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website: Document node lifecycle. Fixes #282.

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Armon Dadgar 2014-08-22 15:54:48 -07:00
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website/source/docs/agent/basics.html.markdown
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@ -95,3 +95,36 @@ may not be important for your use case. For example, for a web server and load
balancer setup, both result in the same action: remove the web node
from the load balancer pool. But for other situations, you may handle
each scenario differently.
## Lifecycle
Every agent in the Consul cluster goes through a lifecycle. Understanding
this lifecycle is useful to building a mental model of an agent's interactions
with a cluster, and how the cluster treats a node.
When an agent is first started, it does not know about any other node in the cluster.
To discover it's peers, it must _join_ the cluster. This is done with the `join`
command or by providing the proper configuration to auto-join on start. Once a node
joins, this information is gossiped to the entire cluster, meaning all nodes will
eventually be aware of each other. If the agent is a server, existing servers will
begin replicating to the new node.
In the case of a network failure, some nodes may be unreachable by other nodes.
In this case, unreachable nodes are marked as _failed_. It is impossible to distinguish
between a network failure and an agent crash, so both cases are handled the same.
Once a node is marked as failed, this information is updated in the service catalog.
There is some nuance here relating, since this update is only possible if the
servers can still [form a quorum](/docs/internals/consensus.html). Once the network
failure recovers, or a crashed agent restarts, the cluster will repair itself,
and unmark a node as failed. The health check in the catalog will also be updated
to reflect this.
When a node _leaves_, it specifies it's intent to do so, and so the cluster
marks that node as having _left_. Unlike the _failed_ case, all of the
services provided by a node are immediately deregistered. If the agent was
a server, replication to it will stop. To prevent an accumulation
of dead nodes, Consul will automatically reap _failed_ nodes out of the
catalog as well. This is currently done on a non-configurable interval
which defaults to 72 hours. Reaping is similar to leaving, causing all
associated services to be deregistered.