Cache me if you can: memcached, caching patterns and best practices by Guillaume Ardaud

Transcript

1. CACHE ME IF YOU CAN Guillaume Ardaud
 @gardaud | gardaud.fr
 gardaud@acm.org Friday April 11th
 PyCon2014
 Montreal
2. MEMCACHED -What is it?
 -How does it work?
 -Patterns & Best Practices
3. Key/Object Store • Give it an object, identified by a key
 • Ask for a key
 • If it isn’t expired, you get the object
4. O(1) EVERYTHING
5. primary data:
 relational database data you can lose, regenerate fast: 
 RAM store data you can lose, regenerate slowly: 
 persistent store Where does memcached fit?
6. Typical memcached API
12. Key names - ASCII based
 - Not crazy long…
 - … but explicit enough
13. Good naming e.g.: 
 json.users.<user_id>
14. Bad naming e.g.: 
 md5(sql_query)
15. HOW DO I LIST ALL KEYS IN MEMCACHED? YOU DON’T (or all keys matching a regex, or all...)
16. it’s a cache not a database
18. memcached docs:
20. memcached is distributed
21. your amazing application memcached box A memcached box B memcached box C memcached client
22. your amazing application memcached box A memcached box B memcached box C memcached client set $KEY send $KEY
 to box C Received!
23. your amazing application memcached box A memcached box B memcached client set $KEY send $KEY
 to box A
24. ~ some memcached subtleties ~
25. EXPIRATION It’s 8am, a request stores an object under the key ‘foo’, with a 2 hour expiration time. What happens at 10am?
26. But what happens when a client tries to access 'foo' at 1pm? …'foo' gets removed. NOTHING
27. Q: Sometimes fetching a key returns None even though it’s not supposed to have expired yet?
28. Memcached has a fixed amount of memory." If you try to store data when memory is full, data will get evicted.
29. How does memcached determine what to evict? Object 1 Object 2 timestamp: 1378399504 timestamp: 137839983 Object 3 timestamp: 1378399939 Object 4 timestamp: 13783992932 Object 5 timestamp: 13783993829
30. Q: Sometimes fetching a key returns None even though it’s not supposed to have expired yet… … AND memcached still has free memory?
32. page 1 page 2 page 3 page 4 page 5 page 6 page 7 ... 1 page = 1MB
33. page chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk 1st_slab_chunk_size = 80 bytes! 2nd_slab_chunk_size = 80 * 1.25 = 100 bytes! 3rd_slab_chunk_size = 100 * 1.25 = 125 bytes! . . .! last_slab_chunk_size = 1048576 bytes Last slab has 1 chunk of 1 MB.
35. page 1 page 2 page 3 page 4 page 5 page 6 page 7 ... class 1 class 2 class 3 class 4 class 5 free page free page
37. If all pages of the needed class are full: get a free page and give it the needed class. If there are no more free pages: 
 the LRU kicks in and evicts data. Each slab class has its own LRU! " If your data needs class 3 and: - there are no free pages - all class 3 pages are full - some class 4 pages have free chunks Data in a class 3 page will still be evicted.
38. Q: Sometimes fetching a key returns None even though it’s not supposed to have expired yet… … AND memcached still has free memory?
39. some useful command line flags memcached -v : verbose output (more verbose: -vv) memcached -M: doesn’t evict when out of memory, but errors " memcached -I1k : change slab page size (1k - 128m) memcached -I1m! memcached -I128m! " memcached -f1.5 : change growth factor (def. 1.25, >1) " " man memcached is your friend
40. Let’s get our hands dirty. common memcached patterns & practices in Python
44. MODEL VERSIONING
45. FETCH STRAIGHT FROM CACHE
47. THUNDERING HERD
49. CACHING LARGE VALUES
50. 2-PHASE FETCH
51. 1. Break big object in smaller chunks (for example chunks of 10 books) PAGINATED CACHE 2. Store each chunk as a separate object in memcached 3. Store the list of keys To fetch the data, fetch the list of keys, iterate through it, and concatenate all fetched lists.
55. Questions? THANKS!
56. Great memcached resources Memcached doc: http://code.google.com/p/memcached/wiki/NewStart?tm=6 " Especially those 2 pages: http://code.google.com/p/memcached/wiki/NewProgrammingTricks http://code.google.com/p/memcached/wiki/NewUserInternals " Various articles/posts about internals etc.: http://stackoverflow.com/questions/6868256/memcached-eviction-prior-to-key- expiry http://www.adayinthelifeof.nl/2011/02/06/memcache-internals/ http://work.tinou.com/2011/04/memcached-for-dummies.html http://returnfoo.com/2012/02/memcached-memory-allocation-and-optimization-2/