# Vert.x Health Checks

This component provides a simple way to expose health checks.

Health checks are used to express the current state of the application in very simple terms: UP or DOWN. The health checks can be used individually, or in combination to Vert.x Web or the event bus.

This component provides a Vert.x Web handler on which you can register procedure testing the health of the application. The handler computes the final state and returns the result as JSON.

## Using Vert.x Health Checks

Notice that you generally need Vert.x Web to use this component. In addition add the following dependency:

- Maven (in your `pom.xml`):

    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-health-check</artifactId>
      <version>3.4.2</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    compile 'io.vertx:vertx-health-check:3.4.2'

### Creating the health check object.

The central object is `HealthChecks`. You can create a new instance using:

    HealthChecks hc = HealthChecks.create(vertx);

    hc.register("my-procedure", future -> future.complete(Status.OK()));

Once you have created this object you can register and unregister procedures. See more about this below.

### Registering the Vert.x Web handler

To create the Vert.x Web handler managing your health check you can either:

- using an existing instance of `HealthChecks`

- let the handler create one instance for you.

    HealthCheckHandler healthCheckHandler1 = HealthCheckHandler.create(vertx);
    HealthCheckHandler healthCheckHandler2 = HealthCheckHandler.createWithHealthChecks(HealthChecks.create(vertx));

    Router router = Router.router(vertx);
    // Populate the router with routes...
    // Register the health check handler
    router.get("/health*").handler(healthCheckHandler1);
    // Or
    router.get("/ping*").handler(healthCheckHandler2);

Procedure registration can be directly made on the `HealthCheckHandler` instance. Alternatively, if you have created the `HealthChecks` instance beforehand, you can register the procedure on this object directly. Registrations and unregistrations can be done at anytime, even after the route registration:

    HealthCheckHandler healthCheckHandler = HealthCheckHandler.create(vertx);

    // Register procedures
    // It can be done after the route registration, or even at runtime
    healthCheckHandler.register("my-procedure-name", future -> {
      // Do the check ....
      // Upon success do
      future.complete(Status.OK());
      // In case of failure do:
      future.complete(Status.KO());
    });

    router.get("/health").handler(healthCheckHandler);

## Procedures

A procedure is a function checking some aspect of the system to deduce the current health. It reports a `Status` indicating whether or not the test has passed or failed. This function must not block and report to the given `Future` whether or not it succeed.

When you register a procedure, you give a name, and the function (handler) executing the check.

Rules deducing the status are the following

- if the future is mark as failed, the check is considered as KO

- if the future is completed successfully but without a `Status`, the check is considered as OK.

- if the future is completed successfully with a `Status` marked as OK, the check is considered as OK.

- if the future is completed successfully with a `Status` marked as KO, the check is considered as KO.

`Status` can also provide additional data:

    HealthCheckHandler healthCheckHandler = HealthCheckHandler.create(vertx);

    // Status can provide addition data provided as JSON
    healthCheckHandler.register("my-procedure-name", future -> {
      future.complete(Status.OK(new JsonObject().put("available-memory", "2mb")));
    });

    healthCheckHandler.register("my-second-procedure-name", future -> {
      future.complete(Status.KO(new JsonObject().put("load", 99)));
    });

    router.get("/health").handler(healthCheckHandler);

Procedures can be organised by groups. The procedure name indicates the group. The procedures are organized as a tree and the structure is mapped to HTTP urls (see below).

    HealthCheckHandler healthCheckHandler = HealthCheckHandler.create(vertx);

    // Register procedures
    // Procedure can be grouped. The group is deduced using a name with "/".
    // Groups can contains other group
    healthCheckHandler.register("a-group/my-procedure-name", future -> {
      //....
    });
    healthCheckHandler.register("a-group/a-second-group/my-second-procedure-name", future -> {
      //....
    });

    router.get("/health").handler(healthCheckHandler);

## HTTP responses and JSON Output

When using the Vert.x web handler, the overall health check is retrieved using a HTTP GET or POST (depending on the route you registered) on the route given when exposing the `HealthCheckHandler`.

If no procedure are registered, the response is `204 - NO CONTENT`, indicating that the system is UP but no procedures has been executed. The response does not contain a payload.

If there is at least one procedure registered, this procedure is executed and the outcome status is computed. The response would use the following status code:

- `200` : Everything is fine

- `503` : At least one procedure has reported a non-healthy state

- `500` : One procedure has thrown an error or has not reported a status in time

The content is a JSON document indicating the overall result (`outcome`). It’s either `UP` or `DOWN`. A `checks` array is also given indicating the result of the different executed procedures. If the procedure has reported additional data, the data is also given:

    {
     "checks" : [
     {
       "id" : "A",
       "status" : "UP"
     },
     {
       "id" : "B",
       "status" : "DOWN",
       "data" : {
         "some-data" : "some-value"
       }
     }
     ],
     "outcome" : "DOWN"
    }

In case of groups/ hierarchy, the `checks` array depicts this structure:

    {
     "checks" : [
     {
       "id" : "my-group",
       "status" : "UP",
       "checks" : [
       {
         "id" : "check-2",
         "status" : "UP",
       },
       {
         "id" : "check-1",
         "status" : "UP"
       }]
     }],
     "outcome" : "UP"
    }

If a procedure throws an error, reports a failure (exception), the JSON document provides the `cause` in the `data` section. If a procedure does not report back before a timeout, the indicated cause is `Timeout`.

## Examples of procedures

This section provides example of common health checks.

### JDBC

This check reports whether or not a connection to the database can be established:

    handler.register("database",
      future -> jdbcClient.getConnection(connection -> {
        if (connection.failed()) {
          future.fail(connection.cause());
        } else {
          connection.result().close();
          future.complete(Status.OK());
        }
      }));

### Service availability

This check reports whether or not a service (here a HTTP endpoint) is available in the service discovery:

    handler.register("my-service",
      future -> HttpEndpoint.getClient(discovery,
        (rec) -> "my-service".equals(rec.getName()),
        client -> {
          if (client.failed()) {
            future.fail(client.cause());
          } else {
            client.result().close();
            future.complete(Status.OK());
          }
        }));

### Event bus

This check reports whether a consumer is ready on the event bus. The protocol, in this example, is a simple ping/pong, but it can be more sophisticated. This check can be used to check whether or not a verticle is ready if it’s listening on a specific event address.

    handler.register("receiver",
      future ->
        vertx.eventBus().send("health", "ping", response -> {
          if (response.succeeded()) {
            future.complete(Status.OK());
          } else {
            future.complete(Status.KO());
          }
        })
    );

## Authentication

When using the Vert.x web handler, you can pass a `AuthProvider` use to authenticate the request. Check <a href="http://vertx.io/docs/#authentication_and_authorisation">Vert.x Auth</a> for more details about available authentication providers.

The Vert.x Web handler creates a JSON object containing:

- the request headers

- the request params

- the form param if any

- the content as JSON if any and if the request set the content type to `application/json`.

The resulting object is passed to the auth provider to authenticate the request. If the authentication failed, it returns a `403 - FORBIDDEN` response.

## Exposing health checks on the event bus

While exposing the health checks using HTTP with the Vert.x web handler is convenient, it can be useful to expose the data differently. This section gives an example to expose the data on the event bus:

    vertx.eventBus().consumer("health",
      message -> healthChecks.invoke(message::reply));