Research of concurrency control protocol based on the main memory database

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1 Research of concurrency control protocol based on the man memory database Abstract Yonghua Zhang * Shjazhuang Unversty of economcs, Shjazhuang, Shjazhuang, Chna Receved 1 October 2014, The concurrent access of data can cause the nconsstency. How to control t effcently s one of the queston of the frst mportance n the database applcaton system. In ths paper a management mechansm called dynamc mult-gran lock and a related concurrency control protocol were gven. Keywords: concurrency access, management mechansm, dynamc mult-gran lock, protocol 1 Concurrency control overvew In the database system, the transactons can be executed serally one by one, that s, there s only one transacton runnng each tme and the other transactons have to wat untl the end of that transacton. In order to fully utlze the system resources and develop the feature of database shared resources, the multple transactons to be executed n parallel should be allowed. In the applcaton system, the process for the multple users to concurrently access the same data s called concurrent access of the data. When the multple users access the database concurrently, there would be the stuaton of multple transactons accessng the same data at the same tme. If no control has been appled n the concurrent operaton, t s possble to access the wrong data, whch would destroy the database consstency. Therefore, the database management system should provde the concurrency control mechansm. When the multple users access the database concurrently, there would be the stuaton of multple transactons accessng the same data at the same tme. If no control has been appled on the concurrent operaton, t s possble to access the wrong data, whch would destroy database consstency. Therefore, the database management system should provde the concurrency control mechansm. For the man memory database system, the data lockng overhead and processng overhead are almost the same. The access speed of memory s much faster than that of the dsk, whch has sgnfcantly reduced the transacton executon tme than the dsk database; correspondngly, the lock occupaton tme has been shortened dramatcally. Therefore, the essental advantages and necessty of fne graned lockng have been lost. Hence the greater gran lock s generally adopted n the man memory database such as relatonshp level or database level, whch has undoubtedly reduced the complexty of concurrency mechansm and the burden n the system, thereby mproves the overall system performance. 2 Concurrent executons of transactons If the concurrent executons of transactons are not reasonably scheduled, the transactonal solaton wll be destroyed n causng the nconsstency of the data storage area. Generally speakng, the concurrency control s not requred f the access data set of the concurrent transactons s not correlated or ntersected, or the wrte set of transactons and the readng set of the other transactons are not ntersected. However, the data storage area s a shared resource. The sharng way of the storage nformaton s to allow the multple user access n modfyng the same data. That means the relatons between the transactons are much close and the access data s ntersected to a great extent. For nstance, f an update operaton and an operaton are conducted concurrently, the wrtng and readng data wll be contradcted. If two or more updates are conducted concurrently, the update nformaton may be lost. Therefore, no correct access data s obtaned wthout the correspondng concurrent access control method. How to control the concurrent access of data effcently s one of the queston of the frst mportance n the database applcaton system. Defnton 1: The defnton of the schedule S s based on the transacton set T={T 1,T 2,...,T n}, whch t s an operaton seres of the multple transactons. Defnton 2: In the schedule S, the operatons of each transacton are not overlapped (namely, n sequence); t wll be the seral schedule. Defnton 3: The schedule S s seralzable. When the conflct S s equvalent to a seral schedule, the seral schedule s generally called conflct equvalence seralzablty. The basc functon of a concurrent controller (or scheduler) s to generate a seral schedule n runnng the transactons. In the man memory database system, the majorty of the concurrent control protocols are based on the seralzable theory. The concurrent control protocol s used to control the schedule of the data of whch ts major * Correspondng author s e-mal: @qq.com 141

