# Vert.x Service Discovery

This component provides an infrastructure to publish and discover various resources, such as service proxies, HTTP endpoints, data sources…​

These resources are called `services`. A `service` is a discoverable functionality. It can be qualified by its type, metadata, and location. So a `service` can be a database, a service proxy, a HTTP endpoint and any other resource you can imagine as soon as you can describe it, discover it and interact with it. It does not have to be a vert.x entity, but can be anything. Each service is described by a `Record`.

The service discovery implements the interactions defined in service-oriented computing. And to some extent, also provides the dynamic service-oriented computing interactions. So, applications can react to arrival and departure of services.

A service provider can:

- publish a service record

- un-publish a published record

- update the status of a published service (down, out of service…​)

A service consumer can:

- lookup services

- bind to a selected service (it gets a `ServiceReference`) and use it

- release the service once the consumer is done with it

- listen for arrival, departure and modification of services.

Consumer would 1) lookup a service record matching their need, 2) retrieve the `ServiceReference` that give access to the service, 3) get a service object to access the service, 4) release the service object once done.

The process can be simplified using service type where you can directly retrieve the service object if you know from which type it is (JDBC client, Http client…​).

As stated above, the central piece of information shared by the providers and consumers are `records`.

Providers and consumers must create their own `ServiceDiscovery` instance. These instances are collaborating in the background (distributed structure) to keep the set of services in sync.

The service discovery supports bridges to import and export services from / to other discovery technologies.

## Using the service discovery

To use the Vert.x service discovery, add the following dependency to the dependencies section of your build descriptor:

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

    <dependency>
    <groupId>io.vertx</groupId>
    <artifactId>vertx-service-discovery</artifactId>
    <version>3.4.1</version>
    </dependency>

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

    compile 'io.vertx:vertx-service-discovery:3.4.1'

## Overall concepts

The discovery mechanism is based on a few concepts explained in this section.

### Service records

A service `Record` is an object that describes a service published by a service provider. It contains a name, some metadata, a location object (describing where is the service). This record is the only object shared by the provider (having published it) and the consumer (retrieve it when doing a lookup).

The metadata and even the location format depend on the `service type` (see below).

A record is published when the provider is ready to be used, and withdrawn when the service provider is stopping.

### Service Provider and publisher

A service provider is an entity providing a service. The publisher is responsible for publishing a record describing the provider. It may be a single entity (a provider publishing itself) or a different entity.

### Service Consumer

Service consumers search for services in the service discovery. Each lookup retrieves `0..n` `Record`. From these records, a consumer can retrieve a `ServiceReference`, representing the binding between the consumer and the provider. This reference allows the consumer to retrieve the service object (to use the service), and release the service.

It is important to release service references to cleanup the objects and update the service usages.

### Service object

The service object is the object that gives access to a service. It can come in various forms, such as a proxy, a client, and may even be non-existent for some service types. The nature of the service object depends on the service type.

Notice that because of the polyglot nature of Vert.x, the service object can differ if you retrieve it from Java, Groovy or another language.

### Service types

Services are just resources, and there are a lot of different kinds of services. They can be functional services, databases, REST APIs, and so on. The Vert.x service discovery has the concept of service types to handle this heterogeneity. Each type defines:

- how the service is located (URI, event bus address, IP / DNS…​) - location

- the nature of the service object (service proxy, HTTP client, message consumer…​) - client

Some service types are implemented and provided by the service discovery component, but you can add your own.

### Service events

Every time a service provider is published or withdrawn, an event is fired on the event bus. This event contains the record that has been modified.

In addition, in order to track who is using who, every time a reference is retrieved with `getReference` or released with `release`, events are emitted on the event bus to track the service usages.

More details on these events below.

### Backend

The service discovery uses a Vert.x distributed data structure to store the records. So, all members of the cluster have access to all the records. This is the default backend implementation. You can implement your own by implementing the `ServiceDiscoveryBackend` SPI. For instance, we provide an implementation based on Redis.

Notice that the discovery does not require Vert.x clustering. In single-node mode, the structure is local. It can be populated with `ServiceImporter`s.

