Appendix B : Implementation of the Data Manager Resource

var t: int # index into the status table # search transaction table for entry for tid. let t be index of entry : : : # write modified objects to the "non-current" copy fa i := 1 to statusTable[t].numDataItems-> ss.write(statusTable[t].dataAddrs[i] + (currentPointers mod 2 + 1), sizeof(data), statusTable[t].memCopy[i]) af free(statusTable[t].memCopy) end prepareToCommit proc commit(tid) var t: int # index into the status table # search transaction table for entry for tid, let t be index of entry : : : # if entry cannot be found, return-transaction has committed already : : : if statusTable[t].transStatus = 'A'-> # transaction hasn't committed yet # replace current pointers of data items by new current pointers fa i := 1 to statusTable[t].numDataItems-> ss.write(statusTable[t].dataAddrs[i], sizeof(int), (currentPointers mod (sizeof(data)+1) + 1)) af statusTable[t].transStatus := 'D' # mark transaction as done fi if statusTable[t].transStatus = 'D'-> # cleanup ss.write(statusTable + t*sizeof(transInfoRec), sizeof(trans), statusTable[t]) lockManager.unlock(tid, statusTable[t].dataAddrs) statusTable[t].transStatus := 'E' # mark table slot as being empty ss.write(statusTable + t*sizeof(transInfoRec), sizeof(trans), statusTable[t]) fi end commit Figure 11: prepareTocommit and commit Operations 24 proc read(tid, dataAddrs, data, numDataItems); # search transaction table for entry for tid. let t be index of entry : : : fa i := 1 to numDataItems-> j = 1; do (statusTable[t].dataAddrs[j] != dataAddrs[i]-> j++ od data[i] = statusTable[t].memCopy[j] af end read Figure 10: read Operation items in the stable store to point to the new version written by prepareToCommit. Following this, the status of the transaction is changed to done ('D') in both the volatile and stable storage versions. The data items are then unlocked. Finally, the transaction status is changed to empty ('E'), with the change being reflected onto stable store as well. Since the transaction manager that coordinates the various data managers may reissue commits when recovering from a failure, the commit operation may be re-executed in part or in total an arbitrary number of times given inopportune failures. Our implementation takes this into account by constructing this operation as a restartable action. The abort operation is similar to the commit and is therefore omitted for brevity. Here we present the rest of the procs exported by the data manager described in Section 4. As outlined there, the data manager keeps track of all in-progress transactions in a status table transStatus. proc startTransaction(tid, dataAddrs, numDataItems) var t: int # index into the status table P(statusTableMutex); # find an empty slot t …