Send heartbeat with a blank log #151
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drmingdrmer
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Heartbeat in standard raft is the way for a leader to assert it is still alive. - A leader send heartbeat at a regular interval. - A follower that receives a heartbeat believes there is an active leader thus it rejects election request(`send_vote`) from another node unreachable to the leader, for a short period. Openraft heartbeat is a blank log Such a heartbeat mechanism depends on clock time. But raft as a distributed consensus already has its own **pseudo time** defined very well. The **pseudo time** in openraft is a tuple `(vote, last_log_id)`, compared in dictionary order. Why it works To refuse the election by a node that does not receive recent messages from the current leader, just let the active leader send a **blank log** to increase the **pseudo time** on a quorum. Because the leader must have the greatest **pseudo time**, thus by comparing the **pseudo time**, a follower automatically refuse election request from a node unreachable to the leader. And comparing the **pseudo time** is already done by `handle_vote_request()`, there is no need to add another timer for the active leader. Other changes: - Feature: add API to switch timeout based events: - `Raft::enable_tick()`: switch on/off election and heartbeat. - `Raft::enable_heartbeat()`: switch on/off heartbeat. - `Raft::enable_elect()`: switch on/off election. These methods make some testing codes easier to write. The corresponding `Config` entries are also added: `Config::enable_tick` `Config::enable_heartbeat` `Config::enable_elect` - Refactor: remove Engine `Command::RejectElection`. Rejecting election now is part of `handle_vote_req()` as blank-log heartbeat is introduced. - Refactor: heartbeat is removed from `ReplicationCore`. Instead, heartbeat is emitted by `RaftCore`. - Fix: when failed to sending append-entries, do not clear `need_to_replicate` flag. - CI: add test with higher network delay. - Doc: explain why using blank log as heartbeat. - Fix: datafuselabs#151
drmingdrmer
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* Feature: use blank log for heartbeat Heartbeat in standard raft is the way for a leader to assert it is still alive. - A leader send heartbeat at a regular interval. - A follower that receives a heartbeat believes there is an active leader thus it rejects election request(`send_vote`) from another node unreachable to the leader, for a short period. Openraft heartbeat is a blank log Such a heartbeat mechanism depends on clock time. But raft as a distributed consensus already has its own **pseudo time** defined very well. The **pseudo time** in openraft is a tuple `(vote, last_log_id)`, compared in dictionary order. Why it works To refuse the election by a node that does not receive recent messages from the current leader, just let the active leader send a **blank log** to increase the **pseudo time** on a quorum. Because the leader must have the greatest **pseudo time**, thus by comparing the **pseudo time**, a follower automatically refuse election request from a node unreachable to the leader. And comparing the **pseudo time** is already done by `handle_vote_request()`, there is no need to add another timer for the active leader. Other changes: - Feature: add API to switch timeout based events: - `Raft::enable_tick()`: switch on/off election and heartbeat. - `Raft::enable_heartbeat()`: switch on/off heartbeat. - `Raft::enable_elect()`: switch on/off election. These methods make some testing codes easier to write. The corresponding `Config` entries are also added: `Config::enable_tick` `Config::enable_heartbeat` `Config::enable_elect` - Refactor: remove Engine `Command::RejectElection`. Rejecting election now is part of `handle_vote_req()` as blank-log heartbeat is introduced. - Refactor: heartbeat is removed from `ReplicationCore`. Instead, heartbeat is emitted by `RaftCore`. - Fix: when failed to sending append-entries, do not clear `need_to_replicate` flag. - CI: add test with higher network delay. - Doc: explain why using blank log as heartbeat. - Fix: #151
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We can change sending heartbeat with a blank log that we can combines heartbeat with
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