## Creating a service discovery instance

Publishers and consumers must create their own `ServiceDiscovery` instance to use the discovery infrastructure:

    ServiceDiscovery discovery = ServiceDiscovery.create(vertx);

    // Customize the configuration
    discovery = ServiceDiscovery.create(vertx,
        new ServiceDiscoveryOptions()
            .setAnnounceAddress("service-announce")
            .setName("my-name"));

    // Do something...

    discovery.close();

By default, the announce address (the event bus address on which service events are sent is: `vertx.discovery .announce`. You can also configure a name used for the service usage (see section about service usage).

When you don’t need the service discovery object anymore, don’t forget to close it. It closes the different discovery importers and exporters you have configured and releases the service references.

You should avoid sharing the service discovery instance, so service usage would represent the right "usages".

## Publishing services

Once you have a service discovery instance, you can publish services. The process is the following:

1. create a record for a specific service provider

2. publish this record

3. keep the published record that is used to un-publish a service or modify it.

To create records, you can either use the `Record` class, or use convenient methods from the service types.

    Record record = new Record()
        .setType("eventbus-service-proxy")
        .setLocation(new JsonObject().put("endpoint", "the-service-address"))
        .setName("my-service")
        .setMetadata(new JsonObject().put("some-label", "some-value"));

    discovery.publish(record, ar -> {
      if (ar.succeeded()) {
        // publication succeeded
        Record publishedRecord = ar.result();
      } else {
        // publication failed
      }
    });

    // Record creation from a type
    record = HttpEndpoint.createRecord("some-rest-api", "localhost", 8080, "/api");
    discovery.publish(record, ar -> {
      if (ar.succeeded()) {
        // publication succeeded
        Record publishedRecord = ar.result();
      } else {
        // publication failed
      }
    });

It is important to keep a reference on the returned records, as this record has been extended by a `registration id`.

## Withdrawing services

To withdraw (un-publish) a record, use:

    discovery.unpublish(record.getRegistration(), ar -> {
      if (ar.succeeded()) {
        // Ok
      } else {
        // cannot un-publish the service, may have already been removed, or the record is not published
      }
    });

## Looking for services

This section explains the low-level process to retrieve services, each service type provide convenient method to aggregates the different steps.

On the consumer side, the first thing to do is to lookup for records. You can search for a single record or all the matching ones. In the first case, the first matching record is returned.

Consumer can pass a filter to select the service. There are two ways to describe the filter:

1. A function taking a `Record` as parameter and returning a boolean (it’s a predicate)

2. This filter is a JSON object. Each entry of the given filter is checked against the record. All entries must exactly match the record. The entry can use the special `*` value to denote a requirement on the key, but not on the value.

Let’s see an example of a JSON filter:

    { "name" = "a" } => matches records with name set to "a"
    { "color" = "*" } => matches records with "color" set
    { "color" = "red" } => only matches records with "color" set to "red"
    { "color" = "red", "name" = "a"} => only matches records with name set to "a", and color set to "red"

If the JSON filter is not set (`null` or empty), it accepts all records. When using functions, to accept all records, you must return true regardless the record.

Here are some examples:

    discovery.getRecord(r -> true, ar -> {
      if (ar.succeeded()) {
        if (ar.result() != null) {
          // we have a record
        } else {
          // the lookup succeeded, but no matching service
        }
      } else {
        // lookup failed
      }
    });

    discovery.getRecord((JsonObject) null, ar -> {
      if (ar.succeeded()) {
        if (ar.result() != null) {
          // we have a record
        } else {
          // the lookup succeeded, but no matching service
        }
      } else {
        // lookup failed
      }
    });

    // Get a record by name
    discovery.getRecord(r -> r.getName().equals("some-name"), ar -> {
      if (ar.succeeded()) {
        if (ar.result() != null) {
          // we have a record
        } else {
          // the lookup succeeded, but no matching service
        }
      } else {
        // lookup failed
      }
    });

    discovery.getRecord(new JsonObject().put("name", "some-service"), ar -> {
      if (ar.succeeded()) {
        if (ar.result() != null) {
          // we have a record
        } else {
          // the lookup succeeded, but no matching service
        }
      } else {
        // lookup failed
      }
    });