2 objectve s to mantan the data consstency and provde data lock and s appled to record the dstrbuton status of the maxmzed concurrency. the data lock. The status of the data lock ncludes read In some cases, t s able to approprately loosen up the locked, wrte locked and unlocked. The data lock has requrements of consstency. The man memory database recorded all the transactons of the locked data. The locked s the shared data nherent n the memory. Whle the data transactons are dvded nto read lock holder and transactons are conducted concurrently n the dfferent wrte lock holder. At the same tme, t wll record all of the threads, t may cause the conflct to access the same data blocked wat transacton lsts n applyng the data lock. In object n the man memory database. The concurrent the lock manager, there s a compatblty lst. When the control protocol s a strategy to solve the conflcts between transacton lock s appled, the lock manager wll the transactons. determne whether to block the lock applcant accordng to The common conflctng modes are stated: the compatblty lst; when the transacton lock s released, read-wrte conflct and wrte wrte conflct. the transacton wll be aroused to obtan the lock when the blocked transacton s allowed to obtan the lock. In terms read-wrte conflct: To access the same data object of the manager, the transacton wll lock the data tem at may cause the conflcts whle a transacton s runnng the the logcal level. The manager wll schedule the gran from read operatons and another transacton s runnng the the dynamcs of the lock. When the conflcts are decreased, wrte operaton. t s suggested to use the table lock; when the conflcts are wrte-wrte conflct: To access the same data object enhanced, the tuple lock s appled. may cause the conflcts whle a transacton s runnng the There s a close relatonshp between the wrte operaton and another transacton s also runnng mplementaton of MMDB lock manager and the database the wrte operaton. organzaton. The MMDB system s composed of data and In the man memory database, to resolve the conflct metadata. Each segment wll correspond to a table n the based on the concurrent control protocol manly depend on database. It s known that there s a drect correlaton two strateges: between the metadata and segment n the table lock, Wat: termnate the conflctng operaton caused by one between the metadata and partton n the tuple lock. transacton and keep the pendng state untl the operaton Whle the transacton submts the lock request and of another transacton s completed. For the concurrent MML receves the lock requests, t needs to dscover the transacton control study n the man memory database, lock gran nformaton n the correspondng segment three knds of watng strateges are derved: control block that wll be converted nto the table lock; f 1) No wat: the watng transacton s mmedately the tuple lock s detected, the tuple lock s appled. If the ended nstead of the completon of another transacton transacton sends the table lock request, the table lock s operaton. appled wthout beng refned. 2) Wound wat: accordng to the arrval tme of each Dynamc Reducton of Lock Gran transacton, the transacton n runnng the data wll be The general condton of the dynamc reducton of lock ended when the transacton n obtanng the data s arrved gran s stated n the followng part. When the lock gran s n an earler way; otherwse, the transacton n obtanng tuple lock, the lock gran nformaton can be set as the table the data wll be ended. lock when all the locks are compatble. Accordng to the 3) Wat de: accordng to the arrval tme of each transacton, the transacton contnues to wat when the transacton nformaton n holdng the lock, t s able to set transacton n obtanng the data s arrved n an earler way. the control block of the table lock. Otherwse, the transacton s ended. Dynamc Increase of Lock Gran Revert: undo the conflctng operaton. When the Whle ncreasng the lock gran, t needs to record the transacton s reset, t s able to go back to the ntal stage. tuple lock requests of the transacton n the actve state. When t needs to use the tuple lock, the memorzed lock 3 Implementaton of man memory database lock wll be converted nto the real tuple lock. In addton, the lock gran nformaton s set as the tuple lock n the table. There are two major approaches to mprove the The locked gran s closely correlated wth the system performance n MMDB. Frst of all, change the database concurrency and the overhead of the concurrent control. In storage structure, put the tuple data and data together, general, the locked gran s greater wth fewer locked acheve a drect access of the metadata and decrease the objects, smaller selectvty, lower concurrency and smaller overhead of the metadata; secondly, the dynamc multgran lock mechansm s appled n the concurrent more locked objects, greater selectvty, hgher overhead; on the contrary, the locked gran s smaller wth operaton. In the lock mechansm, t s requred to obtan concurrency and greater overhead. the correspondng lock before the transactons access the At the logcal level, the dstrbuted database s an entty. data object. There are two basc types of locks: read lock At the physcal level, the dstrbuted database s stored n (shared lock) and wrte lock (exclusve lock). Multple the vared physcal nodes. Therefore, the chef and prompt transactons can share the read lock of the same data rather ssue s to effectvely conduct the concurrent control of the than the wrte lock of the same data. We have adopted the data n the database applcaton system. The current task s management mechansm of the dynamc mult-gran lock to establsh the relable and feasble concurrent control n our research system. The lock manager has developed a strategy n the remote database. 142

