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CSE 414: Transaction Section

2021-11-04

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Announcements

  • Midterm tomorrow!
    • You can either take it in class or on gradescope.
    • Review session today at 4:30 office hours.
  • Homework 4 due Monday!

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ACID

Atomic�Consistent�Isolated�Durable

Everything a concurrent database user wants…

In this topic: delve into the DB Internals required to guarantee ACID

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Atomicity

What? “Atom” - a transaction are indivisible; data operations inside the transaction work in an all or nothing fashion

How? Transaction commit and rollback

Example: Transfer of funds from one account to another involves an atomic transaction that includes operations of debit and credit

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Consistency

What? Only data operations that comply with database validity constraints are allowed.

Once a transaction is completed, it must not leave the database incomplete or inconsistent.

How? Enforcement of consistency rules upon data entry/updation

Example: A database having both first and last name fields will not accept if you enter only one of those - must enter both for the transaction to be complete

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Isolation

What? The appearance of a transaction happening by itself irrespective of what other transactions are happening concurrently

How? User views the database that was present right before the view request is made and any further transactions that are in process after it are ignored

Example: A teller looking up a balance must be isolated from a concurrent transaction involving a withdrawal from the same account

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Durability

What? Once a transaction is complete the information as changed will survive failures of any kind

How? Creation of data “mirrors”

Example: Trying to make an update that leads to a system failure should not tamper with any of the previous data and transactions

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How do ACID properties help us?

  • Ensure that a transaction maintains database integrity
  • ACID compliance matters to important databases ($$$)

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In this topic: delve into the DB Internals required to guarantee ACID

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Txn Perspective

A database is a collection of "elements" that can be written to or read from.

Operation - A read or write to an element (later: or an insert or delete)

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Example: What operation does the following SQL query contain?�SELECT cases FROM Country WHERE cname = ‘France’;

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Txn Perspective

A database is a collection of "elements" that can be written to or read from.

Operation - A read or write to an element (later: or an insert or delete)

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Example: What operation does the following SQL query contain?�SELECT cases FROM Country WHERE cname = ‘France’;

Answer: R(Country) or R(France), depending on whether the "element" is a table or a row (assuming cname is a key). Let's go with the finer-grained element.

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Definitions

Operation -

Transaction -

Schedule -

Serial schedule -�

Serializable schedule - �

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Definitions

Operation - read or write of an element (later: insert or delete)

Transaction - series of operations meant to have the ACID guarantees

Schedule - ordering of the operations of some txns

Serial schedule - schedule where each txn is executed one after another.� No interleaving

Serializable schedule - a schedule that is behaviorally equivalent � to a serial schedule

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Practice - convert SQL to operations

Txn 1:

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

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Txn 2:

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

2b) y = y / 5;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

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Practice - convert SQL to operations

Txn 1:

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

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Txn 2:

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

2b) y = y / 5;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

1a) R(France)

1b)

1c) W(France)

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2a) R(Spain)

2b)

2c) R(France)

2d) W(France)

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Practice - convert SQL to operations

R1(France)

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W1(France)

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R2(Spain)

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R2(France)

W2(France)

What type of schedule is this?

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Definitions

Non-conflicting swap - �

Conflicting swap -

Conflict serializable schedule - �

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Definitions

Non-conflicting swap - a pair of adjacent operations in a schedule � that can be reversed without affecting the schedule's behavior

Conflicting swap - (any two ops in same txn), RW, WR, WW

Conflict serializable schedule - a schedule that can be transformed � into a serial schedule via a series of non-conflicting swaps

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Practice 1 - conflict serializable or not?

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

2b) y = y / 5;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

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Practice 1 - conflict serializable or not?

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

2b) y = y / 5;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

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①

②

1c-2c WR�1c-2d WW 1a-2d RW

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Practice 2 - conflict serializable or not?

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

2b) y = y / 5;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

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Practice 2 - conflict serializable or not?

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

2b) y = y / 5;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

①

②

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Practice 2 - conflict serializable or not?

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

2b) y = y / 5;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

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②

2c-1c RW�1a-2d RW 1c-2d WW

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Future

Tomorrow - learn how a DB can use locks � to prevent executing non-serializable schedules

Next week - learn about the tradeoff� Performance ⇐⇒ Correctness

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Transaction Worksheet!

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Conflict Serializability

Checking for conflict serializability -> precedence graph and cycle checking

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Conflict Serializability

Checking for conflict serializability -> precedence graph and cycle checking

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Serializability

“Conflict serializable” is a stronger constraint than “serializable”

I.e. Any schedule that is conflict serializable must be serializable.

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Serializable

Conflict

Serializable

Easy to check

Not easy to check

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Serializability

S1: w1(Y); w2(Y); w1(X); w2(X); w3(X)

S2: w1(Y); w2(Y); w2(X); w1(X); w3(X)

Are these serializable? Conflict serializable?

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Serializability

S1: w1(Y); w2(Y); w1(X); w2(X); w3(X)

S2: w1(Y); w2(Y); w2(X); w1(X); w3(X)

Conflict Serializable

Serializable (but not conflict serializable)