dropwizard-health

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dropwizard-health

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Provides a health check implementation that performs ongoing monitoring of an application's dependencies and includes an endpoint that can be called by a load balancer to determine if the application is healthy and thus able to receive traffic.

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> ## ℹ️ dropwizard-health has been promoted to a core module and is part of the Dropwizard main project since [Dropwizard 2.1.0](https://github.com/dropwizard/dropwizard/releases/tag/v2.1.0).

# dropwizard-health
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Provides a health check implementation that performs ongoing monitoring of an application's dependencies and includes
an endpoint that can be called by a load balancer to determine if the application is healthy and thus able to receive
traffic.

## Dropwizard Version Support Matrix
dropwizard-health | Dropwizard v1.3.x  | Dropwizard v2.0.x
----------------- | ------------------ | ------------------
v1.3.x            | :white_check_mark: | :white_check_mark:
v1.4.x            | :white_check_mark: | :white_check_mark:
v1.5.x            | :white_check_mark: | :white_check_mark:
v1.6.x            | :white_check_mark: | :white_check_mark:
v1.7.x            | :white_check_mark: | :white_check_mark:

## Usage
Add dependency on library.
```xml
<dependency>
  <groupId>io.dropwizard.modules</groupId>
  <artifactId>dropwizard-health</artifactId>
  <version>${dropwizard-health.version}</version>
</dependency>
```

### Health Checks
In your application's `Configuration` class, add a `HealthConfiguration` object:
```java
public class ExampleConfiguration extends Configuration {
    ...

@Valid
    @NotNull
    @JsonProperty("health")
    private HealthConfiguration healthConfiguration = new HealthConfiguration();

public HealthConfiguration getHealthConfiguration() {
        return healthConfiguration;
    }

public void setHealthConfiguration(final HealthConfiguration healthConfiguration) {
        this.healthConfiguration = healthConfiguration;
    }
}
```

Add a `HealthCheckBundle` to the `Bootstrap` object in your `initialize` method:
```java
bootstrap.addBundle(new HealthCheckBundle<ExampleConfiguration>() {
    @Override
    protected HealthConfiguration getHealthConfiguration(final ExampleConfiguration configuration) {
        return configuration.getHealthConfiguration();
    }
});
```

Configure health checks for any dependencies your application has, such as any databases, caches, queues, etc.
Dropwizard modules that support connecting to a dependency will often register a health check automatically.

Define the following health check configurations in your `config.yml` file:
```yaml
health:
  delayedShutdownHandlerEnabled: true
  shutdownWaitPeriod: 30s
  healthChecks:
    - name: UserDatabase
      critical: true
    - name: UserNotificationTopic
      critical: false
      schedule:
        checkInterval: 2500ms
        downtimeInterval: 10s
        failureAttempts: 2
        successAttempts: 1
    - name: UserCache
      critical: false
```

## Configuration Reference
### Health Configuration
Name | Default | Description
---- | ------- | -----------
delayedShutdownHandlerEnabled | true | Flag indicating whether to delay shutdown to allow already processing requests to complete.
shutdownWaitPeriod | 15 seconds | Amount of time to delay shutdown by to allow already processing requests to complete. Only applicable if `delayedShutdownHandlerEnabled` is true.
healthCheckUrlPaths | \["/health-check"\] | URLs to expose the app's health check on.
healthChecks | [] | A list of configured health checks. See the [Health Check Configuration section](#health-check-configuration) for more details.
initialOverallState | true | Flag indicating whether the overall health state of the application should start as healthy or unhealthy. A value of `true` indicates an initial state of healthy while a value of `false` indicates an initial state of unhealthy.
servlet | default health servlet | The health servlet that is used to generate health check responses. See the [Default Health Servlet section](#default-health-servlet) for more details.

### Health Check Configuration
Name | Default | Description
---- | ------- | -----------
name | (none) | The name of this health check. This must be unique.
type | ready | The type of this health check. This is either `alive` or `ready`. See the [Application Status section](#application-status) for more details.
critical | false | Flag indicating whether this dependency is critical to determine the health of the application. If `true` and this dependency is unhealthy, the application will also be marked as unhealthy.
initialState | true | Flag indicating the initial state to use for this health check. A value of `true` indicates an initial state of healthy while a value of `false` indicates an initial state of unhealthy.
schedule | (none) | The schedule that this health check will be run on. See the [Schedule section](#schedule) for more details.