    // Get all records matching the filter
    discovery.getRecords(r -> "some-value".equals(r.getMetadata().getString("some-label")), ar -> {
      if (ar.succeeded()) {
        List<Record> results = ar.result();
        // If the list is not empty, we have matching record
        // Else, the lookup succeeded, but no matching service
      } else {
        // lookup failed
      }
    });

    discovery.getRecords(new JsonObject().put("some-label", "some-value"), ar -> {
      if (ar.succeeded()) {
        List<Record> results = ar.result();
        // If the list is not empty, we have matching record
        // Else, the lookup succeeded, but no matching service
      } else {
        // lookup failed
      }
    });

You can retrieve a single record or all matching records with `getRecords`. By default, record lookup does include only records with a `status` set to `UP`. This can be overridden:

- when using JSON filter, just set `status` to the value you want (or `*` to accept all status)

- when using function, set the `includeOutOfService` parameter to `true` in `getRecords` .

## Retrieving a service reference

Once you have chosen the `Record`, you can retrieve a `ServiceReference` and then the service object:

    ServiceReference reference1 = discovery.getReference(record1);
    ServiceReference reference2 = discovery.getReference(record2);

    // Then, gets the service object, the returned type depends on the service type:
    // For http endpoint:
    HttpClient client = reference1.getAs(HttpClient.class);
    // For message source
    MessageConsumer consumer = reference2.getAs(MessageConsumer.class);

    // When done with the service
    reference1.release();
    reference2.release();

Don’t forget to release the reference once done.

The service reference represents a binding with the service provider.

When retrieving a service reference you can pass a `JsonObject` used to configure the service object. It can contain various data about the service object. Some service types do not need additional configuration, some require configuration (as data sources):

    ServiceReference reference = discovery.getReferenceWithConfiguration(record, conf);

    // Then, gets the service object, the returned type depends on the service type:
    // For http endpoint:
    JDBCClient client = reference.getAs(JDBCClient.class);

    // Do something with the client...

    // When done with the service
    reference.release();

In the previous examples, the code uses `getAs`. The parameter is the type of object you expect to get. If you are using Java, you can use `get`. However in the other language you must pass the expected type.

## Types of services

A said above, the service discovery has the service type concept to manage the heterogeneity of the different services.

These types are provided by default:

- `HttpEndpoint` - for REST API’s, the service object is a `HttpClient` configured on the host and port (the location is the url).

- `EventBusService` - for service proxies, the service object is a proxy. Its type is the proxies interface (the location is the address).

- `MessageSource` - for message sources (publisher), the service object is a `MessageConsumer` (the location is the address).

- `JDBCDataSource` - for JDBC data sources, the service object is a `JDBCClient` (the configuration of the client is computed from the location, metadata and consumer configuration).

- `RedisDataSource` - for Redis data sources, the service object is a `RedisClient` (the configuration of the client is computed from the location, metadata and consumer configuration).

- `MongoDataSource` - for Mongo data sources, the service object is a `MongoClient` (the configuration of the client is computed from the location, metadata and consumer configuration).

This section gives details about service types in general and describes how to use the default service types.

### Services with no type

Some records may have no type (`ServiceType.UNKNOWN`). It is not possible to retrieve a reference for these records, but you can build the connection details from the `location` and `metadata` of the `Record`.

Using these services does not fire service usage events.

### Implementing your own service type

You can create your own service type by implementing the `ServiceType` SPI:

1. (optional) Create a public interface extending `ServiceType`. This interface is only used to provide helper methods to ease the usage of your type such as `createRecord` methods, `getX` where `X` is the type of service object you retrieve and so on. Check `HttpEndpoint` or `MessageSource` for example

2. Create a class implementing `ServiceType` or the interface you created in step 1. The type has a `name`, and a method to create the `ServiceReference` for this type. The name must match the `type` field of the `Record` associated with your type.

3. Create a class extending `io.vertx.ext.discovery.types.AbstractServiceReference`. You can parameterize the class with the type of service object your are going to return. You must implement `AbstractServiceReference#retrieve()` that creates the service object. This method is only called once. If your service object needs cleanup, also override `AbstractServiceReference#close()`.

