Automated leader election
Starting from version 2.6.1, Tarantool has the built-in functionality managing automated leader election in a replica set. This functionality increases the fault tolerance of the systems built on the base of Tarantool and decreases dependency on external tools for replica set management.
To learn how to configure and monitor automated leader elections, check Managing leader elections.
The following topics are described below:
Leader election and synchronous replication
Leader election and synchronous replication are implemented in Tarantool as
a modification of the Raft
algorithm.
Raft is an algorithm of synchronous replication and automatic leader election.
Its complete description can be found in the corresponding document.
In Tarantool, synchronous replication and leader election
are supported as two separate subsystems.
So it is possible to get synchronous replication
but use an alternative algorithm for leader election.
And vice versa – elect a leader
in the cluster but don’t use synchronous spaces at all.
Synchronous replication has a separate documentation section.
Leader election is described below.
Note
The system behavior can be specified exactly according to the Raft algorithm. To do this:
- Ensure that the user has only synchronous spaces.
- Set the replication.synchro_quorum option to
N / 2 + 1.
- Set the replication.synchro_timeout option to infinity.
- In the replication.election_fencing_mode option, select either the
soft mode (the default)
or the strict mode, which is more restrictive.
Leader election process
Automated leader election in Tarantool helps guarantee that
there is at most one leader at any given moment of time in a replica set.
A leader is a writable node, and all other nodes are non-writable –
they accept read-only requests exclusively.
When the election is enabled, the life cycle of
a replica set is divided into so-called
terms. Each term is described by a monotonically growing number.
After the first boot, each node has its term equal to 1. When a node sees that
it is not a leader and there is no leader available for some time in the replica
set, it increases the term and starts a new leader election round.
Leader election happens via votes. The node that started the election votes
for itself and sends vote requests to other nodes.
Upon receiving vote requests, a node votes for the first of them, and then cannot
do anything in the same term but wait for a leader to be elected.
The node that collected a quorum of votes defined by the replication.synchro_quorum parameter
becomes the leader
and notifies other nodes about that. Also, a split vote can happen
when no nodes received a quorum of votes. In this case,
after a random timeout,
each node increases its term and starts a new election round if no new vote
request with a greater term arrives during this time.
Eventually, a leader is elected.
If any unfinalized synchronous transactions are left from the previous leader,
the new leader finalizes them automatically.
All the non-leader nodes are called followers. The nodes that start a new
election round are called candidates. The elected leader sends heartbeats to
the non-leader nodes to let them know it is alive.
In case there are no heartbeats for the period of replication.timeout * 4,
a non-leader node starts a new election if the following conditions are met:
- The node has a quorum of connections to other cluster members.
- None of these cluster members can see the leader node.
Note
A cluster member considers the leader node to be alive if the member received heartbeats from the leader at least
once during the replication.timeout * 4,
and there are no replication errors (the connection is not broken due to timeout or due to an error).
Terms and votes are persisted by each instance to preserve certain Raft guarantees.
During the election, the nodes prefer to vote for those ones that have the
newest data. So as if an old leader managed to send something before its death
to a quorum of replicas, that data wouldn’t be lost.
When election is enabled, there must be connections
between each node pair so as it would be the full mesh topology. This is needed
because election messages for voting and other internal things need a direct
connection between the nodes.
In the classic Raft algorithm, a leader doesn’t track its connectivity to the rest of the cluster.
Once the leader is elected, it considers itself in the leader position until receiving a new term from another cluster node.
This can lead to a split situation if the other nodes elect a new leader upon losing the connectivity to the previous one.
The issue is resolved in Tarantool version 2.10.0 by introducing the leader fencing mode.
The mode can be switched by the replication.election_fencing_mode configuration parameter.
When the fencing is set to soft or strict, the leader resigns its leadership if it has less than
replication.synchro_quorum of alive connections to the cluster nodes.
The resigning leader receives the status of a follower in the current election term and becomes read-only.
Leader fencing can be turned off by setting the replication.election_fencing_mode configuration parameter to off.
