Transactions n Transaction Concept n Transaction State n

Transactions n Transaction Concept n Transaction State n Implementation of Atomicity and Durability n Concurrent Executions n Serializability n Recoverability n Implementation of Isolation n Transaction Definition in SQL n Testing for Serializability. 1

Transaction Concept n A transaction is a unit of program execution that accesses and possibly updates various data items. n A transaction must see a consistent database. n During transaction execution the database may be inconsistent. n When the transaction is committed, the database must be consistent. n Two main issues to deal with: ê Failures of various kinds, such as hardware failures and system crashes ê Concurrent execution of multiple transactions 2

ACID Properties To preserve integrity of data, the database system must ensure: n Atomicity. Either all operations of the transaction are properly reflected in the database or none are. n Consistency. Execution of a transaction in isolation preserves the consistency of the database. n Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions. ê That is, for every pair of transactions Ti and Tj, it appears to Ti that either Tj, finished execution before Ti started, or Tj started execution after Ti finished. n Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures. 3

Example of Fund Transfer n Transaction to transfer $50 from account A to account B: 1. read(A) 2. A : = A – 50 3. write(A) 4. read(B) 5. B : = B + 50 6. write(B) n Consistency requirement – the sum of A and B is unchanged by the execution of the transaction. n Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result. 4

Example of Fund Transfer (Cont. ) n Durability requirement — once the user has been notified that the transaction has completed (i. e. , the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures. n Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be). Can be ensured trivially by running transactions serially, that is one after the other. However, executing multiple transactions concurrently has significant benefits, as we will see. 5

Transaction State n Active, the initial state; the transaction stays in this state while it is executing n Partially committed, after the final statement has been executed. n Failed, after the discovery that normal execution can no longer proceed. n Aborted, after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. Two options after it has been aborted: ê restart the transaction – only if no internal logical error ê kill the transaction n Committed, after successful completion. 6

Transaction State (Cont. ) 7

Implementation of Atomicity and Durability n The recovery-management component of a database system implements the support for atomicity and durability. n The shadow-database scheme: ê assume that only one transaction is active at a time. ê a pointer called db_pointer always points to the current consistent copy of the database. ê all updates are made on a shadow copy of the database, and db_pointer is made to point to the updated shadow copy only after the transaction reaches partial commit and all updated pages have been flushed to disk. ê in case transaction fails, old consistent copy pointed to by db_pointer can be used, and the shadow copy can be deleted. 8

Implementation of Atomicity and Durability The shadow-database scheme: n Assumes disks to not fail n Useful for text editors, but extremely inefficient for large databases: executing a single transaction requires copying the entire database. 9

Concurrent Executions n Multiple transactions are allowed to run concurrently in the system. Advantages are: ê increased processor and disk utilization, leading to better transaction throughput: one transaction can be using the CPU while another is reading from or writing to the disk ê reduced average response time for transactions: short transactions need not wait behind long ones. n Concurrency control schemes – mechanisms to achieve isolation, i. e. , to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the database 10

Schedules n Schedules – sequences that indicate the chronological order in which instructions of concurrent transactions are executed ê a schedule for a set of transactions must consist of all instructions of those transactions ê must preserve the order in which the instructions appear in each individual transaction. 11

Example Schedules n Let T 1 transfer $50 from A to B, and T 2 transfer 10% of the balance from A to B. The following is a serial schedule (Schedule 1 in the text), in which T 1 is followed by T 2. 12

Example Schedule n Let T 1 and T 2 be the transactions defined previously. The following schedule (Schedule 3 in the text) is not a serial schedule, but it is equivalent to Schedule 1. In both Schedule 1 and 3, the sum A + B is preserved. 13

Example Schedules (Cont. ) n The following concurrent schedule (Schedule 4 in the text) does not preserve the value of the sum A + B. 14

