Chapter 10 : Concurrency Control

Transcription

1 Chapter 10 : Cocurrecy Cotro modified from: Database System Cocepts, 6 th Ed. Siberschatz, Korth ad Sudarsha See for coditios o re-use

2 Ituitio of Lock-based Protocos Trasactios have to acquire ocks o data items before accessig them If a ock is hod by oe trasactio o a data item this restricts the abiity of other trasactios to acquire ocks for that data item By ockig a data item we wat to esure that o access to that data item is possibe that woud ead to o-seriaizabe schedues The trick is to desig a ock mode ad protoco that guaratees that Lock-based cocurrecy protocos are a form of pessimistic cocurrecy cotro mechaism We avoid ever gettig ito a state that ca ead to a o-seriaizabe schedue Aterative cocurrecy cotro mechaism do ot avoid coficts, but determie ater o (at commit time) whether committig a trasactio woud cause a o-seriaizabe schedue to be geerated Optimistic cocurrecy cotro mechaism CS425 Fa 2013 Boris Gavic 10.2 Siberschatz, Korth ad Sudarsha

3 Lock-Based Protocos A ock is a mechaism to cotro cocurret access to a data item Data items ca be ocked i two modes : 1. excusive (X) mode. Data item ca be both read as we as writte. X-ock is requested usig ock-x istructio. 2. shared (S) mode. Data item ca oy be read. S-ock is requested usig ock-s istructio. Lock requests are made to cocurrecy-cotro maager. Trasactio do ot access data items before havig acquired a ock o that data item Trasactios reease their ocks o a data item oy after they have accessed a data item CS425 Fa 2013 Boris Gavic 10.3 Siberschatz, Korth ad Sudarsha

4 Lock-Based Protocos (Cot.) Lock-compatibiity matrix A trasactio may be grated a ock o a item if the requested ock is compatibe with ocks aready hed o the item by other trasactios Ay umber of trasactios ca hod shared ocks o a item, but if ay trasactio hods a excusive ock o the item o other trasactio may hod ay ock o the item. If a ock caot be grated, the requestig trasactio is made to wait ti a icompatibe ocks hed by other trasactios have bee reeased. The ock is the grated. CS425 Fa 2013 Boris Gavic 10.4 Siberschatz, Korth ad Sudarsha

5 Lock-Based Protocos (Cot.) Exampe of a trasactio performig ockig: T 2 : ock-s(a); read (A); uock(a); ock-s(b); read (B); uock(b); dispay(a+b) Lockig as above is ot sufficiet to guaratee seriaizabiity if A ad B get updated i-betwee the read of A ad B, the dispayed sum woud be wrog. A ockig protoco is a set of rues foowed by a trasactios whie requestig ad reeasig ocks. Lockig protocos restrict the set of possibe schedues. CS425 Fa 2013 Boris Gavic 10.5 Siberschatz, Korth ad Sudarsha

6 Pitfas of Lock-Based Protocos Cosider the partia schedue Neither T 3 or T 4 ca make progress executig ock-s(b) causes T 4 to wait for T 3 to reease its ock o B, whie executig ock-x(a) causes T 3 to wait for T 4 to reease its ock o A. Such a situatio is caed a deadock. To hade a deadock oe of T 3 or T 4 must be roed back ad its ocks reeased. CS425 Fa 2013 Boris Gavic 10.6 Siberschatz, Korth ad Sudarsha

7 Pitfas of Lock-Based Protocos (Cot.) The potetia for deadock exists i most ockig protocos. Deadocks are a ecessary evi. Starvatio is aso possibe if the cocurrecy cotro maager is bady desiged. For exampe: A trasactio may be waitig for a X-ock o a item, whie a sequece of other trasactios request ad are grated a S-ock o the same item. The same trasactio is repeatedy roed back due to deadocks. Cocurrecy cotro maagers ca be desiged to prevet starvatio. CS425 Fa 2013 Boris Gavic 10.7 Siberschatz, Korth ad Sudarsha

8 The Two-Phase Lockig Protoco This is a protoco which esures cofict-seriaizabe schedues. Phase 1: Growig Phase trasactio may obtai ocks trasactio may ot reease ocks Phase 2: Shrikig Phase trasactio may reease ocks trasactio may ot obtai ocks The protoco assures seriaizabiity. It ca be proved that the trasactios ca be seriaized i the order of their ock poits (i.e. the poit where a trasactio acquired its fia ock). CS425 Fa 2013 Boris Gavic 10.8 Siberschatz, Korth ad Sudarsha

