Before revision:  Since (n,k) fork-join queues are prevalent in modern popular distributed systems such as Cassandra, MapReduce and Spark, estimating the sojourn time of such queues is critical for these systems' performance measurement and resource plan  .
After revision:  Since (n,k) fork-join queues are prevalent in modern popular distributed systems such as Cassandra, MapReduce and Spark, estimating the sojourn time of such queues is critical for the performance measurement and resource plan .