4. Create a `META-INF/services/io.vertx.servicediscovery.spi.ServiceType` file that is packaged in your jar. In this file, just indicate the fully qualified name of the class created at step 2.

5. Creates a jar containing the service type interface (step 1), the implementation (step 2 and 3) and the service descriptor file (step 4). Put this jar in the classpath of your application. Here you go, your service type is now available.

### HTTP endpoints

A HTTP endpoint represents a REST API or a service accessible using HTTP requests. The HTTP endpoint service objects are `HttpClient` configured with the host, port and ssl.

#### Publishing a HTTP endpoint

To publish a HTTP endpoint, you need a `Record`. You can create the record using `HttpEndpoint.createRecord`.

The next snippet illustrates hot to create a `Record` from `HttpEndpoint`:

    Record record1 = HttpEndpoint.createRecord(
      "some-http-service", // The service name
      "localhost", // The host
      8433, // the port
      "/api" // the root of the service
    );

    discovery.publish(record1, ar -> {
      // ...
    });

    Record record2 = HttpEndpoint.createRecord(
      "some-other-name", // the service name
      true, // whether or not the service requires HTTPs
      "localhost", // The host
      8433, // the port
      "/api", // the root of the service
      new JsonObject().put("some-metadata", "some value")
    );

When you run your service in a container or on the cloud, it may not know its public IP and public port, so the publication must be done by another entity having this info. Generally it’s a bridge.

#### Consuming a HTTP endpoint

Once a HTTP endpoint is published, a consumer can retrieve it. The service object is a `HttpClient` with a port and host configured:

    discovery.getRecord(new JsonObject().put("name", "some-http-service"), ar -> {
      if (ar.succeeded() && ar.result() != null) {
        // Retrieve the service reference
        ServiceReference reference = discovery.getReference(ar.result());
        // Retrieve the service object
        HttpClient client = reference.getAs(HttpClient.class);

        // You need to path the complete path
        client.getNow("/api/persons", response -> {

          // ...

          // Dont' forget to release the service
          reference.release();

        });
      }
    });

You can also use the `HttpEndpoint.getClient` method to combine lookup and service retrieval in one call:

    HttpEndpoint.getClient(discovery, new JsonObject().put("name", "some-http-service"), ar -> {
      if (ar.succeeded()) {
        HttpClient client = ar.result();

        // You need to path the complete path
        client.getNow("/api/persons", response -> {

          // ...

          // Dont' forget to release the service
          ServiceDiscovery.releaseServiceObject(discovery, client);

        });
      }
    });

In this second version, the service object is released using `ServiceDiscovery.releaseServiceObject`, so you don’t need to keep the service reference.

Since Vert.x 3.4.0, another client has been provided. This higher-level client, named `WebClient` tends to be easier to use. You can retrieve a `WebClient` instances using:

    discovery.getRecord(new JsonObject().put("name", "some-http-service"), ar -> {
      if (ar.succeeded() && ar.result() != null) {
        // Retrieve the service reference
        ServiceReference reference = discovery.getReference(ar.result());
        // Retrieve the service object
        WebClient client = reference.getAs(WebClient.class);

        // You need to path the complete path
        client.get("/api/persons").send(
          response -> {

            // ...

            // Dont' forget to release the service
            reference.release();

          });
      }
    });

And, if you prefer the approach using the service type:

    HttpEndpoint.getWebClient(discovery, new JsonObject().put("name", "some-http-service"), ar -> {
      if (ar.succeeded()) {
        WebClient client = ar.result();

        // You need to path the complete path
        client.get("/api/persons")
          .send(response -> {

            // ...

            // Dont' forget to release the service
            ServiceDiscovery.releaseServiceObject(discovery, client);

          });
      }
    });

### Event bus services

Event bus services are service proxies. They implement async-RPC services on top of the event bus. When retrieving a service object from an event bus service, you get a service proxy of the right type. You can access helper methods from `EventBusService`.

Notice that service proxies (service implementations and service interfaces) are developed in Java.

