= Hazelcast Cluster Manager

This is a cluster manager implementation for Vert.x that uses http://hazelcast.com[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:

[source,xml,subs="+attributes"]
----
<dependency>
 <groupId>io.vertx</groupId>
 <artifactId>vertx-hazelcast</artifactId>
 <version>3.9.12</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 managersdo 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-3.9.12.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:3.9.12` 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:

[source,java]
----
ClusterManager mgr = new HazelcastClusterManager();

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

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

[[configcluster]]
== Configuring this cluster manager

=== Configuring with XML

Usually the cluster manager is configured by a file
https://github.com/vert-x3/vertx-hazelcast/blob/master/src/main/resources/default-cluster.xml[`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:

[source]
----
# 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:

[source]
----
# 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.

=== Configuring programmatically

You can specify configuration programmatically if embedding:

[source,java]
----
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!
  }
});
----

It might also be useful to customize an existing XML configuration.
For example, you might want to change the cluster group name and password:

[source,java]
----
Config hazelcastConfig = ConfigUtil.loadConfig();

hazelcastConfig.getGroupConfig()
  .setName("my-cluster-name")
  .setPassword("passwd");

ClusterManager mgr = new HazelcastClusterManager(hazelcastConfig);

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

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

`ConfigUtil#loadConfig` loads Hazelcast config XML and transform it into a `Config` object.
The content is read from:

. the location denoted by the `vertx.hazelcast.config` sysprop, if present, or
. the `cluster.xml` file on the classpath, if present, or
. the default config file

=== Discovery options

Hazelcast supports several different discovery options.
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 discovery differently 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:

[source,java]
----
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:

[source, xml]
----
<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: Hazelcast clients or smart clients are not supported.

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 http://docs.hazelcast.org/docs/3.6/manual/html-single/index.html#system-properties[Hazelcast
system-properties] and
http://docs.hazelcast.org/docs/3.6/manual/html-single/index.html#configuring-with-system-properties[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.

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
`link:../../apidocs/io/vertx/core/VertxOptions.html#setClusterHost-java.lang.String-[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 http://docs.hazelcast.org/docs/3.6.1/manual/html-single/index.html#logging-configuration[hazelcast documentation] for more details.

== Using a different Hazelcast version

You may want to use a different version of Hazelcast. The default version is `3.12.2`. 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:

[source,xml,subs="+attributes"]
----
<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>3.9.12</version>
</dependency>
----

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

On Gradle, you can achieve the same overloading using:

[source]
----
dependencies {
compile ("io.vertx:vertx-hazelcast:3.9.12"){
  exclude group: 'com.hazelcast', module: 'hazelcast'
}
compile "com.hazelcast:hazelcast:ENTER_YOUR_VERSION_HERE"
}
----

== Configuring for Kubernetes

On Kubernetes, Hazelcast should be configured to use the https://github.com/hazelcast/hazelcast-kubernetes[Hazelcast Kubernetes] plugin.

First, add the `io.vertx:vertx-hazelcast:${vertx.version}` and `com.hazelcast:hazelcast-kubernetes:${hazelcast-kubernetes.version}` dependencies to your project.
With Maven it looks like:

[source,xml]
----
<dependency>
 <groupId>io.vertx</groupId>
 <artifactId>vertx-hazelcast</artifactId>
 <version>${vertx.version}</version>
</dependency>
<dependency>
 <groupId>com.hazelcast</groupId>
 <artifactId>hazelcast-kubernetes</artifactId>
 <version>${hazelcast-kubernetes.version}</version>
</dependency>
----

NOTE: If you use a different version of the Hazelcast core library, make sure to use a compatible version of the Kubernetes discovery plugin.

The second step is to configure the discovery plugin inside of your Hazelcast configuration, by either providing a custom `cluster.xml` file or programmatically, as described in <<configcluster>>.

The plugin provides two https://github.com/hazelcast/hazelcast-kubernetes#understanding-discovery-modes[discovery modes]: _Kubernetes API_ and _DNS Lookup_.
Please refer to the plugin project page for pros and cons of both modes.

In this document, we will use _DNS Lookup_ discovery. The following properties have to be changed / added:

[source,xml]
----
<hazelcast>
 <properties>
   <property name="hazelcast.discovery.enabled">true</property> <!--1-->
 </properties>

 <network>
   <join>
     <multicast enabled="false"/> <!--2-->
     <tcp-ip enabled="false" />

     <discovery-strategies>
       <discovery-strategy enabled="true" <!--3-->
           class="com.hazelcast.kubernetes.HazelcastKubernetesDiscoveryStrategy">
         <properties>
           <property name="service-dns">MY-SERVICE-DNS-NAME</property> <!--4-->
         </properties>
       </discovery-strategy>
     </discovery-strategies>
   </join>
 </network>
</hazelcast>
----
<1> Activate Discovery SPI
<2> Deactivate other discoveries
<3> Activate the Kubernetes plugin
<4> Service DNS, usually in the form of `MY-SERVICE-NAME.MY-NAMESPACE.svc.cluster.local` but depends on the Kubernetes distribution