Serializability n Basic Assumption – Each transaction preserves database consistency. n Thus serial execution of a set of transactions preserves database consistency. n A (possibly concurrent) schedule is serializable if it is equivalent to a serial schedule. Different forms of schedule equivalence give rise to the notion of conflict serializability n We ignore operations other than read and write instructions, and we assume that transactions may perform arbitrary computations on data in local buffers in between reads and writes. Our simplified schedules consist of only read and write instructions. 15

Conflict Serializability n Operations oi and oj of transactions Ti and Tj respectively are conflicting if and only if there exists some item x accessed by both oi and oj, and at least one of these operations is write(x). 1. oi = read(x), oj = read(x). 2. oi = read(x), oj = write(x). 3. oi = write(x), oj = read(x). 4. oi = write(x), oj = write(x). oi and oj don’t conflict. They conflict n Intuitively, a conflict between oi and oj forces a (logical) temporal order between them. If oi and oj are consecutive in a schedule and they do not conflict, their results would remain the same even if they had been interchanged in the schedule. 16

Conflict Serializability (Cont. ) n If a schedule S can be transformed into a schedule S´ by a series of swaps of non-conflicting instructions, we say that S and S´ are conflict equivalent. n We say that a schedule S is conflict serializable if it is conflict equivalent to a serial schedule n Example of a schedule that is not conflict serializable: read(x) T 1 T 2 write(x) We are unable to swap instructions in the above schedule to obtain either the serial schedule < T 1, T 2 >, or the serial schedule < T 2, T 1 >. 17

Conflict Serializability (Cont. ) n Schedule below can be transformed into a serial schedule where T 2 follows T 1, by series of swaps of non-conflicting instructions. Therefore Schedule below is conflict serializable. 18

Recoverability Need to address the effect of transaction failures on concurrently running transactions. n Recoverable schedule — if a transaction Tj reads a data items previously written by a transaction Ti , the commit operation of Ti appears before the commit operation of Tj. n The following schedule is not recoverable if T 9 commits immediately after the read n If T 8 should abort, T 9 would have read (and possibly shown to the user) an inconsistent database state. Hence database must ensure that schedules are recoverable. 19

Recoverability (Cont. ) n Cascading rollback – a single transaction failure leads to a series of transaction rollbacks. Consider the following schedule where none of the transactions has yet committed (so the schedule is recoverable) If T 10 fails, T 11 and T 12 must also be rolled back. n Can lead to the undoing of a significant amount of work 20

Recoverability (Cont. ) n Cascadeless schedules — cascading rollbacks cannot occur; for each pair of transactions Ti and Tj such that Tj reads a data item previously written by Ti, the commit operation of Ti appears before the read operation of Tj. n Every cascadeless schedule is also recoverable n It is desirable to restrict the schedules to those that are cascadeless 21

Recoverability (Cont. ) n Strict schedules — Dirty write and reads cannot occur; for each pair of transactions Ti and Tj such that Tj reads or writes a data item previously written by Ti, the commit operation of Ti appears before the read or write operation of Tj. n Every strict schedule is also cascadeless n It is desirable to further restrict the schedules to those that are strict. n Rigorous schedules — For each pair of transactions Ti and Tj conflicting operations of Ti and Ti are separated by a commit operation. n Every rigorous schedule is strict. n It is most desirable to to consider only rigorous schedules 22

Implementation of Isolation n Schedules must be conflict serializable, and recoverable, for the sake of database consistency, and preferably rigorous. n A policy in which only one transaction can execute at a time generates serial schedules, but provides a poor degree of concurrency. . n Concurrency-control schemes tradeoff between the amount of concurrency they allow and the amount of overhead that they incur. n Some schemes allow only conflict-serializable schedules to be generated, while others allow view-serializable schedules that are not conflict-serializable. 23

Transaction Definition in SQL n Data manipulation language must include a construct for specifying the set of actions that comprise a transaction. n In SQL, a transaction begins implicitly. n A transaction in SQL ends by: ê Commit work commits current transaction and begins a new one. ê Rollback work causes current transaction to abort. 24

