# Hazelcast Cluster Manager

This is a cluster manager implementation for Vert.x that uses Hazelcast.

It is the default cluster manager used in the Vert.x distribution, but it can be replaced with another implementation as Vert.x cluster managers are pluggable. This implementation is packaged inside:

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

In Vert.x a cluster manager is used for various functions including:

- Discovery and group membership of Vert.x nodes in a cluster

- Maintaining cluster wide topic subscriber lists (so we know which nodes are interested in which event bus addresses)

- Distributed Map support

- Distributed Locks

- Distributed Counters

Cluster managers do not handle the event bus inter-node transport, this is done directly by Vert.x with TCP connections.

## Using this cluster manager

If you are using Vert.x from the command line, the jar corresponding to this cluster manager (it will be named `vertx-hazelcast-3.4.2.jar` should be in the `lib` directory of the Vert.x installation.

If you want clustering with this cluster manager in your Vert.x Maven or Gradle project then just add a dependency to the artifact: `io.vertx:vertx-hazelcast:3.4.2` in your project.

If the jar is on your classpath as above then Vert.x will automatically detect this and use it as the cluster manager. Please make sure you don’t have any other cluster managers on your classpath or Vert.x might choose the wrong one.

You can also specify the cluster manager programmatically if you are embedding Vert.x by specifying it on the options when you are creating your Vert.x instance, for example:

    ClusterManager mgr = new HazelcastClusterManager();

    VertxOptions options = new VertxOptions().setClusterManager(mgr);

    Vertx.clusteredVertx(options, res -> {
      if (res.succeeded()) {
        Vertx vertx = res.result();
      } else {
        // failed!
      }
    });

## Configuring this cluster manager

Usually the cluster manager is configured by a file `default-cluster.xml` which is packaged inside the jar.

If you want to override this configuration you can provide a file called `cluster.xml` on your classpath and this will be used instead. If you want to embed the `cluster.xml` file in a fat jar, it must be located at the root of the fat jar. If it’s an external file, the directory containing the file must be added to the classpath. For example, if you are using the launcher class from Vert.x, the classpath enhancement can be done as follows:

    # If the cluster.xml is in the current directory:
    java -jar ... -cp . -cluster
    vertx run MyVerticle -cp . -cluster

    # If the cluster.xml is in the conf directory
    java -jar ... -cp conf -cluster

Another way to override the configuration is by providing the system property `vertx.hazelcast.config` with a location:

    # Use a cluster configuration located in an external file
    java -Dvertx.hazelcast.config=./config/my-cluster-config.xml -jar ... -cluster

    # Or use a custom configuration from the classpath
    java -Dvertx.hazelcast.config=classpath:my/package/config/my-cluster-config.xml -jar ... -cluster

The `vertx.hazelcast.config` system property, when present, overrides any `cluster.xml` on the classpath, but if loading from this system property fails, then loading falls back to either `cluster.xml` or the Hazelcast default configuration.

Caution | Configuration of Hazelcast the `-Dhazelcast.config` system property is not supported by Vert.x and should not be used.

The xml file is a Hazelcast configuration file and is described in detail in the documentation on the Hazelcast web-site.

You can also specify configuration programmatically if embedding:

    Config hazelcastConfig = new Config();

    // Now set some stuff on the config (omitted)

    ClusterManager mgr = new HazelcastClusterManager(hazelcastConfig);

    VertxOptions options = new VertxOptions().setClusterManager(mgr);

    Vertx.clusteredVertx(options, res -> {
      if (res.succeeded()) {
        Vertx vertx = res.result();
      } else {
        // failed!
      }
    });

Hazelcast supports several different transports including multicast and TCP. The default configuration uses multicast so you must have multicast enabled on your network for this to work.

For full documentation on how to configure the transport differently or use a different transport please consult the Hazelcast documentation.

## Using an existing Hazelcast cluster

You can pass an existing `HazelcastInstance` in the cluster manager to reuse an existing cluster:

    ClusterManager mgr = new HazelcastClusterManager(hazelcastInstance);
    VertxOptions options = new VertxOptions().setClusterManager(mgr);
    Vertx.clusteredVertx(options, res -> {
      if (res.succeeded()) {
        Vertx vertx = res.result();
      } else {
        // failed!
      }
    });

In this case, vert.x is not the cluster owner and so do not shutdown the cluster on close.

Notice that the custom Hazelcast instance need to be configured with:

    <properties>
      <property name="hazelcast.shutdownhook.enabled">false</property>
    </properties>
    <multimap name="__vertx.subs">
      <backup-count>1</backup-count>
    </multimap>
    <map name="__vertx.haInfo">
      <time-to-live-seconds>0</time-to-live-seconds>
      <max-idle-seconds>0</max-idle-seconds>
      <eviction-policy>NONE</eviction-policy>
      <max-size policy="PER_NODE">0</max-size>
      <eviction-percentage>25</eviction-percentage>
      <merge-policy>com.hazelcast.map.merge.LatestUpdateMapMergePolicy</merge-policy>
    </map>
    <semaphore name="__vertx.*">
      <initial-permits>1</initial-permits>
    </semaphore>

IMPORTANT Do not use Hazelcast clients or smart clients when using high-availability (HA, or fail-over) in your cluster as they do not notify when they leave the cluster and you may loose data, or leave the cluster in an inconsistent state. See https://github.com/vert-x3/vertx-hazelcast/issues/24 for more details.

