High-Performance Parallel Database Processing and Grid Databases- P8

High-Performance Parallel Database Processing and Grid Databases- P8: Parallel databases are database systems that are implemented on parallel computingplatforms. Therefore, high-performance query processing focuses on queryprocessing, including database queries and transactions, that makes use of parallelismtechniques applied to an underlying parallel computing platform in order toachieve high performance.
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High-Performance Parallel Database Processing and Grid Databases- P8330 Chapter 11 Grid Concurrency Control (2) The global transactions currently executing are added to a set, which stores all active transactions. The set of active transactions is represented as Active Trans. (3) The middleware appends a timestamp to every subtransaction of the global transaction before submitting it to the corresponding database. (4) If there are two active global transactions that access more than one database site simultaneously, this creates a potential threat that local databases may schedule the subtransactions in conflicting order. The subtransactions are therefore executed strictly according to the timestamp attached to the subtransaction. Total-order is achieved by executing the conflicting subtransactions according to the timestamp. (5) When all subtransactions of any global transaction complete the execution at all the sites, the transaction terminates and is removed from Active Trans set (see details in Termination Phase). Note: Active trans and Active Trans(DB) are different. The former is the set of currently active global transactions, and the latter is a function that takes the database site as an argument and returns the set of active transac- tions running at that database site.Explanation of Figure 11.3. Line 1 of Figure 11.3 checks the number of sub-transactions of the submitted transaction. If there is only a single subtransaction,then the global transaction can start executing immediately. The global transactionis added in the active set (line 2) and is submitted immediately to the databasefor execution (line 3). If the global transaction has more than one subtransaction,that is, the transaction accesses more than one database site, then total-order mustbe followed. Hence, the timestamp must be appended to all subtransactions of theglobal transaction. The global transaction is added in the active set (line 4). Globaltransactions having only one subtransaction are filtered out from the active set,and the new set (Conflict Active trans) of the conflicting global transactions isformed (line 5). Timestamps are then appended to all subtransactions of the globaltransaction (line 6 and line 7). If the global transaction being submitted conflictswith other active global transactions, it must be submitted to the participant site’squeue to be executed in total-order. Conflict of a submitted global transaction (Ti )with some other active global transaction (T j ) (having more than one active sub-transaction) is checked in line 8. If two global transactions having more than oneactive subtransaction (i.e., global-global conflict) exist, then the global transactionis added in all participating sites’ active transaction sets (Active Trans(DBi )) (line13) and the subtransactions are submitted to the participants’ queue (line 14), tobe strictly executed according to the total-order. If the submitted global transac-tion does not conflict with any other active global transaction (i.e., line 8 is true),then the global transaction is added in the active transaction set of all the partici-pant sites (line 10), and the subtransaction is immediately submitted for scheduling(line 11). Global transactions are said to be conflicting if two global transactions havemore than two active subtransactions executing in different participating sites 11.3 Grid Concurrency Control 331Algorithm: Grid Concurrency Control Algorithm for the submission phaseinput Ti: Transactionvar Active_trans: set of active transactionsvar Conflict_Active_trans: set of active transactionsthat conflict with global transaction being submittedvar Database_accessed [Ti]: database sites being accessedby global transaction TiGenerate timestamp ts: unique timestamp is generated Split_trans(Ti) Database_accessed [Ti] DB_accessed(Ti)1. if Cardinality(Database_accessed [Ti]) D 1 S2. Active_Trans(DBi) Active_Trans(DBi) Ti // Ti has only one subtransaction3. submit subtransaction to DBi else [4. Active_trans Active_Trans(DBi)5. Conflict_Active_trans {Ti jTi 2 Active_trans ^ Cardinality(DB_accessed(Tj)) > 1}6. for each subtransaction of Ti7. Append_TS(Subtransaction) T [8. if Cardinality(Database_accessed[Ti] ( T2 Conflict Active trans DB_accessed(Tj)) Ä 1)9. for each DBi 2 Database_accessed [Ti] S10. Active_Trans(DBi) Active_Trans(DBi) Ti11. submit subtransaction to DBi // Subtransaction executes immediately else12. for e ...