In soft mode, a connection is considered dead if there are no responses for
4 * replication.timeout seconds both on the current leader and the followers.
In strict mode, a connection is considered dead if there are no responses
for 2 * replication.timeout seconds on the current leader and for
4 * replication.timeout seconds on the followers.
This improves chances that there is only one leader at any time.
Fencing applies to the instances that have the replication.election_mode set to “candidate” or “manual”.
There can still be a situation when a replica set has two leaders working independently (so-called split-brain).
It can happen, for example, if a user mistakenly lowered the replication.synchro_quorum below N / 2 + 1.
In this situation, to preserve the data integrity, if an instance detects the split-brain anomaly in the incoming replication data,
it breaks the connection with the instance sending the data and writes the ER_SPLIT_BRAIN error in the log.
Eventually, there will be two sets of nodes with the diverged data,
and any node from one set is disconnected from any node from the other set with the ER_SPLIT_BRAIN error.
Once noticing the error, a user can choose any representative from each of the sets and inspect the data on them.
To correlate the data, the user should remove it from the nodes of one set,
and reconnect them to the nodes from the other set that have the correct data.
Also, if election is enabled on the node, it doesn’t replicate from any nodes except
the newest leader. This is done to avoid the issue when a new leader is elected,
but the old leader has somehow survived and tries to send more changes
to the other nodes.
Term numbers also work as a kind of filter.
For example, if election is enabled on two nodes and node1 has the term number less than node2,
then node2 doesn’t accept any transactions from node1.
Managing leader elections
replication:
election_mode: <string>
election_fencing_mode: <string>
election_timeout: <seconds>
timeout: <seconds>
synchro_quorum: <count>
- replication.election_mode – specifies the role of a node in the leader election
process.
- replication.election_fencing_mode – specifies the leader fencing mode.
- replication.election_timeout – specifies the timeout between election rounds if the
previous round ended up with a split vote.
- replication.timeout – a time interval (in seconds) used by a master to send heartbeat requests to a replica when there are no updates to send to this replica.
- replication.synchro_quorum – a number of replicas that should confirm the receipt of a synchronous transaction before it can finish its commit.
It is important to know that being a leader is not the only requirement for a node to be writable.
The leader should also satisfy the following requirements:
- The database.mode option is set to
rw.
- The leader shouldn’t be in the orphan state.
Nothing prevents you from setting the database.mode option to ro,
but the leader won’t be writable then. The option doesn’t affect the
election process itself, so a read-only instance can still vote and become
a leader.
To monitor the current state of a node regarding the leader election, use the box.info.election function.
Example:
tarantool> box.info.election
---
- state: follower
vote: 0
leader: 0
term: 1
...
The Raft-based election implementation logs all its actions
with the RAFT: prefix. The actions are new Raft message handling,
node state changing, voting, and term bumping.
Leader election doesn’t work correctly if the election quorum is set to less or equal
than <cluster size> / 2. In that case, a split vote can lead to
a state when two leaders are elected at once.
For example, suppose there are five nodes. When the quorum is set to 2, node1
and node2 can both vote for node1. node3 and node4 can both vote
for node5. In this case, node1 and node5 both win the election.
When the quorum is set to the cluster majority, that is
(<cluster size> / 2) + 1 or greater, the split vote is impossible.
That should be considered when adding new nodes.
If the majority value is changing, it’s better to update the quorum on all the existing nodes
before adding a new one.
Also, the automated leader election doesn’t bring many benefits in terms of data
safety when used without synchronous replication.
If the replication is asynchronous and a new leader gets elected,
the old leader is still active and considers itself the leader.
In such case, nothing stops
it from accepting requests from clients and making transactions.
Non-synchronous transactions are successfully committed because
they are not checked against the quorum of replicas.
Synchronous transactions fail because they are not able
to collect the quorum – most of the replicas reject
these old leader’s transactions since it is not a leader anymore.
Leader election and synchronous replication are implemented in Tarantool as a modification of the Raft algorithm. Raft is an algorithm of synchronous replication and automatic leader election. Its complete description can be found in the corresponding document.
