111 lines
5.3 KiB
Plaintext
111 lines
5.3 KiB
Plaintext
---
|
|
layout: docs
|
|
page_title: Bootstrapping a Datacenter
|
|
description: >-
|
|
An agent can run in both client and server mode. Server nodes are responsible
|
|
for running the consensus protocol and storing the cluster state. Before a
|
|
Consul cluster can begin to service requests, a server node must be elected
|
|
leader. Thus, the first nodes that are started are generally the server nodes.
|
|
Bootstrapping is the process of joining these server nodes into a cluster.
|
|
---
|
|
|
|
# Bootstrapping a Datacenter
|
|
|
|
An agent can run in either client or server mode. Server nodes are responsible for running the
|
|
[consensus protocol](/docs/internals/consensus) and storing the cluster state.
|
|
The client nodes are mostly stateless and rely heavily on the server nodes.
|
|
|
|
Before a Consul cluster can begin to service requests,
|
|
a server node must be elected leader. Bootstrapping is the process
|
|
of joining these initial server nodes into a cluster. Read the
|
|
[architecture documentation](/docs/internals/architecture) to learn more about
|
|
the internals of Consul.
|
|
|
|
It is recommended to have three or five total servers per datacenter. A single server deployment is _highly_ discouraged
|
|
as data loss is inevitable in a failure scenario. Please refer to the
|
|
[deployment table](/docs/internals/consensus#deployment-table) for more detail.
|
|
|
|
~> **Note**: In versions of Consul prior to 0.4, bootstrapping was a manual process. For details on using the `-bootstrap` flag directly, see the
|
|
[manual bootstrapping documentation](/docs/install/bootstrapping#manually-join-the-servers).
|
|
Manual bootstrapping with `-bootstrap` is not recommended in
|
|
newer versions of Consul (0.5 and newer) as it is more error-prone.
|
|
Instead you should use automatic bootstrapping
|
|
with [`-bootstrap-expect`](/docs/agent/options#_bootstrap_expect).
|
|
|
|
## Bootstrapping the Servers
|
|
|
|
The recommended way to bootstrap the servers is to use the [`-bootstrap-expect`](/docs/agent/options#_bootstrap_expect)
|
|
configuration option. This option informs Consul of the expected number of
|
|
server nodes and automatically bootstraps when that many servers are available. To prevent
|
|
inconsistencies and split-brain (clusters where multiple servers consider
|
|
themselves leader) situations, you should either specify the same value for
|
|
[`-bootstrap-expect`](/docs/agent/options#_bootstrap_expect)
|
|
or specify no value at all on all the servers. Only servers that specify a value will attempt to bootstrap the cluster.
|
|
|
|
Suppose we are starting a three server cluster. We can start `Node A`, `Node B`, and `Node C` with each
|
|
providing the `-bootstrap-expect 3` flag. Once the nodes are started, you should see a warning message in the service output.
|
|
|
|
```log
|
|
[WARN] raft: EnableSingleNode disabled, and no known peers. Aborting election.
|
|
```
|
|
|
|
The warning indicates that the nodes are expecting 2 peers but none are known yet. Below you will learn how to connect the servers so that one can be
|
|
elected leader.
|
|
|
|
## Creating the Cluster
|
|
|
|
You can trigger leader election by joining the servers together, to create a cluster. You can either configure the nodes to join automatically or manually.
|
|
|
|
### Automatically Join the Servers
|
|
|
|
There are multiple options for joining the servers. Choose the method which best suits your environment and specific use case.
|
|
|
|
- Specify a list of servers with
|
|
[-join](/docs/agent/options#_join) and
|
|
[start_join](/docs/agent/options#start_join)
|
|
options.
|
|
- Specify a list of servers with [-retry-join](/docs/agent/options#_retry_join) option.
|
|
- Use automatic joining by tag for supported cloud environments with the [-retry-join](/docs/agent/options#_retry_join) option.
|
|
|
|
All three methods can be set in the agent configuration file or
|
|
the command line flag.
|
|
|
|
### Manually Join the Servers
|
|
|
|
To manually create a cluster, you should connect to one of the servers
|
|
and run the `consul join` command.
|
|
|
|
```shell-session
|
|
$ consul join <Node A Address> <Node B Address> <Node C Address>
|
|
Successfully joined cluster by contacting 3 nodes.
|
|
```
|
|
|
|
Since a join operation is symmetric, it does not matter which node initiates it. Once the join is successful, one of the nodes will output something like:
|
|
|
|
```log
|
|
[INFO] consul: adding server foo (Addr: 127.0.0.2:8300) (DC: dc1)
|
|
[INFO] consul: adding server bar (Addr: 127.0.0.1:8300) (DC: dc1)
|
|
[INFO] consul: Attempting bootstrap with nodes: [127.0.0.3:8300 127.0.0.2:8300 127.0.0.1:8300]
|
|
...
|
|
[INFO] consul: cluster leadership acquired
|
|
```
|
|
|
|
### Verifying the Cluster and Connect the Clients
|
|
|
|
As a sanity check, the [`consul info`](/commands/info) command
|
|
is a useful tool. It can be used to verify the `raft.num_peers`
|
|
and to view the latest log index under `raft.last_log_index`. When
|
|
running [`consul info`](/commands/info) on the followers, you
|
|
should see `raft.last_log_index` converge to the same value once the
|
|
leader begins replication. That value represents the last log entry that
|
|
has been stored on disk.
|
|
|
|
Now that the servers are all started and replicating to each other, you can
|
|
join the clients with the same join method you used for the servers.
|
|
Clients are much easier as they can join against any existing node. All nodes participate in a gossip
|
|
protocol to perform basic discovery, so once joined to any member of the
|
|
cluster, new clients will automatically find the servers and register
|
|
themselves.
|
|
|
|
-> **Note:** It is not strictly necessary to start the server nodes before the clients; however, most operations will fail until the servers are available.
|