Levels of Consistency in SQL-92 n Serializable — default n Repeatable read — only committed records to be read, repeated reads of same record must return same value. However, aschedulemay not be serializable – it may find some records inserted by a transaction but not find others. n Read committed — only committed records can be read, but successive reads of record may return different (but committed) values. n Read uncommitted — even uncommitted records may be read. Lower degrees of consistency useful for gathering approximate information about the database, e. g. , statistics for query optimizer. 25

Testing for Serializability n Consider some schedule of a set of transactions T 1, T 2, n n . . . , Tn Precedence graph — a direct graph where the vertices are the transactions (names). We draw an arc from Ti to Tj if the two transaction conflict, and Ti accessed the data item on which the conflict arose earlier. We may label the arc by the item that was accessed. Example x y 26

T 1 Example Schedule T 4 T 2 T 3 T 5 read(X) read(Y) read(Z) read(V) read(W) read(Y) write(Z) read(U) 27 read(Y) write(Y) read(Z)

Precedence Graph for Schedule A T 1 T 2 T 4 T 3 28

Test for Conflict Serializability n A schedule is conflict serializable if and only if its precedence graph is acyclic. n Cycle-detection algorithms exist which take order n 2 time, where n is the number of vertices in the graph. (Better algorithms take order n + e where e is the number of edges. ) n If precedence graph is acyclic, the serializability order can be obtained by a topological sorting of the graph. This is a linear order consistent with the partial order of the graph. For example, a serializability order for Schedule A would be T 5 T 1 T 3 T 2 T 4. 29

Illustration of Topological Sorting 30

Concurrency Control vs. Serializability Tests n Testing a schedule for serializability after it has executed is a little too late! n Goal – to develop concurrency control protocols that will assure serializability. They will generally not examine the precedence graph as it is being created; instead a protocol will impose a discipline that avoids nonseralizable schedules. n Tests for serializability help understand why a concurrency control protocol is correct. 31

Concurrency Control n Lock-Based Protocols n Timestamp-Based Protocols n Validation-Based Protocols n Multiple Granularity n Deadlock Handling n Insert and Delete Operations n Concurrency in Index Structures 32

Lock-Based Protocols n A lock is a mechanism to control concurrent access to a data item n Data items can be locked in two modes : 1. exclusive (X) mode. Data item can be both read as well as written. X-lock is requested using lock-X instruction. 2. shared (S) mode. Data item can only be read. S-lock is requested using lock-S instruction. n Lock requests are made to concurrency-control manager. Transaction can proceed only after request is granted. 33

Lock-Based Protocols (Cont. ) n Lock-compatibility matrix n A transaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactions n Any number of transactions can hold shared locks on an item, but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item. n If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held by other transactions have been released. The lock is then granted. 34

Lock-Based Protocols (Cont. ) n Example of a transaction performing locking: T 2: lock-S(A); read (A); unlock(A); lock-S(B); read (B); unlock(B); display(A+B) n Locking as above is not sufficient to guarantee serializability — if A and B get updated in-between the read of A and B, the displayed sum would be wrong. n A locking protocol is a set of rules followed by all transactions while requesting and releasing locks. Locking protocols restrict the set of possible schedules. 35

Pitfalls of Lock-Based Protocols n Consider the partial schedule n Neither T 3 nor T 4 can make progress — executing lock-S(B) causes T 4 to wait for T 3 to release its lock on B, while executing lock-X(A) causes T 3 to wait for T 4 to release its lock on A. n Such a situation is called a deadlock. ê To handle a deadlock one of T 3 or T 4 must be rolled back and its locks released. 36

