Question

I have inherited an app that generates a large array for every user that visit the app. I recently discovered that it is identical for nearly all the users!!

Now I want to somehow make one copy of it so it is not built over and over again. I have thought of a few options and wanted input to see which one is the best:

1) Create a model and shove the data into the database
2) Create a YAML file and have the app load it when it initializes.

I personally like the model idea but a few engineers at work feel as though it does not deserve to be a full model. 97% of the times users will see the same exact thing but 3% of the time users will get a slightly different array (a few elements will have changed).
Any other approaches that I should consider.??..thanks in advance.

Answer 1

Remember that if you store the data in the DB, each request which requires the data will have to execute a DB query to pull it out. If you are running multiple server threads, each thread could have its own copy in memory (if they are all handling requests which require the use of the array). In that case, you wouldn’t be saving any memory (though you might save time from not having to regenerate the array).

If you are running multiple server processes (not threads), and if the array contents change as the application is running, and the changes have to be visible to all the processes, caching in memory won’t work. You will have to use the DB in that case.

From the information in your comment, I suggest you try something like this:

Store the array in your DB, and make sure that the record(s) used have created/updated timestamps. Cache the contents in memory using a constant/global variable/class variable. Also store the last time the cache was updated.

Every time you need to use the array, retrieve the relevant “updated” timestamp from the DB. (You may need to use hand-coded SQL and ModelName.connection.execute to avoid pulling back all the data in the record, which ActiveRecord will probably do.) If the timestamp is later than the last time your cache was updated, pull the array from the DB and update your cache.

Use a Mutex ('require thread') when retrieving/updating the cached data, in case your server setup may use multiple threads. (I don’t think that Passenger does, but I have had problems similar to threading problems when using Passenger+RMagick, so I would still use a Mutex to be safe.)

Wrap all the code which deals with the cached array in a library class (or a class method on the model used to store the data), so the details of cache management don’t spill over into the rest of the application.

Do a little bit of performance testing on the cache setup using Benchmark.measure {}. If a bug in the setup actually made performance worse rather than better, that would be sad…