On-the-Fly Calculation and Verification of Consistent Steering Transactions

Interactive Steering can be a valuable tool for understanding and controlling a distributed computation in real-time. With Interactive Steering, the user may change the state of a computation by adjusting application parameters on-the-fly. In our system, we model both the program’s execution and steering actions in terms of transactions. We define a steering transaction as consistent if its vector time is not concurrent with the vector time of any program transaction. That is, consistent steering transactions occur "between" program transactions, at a point that represents a consistent cut. In this paper, we present an algorithm for verifying the consistency of steering transactions. The algorithm analyzes a record of the program transactions and compares it against the steering transaction; if the time at which the steering transaction was applied is inconsistent, the algorithm generates a vector representing the earliest consistent time at which the steering transaction could have been applied.