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673 high scalability-2009-08-07-Strategy: Break Up the Memcache Dog Pile


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Introduction: Update: Asynchronous HTTP cache validations . A proposed HTTP caching extension: if your application can afford to show slightly out of date content, then stale-while-revalidate can guarantee that the user will always be served directly from the cache, hence guaranteeing a consistent response-time user-experience. Caching is like aspirin for headaches. Head hurts: pop a 'sprin. Slow site: add caching. Facebook must have a lot of headaches because they popped 805 memcached servers between 10,000 web servers and 1,800 MySQL servers and they reportedly have a 99% cache hit rate. But what's the best way for you to cache for your application? It's a remarkably complex and rich topic. Alexey Kovyrin talks about one common caching problem called the Dog Pile Effect in Dog-pile Effect and How to Avoid it with Ruby on Rails . Glenn Franxman also has a Django solution in MintCache . Data is usually cached because it's too expensive to calculate for every hit. Maybe it's a gnarly S


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1 Facebook must have a lot of headaches because they popped 805 memcached servers between 10,000 web servers and 1,800 MySQL servers and they reportedly have a 99% cache hit rate. [sent-6, score-0.433]

2 But what's the best way for you to cache for your application? [sent-7, score-0.266]

3 Maybe it's a gnarly SQL query you want to avoid and a little stale data is OK. [sent-12, score-0.638]

4 Or maybe you have the temerity to write to your database and cause its cache to flush so database caching isn't sufficient at a certain level of scale. [sent-14, score-0.412]

5 Data freshness requires a refrigeration truck or an expiry time on your cache entry that causes stats to be periodically recalculated. [sent-21, score-0.744]

6 Now, what happens when your cached data expires and a 1000 requests simultaneously try to recalculate the expensive to calculate data? [sent-22, score-0.501]

7 And since memcached operations are not atomic it's possible stale data could be cached and you'll serve stale data. [sent-24, score-1.247]

8 No Expire Solution If cache items never expire then there can never be a recalculation storm. [sent-27, score-0.515]

9 This approach can also be used to pre-warm the the cache so a newly brought up system doesn't peg the database. [sent-31, score-0.352]

10 Memcached can still evict your cache item when it starts running out of memory. [sent-33, score-0.543]

11 It uses a LRU (least recently used) policy so your cache item may not be around when a program needs it which means it will have to go without, use a local cache, or recalculate. [sent-34, score-0.47]

12 This approach also doesn't work well for item specific caching. [sent-36, score-0.29]

13 It works for globally calculated items like top N posts, but it doesn't really make sense to periodically cache items for user data when the user isn't even active. [sent-37, score-0.651]

14 Stale Date Solution This solution introduces a stale date in addition to the expiration date. [sent-39, score-0.741]

15 For example, set the item to timeout in 24 hours, but the embedded timeout might be five minutes in the future. [sent-42, score-0.582]

16 On a get from the cache determine if the stale timeout expired and on expiry immediately set a time in the future and re-store the data as is. [sent-43, score-1.276]

17 Fetch data from the DB and update the cache with the latest value. [sent-45, score-0.419]

18 Alexey describes a different two key approach: Create two keys in memcached: MAIN key with expiration time a bit higher than normal + a STALE key which expires earlier. [sent-46, score-0.67]

19 If the stale has expired, re-calculate and set the stale key again. [sent-48, score-1.076]

20 I dislike embedding meta data with data so I like Alexey's approach a bit better, even though it doubles the key space. [sent-49, score-0.536]


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