From cb4e6075344b50cc3606e4d7946a4ad985404c4c Mon Sep 17 00:00:00 2001
From: Michal Maslanka <michal@redpanda.com>
Date: Mon, 20 Nov 2023 16:16:03 +0100
Subject: [PATCH 1/2] r/consensus: log configuration replace event

Signed-off-by: Michal Maslanka <michal@redpanda.com>
---
 src/v/raft/consensus.cc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/v/raft/consensus.cc b/src/v/raft/consensus.cc
index f0696625bfe8..2f056cd038aa 100644
--- a/src/v/raft/consensus.cc
+++ b/src/v/raft/consensus.cc
@@ -1158,10 +1158,16 @@ ss::future<std::error_code> consensus::replace_configuration(
   model::revision_id new_revision,
   std::optional<model::offset> learner_start_offset) {
     return change_configuration(
-      [nodes = std::move(nodes), new_revision, learner_start_offset](
+      [this, nodes = std::move(nodes), new_revision, learner_start_offset](
         group_configuration current) mutable {
+          auto old = current;
           current.set_version(raft::group_configuration::v_5);
           current.replace(nodes, new_revision, learner_start_offset);
+          vlog(
+            _ctxlog.debug,
+            "Replacing current configuration: {} with new configuration: {}",
+            old,
+            current);
 
           return result<group_configuration>{std::move(current)};
       });

From 7bb54d3c5e747f0809e7d9fc74a36e2a8a34e25a Mon Sep 17 00:00:00 2001
From: Michal Maslanka <michal@redpanda.com>
Date: Mon, 20 Nov 2023 16:42:47 +0100
Subject: [PATCH 2/2] r/recovery_stm: stop recovery when follower was already
 updated

Recovery and replicate stms are not synchronized. It may be the case
when both of stms are active at the same time that the same batch is
delivered to the follower twice. In general this batch duplication is
harmless as Raft is not vulnerable for messages redelivery but it may
cause unnecessary truncation and latency increase.

Added a check validating expected log end offset right before sending
recovery append entries request. This will prevent sending the same set
of batches twice to the follower.

Fixes: #14413

Signed-off-by: Michal Maslanka <michal@redpanda.com>
---
 src/v/raft/recovery_stm.cc | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/v/raft/recovery_stm.cc b/src/v/raft/recovery_stm.cc
index c1beda8ae2ce..8f27c802bc2c 100644
--- a/src/v/raft/recovery_stm.cc
+++ b/src/v/raft/recovery_stm.cc
@@ -557,6 +557,18 @@ ss::future<> recovery_stm::replicate(
         _stop_requested = true;
         return ss::now();
     }
+    if (meta.value()->expected_log_end_offset >= _last_batch_offset) {
+        vlog(
+          _ctxlog.trace,
+          "follower expected log end offset is already updated, stopping "
+          "recovery. Expected log end offset: {}, recovery range last offset: "
+          "{}",
+          meta.value()->expected_log_end_offset,
+          _last_batch_offset);
+
+        _stop_requested = true;
+        return ss::now();
+    }
     /**
      * Update follower expected log end. It is equal to the last batch in a set
      * of batches read for this recovery round.