(4) Cassandra uses a weak consistency model to maintain different replicas of an object. When a write request is received in a node, the system routes the requests to the replicas and waits for a quorum of replicas to acknowledge the completion of the write. Read requests are based on the consistency guarantees required by the client. The system can either route the request to the closest replica or send the request to all replicas and wait until a quorum of replicas respond. Cassandra with TTL set up to make it completely soft state, but this is an unusual mode of usage i.e. essentially a transient cache. Soft state might also apply to the chat protocol within Cassandra. New nodes can determine the state of the cluster from the chat messages it receives and this cluster state must be constantly refreshed to detect unresponsive nodes.
(5) In HDFS (Hadoop DFS), applications create a new file by writing the data to it. Once the file is closed, the bytes cannot
the file by reopening the file again. HDFS implements a singlewriter, multiple-reader model. Every time a node opens a file, it is granted a lease for the file, no other client can write to that file. A hit to the NameNode permits the lease to be extended. Once the lease expires and the file is closed, the changes are available to the readers. In Hadoop DFS Queues are allocated a fraction of the capacity of the grid in the sense that a certain capacity of resources will be at their disposal. All applications submitted in the queue will have access to the capacity allocated to the queue. Administrators can configure soft limits and optional hard limits on the capacity allocated to each queue. (6) Haystack: Even when a needle (a photo) in a Haystack is stored in all physical volumes of a logical volume, all updates go through the same logical volume and are applied to all the replicas of a photo. A store machine receives requests to create, modify and delete a photo. These operations are handled by the same store machine.