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Deploying, at an Unusual Scale
Andrew Godwin
October 22, 2011
Programming
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440
Deploying, at an Unusual Scale
A talk I gave at DjangoCon Europe 2011, about Epio's internal architecture at that point.
Andrew Godwin
October 22, 2011
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Transcript
Deploying, At An Unusual Scale Andrew Godwin http://www.flickr.com/photos/whiskeytango/1431343034/ @andrewgodwin
Hi, I'm Andrew. Serial Python developer Django core committer Co-founder
of ep.io
Hi, I'm Andrew. Serial Python developer Django core committer Co-founder
of ep.io Occasional fast talker
""Andrew speaks English like a machine gun speaks bullets."" Reinout
van Rees
We're ep.io Python Platform-as-a-Service Easy deployment, easy upgrades PostgreSQL, Redis,
Celery, and more
Why am I here? Our Architecture How we deploy Django
How varied Django deployments are
Our Architecture
Balancer Runner Runner Runner App 1 App 2 App 3
App 2 App 4 App 1 Databases File Storage Balancer
Oh My God, It's Full of Pairs Everything is redundant
Distributed programming is Hard
Hardware Real colo'd machines Linode EC2 (pretty unreliable) (pretty reliable)
(pretty reliable) IPv6 (as much as we can)
ØMQ We used to use Redis Everything now on ZeroMQ
Eliminates SPOF* * Single Point Of Failure. What a pointless acronym.
ØMQ Usage Redundant location-resolvers (Nexus) REQ/XREP for control messages PUSH/PULL
for stats, logs PUB/SUB for heartbeats, locking
Runners Unsurprisingly, these run the code SquashFS filesystem images Virtualenvs
per app UID & permission isolation, more coming
Logging/Stats All done asynchronously using ØMQ Logs to filesystem (chunked
files) Stats to PostgreSQL database, for now
Loadbalancers Intercept all incoming HTTP requests Look up hostname (or
suffix) HTTP 1.1 compliant
Databases Shared (only for PostgreSQL) Dedicated (uses Runner framework) PostgreSQL
9, damnit
Django in the backend We use the ORM extensively Annoying
settings fiddling in __init__
www.ep.io Runs on ep.io, just like any other app* Provides
JSON API, web UI * Well not quite - App ID 0 is special - but we're working on it
WSGI It's a standard, right?
WSGI It's a standard, right? Well, yes, and it works
fine, but it's not enough for serving a Python app
Static Files CSS, images, JavaScript, etc. Needs a URL and
a directory path
Python & Dependencies Mostly filled by pip/buildout/etc packaging apparently allows
version spec
Deploying Django It makes things consistent, right?
Settings Layouts Vanilla settings.py local_settings.py configs/HOSTNAME.py Many others...
Python Paths Project-level imports App-level imports apps/ directories
Databases If it's SQL, it's PostgreSQL Redis for key-value, MongoDB
soon Some things assume a safe network
HA (High Availability) Not terribly easy with shared DBs PostgreSQL
9's sensible warm standby Redis has SLAVEOF Possibly use DRBD for general solution
Backups High Availability is NOT a backup btrfs for consistent
snapshotting Archived remote syncs No access to backups from servers
Migrations No solution yet for migration/code sync We're working on
it...
Web serving It's not like it's important or anything
gunicorn Small and lightweight Supports long-running requests Pretty stable
nginx Even more lightweight Extremely fast Really, really stable
The Load Balancer Used to be HAProxy Rewritten to custom
Python daemon eventlet used for high throughput Can't use nginx, no HTTP 1.1 for backends
Celery See: Yesterday's Talk Slightly tricky to run many We
use Redis as the backend
Management Commands First off, run as subprocess Then, a custom
PTY module Now, run as pty-wrapping subprocesses
Some General Advice If you're crazy enough to do this
Messaging's Not Enough Having a state to check is handy
Why run one, when you can run two for twice
the price? Redundancy is good. Double redundancy is better.
Always expect the worst Hope you never have to deal
with it.
The more backups, the better. Make sure you have historical
ones, too.
Django is very flexible Sometimes a little too flexible...
Your real problems will emerge later Don't over-optimise up front
for everything
Questions? Andrew Godwin andrew@ep.io @andrewgodwin