3 4 Self-control lock 4.1 OVERVIEW OF SELF-CONTROL LOCK The so-called self-control lock s to dscard the lock manager whle the lock s appled and released through the shared memory operatons. Snce there s no lock management progress, t s able to avod the low effcency n the management process. All the requests and releases are drectly operated n the shared memory whle the ndex does not requre ntra-process communcaton. mechansm. In the tradtonal lock mechansm, there s the correspondng lock manager for each lock to conduct the relevant operatons rather than usng the key component. The lock operaton code can be ndependently conducted and t s able to ensure the specfc LCB consstency through the key component. The communcaton effcency s much hgher than that of nter-process communcaton Recovery When the dfferent records of the same page are locked through the transactons at the dfferent nodes, t s requred to obtan the renewal and cancellng nformaton so as to ensure that the transacton s not qutted for the node falure. To renew the record s temporary whle to cancel the record s to store the nformaton. For nstance, before the record s transferred to another node n one page, the correspondng transacton wll be recorded n the log on account of the renewal and cancellng nformaton for the processed database object. In order to ensure that the renewal record of the transacton s recorded before t s transferred; the transacton can hold a short-term lock untl the renewal nformaton s recorded. FIGURE 1 Database System Structure In terms of the self-lock, the data tems are correlated, namely each data tem corresponds to a LCB. A LCB wll record the current lock types and two transacton tables that one s used to record the current transacton n holdng the lock whle the other s used to record the transacton n watng the lock. Seemngly, there s no bg dfference for the mechansm and the mult-processor developers to realze the semaphore. In fact, there are the bg dfferences: frstly, t s dfferent from the semaphore, the database lock must be controlled so as to ensure the falure atomcty of the transacton; secondly, the database lock s also used to support the other lockng modes other than the shared lock and exclusve lock (Fgure 1). 4.2 IMPLEMENTATION OF SELF-LOCK Operaton of self-lock In vew of the applcaton condtons of the lock, a request ncludes the name and type of the lock. After the hash processng of the name of the lock, t wll be converted nto the correspondng lock control block (LCB). All the updates are conducted through a key component. If the requested type s compatble wth the current type of LCB and there s no conflctng wat process, the record about the process and the request type s added n the holder lsts and t s successfully locked. Otherwse, the record s added nto the wat process lsts and the system wll return back the unapproved nformaton to the requester. The strategy of releasng the lock s smlar. Whle fndng the correspondng LCB, the correspondng record wll be deleted n the holder lst. If there are the compatble lock requests n the watng process, the request wll be approved. The strategy s smlar wth the tradtonal lock Ensure the falure atomcty of the locked space Due to the nfluence of the consstency protocol, LCB s appeared n one node. Whle two transactons at the dfferent nodes obtan the same compatble lock, LCB wll be located at the node n obtanng the lock. Therefore, when one node s broke, some nformaton of the transacton wll be lost rather than all of the LCB. In order to ensure the falure atomcty of the transacton, the measures are taken: before the transacton obtans the lock, t needs to record the transacton dentfer (Tm); n addton, before the lock s requested, t wll temporarly record the name of the lock and apply the transacton s Tm. These logs are used to ensure the consstency of the space whle the node s faled Relevant Performance Analyss The self-lock request can be drectly appled to the shared memory n greatly mprovng ts effcency. In the SD and SM database, the mechansm of the hgh lock gran s smlar. Through the nterrupton rate evaluaton of the large-scale share memory mult-processor system, t s seen that for the general lockng condton, CL s overhead s ncreased wth the conflctng lnear growth. The nterrupton rate s a constant. For the hot lock, the nterrupton performance s almost an order of magntude greater than that of CL. 5 Recoverable user-level spn lock What s spn lock? It s a lock mechansm to protect the shared resources. When the conflcts are reduced, the effcency of the spn lock s much hgher than that of the semaphore. Spn lock s busy watng process n consumng the CPU resources. However, t wll not cause 143

