| Talk: All Things Cached - SF Python 2017 Meetup | |
| =============================================== | |
| * `Python All Things Cached Slides`_ | |
| * Can we have some fun together in this talk? | |
| * Can I show you some code that I would not run in production? | |
| * Great talk by David Beazley at PyCon Israel this year. | |
| * Encourages us to scratch our itch under the code phrase: "It's just a | |
| prototype." Not a bad place to start. Often how it ends :) | |
| Landscape | |
| --------- | |
| * At face value, caches seem simple: get/set/delete. | |
| * But zoom in a little and you find just more and more detail. | |
| Backends | |
| -------- | |
| * Backends have different designs and tradeoffs. | |
| Frameworks | |
| ---------- | |
| * Caches have broad applications. | |
| * Web and scientific communities reach for them first. | |
| I can haz mor memory? | |
| --------------------- | |
| * Redis is great technology: free, open source, fast. | |
| * But another process to manage and more memory required. | |
| :: | |
| $ emacs talk/settings.py | |
| $ emacs talk/urls.py | |
| $ emacs talk/views.py | |
| :: | |
| $ gunicorn --reload talk.wsgi | |
| :: | |
| $ emacs benchmark.py | |
| :: | |
| $ python benchmark.py | |
| * I dislike benchmarks in general so don't copy this code. I kind of stole it | |
| from Beazley in another great talk he did on concurrency in Python. He said | |
| not to copy it so I'm telling you not to copy it. | |
| :: | |
| $ python manage.py shell | |
| .. code-block:: pycon | |
| >>> import time | |
| >>> from django.conf import settings | |
| >>> from django.core.cache import caches | |
| .. code-block:: pycon | |
| >>> for key in settings.CACHES.keys(): | |
| ... caches[key].clear() | |
| :: | |
| >>> while True: | |
| ... !ls /tmp/filebased | wc -l | |
| ... time.sleep(1) | |
| Fool me once, strike one. Feel me twice? Strike three. | |
| ------------------------------------------------------ | |
| * Filebased cache has two severe drawbacks. | |
| 1. Culling is random. | |
| 2. set() uses glob.glob1() which slows linearly with directory size. | |
| DiskCache | |
| --------- | |
| * Wanted to solve Django-filebased cache problems. | |
| * Felt like something was missing in the landscape. | |
| * Found an unlikely hero in SQLite. | |
| I'd rather drive a slow car fast than a fast car slow | |
| ----------------------------------------------------- | |
| * Story: driving down the Grapevine in SoCal in friend's 1960s VW Bug. | |
| Features | |
| -------- | |
| * Lot's of features. Maybe a few too many. Ex: never used the tag metadata and | |
| eviction feature. | |
| Use Case: Static file serving with read() | |
| ----------------------------------------- | |
| * Some fun features. Data is stored in files and web servers are good at | |
| serving files. | |
| Use Case: Analytics with incr()/pop() | |
| ------------------------------------- | |
| * Tried to create really functional APIs. | |
| * All write operations are atomic. | |
| Case Study: Baby Web Crawler | |
| ---------------------------- | |
| * Convert from ephemeral, single-process to persistent, multi-process. | |
| "get" Time vs Percentile | |
| ------------------------ | |
| * Tradeoff cache latency and miss-rate using timeout. | |
| "set" Time vs Percentile | |
| ------------------------ | |
| * Django-filebased cache so slow, can't plot. | |
| Design | |
| ------ | |
| * Cache is a single shard. FanoutCache uses multiple shards. Trick is | |
| cross-platform hash. | |
| * Pickle can actually be fast if you use a higher protocol. Default 0. Up to 4 | |
| now. | |
| * Don't choose higher than 2 if you want to be portable between Python 2 | |
| and 3. | |
| * Size limit really indicates when to start culling. Limit number of items | |
| deleted. | |
| SQLite | |
| ------ | |
| * Tradeoff cache latency and miss-rate using timeout. | |
| * SQLite supports 64-bit integers and floats, UTF-8 text and binary blobs. | |
| * Use a context manager for isolation level management. | |
| * Pragmas tune the behavior and performance of SQLite. | |
| * Default is robust and slow. | |
| * Use write-ahead-log so writers don't block readers. | |
| * Memory-map pages for fast lookups. | |
| Best way to make money in photography? Sell all your gear. | |
| ---------------------------------------------------------- | |
| * Who saw eclipse? Awesome, right? | |
| * Hard to really photograph the experience. | |
| * This is me, staring up at the sun, blinding myself as I hold my glasses and | |
| my phone to take a photo. Clearly lousy. | |
| * Software talks are hard to get right and I can't cover everything related to | |
| caching in 20 minutes. I hope you've learned something tonight or at least | |
| seen something interesting. | |
| Conclusion | |
| ---------- | |
| * Windows support mostly "just worked". | |
| * SQLite is truly cross-platform. | |
| * Filesystems are a little different. | |
| * AppVeyor was about half as fast as Travis. | |
| * check() to fix inconsistencies. | |
| * Caveats: | |
| * NFS and SQLite do not play nice. | |
| * Not well suited to queues (want read:write at 10:1 or higher). | |
| * Alternative databases: BerkeleyDB, LMDB, RocksDB, LevelDB, etc. | |
| * Engage with me on Github, find bugs, complain about performance. | |
| * If you like the project, star-it on Github and share it with friends. | |
| * Thanks for letting me share tonight. Questions? | |
| .. _`Python All Things Cached Slides`: http://bit.ly/dc-2017-slides | |