  Latency is a key service factor for user satisfaction. Consistency is in a
trade-off relation with operation latency in the distributed and replicated
scenario. Existing NoSQL stores guarantee either strong or weak consistencies
but none provides the best consistency based on the response latency. In this
paper, we introduce dConssandra, a NoSQL store enabling users to specify
latency bounds for data access operations. dConssandra dynamically bounds data
access latency by trading off replica consistency. dConssandra is based on
Cassandra. In comparison to Cassandra's implementation, dConssandra has a
staged replication strategy enabling synchronous or asynchronous replication on
demand. The main idea to bound latency by trading off consistency is to
decompose the replication process into minute steps and bound latency by
executing only a subset of these steps. dConssandra also implements a different
in-memory storage architecture to support the above features. Experimental
results for dConssandra over an actual cluster demonstrate that (1) the actual
response latency is bounded by the given latency constraint; (2) greater write
latency bounds lead to a lower latency in reading the latest value; and, (3)
greater read latency bounds lead to the return of more recently written values.