9 The Two-Phase Lockig Protoco (Cot.) Two-phase ockig does ot esure freedom from deadocks Cascadig ro-back is possibe uder two-phase ockig. To avoid this, foow a modified protoco caed strict two-phase ockig (S2PL). Here a trasactio must hod a its excusive ocks ti it commits/aborts. Rigorous two-phase ockig (SS2PL) is eve stricter: here a ocks are hed ti commit/abort. I this protoco trasactios ca be seriaized i the order i which they commit. CS425 Fa 2013 Boris Gavic 10.9 Siberschatz, Korth ad Sudarsha

10 The Two-Phase Lockig Protoco (Cot.) There ca be cofict seriaizabe schedues that caot be obtaied if two-phase ockig is used. However, i the absece of extra iformatio (e.g., orderig of access to data), two-phase ockig is eeded for cofict seriaizabiity i the foowig sese: Give a trasactio T i that does ot foow two-phase ockig, we ca fid a trasactio T j that uses two-phase ockig, ad a schedue for T i ad T j that is ot cofict seriaizabe. CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

11 Lock Coversios Two-phase ockig with ock coversios: First Phase: ca acquire a ock-s o item ca acquire a ock-x o item ca covert a ock-s to a ock-x (upgrade) Secod Phase: ca reease a ock-s ca reease a ock-x ca covert a ock-x to a ock-s (dowgrade) This protoco assures seriaizabiity. But sti reies o the programmer to isert the various ockig istructios. CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

12 Automatic Acquisitio of Locks A trasactio T i issues the stadard read/write istructio, without expicit ockig cas. The operatio read(d) is processed as: if T i has a ock o D the read(d) ese begi if ecessary wait uti o other trasactio has a ock-x o D grat T i a ock-s o D; read(d) ed CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

13 Automatic Acquisitio of Locks (Cot.) write(d) is processed as: if T i has a ock-x o D the write(d) ese begi if ecessary wait uti o other tras. has ay ock o D, if T i has a ock-s o D the upgrade ock o D to ock-x ese grat T i a ock-x o D write(d) ed; A ocks are reeased after commit or abort CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

14 Impemetatio of Lockig A ock maager ca be impemeted as a separate process to which trasactios sed ock ad uock requests The ock maager repies to a ock request by sedig a ock grat messages (or a message askig the trasactio to ro back, i case of a deadock) The requestig trasactio waits uti its request is aswered The ock maager maitais a data-structure caed a ock tabe to record grated ocks ad pedig requests The ock tabe is usuay impemeted as a i-memory hash tabe idexed o the ame of the data item beig ocked CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

15 Lock Tabe T23 T1 T8 T T23 14 T1 T23 Lock tabe aso records the type of ock grated or requested New request is added to the ed of the queue of requests for the data item, ad grated if it is compatibe with a earier ocks Uock requests resut i the request beig deeted, ad ater requests are checked to see if they ca ow be grated If trasactio aborts, a waitig or grated requests of the trasactio are deeted ock maager may keep a ist of ocks hed by each trasactio, to impemet this efficiety 144 T8 CS425 Fa 2013 Boris Gavic grated waitig Siberschatz, Korth ad Sudarsha

16 Deadock Hadig Cosider the foowig two trasactios: T 1 : write (X) T 2 : write(y) write(y) write(x) Schedue with deadock CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

17 Deadock Hadig System is deadocked if there is a set of trasactios such that every trasactio i the set is waitig for aother trasactio i the set. Deadock prevetio protocos esure that the system wi ever eter ito a deadock state. Some prevetio strategies : Require that each trasactio ocks a its data items before it begis executio (predecaratio). Not practica Impose partia orderig of a data items ad require that a trasactio ca ock data items oy i the order specified by the partia order (graph-based protoco). CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

18 More Deadock Prevetio Strategies Foowig schemes use trasactio timestamps for the sake of deadock prevetio aoe. Preemptive: Trasactio hodig a ock is aborted to make ock avaiabe wait-die scheme o-preemptive oder trasactio may wait for youger oe to reease data item. Youger trasactios ever wait for oder oes; they are roed back istead. a trasactio may die severa times before acquirig eeded data item woud-wait scheme preemptive oder trasactio wouds (forces roback) of youger trasactio istead of waitig for it. Youger trasactios may wait for oder oes. may be fewer robacks tha wait-die scheme. CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

