HTTP Balancer

Apache Karaf Cellar is able to expose servlets local to a node on the cluster. It means that a client (browser) can use any node in the cluster, proxying the requests to the node actually hosting the servlets.

Enable HTTP Balancer

To enable Cellar HTTP Balancer, you have to first install the http and http-whiteboard features:

karaf@root()> feature:install http
karaf@root()> feature:install http-whiteboard

Now, we install the cellar-http-balancer feature, actually providing the balancer:

karaf@root()> feature:install cellar-http-balancer

Of course, you can use Cellar to spread the installation of the cellar-http-balancer feature on all nodes in the cluster group:

karaf@root()> cluster:feature-install default cellar-http-balancer

It’s done: the Cellar HTTP Balancer is now enabled. It will expose proxy servlets on nodes.

Balancer in action

To illustrate Cellar HTTP Balancer in action, you need at least a cluster with two nodes.

On node1, we enable the Cellar HTTP Balancer:

karaf@node1()> feature:install http
karaf@node1()> feature:install http-whiteboard
karaf@node1()> feature:repo-add cellar 4.0.0
karaf@node1()> feature:install cellar
karaf@node1()> cluster:feature-install default cellar-http-balancer

Now, we install the webconsole on node1:

karaf@node1()> feature:install webconsole

We can see the "local" servlets provided by the webconsole feature using the http:list command:

karaf@node1()> http:list
ID  | Servlet          | Servlet-Name    | State       | Alias               | Url
------------------------------------------------------------------------------------------------------
101 | KarafOsgiManager | ServletModel-2  | Undeployed  | /system/console     | [/system/console/*]
103 | GogoPlugin       | ServletModel-7  | Deployed    | /gogo               | [/gogo/*]
102 | FeaturesPlugin   | ServletModel-6  | Deployed    | /features           | [/features/*]
101 | ResourceServlet  | /res            | Deployed    | /system/console/res | [/system/console/res/*]
101 | KarafOsgiManager | ServletModel-11 | Deployed    | /system/console     | [/system/console/*]
105 | InstancePlugin   | ServletModel-9  | Deployed    | /instance           | [/instance/*]

You can access to the webconsole using a browser on http://localhost:8181/system/console.

We can see that Cellar HTTP Balancer exposed the servlets to the cluster, using the cluster:http-list command:

karaf@node1()> cluster:http-list default
Alias               | Locations
-----------------------------------------------------------------
/system/console/res | http://172.17.42.1:8181/system/console/res
/gogo               | http://172.17.42.1:8181/gogo
/instance           | http://172.17.42.1:8181/instance
/system/console     | http://172.17.42.1:8181/system/console
/features           | http://172.17.42.1:8181/features

On another node (node2), we install http, http-whiteboard and cellar features:

karaf@node1()> feature:install http
karaf@node1()> feature:install http-whiteboard
karaf@node1()> feature:repo-add cellar 4.0.0
karaf@node1()> feature:install cellar

Note

If you run the nodes on a single machine, you have to provision etc/org.ops4j.pax.web.cfg configuration file containing the org.osgi.service.http.port property with a port number different to 8181. For this example, we use the following etc/org.ops4j.pax.web.cfg file:

org.osgi.service.http.port=8041

On node1, as we installed the cellar-http-balancer using cluster:feature-install command, it’s automatically installed when node2 joins the default cluster group.

We can see the HTTP endpoints available on the cluster using the cluster:http-list command:

karaf@node2()> cluster:http-list default
Alias               | Locations
-----------------------------------------------------------------
/system/console/res | http://172.17.42.1:8181/system/console/res
/gogo               | http://172.17.42.1:8181/gogo
/instance           | http://172.17.42.1:8181/instance
/system/console     | http://172.17.42.1:8181/system/console
/features           | http://172.17.42.1:8181/features

If we take a look on the HTTP endpoints locally available on node2 (using http:list command), we can see the proxies created by Cellar HTTP Balancer:

karaf@node2()> http:list
ID  | Servlet                    | Servlet-Name   | State       | Alias               | Url
---------------------------------------------------------------------------------------------------------------
100 | CellarBalancerProxyServlet | ServletModel-3 | Deployed    | /gogo               | [/gogo/*]
100 | CellarBalancerProxyServlet | ServletModel-2 | Deployed    | /system/console/res | [/system/console/res/*]
100 | CellarBalancerProxyServlet | ServletModel-6 | Deployed    | /features           | [/features/*]
100 | CellarBalancerProxyServlet | ServletModel-5 | Deployed    | /system/console     | [/system/console/*]
100 | CellarBalancerProxyServlet | ServletModel-4 | Deployed    | /instance           | [/instance/*]

You can use a browser on http://localhost:8041/system/console: you will actually use the webconsole from node1, as Cellar HTTP Balancer proxies from node2 to node1.

Cellar HTTP Balancer randomly chooses one endpoint providing the HTTP endpoint.