#### Publishing an event bus service

To publish an event bus service, you need to create a `Record`:

    Record record = EventBusService.createRecord(
        "some-eventbus-service", // The service name
        "address", // the service address,
        "examples.MyService", // the service interface as string
        new JsonObject()
            .put("some-metadata", "some value")
    );

    discovery.publish(record, ar -> {
      // ...
    });

You can also pass the service interface as class:

    Record record = EventBusService.createRecord(
    "some-eventbus-service", // The service name
    "address", // the service address,
    MyService.class // the service interface
    );

    discovery.publish(record, ar -> {
    // ...
    });

#### Consuming an event bus service

To consume an event bus service you can either retrieve the record and then get the reference, or use the `EventBusService` interface that combines the two operations in one call.

When using the reference, you would do something like:

    discovery.getRecord(new JsonObject().put("name", "some-eventbus-service"), ar -> {
    if (ar.succeeded() && ar.result() != null) {
    // Retrieve the service reference
    ServiceReference reference = discovery.getReference(ar.result());
    // Retrieve the service object
    MyService service = reference.getAs(MyService.class);

    // Dont' forget to release the service
    reference.release();
    }
    });

With the `EventBusService` class, you can get the proxy as follows:

    EventBusService.getProxy(discovery, MyService.class, ar -> {
    if (ar.succeeded()) {
    MyService service = ar.result();

    // Dont' forget to release the service
    ServiceDiscovery.releaseServiceObject(discovery, service);
    }
    });

### Message source

A message source is a component sending messages on the event bus on a specific address. Message source clients are `MessageConsumer`.

The location or a message source service is the event bus address on which messages are sent.

#### Publishing a message source

As for the other service types, publishing a message source is a 2-step process:

1. create a record, using `MessageSource`

2. publish the record

    Record record = MessageSource.createRecord(
        "some-message-source-service", // The service name
        "some-address" // The event bus address
    );

    discovery.publish(record, ar -> {
      // ...
    });

    record = MessageSource.createRecord(
        "some-other-message-source-service", // The service name
        "some-address", // The event bus address
        "examples.MyData" // The payload type
    );

In the second record, the type of payload is also indicated. This information is optional.

In java, you can use `Class` parameters:

    Record record1 = MessageSource.createRecord(
    "some-message-source-service", // The service name
    "some-address", // The event bus address
    JsonObject.class // The message payload type
    );

    Record record2 = MessageSource.createRecord(
    "some-other-message-source-service", // The service name
    "some-address", // The event bus address
    JsonObject.class, // The message payload type
    new JsonObject().put("some-metadata", "some value")
    );

#### Consuming a message source

On the consumer side, you can retrieve the record and the reference, or use the `MessageSource` class to retrieve the service is one call.

With the first approach, the code is the following:

    discovery.getRecord(new JsonObject().put("name", "some-message-source-service"), ar -> {
      if (ar.succeeded() && ar.result() != null) {
        // Retrieve the service reference
        ServiceReference reference = discovery.getReference(ar.result());
        // Retrieve the service object
        MessageConsumer<JsonObject> consumer = reference.getAs(MessageConsumer.class);

        // Attach a message handler on it
        consumer.handler(message -> {
          // message handler
          JsonObject payload = message.body();
        });

        // ...
        // when done
        reference.release();
      }
    });

When, using `MessageSource`, it becomes:

    MessageSource.<JsonObject>getConsumer(discovery, new JsonObject().put("name", "some-message-source-service"), ar -> {
      if (ar.succeeded()) {
        MessageConsumer<JsonObject> consumer = ar.result();

        // Attach a message handler on it
        consumer.handler(message -> {
          // message handler
          JsonObject payload = message.body();
        });
        // ...

        // Dont' forget to release the service
        ServiceDiscovery.releaseServiceObject(discovery, consumer);

      }
    });

### JDBC Data source

Data sources represents databases or data stores. JDBC data sources are a specialization for databases accessible using a JDBC driver. The client of a JDBC data source service is a `JDBCClient`.

