h1. Cellar cloud discovery

Cellar relies on Hazelcast in order to discover cluster nodes. This can happen either by using multicast or by unicast (specifying the ip address of each node).
Unfortunately multicast is not allowed in most IaaS providers and specifying the all the ip addresses is not very flexible, since in most cases they are not known in advance.

Cellar solves this problem using a cloud discovery service powered by jclouds.

h2. Cloud discovery service

Most cloud providers among other provide cloud storage. Cellar uses the cloud storage via jclouds, in order to put there the ip addresses of each node so that Hazelcast can found them.
This approach is also called blackboard and in other words is the process where each nodes registers itself in a common storage, so that other nodes know its existence.


h2. Installing Cellar cloud discovery service

To install the cloud discovery service simply the appropriate jclouds provider and then install cellar-cloud feature. For the rest of this manual I will use amazon s3 as an example, but it applies to any provider supported by jclouds.

{code}
features:install jclouds-aws-s3
features:install cellar-cloud
{code}

Once the feature is installed, it requires you to create a configuration that contains credentials and type of the cloud storage (aka blobstore).
To do that add a configuration file under etc with the name org.apache.karaf.cellar.cloud-<provider>.cfg and put there the following information:

provider=aws-s3 (this varries according to the blobstore provider)
identity=<the identity of the blobstore account>
credential=<the credential/password of the blobstore account)
container=<the name of the bucket>
validity=<the amount of time an entry is considered valid, after that time the entry is removed>

After creating the file the service will check for new nodes. If new nodes are found the Hazelcast instance configuration is updated and the instance is restarted.