Pitfalls of Lock-Based Protocols (Cont. ) n The potential for deadlock exists in most locking protocols. Deadlocks are a necessary evil. n Starvation is also possible if concurrency control manager is badly designed. For example: ê A transaction may be waiting for an X-lock on an item, while a sequence of other transactions request and are granted an S-lock on the same item. ê The same transaction is repeatedly rolled back due to deadlocks. n Concurrency control manager can be designed to prevent starvation. 37

The Two-Phase Locking Protocol n This is a protocol which ensures conflict-serializable schedules. n Phase 1: Growing Phase ê transaction may obtain locks ê transaction may not release locks n Phase 2: Shrinking Phase ê transaction may release locks ê transaction may not obtain locks n The protocol assures serializability. It can be proved that the transactions can be serialized in the order of their lock points (i. e. the point where a transaction acquired its final lock). 38

The Two-Phase Locking Protocol (Cont. ) n Two-phase locking does not ensure freedom from deadlocks n Cascading roll-back is possible under two-phase locking. To avoid this, follow a modified protocol called strict two-phase locking. Here a transaction must hold all its exclusive locks till it commits/aborts. n Rigorous two-phase locking is even stricter: here all locks are held till commit/abort. In this protocol transactions can be serialized in the order in which they commit. 39

The Two-Phase Locking Protocol (Cont. ) n There can be conflict serializable schedules that cannot be obtained if two-phase locking is used. n However, in the absence of extra information (e. g. , ordering of access to data), two-phase locking is needed for conflict serializability in the following sense: Given a transaction Ti that does not follow two-phase locking, we can find a transaction Tj that uses two-phase locking, and a schedule for Ti and Tj that is not conflict serializable. 40

Lock Conversions n Two-phase locking with lock conversions: – First Phase: ê can acquire a lock-S on item ê can acquire a lock-X on item ê can convert a lock-S to a lock-X (upgrade) – Second Phase: ê can release a lock-S ê can release a lock-X ê can convert a lock-X to a lock-S (downgrade) n This protocol assures serializability. But still relies on the programmer to insert the various locking instructions. 41

Automatic Acquisition of Locks n A transaction Ti issues the standard read/write instruction, without explicit locking calls. n The operation read(D) is processed as: if Ti has a lock on D then read(D) else begin if necessary wait until no other transaction has a lock-X on D grant Ti a lock-S on D; read(D) end 42

Automatic Acquisition of Locks (Cont. ) n write(D) is processed as: if Ti has a lock-X on D then write(D) else begin if necessary wait until no other trans. has any lock on D, if Ti has a lock-S on D then upgrade lock on D to lock-X else grant Ti a lock-X on D write(D) end; n All locks are released after commit or abort 43

Implementation of Locking n A Lock manager can be implemented as a separate process to which transactions send lock and unlock requests n The lock manager replies to a lock request by sending a lock grant messages (or a message asking the transaction to roll back, in case of a deadlock) n The requesting transaction waits until its request is answered n The lock manager maintains a data structure called a lock table to record granted locks and pending requests n The lock table is usually implemented as an in-memory hash table indexed on the name of the data item being locked 44

Lock n. Table Black rectangles indicate granted locks, white ones indicate waiting requests n Lock table also records the type of lock granted or requested n New request is added to the end of the queue of requests for the data item, and granted if it is compatible with all earlier locks n Unlock requests result in the request being deleted, and later requests are checked to see if they can now be granted n If transaction aborts, all waiting or granted requests of the transaction are deleted ê lock manager may keep a list of locks held by each transaction, to implement this efficiently 45

Graph-Based Protocols n Graph-based protocols are an alternative to two-phase locking n Impose a partial ordering on the set D = {d 1, d 2 , . . . , dh} of all data items. ê If di dj then any transaction accessing both di and dj must access di before accessing dj. ê Implies that the set D may now be viewed as a directed acyclic graph, called a database graph. n The tree-protocol is a simple kind of graph protocol. 46

