cm20145 transactions & serializability chris middup

CM20145CM20145Transactions & Transactions & SerializabilitySerializability

Chris Middup

Lecture PlanLecture Plan

1. Basic Concepts

2. Data, Information & Knowledge

3. Data Models (The E-R Model)

4. The Relational Algebra

5. Introduction to SQL

6. Further SQL (Joins, RA Equivalences)

7. Database Design

8. Further DB Design – Normalisation

9. Architectures and Implementations

10. Integrity and Security

Lecture PlanLecture Plan

11. Ethics and Professional Conduct

12. Legal Issues

13. Transactions

14. Recovery

15. Concurrency Control

16. Storage and File Structure

17. Indexing and Hashing

18. Query Processing & Optimisation

19. XML Databases

20. Revision

A While Ago…A While Ago… Architectures and Implementations

Introductions to Transactions & Storage Architecture concerns:

Speed, Cost, Reliability, Maintainability. Architectural Types:

Centralized, Client/Server, Parallel, Distributed

Integrity and Security Domain Constraints Referential Integrity

Foreign Keys, Cascading Actions Assertions Triggers

Authorization Grant, Revoke, Roles, Audit Trails

Now: Transactions, Concurrency & Recovery

OverviewOverview Transaction Concepts

ACID Possible States

Schedules Serializability

Conflict View Others

Testing for Serializability Precedence Graphs Conflict View

Introduction to TransactionsIntroduction to Transactions A transaction is a unit of program execution that

accesses and possibly updates various data items. A transaction starts with a consistent database. During transaction execution the database may

be inconsistent. When the transaction is committed, the

database must be consistent. Two main issues to deal with:

Failures, e.g. hardware failures and system crashes.

Concurrency, for simultaneous execution of multiple transactions.

©Silberschatz, Korth and Sudarshan

Modifications & additions by S Bird, J Bryson

The ACID TestThe ACID Test Atomicity: Either all operations of the transaction

are properly reflected in the database or none are. Consistency: Execution of a transaction in

isolation preserves the consistency of the database.

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.

Durability: After a transaction completes successfully, the changes it has made to the database persist, even if the system fails.

To preserve integrity of data, the database system must ensure:

Example: A Fund TransferExample: A Fund TransferTransfer $50 from account A to B:

1. read(A)2. A := A – 503. write(A)4. read(B)5. B := B + 506. write(B)

Consistency: the sum of A and B is unchanged by the execution of the transaction.

Atomicity: if the transaction fails after step 3 and before step 6, the system must ensure that no updates are reflected in the database, else an inconsistency will result.

Durability: once the user notified that the transaction complete, the updates to the database by the transaction must persist despite failures.

Isolation: between steps 3-6, no other transaction should access the partially updated database, or it would see an inconsistent state (A + B will be less than it should be).

Transaction StatesTransaction States Active, the initial state; the transaction stays in this

state while it is executing Partially committed, after the final statement has

been executed. Committed, after successful completion.

Failed, after the discovery that normal execution can no longer proceed.

Aborted, after the transaction has been rolled back and the database restored to its state prior to the start of the transaction.

Transaction Definition in SQLTransaction Definition in SQL Data manipulation languages must

include a construct for specifying the set of actions that comprise a transaction.

In SQL, a transaction begins implicitly.

A transaction can be explicitly ended by: Commit work: commits current

transaction and begins a new one. Rollback work: causes current

transaction to abort.

Schedules & ConcurrencySchedules & Concurrency Advantages to Concurrent execution (executing

transactions simultaneously): Increased processor and disk utilization; better throughput.

E.g. one transaction uses CPU while another uses disk. Reduced average response time.

Short transactions need not wait behind long ones.

Concurrency control schemes: Mechanisms to achieve isolation. Control concurrent transactions’ interaction to maintain

database consistency. Schedules:

Def: Sequences that indicate the chronological order in which instructions of concurrent transactions are executed.

A schedule for a set of transactions must consist of all instructions of those transactions.

Must preserve the order in which the instructions appear in each individual transaction.

Example: Serial ScheduleExample: Serial Schedule

Let T1 transfer $50 from A to B, and transfer 10% of the balance from A to B.

In a serial schedule, one transaction just entirely follows the other.

E.g. T2 follows T1.

Example: Concurrent ScheduleExample: Concurrent Schedule Let T1 and T2 be

the transactions defined previously.

This schedule is not a serial schedule, but it is equivalent to the previous schedule.

In both this and the sequential schedule, the sum A + B is preserved.

Concurrency Gone BadConcurrency Gone Bad This concurrent

schedule does not preserve the value of A + B.

BADBAD

Concurrency & SerializabilityConcurrency & Serializability Goal – to develop concurrency control

protocols that will ensure serializability. These protocols impose a discipline that

avoids nonseralizable schedules. A common concurrency control protocol

uses locks. While one transaction is accessing a data

item, no other transaction can modify it. Require a transaction to lock the item before

accessing it.

