# Reliability

Cerbos Hub is designed for high availability. All PDPs continue to operate independently even when Cerbos Hub experiences disruptions.

## Push-based updates

When a PDP connects to Cerbos Hub, it establishes a two-way communication channel used to receive the initial policy bundle and subsequent update notifications. Because there is no polling, all PDPs converge on the same policy version within seconds of a change.

## Disconnection handling

If Cerbos Hub becomes temporarily unavailable:

|     |     |
| --- | --- |
| Running PDPs | Continue serving requests using the last downloaded bundle while attempting to reconnect in the background. Authorization decisions are unaffected. |
| New PDPs | Can start with the last successfully built bundle, served from a separate high-availability fallback service. |

## Local bundle caching

For additional resilience, configure a cache directory to persist bundles to disk:

```yaml
storage:
  driver: hub
  hub:
    remote:
      deploymentID: "..."
      cacheDir: /var/cerbos/hub # Directory to cache downloaded bundles
```

Mount a persistent volume at this path when running in containers or Kubernetes. Cached bundles allow PDPs to restart without network access to Cerbos Hub.

## Offline mode

In disaster recovery scenarios, start the PDP with only the cached bundle:

```shell
docker run --rm --name cerbos 
  -p 3592:3592 -p 3593:3593 
  -e CERBOS_HUB_OFFLINE=true 
  -v /var/cerbos/hub:/var/cerbos/hub 
  ghcr.io/cerbos/cerbos:latest server --config=/conf/.cerbos.yaml
```

The PDP loads the cached bundle and serves requests without connecting to Cerbos Hub.

## Fallback to git

As a last resort, switch the PDP to read policies directly from your Git repository:

```yaml
storage:
  driver: git
  git:
    protocol: https
    url: https://github.com/your-org/policies.git
    branch: main
    checkoutDir: /tmp/cerbos/policies
    updatePollInterval: 60s
```

This bypasses Cerbos Hub entirely, though you lose pre-compilation, testing, and centralized management.

## Monitoring connectivity

Monitor PDP connectivity using the `cerbos_dev_hub_connected` Prometheus metric:

|     |     |
| --- | --- |
| `1` | PDP is connected to Cerbos Hub |
| `0` | PDP is disconnected (using cached bundle) |

Additional metrics for bundle operations:

|     |     |
| --- | --- |
| `cerbos_dev_store_bundle_updates_count` | Number of bundle updates received from Cerbos Hub |
| `cerbos_dev_store_bundle_op_latency` | Time to perform bundle operations |
| `cerbos_dev_store_bundle_fetch_errors_count` | Count of errors during bundle downloads |

See [Observability](https://docs.cerbos.dev/cerbos/latest/configuration/observability) for the full list of available metrics.