Tree Protocol n Only exclusive locks are allowed. n The first lock by Ti may be on any data item. Subsequently, a data Q can be locked by Ti only if the parent of Q is currently locked by Ti. n Data items may be unlocked at any time. 47

Graph-Based Protocols (Cont. ) n The tree protocol ensures conflict serializability as well as freedom from deadlock. n Unlocking may occur earlier in the tree-locking protocol than in the two-phase locking protocol. ê shorter waiting times, and increase in concurrency ê protocol is deadlock-free, no rollbacks are required ê the abort of a transaction can still lead to cascading rollbacks. (this correction has to be made in the book also. ) n However, in the tree-locking protocol, a transaction may have to lock data items that it does not access. ê increased locking overhead, and additional waiting time ê potential decrease in concurrency n Schedules not possible under two-phase locking are possible under tree protocol, and vice versa. 48

Timestamp-Based Protocols n Each transaction is issued a timestamp when it enters the system. If an old transaction Ti has time-stamp TS(Ti), a new transaction Tj is assigned time-stamp TS(Tj) such that TS(Ti) <TS(Tj). n The protocol manages concurrent execution such that the time- stamps determine the serializability order. n In order to assure such behavior, the protocol maintains for each data Q two timestamp values: ê W-timestamp(Q) is the largest time-stamp of any transaction that executed write(Q) successfully. ê R-timestamp(Q) is the largest time-stamp of any transaction that executed read(Q) successfully. 49

Timestamp-Based Protocols (Cont. ) n The timestamp ordering protocol ensures that any conflicting read and write operations are executed in timestamp order. n Suppose a transaction Ti issues a read(Q) 1. If TS(Ti) W-timestamp(Q), then Ti needs to read a value of Q that was already overwritten. Hence, the read operation is rejected, and Ti is rolled back. 2. If TS(Ti) W-timestamp(Q), then the read operation is executed, and R-timestamp(Q) is set to the maximum of Rtimestamp(Q) and TS(Ti). 50

Timestamp-Based Protocols (Cont. ) n Suppose that transaction Ti issues write(Q). n If TS(Ti) < R-timestamp(Q), then the value of Q that Ti is producing was needed previously, and the system assumed that value would never be produced. Hence, the write operation is rejected, and Ti is rolled back. n If TS(Ti) < W-timestamp(Q), then Ti is attempting to write an obsolete value of Q. Hence, this write operation is rejected, and Ti is rolled back. n Otherwise, the write operation is executed, and W- timestamp(Q) is set to TS(Ti). 51

Example Use of the Protocol A partial schedule for several data items for transactions with timestamps 1, 2, 3, 4, 5 T 1 read(Y) read(X) T 2 T 3 read(Y) write(Z) read(X) abort write(Z) abort 52 T 4 T 5 read(X) read(Z) write(Y) write(Z)

Correctness of Timestamp-Ordering Protocol n The timestamp-ordering protocol guarantees serializability since all the arcs in the precedence graph are of the form: transaction with smaller timestamp transaction with larger timestamp Thus, there will be no cycles in the precedence graph n Timestamp protocol ensures freedom from deadlock as no transaction ever waits. n But the schedule may not be cascade-free, and may not even be recoverable. 53

Recoverability and Cascade Freedom n Problem with timestamp-ordering protocol: ê Suppose Ti aborts, but Tj has read a data item written by Ti ê Then Tj must abort; if Tj had been allowed to commit earlier, the schedule is not recoverable. ê Further, any transaction that has read a data item written by Tj must abort ê This can lead to cascading rollback --- that is, a chain of rollbacks n Solution: ê A transaction is structured such that its writes are all performed at the end of its processing ê All writes of a transaction form an atomic action; no transaction may execute while a transaction is being written ê A transaction that aborts is restarted with a new timestamp 54

