## Vert.x Discovery Backend - Zookeeper

The service discovery has a plug-able backend using the `ServiceDiscoveryBackend` SPI. This is an implementation of the SPI based on Apache Zookeeper.

## Using the Zookeeper backend

To use the Zookeeper 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-zookeeper</artifactId>
     <version>3.6.3</version>
    </dependency>

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

    compile 'io.vertx:vertx-service-discovery-backend-zookeeper:3.6.3'

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

## Configuration

There is a single mandatory configuration attribute: `connection`. Connection is the Zookeeper connection string.

Here is an example:

    var ServiceDiscovery = require("vertx-service-discovery-js/service_discovery");
    ServiceDiscovery.create(vertx, {
      "backendConfiguration" : {
        "connection" : "127.0.0.1:2181"
      }
    });

Additionally you can configure:

- `maxRetries`: the number of connection attempt, 3 by default

- `baseSleepTimeBetweenRetries`: the amount of milliseconds to wait between retries (exponential backoff strategy). 1000 ms by default.

- `connectionTimeoutMs`: the connection timeout in milliseconds. Defaults to 1000.

- `canBeReadOnly` : whether or not the backend support the read-only mode (defaults to false)

- `basePath`: the Zookeeper path in which the service records are stored. Default to `/services`.

- `ephemeral`: whether or not the created nodes are ephemeral nodes (see https://zookeeper.apache.org/doc/r3.4.5/zookeeperOver.html#Nodes+and+ephemeral+nodes). `false` by default

- `guaranteed`: whether or not to guarantee the node deletion even in case of failure. `false` by default

    var ServiceDiscovery = require("vertx-service-discovery-js/service_discovery");
    ServiceDiscovery.create(vertx, {
      "backendConfiguration" : {
        "connection" : "127.0.0.1:2181",
        "ephemeral" : true,
        "guaranteed" : true,
        "basePath" : "/services/my-backend"
      }
    });

## How are stored the records

The records are stored in individual nodes structured as follows:

    basepath (/services/)
      |
      |- record 1 registration id => the record 1 is the data of this node
      |- record 2 registration id => the record 2 is the data of this node