One of the key components in clustering is shared storage. Storage is accessible to all nodes, and all nodes can read and write in parallel to the shared storage, depending on the configuration of the cluster. Some configurations allow the storage to be accessible by all the nodes at all times; some allow sharing only during failovers.
More importantly, the application will see the cluster as a single system image and not as multiple connected machines. The cluster manager does not expose the system to the application, and the cluster is transparent to the application. Sharing the data among the available nodes is a fundamental concept in scaling and is achieved using several types of architectures.