Doc: add change log 0.8.2

Author: drmingdrmer

change-log.md

@@ -1,3 +1,77 @@
+## v0.8.2
+
+### Changed:
+
+-   Changed: [342d0de2](https://github.com/datafuselabs/openraft/commit/342d0de2b885388dfa5de64430384bd3275d3697) rename variants in ChangeMembers, add `AddVoters`; by 张炎泼; 2023-03-01
+
+    Rename `ChangeMembers::AddVoter` to `AddVoterIds`, because it just
+    updates voter ids.
+
+    Rename `ChangeMembers::RemoveVoter` to `RemoveVoters`.
+
+    Add `ChangeMembers::AddVoters(BTreeMap)` to add voters with
+    corresponding `Node`, i.e., it adds nodes as learners and update
+    the voter-ids in a `Membership`.
+
+### Added:
+
+-   Added: [50821c37](https://github.com/datafuselabs/openraft/commit/50821c37035850dba9e237d9e7474e918f2bd410) impl PartialEq for Entry; by 张炎泼; 2023-03-02
+
+### Fixed:
+
+-   Fixed: [97fa1581](https://github.com/datafuselabs/openraft/commit/97fa15815a7d51c35a3a613b11defbf5f49cf4c1) discard blank log heartbeat, revert to the standard heartbeat; by 张炎泼; 2023-03-04
+
+    - https://github.com/datafuselabs/openraft/issues/698
+
+    The blank log heartbeat design has two problems:
+
+    - The heartbeat that sends a blank log introduces additional I/O, as a follower has to persist every log to maintain correctness.
+
+    - Although `(term, log_index)` serves as a pseudo time in Raft, measuring whether a node has caught up with the leader and is capable of becoming a new leader, leadership is not solely determined by this pseudo time.
+      Wall clock time is also taken into account.
+
+      There may be a case where the pseudo time is not upto date but the clock time is, and the node should not become the leader.
+      For example, in a cluster of three nodes, if the leader (node-1) is busy sending a snapshot to node-2(it has not yet replicated the latest logs to a quorum, but node-2 received message from the leader(node-1), thus it knew there is an active leader), node-3 should not seize leadership from node-1.
+      This is why there needs to be two types of time, pseudo time `(term, log_index)` and wall clock time, to protect leadership.
+
+      In the follow graph:
+      - node-1 is the leader, has 4 log entries, and is sending a snapshot to
+          node-2,
+      - node-2 received several chunks of snapshot, and it perceived an active
+          leader thus extended leader lease.
+      - node-3 tried to send vote request to node-2, although node-2 do not have
+          as many logs as node-3, it should still reject node-3's vote request
+          because the leader lease has not yet expired.
+
+      In the obsolete design, extending pseudo time `(term, index)` with a
+      `tick`, in this case node-3 will seize the leadership from node-2.
+
+      ```text
+      Ni: Node i
+      Ei: log entry i
+
+      N1 E1 E2 E3 E4
+            |
+            v
+      N2    snapshot
+            +-----------------+
+                     ^        |
+                     |        leader lease
+                     |
+      N3 E1 E2 E3    | vote-request
+      ---------------+----------------------------> clock time
+                     now
+
+      ```
+
+    The original document is presented below for reference.
+
+-   Fixed: [b5caa44d](https://github.com/datafuselabs/openraft/commit/b5caa44d1aac0b539180c1c490f0883dcc83048a) Wait::members() should not count learners as members; by 张炎泼; 2023-03-04
+
+    `Wait::members()` waits until membership becomes the expected value.
+    It should not check against all nodes.
+    Instead, it should only check voters, excluding learners.
+
 ## v0.8.1
 
 ### Added:

change-log/v0.8.2.md

@@ -0,0 +1,71 @@
+### Changed:
+
+-   Changed: [342d0de2](https://github.com/datafuselabs/openraft/commit/342d0de2b885388dfa5de64430384bd3275d3697) rename variants in ChangeMembers, add `AddVoters`; by 张炎泼; 2023-03-01
+
+    Rename `ChangeMembers::AddVoter` to `AddVoterIds`, because it just
+    updates voter ids.
+
+    Rename `ChangeMembers::RemoveVoter` to `RemoveVoters`.
+
+    Add `ChangeMembers::AddVoters(BTreeMap)` to add voters with
+    corresponding `Node`, i.e., it adds nodes as learners and update
+    the voter-ids in a `Membership`.
+
+### Added:
+
+-   Added: [50821c37](https://github.com/datafuselabs/openraft/commit/50821c37035850dba9e237d9e7474e918f2bd410) impl PartialEq for Entry; by 张炎泼; 2023-03-02
+
+### Fixed:
+
+-   Fixed: [97fa1581](https://github.com/datafuselabs/openraft/commit/97fa15815a7d51c35a3a613b11defbf5f49cf4c1) discard blank log heartbeat, revert to the standard heartbeat; by 张炎泼; 2023-03-04
+
+    - https://github.com/datafuselabs/openraft/issues/698
+
+    The blank log heartbeat design has two problems:
+
+    - The heartbeat that sends a blank log introduces additional I/O, as a follower has to persist every log to maintain correctness.
+
+    - Although `(term, log_index)` serves as a pseudo time in Raft, measuring whether a node has caught up with the leader and is capable of becoming a new leader, leadership is not solely determined by this pseudo time.
+      Wall clock time is also taken into account.
+
+      There may be a case where the pseudo time is not upto date but the clock time is, and the node should not become the leader.
+      For example, in a cluster of three nodes, if the leader (node-1) is busy sending a snapshot to node-2(it has not yet replicated the latest logs to a quorum, but node-2 received message from the leader(node-1), thus it knew there is an active leader), node-3 should not seize leadership from node-1.
+      This is why there needs to be two types of time, pseudo time `(term, log_index)` and wall clock time, to protect leadership.
+
+      In the follow graph:
+      - node-1 is the leader, has 4 log entries, and is sending a snapshot to
+          node-2,
+      - node-2 received several chunks of snapshot, and it perceived an active
+          leader thus extended leader lease.
+      - node-3 tried to send vote request to node-2, although node-2 do not have
+          as many logs as node-3, it should still reject node-3's vote request
+          because the leader lease has not yet expired.
+
+      In the obsolete design, extending pseudo time `(term, index)` with a
+      `tick`, in this case node-3 will seize the leadership from node-2.
+
+      ```text
+      Ni: Node i
+      Ei: log entry i
+
+      N1 E1 E2 E3 E4
+            |
+            v
+      N2    snapshot
+            +-----------------+
+                     ^        |
+                     |        leader lease
+                     |
+      N3 E1 E2 E3    | vote-request
+      ---------------+----------------------------> clock time
+                     now
+
+      ```
+
+    The original document is presented below for reference.
+
+-   Fixed: [b5caa44d](https://github.com/datafuselabs/openraft/commit/b5caa44d1aac0b539180c1c490f0883dcc83048a) Wait::members() should not count learners as members; by 张炎泼; 2023-03-04
+
+    `Wait::members()` waits until membership becomes the expected value.
+    It should not check against all nodes.
+    Instead, it should only check voters, excluding learners.