In Tarantool, synchronous replication and leader election are supported as two separate subsystems. So it is possible to get synchronous replication but use an alternative algorithm for leader election. And vice versa – elect a leader in the cluster but don’t use synchronous spaces at all. Synchronous replication has a separate documentation section. Leader election is described below.
Note
The system behavior can be specified exactly according to the Raft algorithm. To do this:
- Ensure that the user has only synchronous spaces.
- Set the replication.synchro_quorum option to
N / 2 + 1. - Set the replication.synchro_timeout option to infinity.
- In the replication.election_fencing_mode option, select either the
softmode (the default) or thestrictmode, which is more restrictive.
Leader election process
Automated leader election in Tarantool helps guarantee that
there is at most one leader at any given moment of time in a replica set.
A leader is a writable node, and all other nodes are non-writable –
they accept read-only requests exclusively.
When the election is enabled, the life cycle of
a replica set is divided into so-called
terms. Each term is described by a monotonically growing number.
After the first boot, each node has its term equal to 1. When a node sees that
it is not a leader and there is no leader available for some time in the replica
set, it increases the term and starts a new leader election round.
Leader election happens via votes. The node that started the election votes
for itself and sends vote requests to other nodes.
Upon receiving vote requests, a node votes for the first of them, and then cannot
do anything in the same term but wait for a leader to be elected.
The node that collected a quorum of votes defined by the replication.synchro_quorum parameter
becomes the leader
and notifies other nodes about that. Also, a split vote can happen
when no nodes received a quorum of votes. In this case,
after a random timeout,
each node increases its term and starts a new election round if no new vote
request with a greater term arrives during this time.
Eventually, a leader is elected.
If any unfinalized synchronous transactions are left from the previous leader,
the new leader finalizes them automatically.
All the non-leader nodes are called followers. The nodes that start a new
election round are called candidates. The elected leader sends heartbeats to
the non-leader nodes to let them know it is alive.
In case there are no heartbeats for the period of replication.timeout * 4,
a non-leader node starts a new election if the following conditions are met:
- The node has a quorum of connections to other cluster members.
- None of these cluster members can see the leader node.
Note
A cluster member considers the leader node to be alive if the member received heartbeats from the leader at least
once during the replication.timeout * 4,
and there are no replication errors (the connection is not broken due to timeout or due to an error).
Terms and votes are persisted by each instance to preserve certain Raft guarantees.
During the election, the nodes prefer to vote for those ones that have the
newest data. So as if an old leader managed to send something before its death
to a quorum of replicas, that data wouldn’t be lost.
When election is enabled, there must be connections
between each node pair so as it would be the full mesh topology. This is needed
because election messages for voting and other internal things need a direct
connection between the nodes.
In the classic Raft algorithm, a leader doesn’t track its connectivity to the rest of the cluster.
Once the leader is elected, it considers itself in the leader position until receiving a new term from another cluster node.
This can lead to a split situation if the other nodes elect a new leader upon losing the connectivity to the previous one.
The issue is resolved in Tarantool version 2.10.0 by introducing the leader fencing mode.
The mode can be switched by the replication.election_fencing_mode configuration parameter.
When the fencing is set to soft or strict, the leader resigns its leadership if it has less than
replication.synchro_quorum of alive connections to the cluster nodes.
The resigning leader receives the status of a follower in the current election term and becomes read-only.
Leader fencing can be turned off by setting the replication.election_fencing_mode configuration parameter to off.
In soft mode, a connection is considered dead if there are no responses for
4 * replication.timeout seconds both on the current leader and the followers.
In strict mode, a connection is considered dead if there are no responses
for 2 * replication.timeout seconds on the current leader and for
4 * replication.timeout seconds on the followers.
This improves chances that there is only one leader at any time.
Fencing applies to the instances that have the replication.election_mode set to “candidate” or “manual”.
There can still be a situation when a replica set has two leaders working independently (so-called split-brain).
It can happen, for example, if a user mistakenly lowered the replication.synchro_quorum below N / 2 + 1.
