Question

I am working on an database class based on pdo, the database used is a mysql database, but i want to cache the database to get more performance. Would it be a good idea to cache the database in an SQlite in-memory database and do the SELECT’s on this (and update the db on a INSERT, UPDATE, …) db?
But, when i write ” cache = new PDO(‘sqlite:memory’); ” does it create a new one or does it use the existent db? also, if one server hosts multiple websites, how could the use different in-memory databases?

Answer 1

Each SQLite in-memory database is connection specific. Even when you open two connections to :memory: within the same process two separate in-memory databases are created. So the answer is that a new database is always created and it never uses an existing database.

Be cautious to use correct name to create in-memory database. Any other name than :memory: (like :memorry: or memory or /:memory:) doesn’t actually create in-memory database but a regular file based database. This happens silently.

The performance gain you will get with the suggested caching is likely to be small. If you do selects on MySQL and the computer has plenty of memory, the data is likely to be already in RAM due to OS disk buffering. So adding another layer of RAM in the form of SQLite in-memory database won’t help much.

The suggested caching leads easily to complex or incorrect caching logic if the system is not read only. If some process does a data update, it must update its cache, update MySQL database and you also have to create some mechanism to tell to other processes that their caches are no longer valid and they must read the data from MySQL. Otherwise your data soon becomes a mess. And think what happens when two processes decide to update same data same time but to different values? You have a conflict.

To get more performance I would first tune the server and MySQL. If it is not enough I would add a cache like Memcached that is not application process specific but a separate process. Even then you have to be careful to build the application logic in a way that data remains consistent on updates. If you use MySQL only and do correct transactions with InnoDB then MySQL does just that for you.