vanishinggradient

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?

Postgres Cheatsheet

This is a collection of the most common commands I run while administering Postgres databases. The variables shown between the open and closed tags, "<" and ">", should be replaced with a name you choose. Postgres has multiple shortcut functions, starting with a forward slash, "". Any SQL command that is not a shortcut, must end with a semicolon, ";". You can use the keyboard UP and DOWN keys to scroll the history of previous commands you've run.

Setup

installation, Ubuntu

http://www.postgresql.org/download/linux/ubuntu/ https://help.ubuntu.com/community/PostgreSQL

PSQL

Magic words:

psql -U postgres

Some interesting flags (to see all, use -h or --help depending on your psql version):

-E: will describe the underlaying queries of the \ commands (cool for learning!)
-l: psql will list all databases and then exit (useful if the user you connect with doesn't has a default database, like at AWS RDS)

# Install django-extensions
# http://django-extensions.readthedocs.io/en/latest/installation_instructions.html
pip3 install django-extensions

# Add django-extensions INSTALLED_APPS in siteapp > settings.py
# INSTALLED_APPS = (
#    ...
#    'django_extensions',
# )

	""" Radon Transform as described in Birkfellner, Wolfgang. Applied Medical Image Processing: A Basic Course. [p. 344] """
	from scipy import misc
	import numpy as np
	import matplotlib.pyplot as plt

	def discrete_radon_transform(image, steps):
	R = np.zeros((steps, len(image)), dtype='float64')
	for s in range(steps):
	rotation = misc.imrotate(image, -s*180/steps).astype('float64')
	R[:,s] = sum(rotation)

	Latency Comparison Numbers (~2012)
	----------------------------------
	L1 cache reference 0.5 ns
	Branch mispredict 5 ns
	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 us
	Send 1K bytes over 1 Gbps network 10,000 ns 10 us
	Read 4K randomly from SSD* 150,000 ns 150 us ~1GB/sec SSD