In this situation, to preserve the data integrity, if an instance detects the split-brain anomaly in the incoming replication data,
it breaks the connection with the instance sending the data and writes the ER_SPLIT_BRAIN error in the log.
Eventually, there will be two sets of nodes with the diverged data,
and any node from one set is disconnected from any node from the other set with the ER_SPLIT_BRAIN error.
Once noticing the error, a user can choose any representative from each of the sets and inspect the data on them.
To correlate the data, the user should remove it from the nodes of one set,
and reconnect them to the nodes from the other set that have the correct data.
Also, if election is enabled on the node, it doesn’t replicate from any nodes except
the newest leader. This is done to avoid the issue when a new leader is elected,
but the old leader has somehow survived and tries to send more changes
to the other nodes.
Term numbers also work as a kind of filter.
For example, if election is enabled on two nodes and node1 has the term number less than node2,
then node2 doesn’t accept any transactions from node1.
Managing leader elections
replication:
election_mode: <string>
election_fencing_mode: <string>
election_timeout: <seconds>
timeout: <seconds>
synchro_quorum: <count>
- replication.election_mode – specifies the role of a node in the leader election
process.
- replication.election_fencing_mode – specifies the leader fencing mode.
- replication.election_timeout – specifies the timeout between election rounds if the
previous round ended up with a split vote.
- replication.timeout – a time interval (in seconds) used by a master to send heartbeat requests to a replica when there are no updates to send to this replica.
- replication.synchro_quorum – a number of replicas that should confirm the receipt of a synchronous transaction before it can finish its commit.
It is important to know that being a leader is not the only requirement for a node to be writable.
The leader should also satisfy the following requirements:
- The database.mode option is set to
rw.
- The leader shouldn’t be in the orphan state.
Nothing prevents you from setting the database.mode option to ro,
but the leader won’t be writable then. The option doesn’t affect the
election process itself, so a read-only instance can still vote and become
a leader.
To monitor the current state of a node regarding the leader election, use the box.info.election function.
Example:
tarantool> box.info.election
---
- state: follower
vote: 0
leader: 0
term: 1
...
The Raft-based election implementation logs all its actions
with the RAFT: prefix. The actions are new Raft message handling,
node state changing, voting, and term bumping.
Leader election doesn’t work correctly if the election quorum is set to less or equal
than <cluster size> / 2. In that case, a split vote can lead to
a state when two leaders are elected at once.
For example, suppose there are five nodes. When the quorum is set to 2, node1
and node2 can both vote for node1. node3 and node4 can both vote
for node5. In this case, node1 and node5 both win the election.
When the quorum is set to the cluster majority, that is
(<cluster size> / 2) + 1 or greater, the split vote is impossible.
That should be considered when adding new nodes.
If the majority value is changing, it’s better to update the quorum on all the existing nodes
before adding a new one.
Also, the automated leader election doesn’t bring many benefits in terms of data
safety when used without synchronous replication.
If the replication is asynchronous and a new leader gets elected,
the old leader is still active and considers itself the leader.
In such case, nothing stops
it from accepting requests from clients and making transactions.
Non-synchronous transactions are successfully committed because
they are not checked against the quorum of replicas.
Synchronous transactions fail because they are not able
to collect the quorum – most of the replicas reject
these old leader’s transactions since it is not a leader anymore.
Automated leader election in Tarantool helps guarantee that there is at most one leader at any given moment of time in a replica set. A leader is a writable node, and all other nodes are non-writable – they accept read-only requests exclusively.
When the election is enabled, the life cycle of a replica set is divided into so-called terms. Each term is described by a monotonically growing number. After the first boot, each node has its term equal to 1. When a node sees that it is not a leader and there is no leader available for some time in the replica set, it increases the term and starts a new leader election round.
Leader election happens via votes. The node that started the election votes for itself and sends vote requests to other nodes. Upon receiving vote requests, a node votes for the first of them, and then cannot do anything in the same term but wait for a leader to be elected.
