# DB2 - lesson 01
#### Paraboschi
##### 6 October 2015
### 1. Transaction Model
#### Transaction Properties and Mechanisms
- ##### Atomicity
    - Abort-rollback-restart
    - Commit protocols
- ##### Consistency
    - Integrity checking of the DBMS
- ##### Isolation
    - Concurrency control
- ##### Durability
    - Recovery management

#### Corresponding modules
- ##### Atomicity and Durability
    - Reliability Manager
- ##### Isolation
    - Concurrency Control System
- ##### Consistency
    - Integrity Control System at query execution time (with DDL Compilers support)
-------
### 2. Concurrency
#### Advantages of Concurrency

Modern computers are capable of executing different processes in a concurrent way,
Concurrency is also important in DBMS because it allows multiple Transactions at the same time

ex: Two students enroll contemporarily on the Politecnico site.

Concurrency is made by
- time sharing on a single processor
- or sharing the processes on multiple CPU cores.

#### Concurrenct executions
A problem may rise when we heve two or more concurrent operations modifying the same data.

There are several ways of managing the problem:

- ##### Serial executions
The two operations are queued and executed separately in time.
- ##### Interleaved execution

- ##### Nested execution
###Concurrency Problems
####Lost update
```
T1: R(D,V1)
V1 = V1 + 3
T2: R(D,V2)
V2 = V2 + 6
T1: W(V1,D)     D=103
T2: W(V2,D)     D=106!
```
####Dirty read
```
T1: R(X,V1)
T1: V1 = V1+3
T1: W(V1,X)     D=103
T2:R(X,V2)
T1: ROLLBACK
T2: V2 = V2+6
T2: W(V2,D)     D=109
```
####Non-repeatable read
```
T1: R(D,V1)
T2: R(D,V2)
T2: V2 = V2 + 6
T2: W(V2,D)      D=106
T1: R(D,V3)     V3<>V1!
```
####Ghost update
```
T1: R(D,V1)
T2: R(D,V2)
T2: V2 = V2 + 6
T2: W(V2,D)      D=106
T1: R(D,V3)     V3<>V1!
```