Using Redis to cache SQL result

Question

I have a SQL-based application and I like to cache the result using Redis. You can think of the application as an address book with multiple SQL tables. The application performs

Answer 1

We met the same problem and we chose to do same thing you are thinking of: remove all query caches affected by the table. It is not ideal like your said but fortunately our "write" is not as high as 40% so it's ok so far. That's the nature of query based caching. As an alternative you can add entity based caching. Instead of caching the search result only, cache the entire table and do the search inside memory. We use C# LINQ so we can do pretty common queries in memory but if the search is too complicated then you are out of luck.

Answer 2

Here's a surprising thing when it comes to PHP and MySQL (I am not sure about other languages) - not caching stuff into memcached or Redis is actually faster. Much faster. Basically, if you just built your app and queried MySQL - you'd get more out of it.

Now for the "why" part.

InnoDB, the default engine, is a superb engine. Specifically, it's memory management (allocation and what not) is superior to any memory storage solutions. That's a fact, you can look it up or take my word for it - it will, at least, perform as good as Redis.

Now what happens in your app - you query MySQL and cache the result into redis. However, MySQL is also smart enough to keep cached results. What you just did is create an additional file descriptor that's required to connect to Redis. You also used some storage (RAM) to cache the result that MySQL already cached.

Here comes another interesting part - the preferred way of serving PHP scripts is by using php-fpm - it's much quicker than any mod_* crap out there. Down to the core, php-fpm is a supervisor process that spawns child processes. They don't shut down after the script is served, which means they cache connections to MySQL - connect once, use multiple times. Basically, if you serve scripts using php-fpm, they will reuse the already established connection to MySQL, meaning that you won't be opening and closing connections for each request - this is extremely resource friendly and it lets you have lightning fast connection to MySQL. MySQL, being memory efficient and having the cached result is much quicker than Redis.

Now what does all of this mean for you - having a proper setup lets you have small code that's simple, easy, doesn't involve Redis and eliminates all the problems that you might have with cache invalidation and what not and you won't waste your memory to contain the same data twice.

Ingredients you need for this to work:

• php-fpm
• MySQL and InnoDB based tables and most of all - sufficient RAM and tweaked innodb_buffer_pool_size variable. That one controls how much RAM InnoDB is allowed to allocate for its purposes - the larger the better.

You eliminated Redis from the game, you kept your code simple and easy to maintain, you didn't duplicate data, you didn't introduce additional system to the play and you let software that's meant to take care of data do its job. Pretty cheap trade-off for maximum usefulness, even if you compile all the software from scratch - it won't take more than an hour or so to get it up and running.

Or, you can just ignore what I wrote and look for a solution using Redis.