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54 changes: 34 additions & 20 deletions README.md
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# Spring Boot Async Health Indicator

Async Health Indicator for [spring-boot-actuator](https://docs.spring.io/spring-boot/docs/current/reference/html/actuator.html) >=2.2.0 gives [Health Indicator](https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/actuate/health/HealthIndicator.html) the ability to get refreshed on a background [ScheduledThreadPoolExecutor](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledThreadPoolExecutor.html) using the annotation `@AsyncHealth`.
Async Health Indicator for [spring-boot-actuator](https://docs.spring.io/spring-boot/docs/current/reference/html/actuator.html) >=2.2.0 gives [Health Indicator](https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/actuate/health/HealthIndicator.html) the ability to get refreshed asynchronously on a background ThreadPoolExecutor using the annotation `@AsyncHealth`.

##### When annotating a `HealthIndicator` with `@AsyncHealth`:

The `health()` method is invoked on application startup and with the given delay (in second) between the termination of one execution and the commencement of the next.
The `health()` method is invoked on application startup and with the configured `refreshRate` between the termination of one execution and the commencement of the next.
Calling the `/health` endpoint will not invoke the `health()` method but return the last `Health` calculated asynchronously. *(`Status.UNKNOWN` is returned if the `/health` endpoint is called before the first `HealthIndicator.health()` check is completed (likely to occur on application startup))*

The last `Health` calculated will be returned when calling the `/health` endpoint.
The duration of the `health()` method is monitored. If it exceeds the configured `timeout`, any call to `/health` will return this HealthIndicator as `DOWN` until the next `health()` method completion.

The following details are added to the `Health`:
The following details are added to the pre-existing `Health` details:

- `lastChecked` (`LocalDateTime`): Defines when the health was calculated.
- `lastChecked` (`LocalDateTime`): Defines when that last `Health` started execution.
- `lastDuration` (Duration in ms): Defines how long it took to calculate that last `Health`.
- `reason` (enum): Added only if the Health Indicator is marked`DOWN`: `Exception` or `Timeout`.

## Advantages over synchronous Indicators

- Expensive Health Indicators that do not have to be calculated every time `/health` is called can run on their own schedule.
- The call to `/health` is now super fast as it returns pre-calculated `Health`.
- The call to `/health` is instantaneous as it returns pre-calculated `Health`.
- Multiple Health Indicators can run in parallel.
- Timeouts can be controlled per Health Indicator.

## Usage

Expand All @@ -30,40 +35,49 @@ This module is auto-configured.
<dependency>
<groupId>com.teketik</groupId>
<artifactId>async-health-indicator</artifactId>
<version>boot2-v1.0</version>
<version>boot2-v1.1</version>
</dependency>
```
- Annotate any `HealthIndicator` with `@AsyncHealth($REFRESH_RATE)`
- Annotate any `HealthIndicator` with `@AsyncHealth(refreshRate = $REFRESH_RATE, timeout = $TIMEOUT)`

`$REFRESH_RATE` = Fixed delay in seconds between the termination of the `health()` execution and the commencement of the next (default 1).

`$REFRESH_RATE` = Fixed delay in seconds between the termination of the `health()` execution and the commencement of the next (default 1)
`$TIMEOUT`= The maximum time in seconds that the `health()` execution can take before being considered `DOWN` (default 10).

## Regarding Timeout

When a `health()` method duration exceeds the configured `timeout`, the thread running it is `interrupted`with the hope that the method will fail with an exception (causing it to be `DOWN`) and free up the thread.
Unfortunately, most I/O calls are not interruptible and the thread may continue to execute the method until it times out (according to the libraries and configuration used).
If that happens, you will observe the `timeout` error message printed for each `/health` hit until that method times out like:
```
ERROR AsyncHealthIndicator : HealthIndicator[name=myIndicator] is taking too long to execute [duration=2121ms][timeout=2s]
ERROR AsyncHealthIndicator : HealthIndicator[name=myIndicator] is taking too long to execute [duration=3189ms][timeout=2s]
```

## ScheduledThreadPool Configuration
It is therefore recommended to ensure that your `health()` methods can time out naturally within an acceptable window (matching the configured `timeout`)

## ThreadPool Configuration

| Property | Description | Default |
| -------- | ----------- | ------- |
| `management.health.async.pool-size` | Number of threads calculating the `health()` methods. (In case you have multiple long-running `health()` methods, it is advised to increase this number so that those indicators may run in parallel and not block others). | 1 |
| `management.health.async.pool.max-size` | Number of maximum threads calculating the `health()` methods. (Note that this max will likely be reached on application startup when all indicators are starting up but will likely size down when different durations allow threads to be reused more efficiently). | 10 |
| `management.health.async.pool.keep-alive` | Maximum time that excess idle threads will wait for new tasks before terminating.| 10 |


## Logging


- On application startup, all cached health indicators are logged under logger `com.teketik.spring.health.AsyncHealthIndicatorAutoConfiguration` as `INFO`:

*Example*: `Using a AsyncHealthIndicator for myIndicator with [refreshRate=2s]`
*Example*: `Initializing AsyncHealthIndicator[name=myIndicator][refreshRate=3s][timeout=2s]`

- All `@AsyncHealth` annotated `HealthIndicator`s have details logged under logger `com.teketik.spring.health.AsyncHealthIndicator` as `TRACE`:
- All `@AsyncHealth` annotated `HealthIndicator`s have details logged under logger `com.teketik.spring.health.AsyncHealthIndicator` as `DEBUG`:

*Example*: `myIndicator computed in 247ms is UP {detailKey=detailValue}`
*Example*: `HealthIndicator[name=myIndicator][duration=147ms][status=UP {detailKey=detailValue}]`


## Limitations

Only implementations of `HealthIndicator` are currently supported. `Composites` are not. (Let me know if you need that!).

## Notes

- `HealthIndicator` will return `Status.UNKNOWN` if the `/health` endpoint is called before the first `HealthIndicator.health()` check is completed. (likely to occur on application startup).

- There is no built-in timeout system. Make sure that your `health()` methods do timeout or they may block every other health checks (if the pool has less threads than you have indicators).


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