Not as easy as it sounds… Topic of Lecture 15!

RecoverabilityRecoverability

Recoverable schedule: if a transaction Tj reads a data item previously written by a transaction Ti , the commit operation of Ti appears before the commit operation of Tj

A Database must ensure that schedules are recoverable!

How do we address failures when we are running concurrent transactions?

SerializabilitySerializability Basic Assumption: Each transaction, on

its own, preserves database consistency. That is, serial execution of transactions

preserves database consistency. A (possibly concurrent) schedule is

serializable if it is equivalent to a serial schedule.

Different forms of equivalence lead to different kinds of serializability: conflict and view. Recovery is the point of serializability.

Serialization makes recovery easier, but can slow down throughput.

Conflict SerializabilityConflict Serializability Instructions li and lj of transactions Ti and Tj

respectively, conflict iff there exists some item Q accessed by both li and lj, and at least one of these instructions wrote Q.1. li = read(Q), lj = read(Q). li and lj don’t conflict.2. li = read(Q), lj = write(Q). They conflict.3. li = write(Q), lj = read(Q). They conflict4. li = write(Q), lj = write(Q). They conflict

A conflict between li and lj forces a (logical) temporal order between them. If li and lj are consecutive in a schedule and

they do not conflict, their results would remain the same even if their order were reversed.

Bottom Line: an order is forced on two transactions iff they both access the same data and at least one changes it.

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.

We say that a schedule S is conflict serializable if it is conflict equivalent to a serial schedule.

Example of a schedule that is not conflict serializable:

T3 T4

read(Q) write(Q)

write(Q)

Conflict Serializability (2)Conflict Serializability (2)

Conflict Serializability (3)Conflict Serializability (3) This concurrent

schedule can be transformed into the serial one (where T2 follows T1) by a series of swaps of non-conflicting instructions.

Therefore it is conflict serializable.

View SerializabilityView Serializability Let S and S´ be two schedules with the same set

of transactions. S and S´ are view equivalent if the following three conditions are met, where Q is a data item and Ti is a transaction:1. If Ti reads the initial value of Q in schedule S, then Ti in

schedule S´ must also read the initial value of Q.2. If Ti executes read(Q) in schedule S, and that value was

produced by transaction Tj (if any), then transaction Ti must in schedule S´ also read the value of Q that was produced by transaction Tj.

3. The transaction (if any) that performs the final write(Q) operation in schedule S (for any data item Q) must perform the final write(Q) operation in schedule S´.

View equivalence is a bit cleverer than conflict equiv. about the use of reads and writes

View Serializability (2)View Serializability (2) A schedule S is view serializable if it is

view equivalent to a serial schedule. Every conflict serializable schedule is

also view serializable. Some schedules are view-serializable

but not conflict serializable (see below). Every view serializable schedule that is

not conflict serializable has blind writes.

Consider some schedule of a set of transactions T1, T2, ..., Tn

Precedence graph: a directed graph where the vertices are transaction names.

We draw an arc from Ti to Tj if the two transactions conflict, and Ti accessed the data item before Tj

We may label the arc by the item that was accessed.

Example:

Testing for SerializabilityTesting for Serializability

x

y

Schedule & Precedence GraphSchedule & Precedence GraphT1 T2 T3 T4 T5

read(X)read(Y)read(Z)

read(V)read(W)read(W)

read(Y)write(Y)

write(Z)read(U)

read(Y)write(Y)read(Z)write(Z)

read(U)write(U)

T

3

T

4

T

1

T

2

y

z

yz

Testing Conflict SerializabilityTesting Conflict Serializability A schedule is conflict

serializable if and only if its precedence graph is acyclic.

Cycle-detection algorithms exist which take order n2 time, where n is the number of vertices in the graph.

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, one serializability order for this graph is : T1 T2 T4 T3 T5

(note there’s another!)

Example of an acyclic precedence graph

Testing View SerializabilityTesting View Serializability The precedence graph test for conflict

serializability must be modified to apply to a test for view serializability.

The problem of checking if a schedule is view serializable is NP-complete. This means the existence of an efficient algorithm is unlikely.

However, practical algorithms that just check some sufficient conditions for view serializability can still be used.

SummarySummary Transaction Concepts





Reading & ExercisesReading & Exercises

Reading Silberschatz Ch: 15. Connolly & Begg 20.1 – 20.2.2 (very

clear!)

Exercises: Silberschatz 15.1, 15.5-9. Connolly & Begg 20.1-3, 20.18-19

cm20145 transactions & serializability chris middup

Documents

transaction execution

transaction definition

transaction stays

transaction complete

individual transaction

transaction states active

set of transactions

short transactions