13 Transactions

single logical unit of work formed by a set of operations.

Operations¶

• read(a)
• write(a)

where a is a resource.

Conflicts¶

occurs when resources are in shareable mode.

2 operations conflict if they are

• on the same object/resource
• by different transactions
• atleast 1 transaction is a write

Transaction States¶

1. active
2. partially-commited
3. committed
4. failed
5. aborted
6. terminated

flowchart LR

a[Active]
pc[Partially-Committed]
f[Failed]
ab[Aborted]
t[Terminated]

a -->
|R/W| pc -->
|Permanently Store| Committed -->
t

a -->
|Failure|f -->
|Roll Back|ab --> t

pc -->|Failure| f

ACID¶

Good features of a database. Relational databases are ACID-compliant, but NoSQL aren’t

Atomicity¶

Transaction status should be binary - occured/not occured. No transaction must not occur partially.

If any sub-steps of a transaction(Operation/Query) fails, the whole transaction must fail and the database must be in the same state as the original.

Consistency¶

Correct data is ensured through constraints.

Isolation¶

Concurrent-Execution Safe

Simultaneous transactions must be considered as multiple sequential transactions.

Transactions must be one-one.

Durability¶

Committed transactions must be stored to a non-volatile memory, to prevent loss of data.

Commit¶

Storing update into permanent memory.

Concurrency Problems¶

Conflicts that occur when simultaneous transactions occur.

Solutions¶

Before any write occurs, always do a read.

Types of Schedules¶

Serial¶

Schedule where each transaction occurs one after the other.

Parallel¶

Multiple schedules happen at the same time?

Concurrent/Interleaved¶

Helps improve Throughput

Schedule where each transaction overlapping over each other.

Serializable¶

Non-serial schedule that can be converted into serial schedule.

Conflict Serializable¶

Serializable schedule that is possible, only after removing conflicts.

Equivalent¶

Represented as \(S \equiv S'\)

A serial and non-serial schedule which can be converted to each other.

Conflict Equivalent¶

Conflict Serializable schedule, such that conflicting-pairs occur in the same way as initially.

View Serializable¶

2 schedules \(S, S'\) that meet the following criteria. The transaction that

1. performs first read for a resource in \(S\) must do the same in \(S'\)
2. reads resource written by another transaction in \(S\) must do the same in \(S'\)
3. performs final write for a resource in \(S\) must do the same in \(S'\)

Eliminating Conflicts¶

Precedence Graph¶

Shows which transaction is dependent on which other transactions(s).

flowchart RL

t1((T1))
t2((T2))
t3((T3))
t2 -->|z| t1
t2 -->|y| t3

t3 -->|x| t1

Interpretations¶

Serializability¶

The above example is serializable.

Order¶

Order will start from the transaction with indegree = 0

In above example, order will be \(T_2 \to T_3 \to T_1\)

Blind Write¶

without even reading the value, you are writing.

Throughput¶

Work done per unit time.

2 Phase Locking Concurrency Control¶

Lock Manager¶

Manages locks on data items

Lock Table¶

used by lock manager blah blah

Graph¶

Types¶

Well Formed Transaction¶

• Data item must be locked before reading/writing.
• Should not try to
  • unlock free resource
  • lock already-locked resource

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