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docs: fix leader action transition descriptions (#28306)

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* and more clarification about member states
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Johannes Rudolph 2020-01-24 11:16:07 +01:00 • committed by Patrik Nordwall
parent 35259f1d16
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akka-docs/src/main/paradox/typed/cluster-membership.md
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@ -61,57 +61,71 @@ All nodes that previously detected it as unreachable has detected it as reachabl
## Membership Lifecycle
A node begins in the `joining` state. Once all nodes have seen that the new
node is joining (through gossip convergence) the `leader` will set the member
state to `up`.
A node is introduced to the cluster by invoking the `join` action which puts the node in the `joining` state.
Once all nodes have seen that the new node is joining (through @ref[gossip convergence](cluster-concepts.md#gossip-convergence)) the
@ref[`leader`](cluster-concepts.md#leader) will set the member state to `up`.
If a node is leaving the cluster in a safe, expected manner, for example through @ref[coordinated shutdown](../coordinated-shutdown.md),
it switches to the `leaving` state. Once the leader sees the convergence on the node in the
it invokes the `leave` action which switches it to the `leaving` state. Once the leader sees the convergence on the node in the
`leaving` state, the leader will then move it to `exiting`. Once all nodes
have seen the exiting state (convergence) the `leader` will remove the node
from the cluster, marking it as `removed`.
If a node is `unreachable` then gossip convergence is not possible and therefore
any `leader` actions are also not possible (for instance, allowing a node to
become a part of the cluster). To be able to move forward the state of the
`unreachable` nodes must be changed. It must become `reachable` again or marked
as `down`. If the node is to join the cluster again the actor system must be
restarted and go through the joining process again. If new incarnation of the unreachable
node tries to rejoin the cluster old incarnation will be marked as `down` and new
incarnation can rejoin the cluster without manual intervention.
most `leader` actions are impossible (for instance, allowing a node to
become a part of the cluster). To be able to move forward, the node must become `reachable` again
or the node must be explicitly "downed". This is required because the state of an unreachable node is unknown and the
cluster cannot know if the node has crashed or is only temporarily unreachable because of network issues or GC pauses.
See the section about [User Actions](#user-actions) below for ways a node can be downed.
The actor system on a node that exited or was downed cannot join the cluster again. In particular, a node that was downed while being
unreachable and then regains connectivity cannot rejoin the cluster. Instead, the process has to be restarted on the
node, creating a new actor system that can go through the joining process again.
A special case is a node that was restarted without going through the leaving or downing process e.g. because the machine
hosting the node was unexpectedly restarted. When the new instance of the node tries to rejoin the cluster, the cluster
might still track the old instance as unreachable. In this case, however, it is clear that the old node is gone because
the new instance will have the same address (host and port) as its old instance. In this case, the previous instance
will be automatically marked as `down` and the new instance can rejoin the cluster without manual intervention.
## Leader
The purpose of the `leader` is to confirm state changes when convergence is reached. The `leader` can be determined
by each node unambiguously after gossip convergence. Any node might be
required to take the role of the `leader` depending on the current cluster composition.
Without convergence, different nodes might have different views about which node is the leader.
Therefore, most leader actions are only allowed if there is convergence to ensure that all nodes agree about the current
state of the cluster and state changes are originated from a single node. Most regular state changes like changing
a node from `joining` to `up` are of that kind.
Other situations require that an action is taken even if convergence cannot be reached currently. Notably, convergence
cannot be reached if one or more nodes in the cluster are currently unreachable as determined by the
@ref[failure detector](cluster-concepts.md#failure-detector). In such a case, the cluster might be partitioned (a split brain
scenario) and each partition might have its own view about which nodes are reachable and which are not. In this case,
a node on each side of the partition might view itself as the leader of the reachable nodes. Any action that the leader
performs in such a case must be designed in a way that all concurrent leaders would come to the same conclusion (which
might be impossible in general and only feasible under additional constraints). The most important case of that kind is a split
brain scenario where nodes need to be downed, either manually or automatically, to bring the cluster back to convergence.
See the [Lightbend Split Brain Resolver](https://doc.akka.io/docs/akka-enhancements/current/split-brain-resolver.html)
for an implementation of that.
Another transition that is possible without convergence is marking members as `WeaklyUp` as described in the next section.
<a id="weakly-up"></a>
## WeaklyUp Members
If a node is `unreachable` then gossip convergence is not possible and therefore any
`leader` actions are also not possible. However, we still might want new nodes to join
the cluster in this scenario.
If a node is `unreachable` then gossip convergence is not
possible and therefore most `leader` actions are impossible. By enabling
`akka.cluster.allow-weakly-up-members` (which is enabled by default), joining nodes can be promoted to `WeaklyUp`
even while convergence is not yet reached. Once gossip convergence can be established again, the leader will move
`WeaklyUp` members to `Up`.
`Joining` members will be promoted to `WeaklyUp` and become part of the cluster if
convergence can't be reached. Once gossip convergence is reached, the leader will move `WeaklyUp`
members to `Up`.
This feature is enabled by default, but it can be disabled with configuration option:
```
akka.cluster.allow-weakly-up-members = off
```
You can subscribe to the `WeaklyUp` membership event to make use of the members that are
in this state, but you should be aware of that members on the other side of a network partition
have no knowledge about the existence of the new members. You should for example not count
`WeaklyUp` members in quorum decisions.
As mentioned before, if a node is `unreachable` then gossip convergence is not
possible and therefore any `leader` actions are also not possible. By enabling
`akka.cluster.allow-weakly-up-members` (enabled by default) it is possible to
let new joining nodes be promoted while convergence is not yet reached. These
`Joining` nodes will be promoted as `WeaklyUp`. Once gossip convergence is
reached, the leader will move `WeaklyUp` members to `Up`.
Note that members on the other side of a network partition have no knowledge about
the existence of the new members. You should for example not count `WeaklyUp`
members in quorum decisions.
You can subscribe to the `WeaklyUp` membership event to make use of the members
that are in this state, but you should be aware of that members on the other
side of a network partition have no knowledge about the existence of the
new members. You should for example not count `WeaklyUp` members in quorum decisions.
## State Diagrams
@ -133,7 +147,8 @@ startup if a node to join have been specified in the configuration
The `leader` has the duty of confirming user actions to shift members in and out of the cluster:
* joining ⭢ up
* weakly up ⭢ up *(no convergence is required for this leader action to be performed)*
* joining ⭢ weakly up *(no convergence is needed for this leader action to be performed which works even if there are unreachable nodes)*
* weakly up ⭢ up *(after full convergence is reached again)*
* leaving ⭢ exiting
* exiting ⭢ removed
* down ⭢ removed