### Schedule
Name | Default | Description
---- | ------- | -----------
checkInterval | 5 seconds | The interval on which to perform a health check for this dependency while the dependency is in a healthy state.
downtimeInterval | 30 seconds | The interval on which to perform a health check for this dependency while the dependency is in an unhealthy state.
initialDelay | the value of `checkInterval` | The initial delay to use when first scheduling the health check.
failureAttempts | 3 | The threshold of consecutive failed attempts needed to mark a dependency as unhealthy (from a healthy state).
successAttempts | 2 | The threshold of consecutive successful attempts needed to mark a dependency as healthy (from an unhealthy state).

### Default Health Servlet
Name | Default | Description
---- | ------- | -----------
cacheControlEnabled | true | Flag controlling whether a `Cache-Control` header will be included in the health check response or not. Set header value using `cacheControlValue`.
cacheControlValue | "no-store" | The value to be set in the `Cache-Control` header in the health check response. Only used if `cacheControlEnabled` is set to `true`.
contentType | application/json | The value of the `Content-Type` header in the health check response.
healthyValue | {"status":"healthy"} | The value of the body of the health check response when the application is healthy.
unhealthyValue | {"status":"unhealthy"} | The value of the body of the health check response when the application is unhealthy.

## Application Status
There are two types of status that are supported: Alive and Ready

### alive
An `alive` status indicates the application is operating normally and does not need to be restarted to recover from a
stuck state.

Long-running applications can eventually reach a broken state and cannot recover except by being restarted
(e.g. deadlocked threads).

### ready
A `ready` status indicates the application is ready to serve traffic.

Applications can temporarily be unable to serve traffic due to a variety of reasons, for example, an application might
need to build/compute large caches during startup or can critically depend on an external service.

### Query Application Status
https://<hostname>:<port>/health-check?type=<type> (replace `<type>` with `ready` or `alive`; defaults to `ready`)

Healthy
```bash
$ curl -v https://<hostname>:<port>/health-check?type=ready
> GET /health-check?type=ready HTTP/1.1
...
>
< HTTP/1.1 200 OK
< Content-Type: application/json
< Cache-Control: no-store
...
<
{"status": "healthy"}
```

Not Healthy
```bash
$ curl -v https://<hostname>:<port>/health-check?type=ready
> GET /health-check?type=ready HTTP/1.1
...
>
< HTTP/1.1 503 Service Unavailable
< Content-Type: application/json
< Cache-Control: no-store
...
<
{"status": "unhealthy"}
```

## HTTP and TCP Health Checks
Should your service have any dependencies that it needs to perform health checks against that expose either an HTTP or TCP health check interface,
you can use the `HttpHealthCheck` or `TcpHealthCheck` classes to do so easily.

### Usage
You will need to register your health check(s) in your `Application` class `run()` method.

#### HTTP
```java
@Override
public void run(final AppConfiguration configuration, final Environment environment) {
    ...
    environment.healthChecks().register("http-service-dependency", new HttpHealthCheck("http://some-http-dependency.com:8080/health-check"));
}
```
#### TCP
```java
@Override
public void run(final AppConfiguration configuration, final Environment environment) {
    ...
    environment.healthChecks().register("tcp-service-dependency", new TcpHealthCheck("some-tcp-dependency.com", 443));
}
```

## Composite Health Checks
You might find you need a health check that is a composite of more than one other health check. For instance, consider the case where you have
a database and a cache, and if only one of those two are unhealthy, your service can still fulfill a subset of functionality, and thus
should not necessarily be marked down.

Let's say that you have a database health check registered under the name `UserDatabase` and a cache health check under the name `UserCache`.
Below, see an example of how you might create a health check that is a composite of these two checks. Note that there does not exist a
`CompositeHealthCheck` class currently, but it might be a nice addition to this library.

#### Example Composite Health Check
```java
private void registerCompositeHealthCheck(final HealthCheckRegistry healthChecks) {
    final HealthCheck databaseCheck = healthChecks.getHealthCheck("UserDatabase");
    final HealthCheck cacheCheck = healthChecks.getHealthCheck("UserCache");

final UserHealthCheck userHealthCheck = new UserHealthCheck(databaseCheck, cacheCheck);
    healthChecks.register("UserComposite", userHealthCheck);
}
```

#### Example Composite Health Check YAML
```yaml
health:
  delayedShutdownHandlerEnabled: true
  shutdownWaitPeriod: 30s
  healthChecks:
    - name: UserComposite
      critical: true
```

## Future improvements:
* Hooks for health status change events.
* The ability to expose health check data to other modules (for instance, an Admin page may want to show historical health check results).
* More out-of-the-box generally useful health checks implementations, like file-system health checks.

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