DEPARTMENT OF COMPUTER SCINCE AND ENGINEERING
TOPIC:-
Transaction Scheduling in Distributed Real-Time Systems
By:- Mohd Su...
Distributed And Real Time database system
• In a distributed
database system,
information is stored
across a number of
sit...
• A real-time database system (RTDBS) is one whose
basic specification and design criteria must include the
requirement of...
Transactions To Access Data
• Each distributed transaction is a collection of sub-
transactions that execute at various si...
• Transactions are scheduled to access
resources such as CPU and data objects based
on their priorities
• The priorities o...
Goal of scheduling transactions in RTDBS
• To meet the timing constraints
• To ensure the data consistency.
concurrency co...
Optimistic concurrency control
• Popular approach to concurrency control for
RTDBS
• properties of the OCC protocols are n...
there are three possible types of conflicts write-read conflicts,
write-write conflicts , read –write conflicts
adjustment...
• To reduce the number of unnecessary restarts in OCC
protocols,Lee et al (Lee, 1993b) suggested the OCC-TI
protocol which...
The DOCC-DA Protocol
• In the DOCCDA protocol, serialization order is
maintained by assigning timestamps to
transactions
•...
• In each site, a data object table and a
transaction table are maintained.
• The data object table keeps a read timestamp...
RTS.(Dx ) : the latest timestamp among the committed
transactions that have read
the data object Dx ;
WTS.(Dx) : the lates...
Read Phase
• When a sub-transaction of Ti wants to read or
pre-write a data object Dx in its private
workspace, it will fi...
Validation Phase
• When a sub-transaction of Ti receives a
validation packet, it will update its SOT(Ti) if
the received S...
To upgrade the PR-lock to the VR-lock on Dx ; Ti will check the value
of TR(Ti ; Dx )
to ensure that the version of Dx rea...
Write Phase
• For each sub-transaction, if a “Commit” message is
received, Ti will abort those serious conflicting
transac...
Conclusions
• The traditional optimistic approach suffers from
the problem of unnecessary restarts
• This problem affects ...
• new real-time DOCC protocol, called DOCC-DA
• alleviates the problem of unnecessary restart
by dynamic adjustment of ser...
• design of the DOCC-DA protocol is suitable to
distributed environments in the sense that it
reduces the number message p...
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  • Ppt on transaction schduling in distributer real time database system(seminaar)

    1. 1. DEPARTMENT OF COMPUTER SCINCE AND ENGINEERING TOPIC:- Transaction Scheduling in Distributed Real-Time Systems By:- Mohd Suhail B.Tech,C.S.E , 3rd Year
    2. 2. Distributed And Real Time database system • In a distributed database system, information is stored across a number of sites interconnected through a reliable communication network. Each site has a system-wide unique identifier, and sites communicate through messages. IMG:-Real time distributed database system
    3. 3. • A real-time database system (RTDBS) is one whose basic specification and design criteria must include the requirement of meeting the timing constraints of real- time transactions • A transaction must be completed within a specified time, called deadline. • Common applications of RTDBS can be found in international financial market systems, air traffic controlling systems, nuclear power plant management systems and integrated manufacturing systems.
    4. 4. Transactions To Access Data • Each distributed transaction is a collection of sub- transactions that execute at various sites where the requested data objects reside. • Each transaction is assigned a globally unique priority based on its real-time constraint. • A transaction is executed in three phases (Kung, 1981): a read phase, a validation phase and a write phase if the validation is successful. • Validation phase ensures that the execution of a validating transaction preserves serializability
    5. 5. • Transactions are scheduled to access resources such as CPU and data objects based on their priorities • The priorities of transactions can be static and dynamic (Ramamritham, 1993). • dynamic priority assignment achieves better system performance than static one in most cases • Conflict resolution relies on transaction restart
    6. 6. Goal of scheduling transactions in RTDBS • To meet the timing constraints • To ensure the data consistency. concurrency control in database systems can be used to maintain data consistency, there are many protocols for concurrency control, one of them is optimistic concurrency control (occ). It is very popular protocol for RTDBS.
    7. 7. Optimistic concurrency control • Popular approach to concurrency control for RTDBS • properties of the OCC protocols are non- blocking, deadlock-free and fruitful restarts • In OCC protocols, conflict resolution is performed at the end of a transaction execution. Therefore, restarting other conflicting transactions must result in a transaction commitment. We called this fruitful restart
    8. 8. there are three possible types of conflicts write-read conflicts, write-write conflicts , read –write conflicts adjustment are made to resolve these conflicts may be backward adjustment ,forward adjustment . • OCC-FV protocol has a serious problem of unnecessary restarts (Lam, 1995; Lee, 1993b) which is caused by an ineffective validation scheme To Avoid Unnecessary Restarts Dynamic Adjustment of Serialization Order with Thomas’ Write Rule is used Problem of Unnecessary Restarts
    9. 9. • To reduce the number of unnecessary restarts in OCC protocols,Lee et al (Lee, 1993b) suggested the OCC-TI protocol which utilized the notion of dynamic adjustment of serialization order (DASO) and timestamp intervals (Konana, 1997).