Thomas’ Write Rule n Modified version of the timestamp-ordering protocol in which obsolete write operations may be ignored under certain circumstances. n When Ti attempts to write data item Q, if TS(Ti) < W- timestamp(Q), then Ti is attempting to write an obsolete value of {Q}. Hence, rather than rolling back Ti as the timestamp ordering protocol would have done, this {write} operation can be ignored. n Otherwise this protocol is the same as the timestamp ordering protocol. n Thomas' Write Rule allows greater potential concurrency. Unlike previous protocols, it allows some view-serializable schedules that are not conflict-serializable. 55

Validation-Based Protocol n Execution of transaction Ti is done in three phases. 1. Read and execution phase: Transaction Ti writes only to temporary local variables 2. Validation phase: Transaction Ti performs a ``validation test'' to determine if local variables can be written without violating serializability. 3. Write phase: If Ti is validated, the updates are applied to the database; otherwise, Ti is rolled back. n The three phases of concurrently executing transactions can be interleaved, but each transaction must go through the three phases in that order. n Also called as optimistic concurrency control since transaction executes fully in the hope that all will go well during validation 56

Validation-Based Protocol (Cont. ) n Each transaction Ti has 3 timestamps - Start(Ti) : the time when Ti started its execution - Validation(Ti): the time when Ti entered its validation phase - Finish(Ti) : the time when Ti finished its write phase n Serializability order is determined by timestamp given at validation time, to increase concurrency. Thus TS(Ti) is given the value of Validation(Ti). n This protocol is useful and gives greater degree of concurrency if probability of conflicts is low. That is because the serializability order is not pre-decided and relatively less transactions will have to be rolled back. 57

Validation Test for Transaction Tj n If for all Ti with TS (Ti) < TS (Tj) either one of the following condition holds: ê finish(Ti) < start(Tj) ê start(Tj) < finish(Ti) < validation(Tj) and the set of data items written by Ti does not intersect with the set of data items read by Tj. then validation succeeds and Tj can be committed. Otherwise, validation fails and Tj is aborted. n Justification: Either first condition is satisfied, and there is no overlapped execution, or secondition is satisfied and 1. the writes of Tj do not affect reads of Ti since they occur after Ti has finished its reads. 2. the writes of Ti do not affect reads of Tj since Tj does not read any item written by Ti. 58

Schedule Produced by Validation n Example of schedule produced using validation T 14 T 15 read(B) read(A) (validate) display (A+B) read(B) B: - B-50 read(A) A: - A+50 (validate) write (B) write (A) 59

Multiversion Schemes n Multiversion schemes keep old versions of data item to increase concurrency. ê Multiversion Timestamp Ordering ê Multiversion Two-Phase Locking n Each successful write results in the creation of a new version of the data item written. n Use timestamps to label versions. n When a read(Q) operation is issued, select an appropriate version of Q based on the timestamp of the transaction, and return the value of the selected version. n reads never have to wait as an appropriate version is returned immediately. 60

Multiversion Timestamp Ordering n Each data item Q has a sequence of versions <Q 1, Q 2, . . , Qm>. Each version Qk contains three data fields: ê Content -- the value of version Qk. ê W-timestamp(Qk) -- timestamp of the transaction that created (wrote) version Qk ê R-timestamp(Qk) -- largest timestamp of a transaction that successfully read version Qk n when a transaction Ti creates a new version Qk of Q, Qk's W- timestamp and R-timestamp are initialized to TS(Ti). n R-timestamp of Qk is updated whenever a transaction Tj reads Qk, and TS(Tj) > R-timestamp(Qk). 61

Multiversion Timestamp Ordering (Cont) n The multiversion timestamp scheme presented next ensures serializability. n Suppose that transaction Ti issues a read(Q) or write(Q) operation. Let Qk denote the version of Q whose write timestamp is the largest write timestamp less than or equal to TS(Ti). 1. If transaction Ti issues a read(Q), then the value returned is the content of version Qk. 2. If transaction Ti issues a write(Q), and if TS(Ti) < Rtimestamp(Qk), then transaction Ti is rolled back. Otherwise, if TS(Ti) = W-timestamp(Qk), the contents of Qk are overwritten, otherwise a new version of Q is created. n Reads always succeed; a write by Ti is rejected if some other transaction Tj that (in the serialization order defined by the timestamp values) should read Ti's write, has already read a version created by a transaction older than Ti. 62