4 the sleepng of the thread and the user state wll not be long for the lock request, t needs to call the cleanng swtched to the system status. Spn lock s appled when process n releasng the lock or conductng the the lock hold-up tme s shorter wth more CPU resources. correspondng recovery operaton. In order to mplement the spn lock, t needs to set the spnnng tmes and prevent the endless loop. In terms of 5.2 Relevant Performance of spn lock the recursve call, the spn lock s forbdden of whch t wll trgger the dead lock. For many spn locks, the atomc Snce the spn lock has escaped the system, ts operaton operaton s not avalable n obtanng the lock and effcency s hgher than that of the semaphore. If t s regsterng the holdng nformaton. When the key unable to correctly get the holder nformaton, t needs to component s termnated or the processng speed s slow restart the system and recover the nformaton when the down, t s unable to correctly determne the holder. It s holdng process s ended. It s a huge overhead n able to conduct the recoverable user-level spn lock snce overrdng the orgnal advantages of spn lock. Based on t s able to correctly determne the nformaton of the the above algorthms, t has no need to restart the system, holder of the lock and be appled to the re-nstallaton of greatly mprovng the processng effcency based on the the protected data tem and the subsequent lock release dead process and gvng full play to the merts of spn lock. operaton. In ths way, the system rebootng s avoded. 6 Proposal of the protocol 5.1 Implementaton of the recoverable spn lock Frst of all, t needs to add a data structure n each process to record the requred lock. It s called want doman. In ths way, t s helpful to determne the process set n holdng the lock. Durng the process of determnng the holder, the lock may be requested by the other processes n causng the uncertanty of the process set. Therefore, t needs to add a semaphore for the lock n determnng the holder. If t s n progress, the other processes may gve up the request of the lock and want doman. The pseudo code of the correspondng data structure s stated as follows: struct SafeSpnLock{ nt lock; ProcessID owner; nt cleanup_n_progress; }; struct LockAccessRecord{ SafeSpnLock*wants; }; The correspondng cleanng process s stated as follows: Set the lock s cleanup_n_progress to prevent the mssng of the want doman, and get the lock n the cleanng process. 1) Decde all the possble processes n holdng or obtanng the lock 2) Conduct the loop operaton untl ether of the two condtons s satsfed: A. The status of the spn lock becomes clear n beng dle or held by the actve process. B. The process set s empty. The lock s not held by any actve process. If A s avalable, the locked status s mmedately determned. Otherwse, t has to wat for a perod of tme. Some processes are excluded and reorganzed. If A s not satsfed for all the tme, B s fnally satsfed. The lock may be dle or beng held by a dead process. If the lock holder nformaton s no_progress n the dead process, t s known that the process s ended n obtanng the lock and regsterng the nformaton, or releasng the lock. Under such condtons, the lock wll be released at ease; f the holder nformaton s known, t can conduct the recovery operaton and release the lock agan. When there s the deal process or the watng tme s too Accordng to the current research on the man memory database and the characterstcs of man memory database, the matchng concurrent control protocol s proposed n the paper. The protocol model s ntroduced to ensure the seralzablty of the transacton and preventng the other transacton to read the drty data wthout delverng the transacton. Tradtonally, the lock based on the concurrent control protocol ncludes read lock and wrte lock. The share and non-share relatons are appled between the locks. In ths way, there are three knds of the blockng approaches n the system: wrte block read block wrte block In the system model, there s a new relaton between read lock and wrte lock, that s to say the orderly sharng relaton n cancellng the above mentoned frst and thrd blocks. For nstance: n order to cancel the read-wrte block, the other transacton t can stll conduct the wrte lock on the data object when the transacton t j has conducted the read lock on a data object. At ths tme, t s known to be the orderly sharng relaton between t read lock and t j wrte lock. In order to ensure the seralzablty of the transacton, the desgned protocol shall conform to the followng rules: Rule 1: If there s the orderly sharng relaton between the obtaned lock of the transacton t and the holdng lock of the transacton t j, The correspondng t operaton s conducted untl the correspondng t operaton s completed (controlled by the schedulng procedure). t s submtted untl the endng/submsson of t j. The endng of a transacton wll cause the consecutve endng of the transactons n readng the drty data, whch t wll have a great nfluence to the system performance. For the second blockng approach, the orderly sharng relaton s not avalable n our establshed lockng model. Based on the protocol, the transacton can just read the submtted data. Suppose T t1, t2,..., T n s the transacton set of D t 1,2,, n s the access the system ndex and 144

