Concurrency Control In Distributed Database Systems Information Technology Essay

Concurrency Control In Distri pocketlyed Database Systems Information Technology EssayConcurrency harbor is a very important issue in distributed selective informationbase system design. This is because concurrency allows m every(prenominal) exertions to be executing simultaneously such that collection of manipulated info stage is left in a consistent state. Database concurrency image permits exploiters to plan of attack a database in a multi curriculummed fashion charm preserving the illusion that user is executing al iodin on a dedicated system. in m any(prenominal)(prenominal) event, it produces the same rear and has the same output on the database as some serial public presentation of the same effect.Concurrency take c are in distributed system is achieved by a program which is called scheduler. Scheduler help to order the trading trading operations of movement in such a way that the resulting logs is serializable. thither confound deuce geek of the concurren cy get a line that atomic human body 18 going approach and non- engageing approach. In barrier of fix approach, devil-phase fix is astray used and purpose for modify or distributed database system. Before distributed database systems approaching some fiber of database, it moldiness adopt a discarding mechanism such as separately act has to obtain a lock. After that part is locked by early(a) dealing, the opening request result be block and the exercise who making reauest has to carry. Two-phase locking ordain cause the stalemate paradox. This locking induces exalted communication cost because of the blind alley problem.Whereas in term of non-locking approach which is change integrity into cardinal type which ar Timestamp rescript (TO) and serialisation Graph examen (SGT). Timestamp ordering utilizes laughable performance timestamp for determining the serialization order. It divided into iii different modes that be basic, conservative and optimi stic modes. For Serialization Graph Testing, it has been virtually attractive log family unit until 1987 because it is cognise as the largest known class of serializable logs. there argon some limitations that give the sack found in those 2 approaches. However, there argon some solution that can used to sour the problem and will be discuss more detail in the down the stairs.2. Type and the issue of concurrency control2.1 Two -phase lockingTwo -phase locking is a widely used concurrency control technique to synchronize accesses to shared out gunpoint. Each data stop has a lock associated with it. The scheduler will graduation exercise examine the associated lock before a performance (T1) may access a data item. If there is no transaction reconciles the lock so the scheduler will obtains the lock on behalf of T1. When an separate transaction T2 hold the lock, the T1 has to live until T2 to gives up the lock. The scheduler will give the T1 the lock after the T2 dis mission the lock. Scheduler essential ensure that there is only mavin transaction can hold the lock at a time and only sensation transaction can access data item at a time. They are two types of locks associate with data item that is drive locks and salve locks. systema skeletale 1 below show that the transaction T1 and T2 follow the two-phase locking protocol. range 1 Transaction T1 and T2 with 2-phase locking protocolHowever, there is an important and pitiful property of two-phase locking schedulers is that they are subject to stands. For instances, the classic tie-up positioning in which neither of two processes can proceed that is one must release a resource and the other one needs to proceed. anyways that, stand excessively arises when the transaction try to streng consequently read locks to write locks. To ensure that there is no transaction is blocked forever, the scheduler needs a strategy for detecting the deadlock.In addition, two-phase locking is suffer from sensible delays imputable to a node needs to send message to all nodes and must receive acknowl saltationment from all nodes to discover a deadlock. The messages will mass meeting amid nodes to get decision of legal proceeding endue. The whole system will stop since non discover the deadlock immediately and a vivid of distributed system make deadlock treatment very difficult. Moreover, the message that imputable to commit and deadlock treatment make high traffic on network which is makes need of special kind of networking.Time forecast 2 A spatial schedules of T1 and T2 in state of deadlockT1 wait for T2 to release read lock on y2T3 wait for T1 to release read lock on x1T2 wait for T3 to release read lock on z3Figure 3 A wait-for chart for the spatial schedule of Figure 2 in that location are two type of graph that show as above which is a spatial schedule of T1 and T2 in the state of deadlock. some other graph show that the wait for graph for the spatial schedule of the ex cogitation 2 above. Each node of the graph represent the transaction, the edges represent the waiting for relationship.Furthermore, concurrency control also deals with starvation. The starvation reachs when a particular transaction systematically waits or sum uped and never gets a chance to proceed further. In a deadlock resolution it is possible the same transaction may consistently be selected as victim and rolled-back. This type of limitation is inherent in all priority based scheduling mechanism. The wound-wait scheme a junior transaction may always be aborted by a coarse lickning older transaction which may create starvation.Besides that, the two phases in locking method also may cause to sorry read and cascading abort. The problem of dirty read is arises when assume T1 and T2 are penalize interleave. For this condition, the T1 will get the exclusive locks for record. Supposedly, T1 release locks immediately after doing updates while T2 acquires that locks and does its update. If the T1 fail before it commits due to certain of reason, then it will fork out value to original value. T2 will continue to its execution with uncommitted data and this will cause an error on the system result. On the other hand, the problem of cascading abort arises when if a transaction aborts. There may have some other transaction already used data from an aim that the aborted transaction modified it and unlocked