Multiversion Two-Phase Locking n Differentiates between read-only transactions and update transactions n Update transactions acquire read and write locks, and hold all locks up to the end of the transaction. That is, update transactions follow rigorous two-phase locking. ê Each successful write results in the creation of a new version of the data item written. ê each version of a data item has a single timestamp whose value is obtained from a counter ts-counter that is incremented during commit processing. n Read-only transactions are assigned a timestamp by reading the current value of ts-counter before they start execution; they follow the multiversion timestamp-ordering protocol for performing reads. 63

Multiversion Two-Phase Locking (Cont. ) n When an update transaction wants to read a data item, it obtains n n n a shared lock on it, and reads the latest version. When it wants to write an item, it obtains X lock on; it then creates a new version of the item and sets this version's timestamp to . When update transaction Ti completes, commit processing occurs: ê Ti sets timestamp on the versions it has created to ts-counter + 1 ê Ti increments ts-counter by 1 Read-only transactions that start after Ti increments ts-counter will see the values updated by Ti. Read-only transactions that start before Ti increments the ts-counter will see the value before the updates by Ti. Only serializable schedules are produced. 64

Deadlock Handling n Consider the following two transactions: T 1 : write (X) T 2 : write(Y) write(X) n Schedule with deadlock T 1 lock-X on X write (X) T 2 lock-X on Y write (X) wait for lock-X on X wait for lock-X on Y 65

Deadlock Handling n System is deadlocked if there is a set of transactions such that every transaction in the set is waiting for another transaction in the set. n Deadlock prevention protocols ensure that the system will never enter into a deadlock state. Some prevention strategies : ê Require that each transaction locks all its data items before it begins execution (predeclaration). ê Impose partial ordering of all data items and require that a transaction can lock data items only in the order specified by the partial order (graph-based protocol). 66

More Deadlock Prevention Strategies n Following schemes use transaction timestamps for the sake of deadlock prevention alone. n wait-die scheme — non-preemptive ê older transaction may wait for younger one to release data item. Younger transactions never wait for older ones; they are rolled back instead. ê a transaction may die several times before acquiring needed data item n wound-wait scheme — preemptive ê older transaction wounds (forces rollback) of younger transaction instead of waiting for it. Younger transactions may wait for older ones. ê may be fewer rollbacks than wait-die scheme. 67

Deadlock prevention (Cont. ) n Both in wait-die and in wound-wait schemes, a rolled back transactions is restarted with its original timestamp. Older transactions thus have precedence over newer ones, and starvation is hence avoided. n Timeout-Based Schemes : ê a transaction waits for a lock only for a specified amount of time. After that, the wait times out and the transaction is rolled back. ê thus deadlocks are not possible ê simple to implement; but starvation is possible. Also difficult to determine good value of the timeout interval. 68

Deadlock Detection n Deadlocks can be described as a wait-for graph, which consists of a pair G = (V, E), ê V is a set of vertices (all the transactions in the system) ê E is a set of edges; each element is an ordered pair Ti Tj. n If Ti Tj is in E, then there is a directed edge from Ti to Tj, implying that Ti is waiting for Tj to release a data item. n When Ti requests a data item currently being held by Tj, then the edge Ti Tj is inserted in the wait-for graph. This edge is removed only when Tj is no longer holding a data item needed by Ti. n The system is in a deadlock state if and only if the wait-for graph has a cycle. Must invoke a deadlock-detection algorithm periodically to look for cycles. 69

