Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
Migrations Under The Hood
Search
Andrew Godwin
November 14, 2014
Programming
6
12k
Migrations Under The Hood
A talk I gave at Django Under The Hood 2014.
Andrew Godwin
November 14, 2014
Tweet
Share
More Decks by Andrew Godwin
See All by Andrew Godwin
Reconciling Everything
andrewgodwin
1
330
Django Through The Years
andrewgodwin
0
220
Writing Maintainable Software At Scale
andrewgodwin
0
450
A Newcomer's Guide To Airflow's Architecture
andrewgodwin
0
360
Async, Python, and the Future
andrewgodwin
2
680
How To Break Django: With Async
andrewgodwin
1
730
Taking Django's ORM Async
andrewgodwin
0
730
The Long Road To Asynchrony
andrewgodwin
0
660
The Scientist & The Engineer
andrewgodwin
1
780
Other Decks in Programming
See All in Programming
FindyにおけるTakumi活用と脆弱性管理のこれから
rvirus0817
0
530
RDoc meets YARD
okuramasafumi
4
170
Reading Rails 1.0 Source Code
okuramasafumi
0
250
AI時代のUIはどこへ行く?
yusukebe
18
9k
Flutter with Dart MCP: All You Need - 박제창 2025 I/O Extended Busan
itsmedreamwalker
0
150
Swift Updates - Learn Languages 2025
koher
2
490
How Android Uses Data Structures Behind The Scenes
l2hyunwoo
0
480
詳解!defer panic recover のしくみ / Understanding defer, panic, and recover
convto
0
250
Improving my own Ruby thereafter
sisshiki1969
1
160
そのAPI、誰のため? Androidライブラリ設計における利用者目線の実践テクニック
mkeeda
2
1.8k
今だからこそ入門する Server-Sent Events (SSE)
nearme_tech
PRO
3
250
1から理解するWeb Push
dora1998
7
1.9k
Featured
See All Featured
Templates, Plugins, & Blocks: Oh My! Creating the theme that thinks of everything
marktimemedia
31
2.5k
A Modern Web Designer's Workflow
chriscoyier
696
190k
Designing Experiences People Love
moore
142
24k
Testing 201, or: Great Expectations
jmmastey
45
7.7k
The Success of Rails: Ensuring Growth for the Next 100 Years
eileencodes
46
7.6k
Designing for humans not robots
tammielis
253
25k
No one is an island. Learnings from fostering a developers community.
thoeni
21
3.4k
GraphQLとの向き合い方2022年版
quramy
49
14k
Imperfection Machines: The Place of Print at Facebook
scottboms
268
13k
For a Future-Friendly Web
brad_frost
180
9.9k
Large-scale JavaScript Application Architecture
addyosmani
513
110k
Being A Developer After 40
akosma
90
590k
Transcript
Migrations UNDER THE HOOD Andrew Godwin Migrations
Andrew Godwin Author of South migrations library Hi, I'm Author
of 1.7 Django migrations Senior Software Engineer at Eventbrite Apparently at every Django conference
Migrations
Migrations
Other talks are available. This is a technical dive!
The Initial Plan Django Schema backend ORM Hooks South 2
Migration handling User interface
TheBRevisedBPlan Django SchemaBbackend ORMBHooks SouthB2 MigrationBhandling UserBinterface BackportBforB1.4B-B1.6
The Revised Revised Plan Django Schema backend ORM Hooks Migration
handling User interface
Logically Separate SchemaEditor Schema Migrations field.deconstruct() ModelOptions.apps Operations Loader /
Graph Executor Autodetector Optimiser State
Logically Separate SchemaEditor Schema Migrations field.deconstruct() ModelOptions.apps Operations Loader /
Graph Executor Autodetector Optimiser State
myapp/0001 myapp/0002 otherapp/0001 contenttypes/0001 appthree/0001 myapp/0003 otherapp/0002
(insert wavy memory effect!) Why do we need dependencies?
Makes all tables syncdb Sets all constraints Adds all ForeignKeys
Makes appA tables migrations Makes appB tables Adds FKs from
appB to appA Adds FKs from appA to appB
Makes appA tables migrations Makes appB tables Adds FKs from
appB to appA Adds FKs from appA to appB
The obvious stuff. Basic Dependencies
Or your database gets really mopey. FK target add before
FK added
In the general case; some DBs less fussy FK target
delete after FK deleted
Good luck doing it otherwise. Model create before field create
Also after field create Model delete after field alter
Stuff you occasionally forget Django has. Trickier Dependencies
And, of course, the reverse for deletion. M2M through model
before M2M
I'll let you figure out deletion reversals from now on.
MTI parent before MTI child
Index together, too Unique together after fields
Well done if you've actually used this feature. Order with
respect to after target
This happens when you turn proxies into real models. Same
delete before same create
Impossible dependencies
OH PLEASE NO TAKE IT AWAY Swappable models
Your migration dependencies myapp/0001 myapp/0002 otherapp/0001 auth/0001 contenttypes/0001
myapp/0001 myapp/0002 otherapp/0001 auth/0001 contenttypes/0001 thirdapp/0001 Your migration dependencies on
swappable models
what? Your migration dependencies on swappable models myapp/0001 myapp/0002 otherapp/0001
auth/0001 contenttypes/0001 thirdapp/0001 ???
what? Your migration dependencies on swappable models myapp/0001 myapp/0002 otherapp/0001
auth/0001 contenttypes/0001 thirdapp/0001 ??? argh
End of presentation. Click to exit.
They can't change at runtime, for swappable models or anything
else. Dependencies are static
(for a while, the answer was "careful function ordering") How
is it implemented?
no issues depend match shift operations restart
Using dependencies to define boundaries Chop operations into migrations
Optimisation
There's really quite a lot. Even more rules
create model + delete model = nothing create model +
create field = create model create field + alter field = create field alter field + delete field = delete field alter field + alter field = alter field
This time, with collapses rather than swaps Even more shuffling
can't reduce reduce possible check conflicts reduced, reset
Lets us be more verbose and clear with operations Applied
after autodetection
Makes your history a bit shorter Applied after squash
Loading & Graph
Builds graph of all basic migrations in memory Loads from
disk first
Marks applied state on each node Loads from db second
Only if all replaced nodes have same applied state Applies
squashes third
In-order list of all a node's ancestors in the tree
Graph returns "plans"
• Autodetector is slow • Optimizer is not great •
Graph state building is inefficient Room for improvement!
django/db/migrations/autodetector.py Autodetector django/db/migrations/optimizer.py Optimiser reduce()GfunctionGisGentryGforGlogic StartGatG_detect_changes()GandGfollowGcalls django/db/migrations/graph.py Graph django/db/migrations/loader.py
Thank you. Andrew Godwin @andrewgodwin