Concurrency Control Algorithms

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# Concurrency Control Algorithms - PowerPoint PPT Presentation

Concurrency Control Algorithms. Chapter 4. Overview. Introduction B asics Locking schedulers Two-Phase Locking Non-Two-Phase Locking Protocols Write-Only Tree Locking Read/Write Tree Locking. III. Non-locking schedulers Timestamp Ordering Optimistic schedulers BOCC. Overview.

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### Concurrency Control Algorithms

Chapter 4

Overview
• Introduction
• Basics
• Locking schedulers
• Two-Phase Locking
• Non-Two-Phase Locking Protocols
• Write-Only Tree Locking
• III. Non-locking schedulers
• Timestamp Ordering
• Optimistic schedulers
• BOCC
Overview
• Introduction
• Basics
• Locking schedulers
• Two-Phase Locking
• Non-Two-Phase Locking Protocols
• Write-Only Tree Locking
• III. Non-locking schedulers
• Timestamp Ordering
• Optimistic schedulers
• BOCC
CSR – Class of conflict-serializable schedules
• Definition: Conflicts and ConflictRelations
• Let s be a schedule, t, t‘  trans(s), t  t‘.
• Two data operations p  t and q  t‘ are inconflict in s if
• they access the same data item and at least one of them is a write. i.e.,
• (p = r(x) ˄ q = w(x)) ˅ (p = w(x) ˄ q = r(x)) ˅ (p = w(x) ˄ q = w(x))
• {(p, q)} | p, q are in conflict and p occurs before q in s} is the conflict relationof s.

Definition: Conflict Equivalence

Schedules s and s‘ are conflict equivalent (denoted scs‘), if:

op(s) = op(s‘) and conf(s) = conf(s‘).

CSR – Class of conflict-serializable schedules

Definition: Conflict Serializability:

Schedule s is conflict serializableif there is a serial schedule s‘ such that scs‘.

CSR denotes the class of all conflict serializable schedules.

Example:

s1 = r1(x)r2(x)r1(z)w1(x)w2(y) r3(z) w3(y) c1 c2 w3(z) c3  CSR

s2 = r2(x) w2(x)r1(x) r1(y)r2(y) w2(y)c1c2  CSR

t2writes on x before t 1read x

Scheduler Actions and Transaction States

Definition: CSR Safety

For a scheduler S, Gen(S) denotes the set of all schedules that S can generate.

A scheduler is called CSR safeif Gen(S) CSR

All of the following algorithms are CSR safe

Scheduler Classification
• Schedulers can be classified as pessimistic or optimistic:
• optimistic Schedulers:
• also called „aggressiv“, because they mostly let steps pass and rarely block. This bears the danger of „getting stuck“ eventually when the serializability of the output can no longer be guaranteed
• pessimistic Schedulers:
• also called „conservative“, because they mostly block, in extreme, albeit unlikely case, the output could become a serial schedule, if all transactions but one were blocked.
Overview
• Introduction
• basics
• Locking schedulers
• Two-Phase Locking
• Non-Two-Phase Locking Protocols
• Write-Only Tree Locking
• III. Non-locking schedulers
• Timestamp Ordering
• Optimistic schedulers
• BOCC
Locking Schedulers

The Idea behind locking schedules is to synchronize access to shared data by using locks.

• In a nuthsell this means that if a transaction holds a lock on a specific data item, this item is not available to other, concurrent transactions

compatibility of locks:

lock notation:

wl(x)  write lock x

ru(x)  write unlock x

wu(x)  write unlock x

Rules for well-formed locking

LR1:

If ti contains a step of the form ri (x)[wi (x)], then the schedule s also contains a step of the form rli (x)[wli (x)] before the data operation. Moreover s contains a step of the form rui (x)[wui (x)] somewhere after the operation.

LR2:

For each x accessed by ti, schedule s has at most one rli (x) and at most one wli (x) step.

 locks of the same type are set at most once per transaction and per data item.

LR3:

No step of the form rui (.) or wui (.) is redundant (i.e., executed per transaction more than once)

LR4:

If x is held locked by both ti and tj for ti, tj trans(s), i ≠ j, then these locks are not in conflict (i.e., they are compatible)

Two-Phase Locking (2PL) - Definition
• A locking protocol is two-phaseif for every transaction a phase during which locks are set is distinguished from and strictly followed by a phase during which locks are released.

Definition: Two-Phase Locking

A locking protocol is two-phase if for every output s and every transaction ti  trans(s) is true that no qli step follows the first ouistep (o ,q  {r, w}).

Two-Phase Locking (2PL) - Example

2PL Example :

s = w1(x)r2(x)w1(y) w1(z) r3(z) c1w2(y) w3(y) c2w3(z) c3

A 2PL output could be:

wl1(x) w1(x) wl1(y) w1(y) wl1(z) w1(z) wu1(x)rl2(x) r2(x)wu1(y) wu1(z) c1

rl3(z) r3(z) wl2(y) w2(y) wu2(y) ru2(x) c2

wl3(y) w3(y) wl3(z) w3(z) wu3(z) wu3(y) c3

Overview
• Introduction
• basics
• Locking schedulers
• Two-Phase Locking
• Non-Two-Phase Locking Protocols
• Write-Only Tree Locking
• III. Non-locking schedulers
• Timestamp Ordering
• Optimistic schedulers
• BOCC
• Hybrid Schedulers
Non-Two-Phase Locking Protocols

The following two tree-based protocols are geared for transactions that exhibit treelike access patterns . In other cases they are susceptible to degradation.

The data items are viewed as nodes of a tree and accesses have to follow a path down the tree.

On the next slides we will have a look at those two tree-based locking schedulers:

Write-Only Tree Locking

Write-Only Tree Locking (WTL)

The Write-Only Tree Locking protocol uses a tree to

organize the data items.