The node that collected a quorum of votes defined by the replication.synchro_quorum parameter becomes the leader and notifies other nodes about that. Also, a split vote can happen when no nodes received a quorum of votes. In this case, after a random timeout, each node increases its term and starts a new election round if no new vote request with a greater term arrives during this time. Eventually, a leader is elected.
If any unfinalized synchronous transactions are left from the previous leader, the new leader finalizes them automatically.
All the non-leader nodes are called followers. The nodes that start a new election round are called candidates. The elected leader sends heartbeats to the non-leader nodes to let them know it is alive.
In case there are no heartbeats for the period of replication.timeout * 4, a non-leader node starts a new election if the following conditions are met:
- The node has a quorum of connections to other cluster members.
- None of these cluster members can see the leader node.
Note
A cluster member considers the leader node to be alive if the member received heartbeats from the leader at least
once during the replication.timeout * 4,
and there are no replication errors (the connection is not broken due to timeout or due to an error).
Terms and votes are persisted by each instance to preserve certain Raft guarantees.
During the election, the nodes prefer to vote for those ones that have the newest data. So as if an old leader managed to send something before its death to a quorum of replicas, that data wouldn’t be lost.
When election is enabled, there must be connections between each node pair so as it would be the full mesh topology. This is needed because election messages for voting and other internal things need a direct connection between the nodes.
In the classic Raft algorithm, a leader doesn’t track its connectivity to the rest of the cluster. Once the leader is elected, it considers itself in the leader position until receiving a new term from another cluster node. This can lead to a split situation if the other nodes elect a new leader upon losing the connectivity to the previous one.
The issue is resolved in Tarantool version 2.10.0 by introducing the leader fencing mode.
The mode can be switched by the replication.election_fencing_mode configuration parameter.
When the fencing is set to soft or strict, the leader resigns its leadership if it has less than
replication.synchro_quorum of alive connections to the cluster nodes.
The resigning leader receives the status of a follower in the current election term and becomes read-only.
Leader fencing can be turned off by setting the replication.election_fencing_mode configuration parameter to off.
In soft mode, a connection is considered dead if there are no responses for
4 * replication.timeout seconds both on the current leader and the followers.
In strict mode, a connection is considered dead if there are no responses
for 2 * replication.timeout seconds on the current leader and for
4 * replication.timeout seconds on the followers.
This improves chances that there is only one leader at any time.
Fencing applies to the instances that have the replication.election_mode set to “candidate” or “manual”.
There can still be a situation when a replica set has two leaders working independently (so-called split-brain).
It can happen, for example, if a user mistakenly lowered the replication.synchro_quorum below N / 2 + 1.
In this situation, to preserve the data integrity, if an instance detects the split-brain anomaly in the incoming replication data,
it breaks the connection with the instance sending the data and writes the ER_SPLIT_BRAIN error in the log.
Eventually, there will be two sets of nodes with the diverged data,
and any node from one set is disconnected from any node from the other set with the ER_SPLIT_BRAIN error.
Once noticing the error, a user can choose any representative from each of the sets and inspect the data on them. To correlate the data, the user should remove it from the nodes of one set, and reconnect them to the nodes from the other set that have the correct data.
Also, if election is enabled on the node, it doesn’t replicate from any nodes except the newest leader. This is done to avoid the issue when a new leader is elected, but the old leader has somehow survived and tries to send more changes to the other nodes.
Term numbers also work as a kind of filter.
For example, if election is enabled on two nodes and node1 has the term number less than node2,
then node2 doesn’t accept any transactions from node1.
Managing leader elections
replication:
election_mode: <string>
election_fencing_mode: <string>
election_timeout: <seconds>
timeout: <seconds>
synchro_quorum: <count>
- replication.election_mode – specifies the role of a node in the leader election
process.
- replication.election_fencing_mode – specifies the leader fencing mode.
- replication.election_timeout – specifies the timeout between election rounds if the
previous round ended up with a split vote.
- replication.timeout – a time interval (in seconds) used by a master to send heartbeat requests to a replica when there are no updates to send to this replica.
- replication.synchro_quorum – a number of replicas that should confirm the receipt of a synchronous transaction before it can finish its commit.
