# Health checks (/docs/guides/health-checks)



`config.healthCheck` is what tells the host an instance is well. Every instance names one —
there is no default — and it does two jobs: a wake waits for it before handing the caller on,
and liveness is read from it after that.

```json
"healthCheck": {
  "kind": "http",
  "path": "/healthz",
  "intervalMs": 5000,
  "timeoutMs": 2000,
  "gracePeriodMs": 30000,
  "healthyThreshold": 1,
  "unhealthyThreshold": 3
}
```

## The three kinds [#the-three-kinds]

| `kind`           | Healthy means                                                                                                                                      |
| ---------------- | -------------------------------------------------------------------------------------------------------------------------------------------------- |
| `http`           | `path`, requested on `httpPort`, answers 2xx.                                                                                                      |
| `tcp`            | A connection to `httpPort` is accepted. For a port that does not speak HTTP.                                                                       |
| `boot-completed` | The microVM is up and `httpPort` has accepted a connection once. For a guest this host did not build, which answers no path it was not told about. |

`http` and `tcp` carry the timing above: `intervalMs`, `timeoutMs`, `gracePeriodMs`,
`healthyThreshold` and `unhealthyThreshold`. `boot-completed` carries none — the app has 30
seconds to start listening, and nothing is timed after that. An instance that fails its check
`unhealthyThreshold` times in a row is reported `unhealthy`, with why.

<Callout title="Good to know">
  Use `http` when you can. A TCP connect is answered by the guest kernel's accept queue whether
  or not the process behind it will ever read the request, so a program that has stopped
  answering passes `tcp` for as long as it lives.
</Callout>

## Restarts [#restarts]

`config.restartPolicy` is the budget for starting the program again after it exits:

```json
"restartPolicy": {
  "maxRestarts": 5,
  "initialBackoffMs": 500,
  "maxBackoffMs": 30000,
  "backoffFactor": 2,
  "resetAfterMs": 60000
}
```

The wait before each attempt grows from `initialBackoffMs` by `backoffFactor`, capped at
`maxBackoffMs`. A program that stays up for `resetAfterMs` earns the whole budget back. Past
`maxRestarts`, the instance is reported `failed` and is not started again until it is deployed
afresh, under a new `deploymentId`.

[The reference](/docs/reference/desired-state) is every field of a health check and a restart
policy.