[[Undertow_using_as_a_Load_Balancer]]
= Using WildFly as a Load Balancer

WildFly 10 added support for using the Undertow subsystem as a load
balancer. WildFly supports two different approaches, you can either
define a static load balancer, and specify the back end hosts in your
configuration, or use it as a mod_cluster frontend, and use mod_cluster
to dynamically update the hosts.

[[general-overview]]
== General Overview

WildFly uses Undertow's proxy capabilities to act as a load balancer.
Undertow will connect to the back end servers using its built in client,
and proxies requests.

The following protocols are supported:

* `http`
* `ajp`
* `http2`
* `h2c` (clear text HTTP2) +
Of these protocols h2c should give the best performance, if the back end
servers support it.

The Undertow proxy uses async IO, the only threads that are involved in
the request is the IO thread that is responsible for the connection. The
connection to the back end server is made from the same thread, which
removes the need for any thread safety constructs. +
If both the front and back end servers support server push, and HTTP2 is
in use then the proxy also supports pushing responses to the client. In
cases where proxy and backend are capable of server push, but the client
does not support it the server will send a `X-Disable-Push` header to let
the backend know that it should not attempt to push for this request.

=== Load balancer server profiles

WildFly 11 added load balancer profiles for both standalone and domain modes.

==== Example: Start standalone load balancer

[source,options="nowrap"]
----
# configure correct path to WildFly installation
WILDFLY_HOME=/path/to/wildfly

# configure correct IP of the node
MY_IP=192.168.1.1

# run the load balancer profile
$WILDFLY_HOME/bin/standalone.sh -b $MY_IP -bprivate $MY_IP -c standalone-load-balancer.xml
----

It's highly recommended to use private/internal network for communication between load balancer and nodes.
To do this set the correct IP address to the `private` interface (`-bprivate` argument).

==== Example: Start worker node

Run the server with the HA (or Full HA) profile, which has `mod_cluster` component included.
If the UDP multicast is working in your environment, the workers should work out of the box without any change.
If it's not the case, then configure the IP address of the load balancer statically.

[source,options="nowrap"]
----
# configure correct path to WildFly installation
WILDFLY_HOME=/path/to/wildfly
# configure correct IP of the node
MY_IP=192.168.1.2

# Configure static load balancer IP address.
# This is necessary when UDP multicast doesn't work in your environment.
LOAD_BALANCER_IP=192.168.1.1
$WILDFLY_HOME/bin/jboss-cli.sh <<EOT

embed-server -c=standalone-ha.xml
/subsystem=modcluster/proxy=default:write-attribute(name=advertise, value=false)
/socket-binding-group=standard-sockets/remote-destination-outbound-socket-binding=proxy1:add(host=$LOAD_BALANCER_IP, port=8090)
/subsystem=modcluster/proxy=default:list-add(name=proxies, value=proxy1)
EOT

# start the woker node with HA profile
$WILDFLY_HOME/bin/standalone.sh -c standalone-ha.xml -b $MY_IP -bprivate $MY_IP
----

Again, to make it safe, users should configure private/internal IP address into `MY_IP` variable.

[[using-wildfly-as-a-static-load-balancer]]
== Using WildFly as a static load balancer

To use WildFly as a static load balancer the first step is to create a
proxy handler in the Undertow subsystem. For the purposes of this
example we are going to assume that our load balancer is going to load
balance between two servers, `sv1.foo.com` and `sv2.foo.com`, and will be
using the AJP protocol.

The first step is to add a reverse proxy handler to the Undertow
subsystem:

[source,options="nowrap"]
----
/subsystem=undertow/configuration=handler/reverse-proxy=my-handler:add()
----

Then we need to define `outbound-socket-binding`-s for remote hosts:

[source,options="nowrap"]
----
/socket-binding-group=standard-sockets/remote-destination-outbound-socket-binding=remote-host1:add(host=sv1.foo.com, port=8009)
/socket-binding-group=standard-sockets/remote-destination-outbound-socket-binding=remote-host2:add(host=sv2.foo.com, port=8009)
----

and than we add them as hosts to reverse proxy handler:

[source,options="nowrap"]
----
/subsystem=undertow/configuration=handler/reverse-proxy=my-handler/host=host1:add(outbound-socket-binding=remote-host1, scheme=ajp, instance-id=myroute, path=/test)
/subsystem=undertow/configuration=handler/reverse-proxy=my-handler/host=host2:add(outbound-socket-binding=remote-host2, scheme=ajp, instance-id=myroute, path=/test)
----

Now we need to actually add the reverse proxy to a location.
Assuming we are serving the path `/app`:

[source,options="nowrap"]
----
/subsystem=undertow/server=default-server/host=default-host/location=\/app:add(handler=my-handler)
----

This is all there is to it. If you point your browser to http://localhost:8080/app
you should be able to see the proxied content.

The full details of all configuration options available can be found in
the link:wildscribe/subsystem/undertow/index.html[subsystem reference^].