5 set of the transacton t. As well, represents that conducts the read lock on the data object x ED( t ) ; represents that t conducts the wrte lock on x; represents there s the orderly sharng relaton between the holdng read lock of x based on t and the wrte lock of x based on t j. PRI(t ) represents the prorty of the transacton t. The protocol can be brefly descrbed as: when the transacton t T, RL x t,,, WL x, t OS RL x t t T s mplemented, x D( t ), t wll conduct the lockng operaton on the x. In the submttng or endng process, t wll release the holdng locks. When t tres to conduct the wrte lock on x, t can stll obtan the wrte lock on the data object f x s read or wrte locked by the other transactons. As well,, j, j or OS RL x, t j, OS RL x, t,. When t s ready to be submtted, t stll keeps the watng status when Rule 1 s not satsfed or the date of t s not expred. Otherwse, t wll be submtted when Rule 1 s satsfed. If the date of t j s expred, t has the top prorty. It wll be submtted through the transactons when t s ended prevously or has the OS relatons. When t tres to conduct the wrte lock on x, the transacton t can stll obtan the read lock on the data object when x s conducted the read lock through another transacton t j. If x s read locked by t, the data object s wrte blocked through the other transactons, that s to say. PRI t PRI t j If, t can just obtan the read lock when t j s submtted or ended. If PRI t PRI t j, t needs to estmate whether the transacton t can be completed at the deadlne (to judge whether t s completed at the deadlne, t can compare the References present tme wth the flled deadlne of the transacton and the runnng tme estmates). If t j can be completed at the deadlne, t j wll be ended and t wll be allowed to conduct the read lock on x; f t cannot be completed at the deadlne, t wll be ended. It s seen that the ntroducton of the orderly sharng relaton can greatly reduce the blockng stuaton n the system, but the submsson of the current transacton may be postponed. The major desgn objectve of ARTDEIS s to satsfy the tme lmt of the ntatve real-tme transacton rather than the response speed. Therefore, the delay of the submsson does not go aganst ts major desgn prncple or stop the read/wrte operaton of the other transactons. When some hgh prorty transacton tres to end a low prorty transacton, t wll estmate whether the hgh prorty transacton can be completed at the deadlne so that t can prevent the runnng transacton beng ended by the other endng transacton (It s called wastng end). Before the transacton s submtted, all of the databases n conductng the wrte operaton are locked. Therefore, the other transactons cannot read the uncommtted data so as to ensure the seralzablty of the transacton. 7 Summary and nnovatons In ths paper, the author nvestgates the concurrent control mechansm n the tradtonal database system and analyzes the general control approaches. Based on the characterstcs of the man memory database, the matchng concurrent control protocol s proposed to ensure the seralzablty of the transacton. [1] Wang H, Pan Z 2004 Analyss of Data Structure n Man Memory Database Modern Electroncs Technque 27(3) [2] Ma H, Yang B, Yao J 2003 Appled Mult-Dmensonal Index Structure n the Man Memory Database System Computer Engneerng and Applcatons 39(29) [3] Lu Y, Lao G 2003 Appled Recovery System for Real-Tme Man Memory Database Journal of Chnese Computer System 24(3) [4] Wang R, Huang Q, Tang L, L W, Chen Q, Long K 2006 Memory Management of Embedded TCP/IP Protocol Mcrocomputer Informaton [5] Zhou Y 2010 Improved Lockng Protocol n Man Memory Database Computer Systems Applcatons 19(12) [6] Zhang Y, Lan L 2012 Concurrent Access Algorthms Based on the Mult-Gran Lockng Transactons Mcrocomputer & Its Applcatons 05 [7] Sang C 2013 Research on Consstency of Mult-Tenant Database [D]Shandong Unversty 2013 [8] Pands I, Johnson R, Hardavellas N, Alamak A 2010 Data Orented Transacton Executon VLDB [9] Curno C, Jones E P C, Madden S, Balakrshnan H 2011 Workload Aware Database Montorng and Consoldaton SIGMOD [10] Zheng Y, Sang C, Meng X, L Q Tenant Orented Lock Granularty Adjustment Strategy n the Shared Storage Mult-tenant Database ICNDS JDCTA Author, , Shjazhuang, Chna. Current poston, grades: Drector, Engneer n Shjazhuang Unversty Of Economcs. Unversty studes: Shjazhuang Unversty Of Economcs and Xdan Unversty. Scentfc nterest: database and database concurrency control, nternet, data encrypton. Publcatons: 11 papers. 145