19 Deadock prevetio (Cot.) Both i wait-die ad i woud-wait schemes, a roed back trasactios is restarted with its origia timestamp. Oder trasactios thus have precedece over ewer oes, ad starvatio is hece avoided. Timeout-Based Schemes: a trasactio waits for a ock oy for a specified amout of time. After that, the wait times out ad the trasactio is roed back. thus deadocks are ot possibe simpe to impemet; but starvatio is possibe. Aso difficut to determie good vaue of the timeout iterva. CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

20 Deadock Detectio Deadocks ca be described as a wait-for graph, which cosists of a pair G = (V,E), V is a set of vertices (a the trasactios i the system) E is a set of edges; each eemet is a ordered pair T i T j. If T i T j is i E, the there is a directed edge from T i to T j, impyig that T i is waitig for T j to reease a data item. Whe T i requests a data item currety beig hed by T j, the the edge T i T j is iserted i the wait-for graph. This edge is removed oy whe T j is o oger hodig a data item eeded by T i. The system is i a deadock state if ad oy if the wait-for graph has a cyce. Must ivoke a deadock-detectio agorithm periodicay to ook for cyces. CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

21 Deadock Detectio (Cot.) Wait-for graph without a cyce Wait-for graph with a cyce CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

22 Deadock Recovery Whe deadock is detected : Some trasactio wi have to roed back (made a victim) to break deadock. Seect that trasactio as victim that wi icur miimum cost. Roback -- determie how far to ro back trasactio Tota roback: Abort the trasactio ad the restart it. More effective to ro back trasactio oy as far as ecessary to break deadock. Starvatio happes if same trasactio is aways chose as victim. Icude the umber of robacks i the cost factor to avoid starvatio CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

23 Weak Leves of Cosistecy Degree-two cosistecy: differs from two-phase ockig i that S-ocks may be reeased at ay time, ad ocks may be acquired at ay time X-ocks must be hed ti ed of trasactio Seriaizabiity is ot guarateed, programmer must esure that o erroeous database state wi occur] Cursor stabiity: For reads, each tupe is ocked, read, ad ock is immediatey reeased X-ocks are hed ti ed of trasactio Specia case of degree-two cosistecy CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

24 Weak Leves of Cosistecy i SQL SQL aows o-seriaizabe executios Seriaizabe: is the defaut Repeatabe read: aows oy committed records to be read, ad repeatig a read shoud retur the same vaue (so read ocks shoud be retaied) However, the phatom pheomeo eed ot be preveted T1 may see some records iserted by T2, but may ot see others iserted by T2 Read committed: same as degree two cosistecy, but most systems impemet it as cursor-stabiity Read ucommitted: aows eve ucommitted data to be read I may database systems, read committed is the defaut cosistecy eve has to be expicity chaged to seriaizabe whe required set isoatio eve seriaizabe CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

25 Recap Cocurrecy Cotro Pessimistic: Prevet bad thigs from happeig Lockig Protocos Optimistic: Detect that bad thigs have happeed ad resove the probem Two-Phase Lockig (2PL) Two types of ocks: Shared (S) ocks for read-oy access Excusive (X) ocks for write + read access Lock compatibiity Trasactios caot acquire ocks after they have reeased a ock Divides trasactio ito growig ad shrikig phase Esures cofict-seriaizabiity Cascadig robacks are possibe Deadocks are possibe CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

26 Recap Strict Two-Phase Lockig (S2PL) Excusive ocks are hed uti trasactio commit Prevets cascadig robacks Deadocks are sti possibe Strict Strog Two-Phase Lockig (SS2PL) A ocks are hed uti trasactio commit Eabes seriaizabiity i commit order Deadocks Deadock Prevetio Wait-die: Youger trasactio that waits for oder is roed back Woud-wait: If oder waits for youger, the youger is roed back Deadock Detectio Cyce Detectio i Waits-for graph Expesive Timeout CS425 Fa 2013 Boris Gavic Siberschatz, Korth ad Sudarsha

27 Ed of Chapter Thaks to Aa Fekete ad Sudhir Jorwekar for Sapshot Isoatio exampes modified from: Database System Cocepts, 6 th Ed. Siberschatz, Korth ad Sudarsha See for coditios o re-use