In its model read operations are missing, they would

cause problems we will see later.

Thus a transaction can only write data (or read and write

are applied to the same item as collapsed into one

operation) .

Note:

the tree is a virtual data organization only, the relationship between the

data items can be quite different

Under the write-only tree locking protocol, lock requests and releases

must obey the locking rules LR1-LR4 and the following two rules on the

next slide:

Write-Only Tree Locking (WTL)

WTL1:

Ifxis any node in the tree other than the root, wli (x) can be set only if ticurrently holds a write lock on y, where yis parent of x

Example:

if a transaction t = w(c)w(e) wants to acquire wl(c) or wl(e) it has to hold wl(b)

WTL2:

After a wui (x), no further wli (x) is allowed (on the same data item x)

Write-Only Tree Locking (WTL)

Sample Transaction under the WTL protocol:

transaction t = w(d)w(i)w(k)

wl(a)wl(b)wu(a)wl(d)wl(e)wu(b)w(d)wu(d)wl(i)wu(e)w(i)

wl(k)wu(i)w(k)wu(k)

The Read/Write Tree Locking is a generalization of the Write-Only Tree Locking protocol.

Unlike the the Write-Only Tree Locking protocol it supports seperate read operations, too.

Problem:ti locks root before tj does, but tjpasses ti within a “read zone”

Example:

rl1(a) rl1(b) r1(a) r1(b) wl1(a)w1(a) wl1(b) ul1(a)rl2(a)r2(a)

w1(b) rl1(e) ul1(b)rl2(b) r2(b) ul2(a) rl2(e) rl2(i) ul2(b) r2(e)r1(e)

r2(i) wl2(i)w2(i)wl2(k) ul2(e) ul2(i)rl1(i) ul1(e)r1(i) ...

appears to follow TL rules but  CSR

This zones are called pitfall.

Definition: pitfall

For transaction t with read set RS(t) and write set WS(t) let C1, ..., Cm be the connected components of RS(t).

A pitfallof t is a set of the form Ci {x  WS(t) | x is a child or parent of some y  Ci}.

Under the RWTL lock requests and releases must obey LR1 - LR4, WTL1, WTL2, and the two-phase property within each pitfall.

Example:

transaction t with RS(t) = {f, i, g} and

WS(t) = {c, l, j, k, o}

has pitfalls pf1={c, f, i, l, j} and pf2={g, c, k}.

Overview
• Introduction
• basics
• Locking schedulers
• Two-Phase Locking
• Non-Two-Phase Locking Protocols
• Write-Only Tree Locking
• III. Non-locking schedulers
• Timestamp Ordering
• Optimistic schedulers
• BOCC
• Hybrid Schedulers
Nonlocking Schedulers

The next two protocols are alternatives to locking schedulers.

They guarantee safety of their output without using locks.

The first one is the Basic Time Stamp Ordering, which counts to

the pessimistic protocols and with the BOCC protocol we will also

see an optimistic scheduler

Timestamp Ordering (TO)

Timestamp Ordering protocols get rid of locks and use timestamps instead.

Timestamp Ordering Rule (TO rule):

Each transaction tiis assigned a unique timestamp ts(ti)

(e.g., the time of ti‘s beginning).

If pi(x) and qj(x) are in conflict, then the following must hold:

pi(x) is executed beforeqj(x) iffts(ti) < ts(tj).

Basic Timestamp Odering (BTO)
• Basic timestamp ordering protocol (BTO):
• For each data item x maintain
• max-r-scheduled(x): the value of the largest timestamp of a read operation on x already sent to the scheduler
• max-w-scheduled(x): the value of the largest timestamp of a write operation on x already sent to the scheduler
• Operation pi(x) is compared to max-q (x) for each conflicting q:
• if ts(ti) < max-q (x) for some q then abort ti
• else schedule pi(x) for execution and set max-p (x) to ts(ti)
Basic Timestamp Ordering (BTO)

BTO Example:

s = r1(x)w2(x) r3(y) w2(y) c2 w3(z) c3r1(z) c1

r1(x)w2(x) r3(y) a2 w3(z) c3a1

Overview
• Introduction
• basics
• Locking schedulers
• Two-Phase Locking
• Non-Two-Phase Locking Protocols
• Write-Only Tree Locking
• III. Non-locking schedulers
• Timestamp Ordering
• Optimistic schedulers
• BOCC
• Hybrid Schedulers
Optimistic Protocols
• In some scenarios optmistic protocols can do a better job then pessimistic.
• A product catalog application where 99% of the transactions are just read price information and descriptions of products. From time to time prices are updated or new products are added, but this occurs with a very low frequency compared to the read events.
•  A 2PL protocol for example would waste a considerable amount of time for managing locks, instead of reading data items.

optimistic schedulers do a good job in cases were conflicts aren‘t frequent

The three phases of a optimistic scheduler

The transaction is executed, but all writes applied to a workspace private to the transaction only (not the database)

2. Validation phase:

The scheduler tests if its execution has been „correct“ in the sense of conflict serializability and whether the result can be copied to database – if not the transaction is aborted, otherwise the next phase is entered

3. Write phase:

The workspace contents are transferred into the database to conclude the transaction

BOCC

Under backward-oriented optimistic concurrency control (BOCC),

a transaction under validation executes a conflict test against all those transactions that are already committed.

• BOCC validationof tj:
• compare tjto all previously committed ti
• accept tj if one of the following holds
• ti has ended before tjhas started, or
• RS(tj)  WS(ti) =  and ti has validated before tj
BOCC

Example:

Execution of BOCC

Overview
• Introduction
• Basics
• Locking schedulers
• Two-Phase Locking
• Non-Two-Phase Locking Protocols
• Write-Only Tree Locking