Changing existing Core Data Image Store from Transformable to Binary Data/Allows External Storage

Question

I am about to undertake the daunting project of converting my live (i.e. already on the app store for a number of years) app from Transformable to Binary Data store for images in Core Data.

I have many users with very large databases that store lots of images. This has really slowed down the Backup/Restore process, and probably caused some other behind-the-scenes issues as well. I didn't know any better when I set it up this way years ago.

How can I undergo this process so as not to lose even one of my customer's images? If it were just me and my own data, I'm sure I could get things working. But I want to be sure to do this properly, step by step, and I knew that this community could be a big help in that area. I really don't know where to start for the existing images.

Basically, I am looking for 1) steps to take, so as not to miss a beat. and 2) general advice, warnings, etc. in this process. I really need a clean migration when this version goes live.

Thanks in advance to anyone who can help.

Answer 1

Transformable data type is really just binary under the covers with some additional metadata. Have you tested a simple lightweight migration on an existing store? I suspect the migration would work and would leave the existing data in the store.

If you are looking to get the existing binary data actually moved out of the SQLite file then you are looking at something a bit more involved.

A heavy migration will accomplish what you are looking for but if the stores are large it may take took long and potentially not provide enough feedback for a good user experience. I personally do not use heavy migrations, ever, on IOS but it will accomplish your goal.

An export/import will also work. I generally recommend export/import when a lightweight migration won't work. It involves a medium amount of code but in the end you own the code, understand the entire process and can tweak it to your exact needs.

Answer 2

One piece of advice: don't use "Allows External Storage", especially if you plan to use iCloud syncing with Core Data in the future. Reference: http://www.objc.io/issue-10/icloud-core-data.html

Instead, you might want to consider moving the images into their own files, and saving the URL to those files inside your database instead. You will have to work out how best to do the migration: lightweight migration is probably not an option if you go down this route.