== 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`):

[source,xml,subs="+attributes"]
----
<dependency>
  <groupId>io.vertx</groupId>
  <artifactId>vertx-service-discovery-backend-zookeeper</artifactId>
  <version>3.4.2</version>
</dependency>
----

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

[source,groovy,subs="+attributes"]
----
compile 'io.vertx:vertx-service-discovery-backend-zookeeper:3.4.2'
----

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:

[source,js]
----
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

[source,js]
----
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:

[source]
----
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
----