Oleksandr Matsko dragon-lex

System Design Cheatsheet

Picking the right architecture = Picking the right battles + Managing trade-offs

Basic Steps

Clarify and agree on the scope of the system

User cases (description of sequences of events that, taken together, lead to a system doing something useful)
- Who is going to use it?
- How are they going to use it?

Latency numbers every programmer should know

L1 cache reference ......................... 0.5 ns
Branch mispredict ............................ 5 ns                     on recent CPU
L2 cache reference ........................... 7 ns                     14x L1 cache
Mutex lock/unlock ........................... 25 ns
Main memory reference ...................... 100 ns                     20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy ............. 3,000 ns  =   3 µs
Send 2K bytes over 1 Gbps network ....... 20,000 ns  =  20 µs
SSD random read ........................ 150,000 ns  = 150 µs

Read 1 MB sequentially from memory ..... 250,000 ns = 250 µs 4X memory

Tools and libraries which I prefer to use in Python stack

Development Environment

Ubuntu Linux for both development and production
Git version control system
Jenkins CI build server (but recently I'm moving to git server-side hooks with ssh from git server to dev/stage/prod servers)
Redmine bug tracking system
JetBrains PyCharm IDE

Production Environment

Latency numbers every programmer should know

L1 cache reference ......................... 0.5 ns
Branch mispredict ............................ 5 ns
L2 cache reference ........................... 7 ns
Mutex lock/unlock ........................... 25 ns
Main memory reference ...................... 100 ns             
Compress 1K bytes with Zippy ............. 3,000 ns  =   3 µs
Send 2K bytes over 1 Gbps network ....... 20,000 ns  =  20 µs
SSD random read ........................ 150,000 ns  = 150 µs

Read 1 MB sequentially from memory ..... 250,000 ns = 250 µs

	import logging.config
	import os
	from django.utils.log import DEFAULT_LOGGING

	# Disable Django's logging setup
	LOGGING_CONFIG = None

	LOGLEVEL = os.environ.get('LOGLEVEL', 'info').upper()

	logging.config.dictConfig({

	class A(models.Model):
	things = models.ManyToManyField("B", through=ThroughModel)

	class B(models.Model):
	text = models.TextField()

	class ThroughModel(models.Model):
	a = models.ForeignKey(A)
	b = models.ForeignKey(B)
	extra = models.BooleanField()

	{#
	Source: http://dev.dbl-a.com/symfony-2-0/symfony2-and-twig-pagination/
	Updated by: Simon Schick <simonsimcity@gmail.com>

	Parameters:
	* currentFilters (array) : associative array that contains the current route-arguments
	* currentPage (int) : the current page you are in
	* paginationPath (string) : the route name to use for links
	* showAlwaysFirstAndLast (bool) : Always show first and last link (just disabled)
	* lastPage (int) : represents the total number of existing pages

	# Something in lines of http://stackoverflow.com/questions/348630/how-can-i-download-all-emails-with-attachments-from-gmail
	# Make sure you have IMAP enabled in your gmail settings.
	# Right now it won't download same file name twice even if their contents are different.

	import email
	import getpass, imaplib
	import os
	import sys

	detach_dir = '.'

	# -- coding: utf-8 --
	"""
	Easy change settings with uncomment right string ang comment wrong.

	Iterate over files in ``env.settings_files``, comment lines with
	wrong marker and uncomment with right marker.

	Usage
	=====