WebFund 2016W Lecture 11

Notes

In-class Notes

Lecture 11
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Databases

/var/spool/mail
  - where email is stored
  - one file per user

my mail inbox in /var/spool/mail/soma

Who accesses this file?
  - mail client
  - mail server

What happens if the mail server delivers mail while I'm going
through my email (deleting, filing)
 - unless you lock it, you lose mail, mail client will overwrite
   inbox and delete new messages
 - solution: lock the inbox
     create a soma.lock file
     (inside it has the process ID of the accessing process)

This is a bad idea that does not scale
Instead, use a database
 - concurrent access
 - data persistence



What kind of database?
 - key-value stores
    like a filesystem for very small files,
    with search
 - document store  <-- we'll use this
    values have structure
    can index on values
 - relational database
    sets of tables
    tables have rows and columns
    relations between tables
    example: names and addresses
      - rows are individual people
      - columns are first name, last name, address, city, etc.
    relation example: customers and purchases
      - each purchase (invoice) has item purchased, price, and
        customer ID
      - customer table connects customer ID and name, address, etc

We're going to use MongoDB, a document store, not a relational
database.
  - stores JSON-like documents
  - scriptable in JavaScript

If we were to use a relational database, we'd have to learn SQL
  - Structured Query Language
  - many man implementations
     Oracle, MySQL (MariaDB), PostgreSQL, MS SQL Server, SQLite, etc
  - you mainly need a relational database to handle transactions

Transaction
 - what if a change to a database involves multiple tables?
 - a transaction ensures all tables are changed or none are

Classic example: airline tickets

Tables:
 - seat reservations
 - payment
 - customer info

When you buy a ticket, you want to change all three

A transaction ensures atomic behavior
  all happen or none happen

MongoDB        Relational DB
Document   =>  row
Collection =>  table