The `MY-SERVICE-DNS-NAME` value must be a *headless* Kubernetes service name that will be used by Hazelcast to identify all cluster members.
A headless service can be created with:

[source,yaml]
----
apiVersion: v1
kind: Service
metadata:
 namespace: MY-NAMESPACE
 name: MY-SERVICE-NAME
spec:
 selector:
   component: MY-SERVICE-NAME # <1>
 clusterIP: None
 ports:
 - name: hz-port-name
   port: 5701
   protocol: TCP
----
<1> Cluster members selected by label

Eventually, attach the `component` label to all deployments that should be part of the cluster:

[source,yaml]
----
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
 namespace: MY-NAMESPACE
spec:
 template:
   metadata:
     labels:
       component: MY-SERVICE-NAME
----

Further configuration details are available on the https://github.com/hazelcast/hazelcast-kubernetes[Hazelcast Kubernetes Plugin page].

=== Rolling updates

During rolling updates, it is recommended to replace pods one by one.

To do so, we must configure Kubernetes to:

* never start more than one new pod at once
* forbid more than one unavailable pod during the process

[source,yaml]
----
spec:
 strategy:
   type: Rolling
   rollingParams:
     updatePeriodSeconds: 10
     intervalSeconds: 20
     timeoutSeconds: 600
     maxUnavailable: 1 <1>
     maxSurge: 1 <2>
----
<1> the maximum number of pods that can be unavailable during the update process
<2> the maximum number of pods that can be created over the desired number of pods

Also, the pod readiness probe must take the cluster state into account.
Please refer to the <<one-by-one, cluster administration>> section for details on how to implement a readiness probe with link:../../vertx-health-check/java/[Vert.x Health Checks].

== Cluster administration

The Hazelcast cluster manager works by turning Vert.x nodes into members of a Hazelcast cluster.
As a consequence, Vert.x cluster manager administration should follow the Hazelcast management guidelines.

First, let's take a step back and introduce data partitioning and split-brain syndrome.

=== Data partitioning

Each Vert.x node holds pieces of the clustering data: eventbus subscriptions, async map entries, clustered counters... etc.

When a member joins or leaves the cluster, Hazelcast migrates data partitions.
In other words, it moves data around to accomodate the new cluster topology.
This process may take some time, depending on the amount of clustered data and number of nodes.

=== Split-brain syndrome

In a perfect world, there would be no network equipment failures.
Reality is, though, that sooner or later your cluster will be divided into smaller groups, unable to see each others.

Hazelcast is capable of merging the nodes back into a single cluster.
But just as with data partition migrations, this process may take some time.
Before the cluster is fully functional again, some eventbus consumers might not be able to get messages.
Or high-availability may not be able to redeploy a failing verticle.

[NOTE]
====
It is difficult (if possible at all) to make a difference between a network partition and:

- long GC pauses (leading to missed heartbeats checks),
- many nodes being killed forcefully, at-once, because you are deploying a new version of your application
====

=== Recommendations

Considering the common clustering issues discussed above, it is recommended to stick to the following good practices.

==== Graceful shutdown

Avoid stopping members forcefully (e.g, `kill -9` a node).

Of course process crashes are inevitable, but a graceful shutdown helps to get the remaining nodes in a stable state faster.

[[one-by-one]]
==== One node after the other

When rolling a new version of your app, scaling-up or down your cluster, add or remove nodes one after the other.

Stopping nodes one by one prevents the cluster from thinking a network partition occured.
Adding them one by one allows for clean, incremental data partition migrations.

The cluster safety can be verified with link:../../vertx-health-check/java/[Vert.x Health Checks]:

[source,java]
----
Handler<Promise<Status>> procedure = ClusterHealthCheck.createProcedure(vertx);
HealthChecks checks = HealthChecks.create(vertx).register("cluster-health", procedure);
----

After creation, the health check can be exposed over HTTP with a link:../../vertx-web/java/[Vert.x Web] router handler:

[source,java]
----
Router router = Router.router(vertx);
router.get("/readiness").handler(HealthCheckHandler.createWithHealthChecks(checks));
----

==== Using Lite Members

To minimize the time a Vert.x cluster spends accomodating a new topology, you may use external data nodes and mark Vert.x nodes as https://docs.hazelcast.org/docs/latest/manual/html-single/#enabling-lite-members[_Lite Members_].

_Lite Members_ participate in a Hazelcast cluster like regular members, but they do not own any data partition.
Therefore, Hazelcast does not need to migrate partitions when such members are added or removed.

IMPORTANT: You must start the external data nodes beforehand as Hazelcast won't create a cluster with _Lite Members_ only.

To start an external node, you can use the Hazelcast distribution start script, or proceed programmatically.

Vert.x nodes can be marked as _Lite Members_ in the XML configuration:

[source,xml]
----
<lite-member enabled="true"/>
----

You can also do it programmatically:

[source,java]
----
Config hazelcastConfig = ConfigUtil.loadConfig()
  .setLiteMember(true);

ClusterManager mgr = new HazelcastClusterManager(hazelcastConfig);

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

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