#### Publishing a JDBC service

As for the other service types, publishing a JDBC data source is a 2-step process:

1. create a record, using `JDBCDataSource`

2. publish the record

    Record record = JDBCDataSource.createRecord(
        "some-data-source-service", // The service name
        new JsonObject().put("url", "some jdbc url"), // The location
        new JsonObject().put("some-metadata", "some-value") // Some metadata
    );

    discovery.publish(record, ar -> {
      // ...
    });

As JDBC data sources can represent a high variety of databases, and their access is often different, the record is rather unstructured. The `location` is a simple JSON object that should provide the fields to access the data source (JDBC url, username…​). The set of fields may depend on the database but also on the connection pool used in front.

#### Consuming a JDBC service

As stated in the previous section, how to access a data source depends on the data source itself. To build the `JDBCClient`, you can merge configuration: the record location, the metadata and a json object provided by the consumer:

    discovery.getRecord(
        new JsonObject().put("name", "some-data-source-service"),
        ar -> {
          if (ar.succeeded() && ar.result() != null) {
            // Retrieve the service reference
            ServiceReference reference = discovery.getReferenceWithConfiguration(
                ar.result(), // The record
                new JsonObject().put("username", "clement").put("password", "*****")); // Some additional metadata

            // Retrieve the service object
            JDBCClient client = reference.getAs(JDBCClient.class);

            // ...

            // when done
            reference.release();
          }
        });

You can also use the `JDBCClient` class to the lookup and retrieval in one call:

    JDBCDataSource.<JsonObject>getJDBCClient(discovery,
        new JsonObject().put("name", "some-data-source-service"),
        new JsonObject().put("username", "clement").put("password", "*****"), // Some additional metadata
        ar -> {
          if (ar.succeeded()) {
            JDBCClient client = ar.result();

            // ...

            // Dont' forget to release the service
            ServiceDiscovery.releaseServiceObject(discovery, client);

          }
        });

### Redis Data source

Redis data sources are a specialization for Redis persistence databases. The client of a Redis data source service is a `RedisClient`.

#### Publishing a Redis service

Publishing a Redis data source is a 2-step process:

1. create a record, using `RedisDataSource`

2. publish the record

    Record record = RedisDataSource.createRecord(
      "some-redis-data-source-service", // The service name
      new JsonObject().put("url", "localhost"), // The location
      new JsonObject().put("some-metadata", "some-value") // Some metadata
    );

    discovery.publish(record, ar -> {
      // ...
    });

The `location` is a simple JSON object that should provide the fields to access the Redis data source (url, port…​).

#### Consuming a Redis service

As stated in the previous section, how to access a data source depends on the data source itself. To build the `RedisClient`, you can merge configuration: the record location, the metadata and a json object provided by the consumer:

    discovery.getRecord(
      new JsonObject().put("name", "some-redis-data-source-service"), ar -> {
        if (ar.succeeded() && ar.result() != null) {
          // Retrieve the service reference
          ServiceReference reference = discovery.getReference(ar.result());

          // Retrieve the service instance
          RedisClient client = reference.getAs(RedisClient.class);

          // ...

          // when done
          reference.release();
        }
      });

You can also use the `RedisDataSource` class to the lookup and retrieval in one call:

    RedisDataSource.getRedisClient(discovery,
      new JsonObject().put("name", "some-redis-data-source-service"),
      ar -> {
        if (ar.succeeded()) {
          RedisClient client = ar.result();

          // ...

          // Dont' forget to release the service
          ServiceDiscovery.releaseServiceObject(discovery, client);

        }
      });

### Mongo Data source

Mongo data sources are a specialization for MongoDB databases. The client of a Mongo data source service is a `MongoClient`.

#### Publishing a Mongo service

Publishing a Mongo data source is a 2-step process:

1. create a record, using `MongoDataSource`

2. publish the record

    Record record = MongoDataSource.createRecord(
      "some-data-source-service", // The service name
      new JsonObject().put("connection_string", "some mongo connection"), // The location
      new JsonObject().put("some-metadata", "some-value") // Some metadata
    );

    discovery.publish(record, ar -> {
      // ...
    });

The `location` is a simple JSON object that should provide the fields to access the Redis data source (url, port…​).

