Caching COUNT with PHP and Redis | Hacker News Reader

Caching COUNT with PHP and Redis

forrst.com

26 pointscalebrown12 comments

> if for some reason the memory cache diverges from the true count, we just wipe out the memory cache and the app takes care of regenerating it automatically

Warning, this is a myth: once you starte relying on caching to serve your amount of traffic, flushing the cache will have the result of taking the site down. This is why persistence is an absolute requirement of a serious caching server IMHO. With Redis when things may be in desync it's better to selectively remove entries with (RANDOMKEY+DEL) at a rate that the system is able to handle.

Also given that you are using Redis that has atomic increments, why not going the extra mile and issuing INCR/DECR operations when something is added/removed?

I should have been more specific--technically the cache would be wiped for a specific post, never for the entire db of posts. But you're totally right about the persistence --that's why something like redis made sense to me.

Useful tip about randomkey and del, thank you.

I def. could switch it over to an incr/decr setup. Maybe I'll do that tonight.

Doesn’t this seem too basic to have been voted up so much? That is, compared to the rest of the technical posts that are voted up.

And perhaps MemcacheDB (http://memcachedb.org/) is a better bet if you don’t need Redis’s list and set operations. But I don’t understand why the cache needs to be persistent.

Two things occur:

1. This is a better fit for memcache than redis, since you can get the counts to expire automatically

2. If you are fetching paginated results with a LIMIT (and you usually are) then you can use the SQL_CALC_FOUND_ROWS prefix on your SELECT to get these counts for "free" from MySQL without needing to do your own caching (see http://dev.mysql.com/doc/refman/5.0/en/select.html )

Assuming good indexing with non-enormous tables, he could use subqueries to get the COUNT() data at the same time that he pulls the records themselves. And, as he notes, he could have a count field in MySQL, caching at that level instead of in Redis.

Redis is great, maybe memcached would serve better for this pupose as some of the comments have mentioned. I just want to point out one thing about his multiple count queries per page.

You can also do a grouped count query in a single query, by using something like:

SELECT COUNT(*) FROM comments WHERE GROUP BY post_id;

From that he gets an array of all the counts, so he can look those up easily.

And that result can also be cached as well, of course.

MySQL already caches these. You've added a cache on top of a cache. Is a round trip to the database so slow?

if you are doing 15-20 COUNTS per page (as he said he was) even if you are on a fast local network those few milliseconds of latency can probably add up, also he said he was directly managing updating/invalidating the count cache entries which is probably more effective (because the caching strategy is so specific) than whatever more generic strategy mysql is using.

I was under the perhaps false impression that count star queries with or without where conditions are not cached on innodb tables.

Not exactly. On isam table the count(*) queries can sometimes directly lookup the answer from the index, or the table statistics.

On innodb, because of transactions, this is not possible, and it needs to actually count the rows.

BUT, the query itself is still cached - at least until the underlying table gets updated, which invalidates the cache (I'm not sure of the exact invalidation strategy, but it's part of MySQL and doesn't depend on the engine).

Updating the table invalidates all cached queries that refer to that table, making the query cache useless for any site that gets a decent amount of updates.

Depending on your database engine and WHERE conditions it may not be cached as you assume it to be.

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