# High Availability

High Availability (HA) mode solves for horizontal scalability and automatic failover
within a single region. When in HA mode, Coder continues using a single Postgres
endpoint. [GCP](https://cloud.google.com/sql/docs/postgres/high-availability), [AWS](https://docs.aws.amazon.com/prescriptive-guidance/latest/saas-multitenant-managed-postgresql/availability.html),
and other cloud vendors offer fully-managed HA Postgres services that pair
nicely with Coder.

For Coder to operate correctly, every node must be within 10ms of each other
and Postgres. We make a best-effort attempt to warn the user when inter-Coder
latency is too high, but if requests start dropping, this is one metric to investigate.
Note that this latency requirement applies _only_ to Coder services. Coder will
operate correctly even with few seconds of latency on
workspace <-> Coder and user <-> Coder connections.

## Setup

Coder automatically enters HA mode when multiple instances simultaneously connect
to the same Postgres endpoint.

HA brings one configuration variable to set in each Coder
node: `CODER_DERP_SERVER_RELAY_URL`. The HA nodes use these URLs to communicate
with each other. Inter-node communication is only required while using the
embedded relay (default). If you're using [custom relays](../networking/index.md#custom-relays), Coder ignores `CODER_DERP_SERVER_RELAY_URL` since Postgres is the sole rendezvous for the Coder nodes.

`CODER_DERP_SERVER_RELAY_URL` will never be `CODER_ACCESS_URL` because
`CODER_ACCESS_URL` is a load balancer to all Coder nodes.

Here's an example 3-node network configuration setup:

| Name      | `CODER_ADDRESS` | `CODER_DERP_SERVER_RELAY_URL` | `CODER_ACCESS_URL`       |
| --------- | --------------- | ----------------------------- | ------------------------ |
| `coder-1` | `*:80`          | `http://10.0.0.1:80`          | `https://coder.big.corp` |
| `coder-2` | `*:80`          | `http://10.0.0.2:80`          | `https://coder.big.corp` |
| `coder-3` | `*:80`          | `http://10.0.0.3:80`          | `https://coder.big.corp` |

## Kubernetes

If you installed Coder via
[our Helm Chart](../install/kubernetes.md#install-coder-with-helm), just
increase `coder.replicaCount` in `values.yaml`.

If you installed Coder into Kubernetes by some other means, insert the relay URL
via the environment like so:

```yaml
env:
  - name: POD_IP
    valueFrom:
      fieldRef:
        fieldPath: status.podIP
  - name: CODER_DERP_SERVER_RELAY_URL
    value: http://$(POD_IP)
```

Then, increase the number of pods.

## Up next

- [Networking](../networking/index.md)
- [Kubernetes](../install/kubernetes.md)
- [Enterprise](../enterprise.md)