it. If this happen, any such transaction will also have to be aborts.2.2 Timestamp ordering techniqueTimestamp is a monotonically increase variable indicating the age of an operation or a transaction. The larger of the timestamp value indicate that more recent event or operation. Timestamp ordering technique concurrency mechanism were considered suitable for distributed database systems since transaction to be rolled back can be determined locally at each grade. It involves a uncomparable transaction timestamps in place of conventional locks. This technique is based on the idea that an operation is allowed to proceed only if all the trothing operations of older transaction have already been processed. For instance, when a transaction accesses an item the system will check whether the transaction is older than the last one which is accesses the item. If this is the case that transaction proceeds, otherwise ordering is violated and the transaction is aborted. Besides that, the serializability of the transaction is preserved and requires knowledge slightly which transaction are young than the others.In the implementation of a distributed timing system, each site in the distributed system contains a local time or a counter. This clock assumed to tick at least once between any two events. This event in spite of appearance the site is totally ordered. If the total ordering of event at different sites, it need to assigned a unique number to each site and the number is concatenated as least square bits to the current value of local bl ock. Moreover, each message contains the information about the local time of their site of origin at which the message is sent. There are many concurrency control method on timestamp ordering technique. iodin of the method is basic timestamp ordering (BTO) which is for each data item x, the largest timestamp of any write operation on data item x and the largest timestamp of any read operation on data item x which are denoted by R_TS (x) and W_TS (x) respectively.The basic timestamp ordering technique is easy to distribute and the transaction to be aborted will immediately be recognized when the operation are being scheduled. This type of method will not deal with deadlock problem because the locks are not used and operations are blocked. Hence, an atomic commitment mechanism is necessary to provide the reliability.3. Improvement of issue concurrency control3. 1standstill resolutionThe preceding implementation of two-phase locking makes the transaction to wait for unobtainable lock s. When the waiting is uncontrolled will cause a deadlock. The situation of deadlock can be characterized by wait-for graph, that graph indicates which transaction is waiting for which other transaction. There are three type of general technique are available for deadlock resolution that are deadlock detection, deadlock ginmill and timeout strategies.3.1.1 Deadlock DetectionDeadlock detection is a transaction wait for each other in an uncontrolled manner and is only aborted if a deadlock occurs. It can be detected by explicitly constructing a wait-for graph and meddling it for bicycles. Victim which is one transaction on the cycle will abort when cycle is found and thereby breaking the deadlock. There are a few victim selection criteria will be consider. First is the Current Blocker, this current blocker will pick the transaction that blocked the most recently. Secondly, Random Blocker which is a process of picks a transaction at random from the participants in the deadlock cycle . Third is the arcminute Locks that is to pick a transaction that is holding the fewest locks. Fourth is the Youngest which is picked the transaction with the most recent initial startup time. Lastly is the Min crop and accountable to pick the transaction that has consumed the least amount of physical resources (CPU + I/O time) since it first began running.In order to minimize the cost of restarting the victim, unremarkably the victim is based on the amount of resource that use by each of transaction on the cycle. Each of the two-phase lockings scheduler can be soft construct the waits-for graph based on the waits-for relationships local to that scheduler. But this is not efficient to characterize all deadlocks in the distributed system. To increase the efficiency, more international wait-for graphs have to combine with local waits-for graph. For the centralized two-phase locking will not have this type of problem since there only consist of one scheduler.In order to construct global waits-for graph, there consist of two techniques which are hierarchical and centralized deadlock detection. For the hierarchical deadlock detection, the database sites are organized into hierarchy with deadlock detector to each node of the hierarchy. Deadlock divided into many sites. Deadlock involve a single site are detected at that. Whereas deadlock involve more than two sites of the same region detected by the regional deadlock detector.On the other hand, one site is designated the deadlock detector in the centralized deadlock detection. Every few minutes, each scheduler has to send its local wait-for graph to the deadlock detector. Then the deadlock detector combines the local graph into a system wide waits-for graph by constructing the union of the local graph. Although some(prenominal) of the technique mention above differ in detail but it involve periodic transmission of local waits-for information to one or more deadlock detector sites.3.1.2Deadlock PreventionDeadlo ck prevention is a cautious scheme in which transaction is restarted when the system is afraid that deadlock to be occurring. In the process of deadlock prevention, the scheduler will try on the requesting transaction which is name (T1) and the transaction that currently owns by the lock (T2) when a lock request is denied. When T1 and T2 pass the test, T1 is permitted to wait for T2 as usual. Otherwise, one of the two is aborted. There are a few prevention algorithms that are Wound-Wait, Wait-Die, Immediate-Restart and Running Priority. In the Wait-Die prevention algorithm, if a lock request from transaction T1 leads to a involution with another transaction T2, then T1 started time earlier than T2 and block the T1 otherwise will restarted T1. The deadlock prevention will know as nonpreemptive if T1 is restarted. By this technique deadlock