The delegate must satisfy conditions below:
+    // 1. Must be 'recent_active'
+    // 2. The progress state should be 'Replicate' but not 'paused'


Seems Probe needs also to be allowed? I think calling is_paused here is fine.

Probe might not be suitable for a delegate in the leader's view because the Probe member can probably just recover from a crash or is suffering the network partition.

hicqu · 2019-12-05T11:15:53Z

src/raft.rs

+    }
+
+    #[inline]
+    fn is_follower_replication_enabled(&self) -> bool {


I think it's not accurate. Every peer can't know follower replication is enabled or not. It just choose a strategy after receives followers' messages.

We can add some comments to make this clear.

harness/src/interface.rs

harness/src/network.rs

harness/tests/integration_cases/test_raft_follower_replication.rs

src/raft.rs

src/progress/progress_set.rs

Signed-off-by: Fullstop000 <[email protected]>

TennyZhuang · 2021-08-20T08:44:20Z

Any updates?

Xuanwo · 2021-08-24T05:49:51Z

This PR is discontinued for so long, any updates here? Do we have a plan to get it merged?

Fullstop000 changed the title ~~[WIP] Introduce follower replication~~ Introduce follower replication Aug 27, 2019

Hoverbear requested review from hicqu, Hoverbear and BusyJay September 9, 2019 19:00

Fullstop000 force-pushed the follower_replication branch 4 times, most recently from ae47fee to 3502119 Compare October 29, 2019 09:45

Hoverbear assigned Fullstop000 Nov 7, 2019

Hoverbear added the Feature Related to a major feature. label Nov 7, 2019

Fullstop000 mentioned this pull request Nov 14, 2019

Introduce Follower Replication tikv/rfcs#33

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Fullstop000 force-pushed the follower_replication branch from 3502119 to 0902e2f Compare November 19, 2019 03:00

hicqu mentioned this pull request Nov 25, 2019

Incubating Program: Follower Read With Applied Index pingcap/community#86

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9 tasks

hicqu reviewed Dec 2, 2019

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Signed-off-by: Fullstop000 <[email protected]>

Fullstop000 force-pushed the follower_replication branch 2 times, most recently from 88f583e to 1da637c Compare February 25, 2020 08:31

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Signed-off-by: Fullstop000 <[email protected]>

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Introduce follower replication #249

Introduce follower replication #249

Fullstop000 commented Jul 1, 2019 •

edited by hicqu

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Xuanwo commented Aug 24, 2021

Introduce follower replication #249

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Introduce follower replication #249

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Fullstop000 commented Jul 1, 2019 • edited by hicqu Loading

New features

TODOs

Problems

Fullstop000 commented Sep 5, 2019

Hoverbear commented Sep 9, 2019

Fullstop000 commented Sep 10, 2019

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