#### Consuming a Mongo service

As stated in the previous section, how to access a data source depends on the data source itself. To build the `MongoClient`, you can merge configuration: the record location, the metadata and a json object provided by the consumer:

    discovery.getRecord(
      new JsonObject().put("name", "some-data-source-service"),
      ar -> {
        if (ar.succeeded() && ar.result() != null) {
          // Retrieve the service reference
          ServiceReference reference = discovery.getReferenceWithConfiguration(
            ar.result(), // The record
            new JsonObject().put("username", "clement").put("password", "*****")); // Some additional metadata

          // Retrieve the service object
          MongoClient client = reference.get();

          // ...

          // when done
          reference.release();
        }
      });

You can also use the `MongoDataSource` class to the lookup and retrieval in one call:

    MongoDataSource.<JsonObject>getMongoClient(discovery,
      new JsonObject().put("name", "some-data-source-service"),
      new JsonObject().put("username", "clement").put("password", "*****"), // Some additional metadata
      ar -> {
        if (ar.succeeded()) {
          MongoClient client = ar.result();

          // ...

          // Dont' forget to release the service
          ServiceDiscovery.releaseServiceObject(discovery, client);

        }
      });

## Listening for service arrivals and departures

Every time a provider is published or removed, an event is published on the vertx.discovery.announce address. This address is configurable from the `ServiceDiscoveryOptions`.

The received record has a `status` field indicating the new state of the record:

- `UP` : the service is available, you can start using it

- `DOWN` : the service is not available anymore, you should not use it anymore

- `OUT_OF_SERVICE` : the service is not running, you should not use it anymore, but it may come back later.

## Listening for service usage

Every time a service reference is retrieved (`bind`) or released (`release`), an event is published on the vertx .discovery.usage address. This address is configurable from the `ServiceDiscoveryOptions`.

It lets you listen for service usage and map the service bindings.

The received message is a `JsonObject` containing:

- the record in the `record` field

- the type of event in the `type` field. It’s either `bind` or `release`

- the id of the service discovery (either its name or the node id) in the `id` field

This `id` is configurable from the `ServiceDiscoveryOptions`. By default it’s "localhost" on single node configuration and the id of the node in clustered mode.

You can disable the service usage support by setting the usage address to `null` with `setUsageAddress`.

## Service discovery bridges

Bridges let you import and export services from / to other discovery mechanism such as Docker, Kubernates, Consul…​ Each bridge decides how the services are imported and exported. It does not have to be bi-directional.

You can provide your own bridge by implementing the `ServiceImporter` interface and register it using `registerServiceImporter`.

The second parameter can provide an optional configuration for the bridge.

When the bridge is registered the

{@link io.vertx.servicediscovery.spi.ServiceImporter#start)} method is called. It lets you configure the bridge. When the bridge is configured, ready and has imported / exported the initial services, it must complete the given `Future`. If the bridge starts method is blocking, it must use an `executeBlocking` construct, and complete the given future object.

When the service discovery is stopped, the bridge is stopped. The `close` method is called that provides the opportunity to cleanup resources, removed imported / exported services…​ This method must complete the given `Future` to notify the caller of the completion.

Notice than in a cluster, only one member needs to register the bridge as the records are accessible by all members.

## Additional bridges

In addition of the bridges supported by this library, Vert.x Service Discovery provides additional bridges you can use in your application.

### Consul bridge

This discovery bridge imports services from Consul into the Vert.x service discovery.

The bridge connects to a Consul agent (server) and periodically scan for services:

- new services are imported

- services in maintenance mode or that has been removed from consul are removed

This bridge uses the HTTP API for Consul. It does not export to Consul and does not support service modification.

The service type is deduced from `tags`. If a `tag` matches a known service type, this service type will be used. If not, the service is imported as `unknown`. Only `http-endpoint` is supported for now.

