Updates to the Adding/Removing Servers Guide (#5004)
* added a new section for adding servers, updated section titles, and added code snippets. * Fixing typos * fixing typos * Addressing some of Paul's feedback. * Updated the outage recovery recommendation
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---
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layout: "docs"
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page_title: "Adding/Removing Servers"
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page_title: "Adding & Removing Servers"
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sidebar_current: "docs-guides-servers"
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description: |-
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Consul is designed to require minimal operator involvement, however any changes to the set of Consul servers must be handled carefully. To better understand why, reading about the consensus protocol will be useful. In short, the Consul servers perform leader election and replication. For changes to be processed, a minimum quorum of servers (N/2)+1 must be available. That means if there are 3 server nodes, at least 2 must be available.
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---
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# Adding/Removing Servers
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# Adding & Removing Servers
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Consul is designed to require minimal operator involvement, however any changes
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to the set of Consul servers must be handled carefully. To better understand
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@ -18,13 +18,16 @@ That means if there are 3 server nodes, at least 2 must be available.
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In general, if you are ever adding and removing nodes simultaneously, it is better
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to first add the new nodes and then remove the old nodes.
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## Adding New Servers
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In this guide, we will cover the different methods for adding and removing servers.
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Adding new servers is generally straightforward. Simply start the new
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agent with the `-server` flag. At this point, the server will not be a member of
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## Manually Add a New Server
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Manually adding new servers is generally straightforward, start the new
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agent with the `-server` flag. At this point the server will not be a member of
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any cluster, and should emit something like:
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```text
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```sh
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consul agent -server
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[WARN] raft: EnableSingleNode disabled, and no known peers. Aborting election.
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```
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@ -32,14 +35,50 @@ This means that it does not know about any peers and is not configured to elect
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This is expected, and we can now add this node to the existing cluster using `join`.
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From the new server, we can join any member of the existing cluster:
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```text
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$ consul join <Node Address>
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```sh
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$ consul join <Existing Node Address>
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Successfully joined cluster by contacting 1 nodes.
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```
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It is important to note that any node, including a non-server may be specified for
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join. The gossip protocol is used to properly discover all the nodes in the cluster.
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Once the node has joined, the existing cluster leader should log something like:
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join. Generally, this method is good for testing purposes but not recommended for production
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deployments. For production clusters, you will likely want to use the agent configuration
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option to add additional servers.
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## Add a Server with Agent Configuration
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In production environments, you should use the [agent configuration](https://www.consul.io/docs/agent/options.html) option, `retry_join`. `retry_join` can be used as a command line flag or in the agent configuration file.
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With the Consul CLI:
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```sh
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$ consul agent -retry-join=["52.10.110.11", "52.10.110.12", "52.10.100.13"]
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```
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In the agent configuration file:
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```sh
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{
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"bootstrap": false,
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"bootstrap_expect": 3,
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"server": true,
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"retry_join": ["52.10.110.11", "52.10.110.12", "52.10.100.13"]
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}
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```
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[`retry_join`](https://www.consul.io/docs/agent/options.html#retry-join)
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will ensure that if any server loses connection
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with the cluster for any reason, including the node restarting, it can
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rejoin when it comes back. In additon to working with static IPs, it
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can also be useful for other discovery mechanisms, such as auto joining
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based on cloud metadata and discovery. Both servers and clients can use this method.
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### Server Coordination
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To ensure Consul servers are joining the cluster properly, you should monitor
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the server coordination. The gossip protocol is used to properly discover all
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the nodes in the cluster. Once the node has joined, the existing cluster
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leader should log something like:
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```text
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[INFO] raft: Added peer 127.0.0.2:8300, starting replication
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last_log_term = 21
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last_snapshot_index = 40966
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last_snapshot_term = 20
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num_peers = 2
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num_peers = 4
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state = Leader
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term = 21
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...
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the possibility of data loss in case the existing servers fail while bringing
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the new servers up-to-date.
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## Removing Servers
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## Manually Remove a Server
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Removing servers must be done carefully to avoid causing an availability outage.
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For a cluster of N servers, at least (N/2)+1 must be available for the cluster
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to function. See this [deployment table](/docs/internals/consensus.html#toc_4).
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If you have 3 servers, and 1 of them is currently failed, removing any servers
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If you have 3 servers and 1 of them is currently failing, removing any other servers
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will cause the cluster to become unavailable.
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To avoid this, it may be necessary to first add new servers to the cluster,
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can handle a node leaving, the actual process is simple. You simply issue a
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`leave` command to the server.
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```sh
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consul leave
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```
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The server leaving should contain logs like:
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```text
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At this point the node has been gracefully removed from the cluster, and
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will shut down.
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If the leader is affected by an outage, then [manual recovery](/docs/guides/outage.html#manual-recovery-using-peers-json) needs to be done.
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To remove all agents that accidentally joined the wrong set of servers, clear out the contents of the data directory (`-data-dir`) on both client and server nodes.
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These graceful methods to remove servres assumse you have a healthly cluster.
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If the cluster has no leader due to loss of quorum or data corruption, you should
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plan for [outage recovery](/docs/guides/outage.html#manual-recovery-using-peers-json).
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!> **WARNING** Removing data on server nodes will destroy all state in the cluster
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## Forced Removal
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## Manual Forced Removal
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In some cases, it may not be possible to gracefully remove a server. For example,
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if the server simply fails, then there is no ability to issue a leave. Instead,
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leave the cluster. However, if this is not a possibility, then the `force-leave` command
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can be used to force removal of a server.
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```sh
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consul force-leave <node>
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```
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This is done by invoking that command with the name of the failed node. At this point,
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the cluster leader will mark the node as having left the cluster and it will stop attempting to replicate.
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## Summary
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In this guide we learned the straightforward process of adding and removing servers including;
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manually adding servers, adding servers through the agent configuration, gracefully removing
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servers, and forcing removal of servers. Finally, we should restate that manually adding servers
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is good for testing purposes, however, for production it is recommended to add servers with
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the agent configuration.
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