are impossible since there is only one transaction can be blocked by a younger transaction.In the Wound-Wait algorithm, if T1 started running be fore T2 then restarted T2 otherwise blocked T1. This type of algorithm is known as preemptive which is older transaction run though the system by killing any one that they conflict with and continues waiting onlt for older conflicting transaction. The scheduler must ensure that T1 wait for T2 so that deadlock cannot occur. There is a better approach can be assign priorities to transaction and to test priorities to decide whether T1 can wait for T2. If T1 has the frown priority than T2 then T1 will wait for T2. When they have same priorities, T1 cannot wait for T2 or vice versa. This test will prevents the deadlock to occur since every edge in the waits-for graph T1 has a lower priority than T2. In addition, the Immediate-Restart algorithm also will overcome the deadlock by on the buttonly restart T1 since there is no transaction is ever blocked.Besides that, preordering of resources is a type of deadlock avoidance technique that used to avoid restarts al unitedly. It requires pre declaration of locks which destine that each transaction obtains all its locks before execution. The priority of transaction is the number of the highest number lock on it and the data item are numbered and each transaction request locks one at a time in numeric order. Although this techniques can avoid the deadlock occur but it forces locks to be obtained sequentially which is tends to increase response time.3.1.3 Timeout strategicIn the timeout strategic, a transaction whose lock request cannot be granted is simply placed in the blocked queue. When the wait time exceeds some threshold value then the transaction will restarted. Timeout will restart transaction that involves in deadlocks in the detection strategies whereas in prevention strategic it may also restarted some transaction that are not involved in any cycle.3.2 unrelenting 2 Phase lock (S2 PL)In order to prevent cascading problem and dirty read, Strict 2 Phase Locking mechanism is apply. The penalize transaction hold s all its lock to the every end until it is committed or aborts. For the dirty read, the executed transaction does not release any of exclusive locks until it commits or aborts. S2PL has important role in the two phase locking problems. In first time transaction it need a lock on a data item, acquires it and all locks of a transaction released together when the transaction terminates and has a few resource wasting. Figure 4 below show the S2PL mechanism.Figure 4 S2PL mechanism3.3 Ordering by Serialization Number (OSN) methodThere exists a new method for concurrency control in distributed system which increases the level of synchronous execution of transaction and known as ordering by serialization number (OSN). The deadlock is prevented by this method since it works in the certifier mode and uses time interval technique in conjunction. In order to provide serializability, it combines with time interval technique with hornswoggle term locks. Scheduler is distributed and the standar d transaction execution policy is assumed. The read and write operations are issued continuously during transaction execution. The write operations are performed on the private copies of the data that issue by the transaction. The transaction is certified and its operation will applied to database when a validation test is passed by the transaction at the end of transaction. Otherwise, the transaction will restart if the test validation is not passed.To find the serialization number for a transaction, the largest serialization number of certified transaction will read item x. The largest serialization number of certified transaction which have written item x will record along the data item x which is known as RSN(x) and WSN(x). There consist of four types of short term locks in this OSN method that are R-lock, W-lock, Certified Read lock, Certified pull through lock. The R-lock and W-lock normally used when read or write the data item. It responsible to protect the data item forms two conflicting operations of concurrent transaction. Whereas Certified Read lock (CR-lock) and Certified Write lock (CW-lock) are used during the validation test while a transaction is searching for a valid time interval. These lock are held until either the transactions operation are applied to the database or the transaction is aborted.The deadlock of short term locks is prevented by a few steps. Firstly, R-lock or W-lock is obtained at any order. Since transaction do not require any other lock before releasing a particular R-lock or W-lock deadlock does not appear. Secondly, employing the preordering for deadlock avoidance can prevent the deadlock of the CR-lock and CW-lock respectively. Although this method requires the access set of transaction to known in advance but this loss eliminated in the OSN method. This is because CR-lock and CW-lock requires by the time of certification. Besides that, if a transaction requires twain CR-lock and CW-lock on data item x the system wil l grants them both when both of these locks are requires at once.4. ConclusionIn this study, the performance of concurrency control protocol on top of a comprehensively modeled high-speed network is evaluated. It found that there exist of some issue within the concurrency protocol such as deadlock and starvation in the two-phase locking. The weaknesses that occur in another concurrency protocol that is timestamp ordering.By the way, there are also discussing about the effective way to make improvement of the concurrency control by using three types of deadlock resolution to solving the deadlock problem. The deadlock resolution included deadlock detection, deadlock prevention and timeout strategic. Furthermore, a precise definition of deadlock in term of existence interval of an edge is given and these help provide a better understanding of deadlock detection is distributed system. Besides, this paper also discuss the complete specification of a secure version of two phase locking p rotocol. The interaction between the protocol and the standard failure recovery procedures were discuss and modification to the commit and restart procedures were proposed.5. ReferenceAbdou R. Ali and Hany M. 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