#### Using the bridge

To use this Vert.x discovery bridge, add the following dependency to the dependencies section of your build descriptor:

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

    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-service-discovery-bridge-consul</artifactId>
      <version>3.4.1</version>
    </dependency>

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

    compile 'io.vertx:vertx-service-discovery-bridge-consul:3.4.1'

Then, when creating the service discovery registers this bridge as follows:

    ServiceDiscovery.create(vertx)
        .registerServiceImporter(new ConsulServiceImporter(),
            new JsonObject()
                .put("host", "localhost")
                .put("port", 8500)
                .put("scan-period", 2000));

You can configure the:

- agent host using the `host` property, it defaults to `localhost`

- agent port using the `port` property, it defaults to 8500

- scan period using the `scan-period` property. The time is set in ms, and is 2000 ms by default

### Kubernetes bridge

This discovery bridge imports services from Kubernetes (or Openshift v3) into the Vert.x service discovery.

Kubernetes services are mapped to `Record`. This bridge only supports the importation of services from kubernetes in vert.x (and not the opposite).

`Record` are created from Kubernetes Service. The service type is deduced from the `service.type` label. If not set, the service is imported as `unknown`. Only `http-endpoint` are supported for now.

#### Using the bridge

To use this Vert.x discovery bridge, add the following dependency to the dependencies section of your build descriptor:

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

    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-service-discovery-bridge-kubernetes</artifactId>
      <version>3.4.1</version>
    </dependency>

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

    compile 'io.vertx:vertx-service-discovery-bridge-kubernetes:3.4.1'

#### Configuring the bridge

The bridge is configured using:

- the oauth token (using the content of `/var/run/secrets/kubernetes.io/serviceaccount/token` by default)

- the namespace in which the service are searched (defaults to `default`).

Be aware that the application must have access to Kubernetes and must be able to read the chosen namespace.

#### The Service to Record mapping

The record is created as follows:

- the service type is deduced from the `service.type` label. If this label is not set the service type is set to `unknown`

- the record’s name is the service’s name

- the labels of the service are mapped to metadata

- in addition are added: `kubernetes.uuid`, `kubernetes.namespace`, `kubernetes.name`

- the location is deduced from the first port of the service

For HTTP endpoints, the `ssl` (`https`) attribute is set to `true` if the service has the `ssl` label set to `true`.

#### Dynamics

The bridge imports all services on `start` and removes them on `stop`. In between it watches the Kubernetes services and add the new ones and removes the deleted ones.

Unresolved directive in index.adoc - include::zookeeper-bridge.adoc[]

### Docker Links bridge

This discovery bridge imports services from Docker Links into the Vert.x service discovery.

When you link a Docker container to another Docker container, Docker injects a set of environment variables. This bridge analyzes these environment variables and imports service record for each link. The service type is deduced from the `service.type` label. If not set, the service is imported as `unknown`. Only `http-endpoint` are supported for now.

As the links are created when the container starts, the imported records are created when the bridge starts and do not change afterwards.

#### Using the bridge

To use this Vert.x discovery bridge, add the following dependency to the dependencies section of your build descriptor:

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

    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-service-discovery-bridge-docker-links</artifactId>
      <version>3.4.1</version>
    </dependency>

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

    compile 'io.vertx:vertx-service-discovery-bridge-docker-links:3.4.1'

Then, when creating the service discovery, registers this bridge as follows:

    ServiceDiscovery.create(vertx)
        .registerServiceImporter(new DockerLinksServiceImporter(), new JsonObject());

The bridge does not need any further configuration.

## Additional backends

In addition of the backend supported by this library, Vert.x Service Discovery provides additional backends you can use in your application.

### Redis backend

The service discovery has a plug-able backend using the `ServiceDiscoveryBackend` SPI.

This is an implementation of the SPI based on Redis.

#### Using the Redis backend

To use the Redis backend, add the following dependency to the dependencies section of your build descriptor:

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

    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-service-discovery-backend-redis</artifactId>
      <version>3.4.1</version>
    </dependency>

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

    compile 'io.vertx:vertx-service-discovery-backend-redis:3.4.1'

Be aware that you can have only a single implementation of the SPI in your classpath. If none, the default backend is used.

#### Configuration

The backend is based on the vertx-redis-client. The configuration is the client configuration as well as `key` indicating in which key on Redis the records are stored.

Here is an example:

    ServiceDiscovery.create(vertx, new ServiceDiscoveryOptions()
        .setBackendConfiguration(
            new JsonObject()
                .put("host", "127.0.0.1")
                .put("key", "records")
        ));