Computer Science 101

gistfile1.txt

## Way of presenting idea
- Flowcharts
- Pseudocode
- Math Model

## Complexity
Time Complexity.
Memory Complexity

## Strategy
###Heuristics
- Greed
never come back to previous choices.
Take the best choice at each step and don't question it later.

Linked List vs Array
Linked List > Array when
- need insertions/deletions in the list to be extremely fast
- no need random, unordered access to the data
- no need insert/delete item in the middle of list
- can't evaluate the exact size of the list

Array > Linked List
- frequently need random, unordered access to the data
- extreme performace to access the items
- number of items doens't change during execution.

### Distributed Database
- enormous data. Finding a single computer with much storage space is impractical
- Database system that process several thousand queryes per second.
- Mission-critical. fault-tolerated

#### Single-Master Replication
One computer is the amster and receive all queries to the db. each
slave has a replica of the db. When the master recieve write queries, it forward them to slaves.
If the master crash, the slave machines can coordiante and elect a new master.

#### Multi-Master Replication
For massive amount of simultaneous write queries.
All computer in the cluster become masters. a load balancer is used to distribue 
incoming read and write quries equallt among the machines. Each propagate write
queries among themseles and each has a copy of the entire db.

#### Sharding
Partition the DB among computers. A query router forwrds queries to the relevant one.
Problem: if a machine in the cluster fails, the part of data will be unavailable.
Sharding with Replication to rescue

#### Data Consistency in Dsitributed Database
CAP (Consistency, Availalbity, Partition)

### Computer
#### Architecture
Memory: Ram
Processor: CPU

##### Memory
Memory is devied into many cells. each cell store a tiny amount of data, and
has a numerical address. Reading or writing data in that memory is done through
operation affecting only that cell.

We transmite cell address through wires as binary numbers. each wire transmits
a binary digit. High voltage -> 1, low voltage -> 0

##### CPU
Registers: internal memory cells
cpu perform simple math operations with number stored in these registers.
cpu can also move data between ram and these registers.
The collection of all operations a CPU can do is called its instruction set.
Each operation in the instruction set is assigned a number.
Compute code is essentially a sequence of numbers representing CPU operations
These operations are stored as numbers in RAM.
We store input/output data, partial calculations, and computer code all mixed in RAM.

#### Why we can't insert PS CD into destop computer and play?
It's because of CPU architectures.
x86 is the most standard nowadays
64-bit architectue means CPU coul operate binary number of up to 64 digits in a
single machine instruction.

Emulators
the intructions are decoed by the emulator program and executed within the emulatede
machine.

### Compilers
Compiler is a program to translate our Programming languages as machine instructions
a CPU can run.

### Operating Systems
Program reply on OS to execute. because a program can then work effortlessly with
different hard-ward. Program make special system calls. Compilers translate the
IO command to system calls.