It is important to know that being a leader is not the only requirement for a node to be writable.
The leader should also satisfy the following requirements:
- The database.mode option is set to
rw.
- The leader shouldn’t be in the orphan state.
Nothing prevents you from setting the database.mode option to ro,
but the leader won’t be writable then. The option doesn’t affect the
election process itself, so a read-only instance can still vote and become
a leader.
To monitor the current state of a node regarding the leader election, use the box.info.election function.
Example:
tarantool> box.info.election
---
- state: follower
vote: 0
leader: 0
term: 1
...
The Raft-based election implementation logs all its actions
with the RAFT: prefix. The actions are new Raft message handling,
node state changing, voting, and term bumping.
Leader election doesn’t work correctly if the election quorum is set to less or equal
than <cluster size> / 2. In that case, a split vote can lead to
a state when two leaders are elected at once.
For example, suppose there are five nodes. When the quorum is set to 2, node1
and node2 can both vote for node1. node3 and node4 can both vote
for node5. In this case, node1 and node5 both win the election.
When the quorum is set to the cluster majority, that is
(<cluster size> / 2) + 1 or greater, the split vote is impossible.
That should be considered when adding new nodes.
If the majority value is changing, it’s better to update the quorum on all the existing nodes
before adding a new one.
Also, the automated leader election doesn’t bring many benefits in terms of data
safety when used without synchronous replication.
If the replication is asynchronous and a new leader gets elected,
the old leader is still active and considers itself the leader.
In such case, nothing stops
it from accepting requests from clients and making transactions.
Non-synchronous transactions are successfully committed because
they are not checked against the quorum of replicas.
Synchronous transactions fail because they are not able
to collect the quorum – most of the replicas reject
these old leader’s transactions since it is not a leader anymore.
replication:
election_mode: <string>
election_fencing_mode: <string>
election_timeout: <seconds>
timeout: <seconds>
synchro_quorum: <count>
- replication.election_mode – specifies the role of a node in the leader election process.
- replication.election_fencing_mode – specifies the leader fencing mode.
- replication.election_timeout – specifies the timeout between election rounds if the previous round ended up with a split vote.
- replication.timeout – a time interval (in seconds) used by a master to send heartbeat requests to a replica when there are no updates to send to this replica.
- replication.synchro_quorum – a number of replicas that should confirm the receipt of a synchronous transaction before it can finish its commit.
It is important to know that being a leader is not the only requirement for a node to be writable. The leader should also satisfy the following requirements:
- The database.mode option is set to
rw. - The leader shouldn’t be in the orphan state.
Nothing prevents you from setting the database.mode option to ro,
but the leader won’t be writable then. The option doesn’t affect the
election process itself, so a read-only instance can still vote and become
a leader.
To monitor the current state of a node regarding the leader election, use the box.info.election function.
Example:
tarantool> box.info.election
---
- state: follower
vote: 0
leader: 0
term: 1
...
The Raft-based election implementation logs all its actions
with the RAFT: prefix. The actions are new Raft message handling,
node state changing, voting, and term bumping.
Leader election doesn’t work correctly if the election quorum is set to less or equal
than <cluster size> / 2. In that case, a split vote can lead to
a state when two leaders are elected at once.
For example, suppose there are five nodes. When the quorum is set to 2, node1
and node2 can both vote for node1. node3 and node4 can both vote
for node5. In this case, node1 and node5 both win the election.
When the quorum is set to the cluster majority, that is
(<cluster size> / 2) + 1 or greater, the split vote is impossible.
That should be considered when adding new nodes. If the majority value is changing, it’s better to update the quorum on all the existing nodes before adding a new one.
Also, the automated leader election doesn’t bring many benefits in terms of data safety when used without synchronous replication. If the replication is asynchronous and a new leader gets elected, the old leader is still active and considers itself the leader. In such case, nothing stops it from accepting requests from clients and making transactions. Non-synchronous transactions are successfully committed because they are not checked against the quorum of replicas. Synchronous transactions fail because they are not able to collect the quorum – most of the replicas reject these old leader’s transactions since it is not a leader anymore.