Deadlock Detection (Cont. ) Wait-for graph with a cycle Wait-for graph without a cycle 70

Deadlock Recovery n When deadlock is detected : ê Some transaction will have to rolled back (made a victim) to break deadlock. Select that transaction as victim that will incur minimum cost. ê Rollback -- determine how far to roll back transaction Ø Total rollback: Abort the transaction and then restart it. Ø More effective to roll back transaction only as far as necessary to break deadlock. ê Starvation happens if same transaction is always chosen as victim. Include the number of rollbacks in the cost factor to avoid starvation 71

Insert and Delete Operations n If two-phase locking is used : ê A delete operation may be performed only if the transaction deleting the tuple has an exclusive lock on the tuple to be deleted. ê A transaction that inserts a new tuple into the database is given an X -mode lock on the tuple n Insertions and deletions can lead to the phantom phenomenon. ê A transaction that scans a relation (e. g. , find all accounts in Perryridge) and a transaction that inserts a tuple in the relation (e. g. , insert a new account at Perryridge) may conflict in spite of not accessing any tuple in common. ê If only tuple locks are used, non-serializable schedules can result: the scan transaction may not see the new account, yet may be serialized before the insert transaction. 72

Insert and Delete Operations (Cont. ) n The transaction scanning the relation is reading information that indicates what tuples the relation contains, while a transaction inserting a tuple updates the same information. ê The information should be locked. n One solution: ê Associate a data item with the relation, to represent the information about what tuples the relation contains. ê Transactions scanning the relation acquire a shared lock in the data item, ê Transactions inserting or deleting a tuple acquire an exclusive lock on the data item. (Note: locks on the data item do not conflict with locks on individual tuples. ) n Above protocol provides very low concurrency for insertions/deletions. n Index locking protocols provide higher concurrency while preventing the phantom phenomenon, by requiring locks on certain index buckets. 73

Index Locking Protocol n Every relation must have at least one index. Access to a relation must be made only through one of the indices on the relation. n A transaction Ti that performs a lookup must lock all the index buckets that it accesses, in S-mode. n A transaction Ti may not insert a tuple ti into a relation r without updating all indices to r. n Ti must perform a lookup on every index to find all index buckets that could have possibly contained a pointer to tuple ti, had it existed already, and obtain locks in X-mode on all these index buckets. Ti must also obtain locks in X-mode on all index buckets that it modifies. n The rules of the two-phase locking protocol must be observed. 74

Weak Levels of Consistency n Degree-two consistency: differs from two-phase locking in that S-locks may be released at any time, and locks may be acquired at any time ê X-locks must be held till end of transaction ê Serializability is not guaranteed, programmer must ensure that no erroneous database state will occur n Cursor stability: ê For reads, each tuple is locked, read, and lock is immediately released ê X-locks are held till end of transaction ê Special case of degree-two consistency 75

Concurrency in Index Structures n Indices are unlike other database items in that their only job is to n n n help in accessing data. Index-structures are typically accessed very often, much more than other database items. Treating index-structures like other database items leads to low concurrency. Two-phase locking on an index may result in transactions executing practically one-at-a-time. It is acceptable to have nonserializable concurrent access to an index as long as the accuracy of the index is maintained. In particular, the exact values read in an internal node of a B+-tree are irrelevant so long as we land up in the correct leaf node. There are index concurrency protocols where locks on internal nodes are released early, and not in a two-phase fashion. 76

Concurrency in Index Structures n Example of index concurrency protocol: n Use crabbing instead of two-phase locking on the nodes of the B+-tree, as follows. During search/insertion/deletion: ê First lock the root node in shared mode. ê After locking all required children of a node in shared mode, release the lock on the node. ê During insertion/deletion, upgrade leaf node locks to exclusive mode. ê When splitting or coalescing requires changes to a parent, lock the parent in exclusive mode. n Above protocol can cause excessive deadlocks. Better protocols are available; 77