IMPORTANT Make sure Hazelcast is started before and shut down after Vert.x. Also, the Hazelcast shutdown hook should be disabled (see xml example, or via system property).

### Changing timeout for failed nodes

By default a node will be removed from the cluster if Hazelcast didn’t receive a heartbeat for 300 seconds. To change this value `hazelcast.max.no.heartbeat.seconds` system property such as in:

    -Dhazelcast.max.no.heartbeat.seconds=5

Afterwards a node will be removed from the cluster after 5 seconds without a heartbeat.

See Hazelcast system-properties and configuring Hazelcast with system properties for the other properties you can configure.

## Using Hazelcast async methods

Hazelcast’s `IMap` and `IAtomicLong` interfaces can be used with async methods returning `ICompletableFuture<V>`, a natural fit for Vert.x threading model. Even though these interfaces have been available for a long time, they are not provided by the public `HazelcastInstance` API.

The default behavior of the `HazelcastClusterManager` is to use the public API. Supplying the option `-Dvertx.hazelcast.async-api=true` on JVM startup, will indicate that the async Hazelcast API methods will be used to communicate with the hazelcast cluster. Effectively, this means that when this option is enabled, execution of all `Counter` operations and `AsyncMap.get`,`AsyncMap.put` and `AsyncMap.remove` operations will occur in the calling thread (the event loop), instead of a worker thread with `vertx.executeBlocking`.

## Trouble shooting clustering

If the default multicast configuration is not working here are some common causes:

### Multicast not enabled on the machine.

It is quite common in particular on OSX machines for multicast to be disabled by default. Please google for information on how to enable this.

### Using wrong network interface

If you have more than one network interface on your machine (and this can also be the case if you are running VPN software on your machine), then Hazelcast may be using the wrong one.

To tell Hazelcast to use a specific interface you can provide the IP address of the interface in the `interfaces` element of the configuration. Make sure you set the `enabled` attribute to `true`. For example:

    <interfaces enabled="true">
      <interface>192.168.1.20</interface>
    </interfaces>

When running Vert.x is in clustered mode, you should also make sure that Vert.x knows about the correct interface. When running at the command line this is done by specifying the `cluster-host` option:

    vertx run myverticle.js -cluster -cluster-host your-ip-address

Where `your-ip-address` is the same IP address you specified in the Hazelcast configuration.

If using Vert.x programmatically you can specify this using `setClusterHost`.

### Using a VPN

This is a variation of the above case. VPN software often works by creating a virtual network interface which often doesn’t support multicast. If you have a VPN running and you do not specify the correct interface to use in both the hazelcast configuration and to Vert.x then the VPN interface may be chosen instead of the correct interface.

So, if you have a VPN running you may have to configure both the Hazelcast and Vert.x to use the correct interface as described in the previous section.

### When multicast is not available

In some cases you may not be able to use multicast as it might not be available in your environment. In that case you should configure another transport, e.g. TCP to use TCP sockets, or AWS when running on Amazon EC2.

For more information on available Hazelcast transports and how to configure them please consult the Hazelcast documentation.

### Enabling logging

When trouble-shooting clustering issues with Hazelcast it’s often useful to get some logging output from Hazelcast to see if it’s forming a cluster properly. You can do this (when using the default JUL logging) by adding a file called `vertx-default-jul-logging.properties` on your classpath. This is a standard java.util.logging (JUL) configuration file. Inside it set:

    com.hazelcast.level=INFO

and also

    java.util.logging.ConsoleHandler.level=INFO
    java.util.logging.FileHandler.level=INFO

## Hazelcast logging

The logging backend used by Hazelcast is `jdk` by default (understand JUL). If you want to redirect the logging to another library, you need to set the `hazelcast.logging.type` system property such as in:

    -Dhazelcast.logging.type=slf4j

See the hazelcast documentation for more details.

## Using a different Hazelcast version

You may want to use a different version of Hazelcast. The default version is `3.6.3`. To do so, you need to:

- put the version you want in the application classpath

- if you are running a fat jar, configure your build manager to use the right version

In this later case, you would need in Maven:

    <dependency>
      <groupId>com.hazelcast</groupId>
      <artifactId>hazelcast</artifactId>
      <version>ENTER_YOUR_VERSION_HERE</version>
    </dependency>
    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-hazelcast</artifactId>
      <version>3.4.2</version>
    </dependency>

Depending on the version, you may need to exclude some transitive dependencies.

On Gradle, you can achieve the same overloading using:

    dependencies {
     compile ("io.vertx:vertx-hazelcast:3.4.2"){
       exclude group: 'com.hazelcast', module: 'hazelcast'
     }
     compile "com.hazelcast:hazelcast:ENTER_YOUR_VERSION_HERE"
    }