    10. 10. The DOCC-DA Protocol • In the DOCCDA protocol, serialization order is maintained by assigning timestamps to transactions • The timestamp of each transaction, called the serialization order timestamp (SOT), • SOT of a transaction indicates its relative position in the serialization order. • Each sub-transaction of a transaction Ti will also carry its own SOT(Ti) at its local site to perform its local validation
    11. 11. • In each site, a data object table and a transaction table are maintained. • The data object table keeps a read timestamp and a write timestamp for each data object in the local database • They are defined as follows
    12. 12. RTS.(Dx ) : the latest timestamp among the committed transactions that have read the data object Dx ; WTS.(Dx) : the latest timestamp among the committed transactions that have written the data object Dx : The transaction table at each site maintains the following information for each local active transaction or sub-transaction Ti : RS.(Ti) : the read set of Ti ; WS.(Ti) : the write set of Ti ; SOT.(Ti ) :0 the serialization order timestamp of Ti ; TR.(Ti ; Dx ) : the value of WTS.(Dx ) of the data object Dx when Ti reads Dx ;
    13. 13. Read Phase • When a sub-transaction of Ti wants to read or pre-write a data object Dx in its private workspace, it will first obtain the PR-lock or PW- lock respectively. These locks will be granted if there is no VR-lock or VW-lock. In the read phase, there is no need for Ti to detect conflicts. However, the write timestamp of each data object read will be recorded. That is, if Ti wants to read Dx , the value of WTS.( Dx ) will be recorded into TR.(Ti ; Dx ). If Ti wants to write Dx , the new value of Dx will be pre-written into its private workspace.
    14. 14. Validation Phase • When a sub-transaction of Ti receives a validation packet, it will update its SOT(Ti) if the received SOT(Ti ) is earlier. Afterwards, Ti will upgrade its local PR-locks and PW-locks to the VR-locks and VW-locks respectively one by one. If there is a VR-lock or VW-lock being held by another transaction, Ti will be blocked until the lock is released.
    15. 15. To upgrade the PR-lock to the VR-lock on Dx ; Ti will check the value of TR(Ti ; Dx ) to ensure that the version of Dx read by Ti is the one written by a committed transaction whose serialization order precedes that of Ti , i.e. TR(Ti ; Dx ) is earlier than SOT(Ti ). If TR(Ti ; Dx ) is later than SOT(Ti ); Ti will send “Abort” message to the parent transaction that will globally abort the whole transaction because a committed transaction has invalidated the value of Dx that Ti has read. Otherwise, the VR-lock will be granted. If there is a PW lock held by another transaction, Ti will either create or update the token for the conflicting transaction and set the FOR flag in the token.
    16. 16. Write Phase • For each sub-transaction, if a “Commit” message is received, Ti will abort those serious conflicting transactions in the SERIOUS set. For those transactions in BTRAN set, their SOT values are updated to SOT(Ti ) " where " is a sufficiently small value. Then Ti will update the values of RTS.(Dx ) and WTS.(Dx ) to the received SOT.(Ti) value on Dx held by its VR-lock or VW-lock. Finally, the prewritten data objects will be made permanent in the database following Thomas’ write rule. Finally, all its VR-locks and VW-locks will be released. On the other hand, if “Abort” message is received, all its locks are released and all prewritten data objects are disregarded
    17. 17. Conclusions • The traditional optimistic approach suffers from the problem of unnecessary restarts • This problem affects the performance of RTDBS as transaction restarts can significantly increase the system workload and intensify resource and data contention • Transaction restarts in distributed RTDBS make a transaction more difficult to meet its deadline than in a centralized one because of the communication overhead
    18. 18. • new real-time DOCC protocol, called DOCC-DA • alleviates the problem of unnecessary restart by dynamic adjustment of serialization orders of the concurrent executing conflicting transactions with respect to a validating transaction. • Under the DOCC-DA protocol, only those transactions with serious conflict with the validating transaction will be restarted
    19. 19. • design of the DOCC-DA protocol is suitable to distributed environments in the sense that it reduces the number message passing between different sites by using a new distributed circular validation scheme • DOCC-DA protocol gives a greater performance gain when transactions have a large proportion of read operations. by the use of DASO

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