When caching images and data should you use Internal or External Storage?

Question

I've read this Android save app settings/data in Internal/External Storage but I don't think i've really got an answer.

I'm developing an app where I should store some "cache" data, like a dozen of images and some strings (json).

Initially I was storing all that on the sdcard, (external storage) but later i thought that this could be deprecated by the SDCard deprecation in most recent devices.

After reading a bit, I understood that external storage is not only sdcard, but "a removable storage media (such as an SD card) or an internal (non-removable) storage" so it should not be deprecated but...it's shared space, and there is not ownership over the files stored there, so the first problem was that I was unable to delete them when the app was deleted.

So I changed to the Internal Storage, to avoid having the files/images "public" and also having them removed after app deletion.

Is this the recommended approach?

On devices older with low internal storage but with a lot of space in the SDcard is this a good approach?

My application supports from 1.6 to 4.0 (so far) so I have a lot of legacy devices... and I must have the app working (well) on all.

Looking forward for some interesting answers!

Answer 1

It depends on the type of data you are wanting to store.

You mention it's cached data so myassumption is that it should not matter if for some reason it all disappears. This leads me to believe that you should be using the getCacheDir(). In this case the system will remove files if the cache becomes too big so devices with low internal storage shouldn't present a problem (although it is recommended to manage this your self anyway), it's relatively secure and it will be managed by the app so if there is an uninstall it will be removed.

getExternalCacheDir() was introduced in 2.2 so isn't any use to you unless you would like to detect the version and switch between the 2 caching directories getExternalCacheDir() doesn't provide security so data could be accessed by anyway with access to the SD card. The only reason I could think you might want to do this is because of size of cache you desire but from your description the data doesn't seem excessive.

UPDATED from comment:

although this is a specific case where it's cache...but I don't want it to be deleted whenever the system wants. It's the kind of cache that I need the to app decide when to purge. What is the main concern of storing in on "normal" internal storage without being on the cache dir?

If you get to the stage where the system is cleaning up internal cached data because storage is so low then you should probably leave it to clean up this sort of app data. By using standard internal data storage you are bypassing this safe guard which would probably create more unpleasant problems than having app data deleted.

If your data is of high importance then I would suggest trying to identify specific data that is more important and managing that separately. If this data you identify needs to be secure then internal storage using files or a db (depending on the data type) seems like your only real option but you would have to be wary of this data building up.

UPDATED from comment

What do you think about using SharedPreferences to save string data? Is there a limit on SharedPreference string saved size? Is it a (good) possibility?

I have used shared preferences to store relatively big json strings in the past with no problem, I find it's simpler than using the databases for primitive data types (and strings) where there are limited values to save. However when you have images or lot's of values, management becomes more complex. Also you will have the same problem as you would with standard internal storage in terms of storage space.

Answer 2

I would keep images on the external storage, probably in a "hidden" folder (adding a dot at the beggining of the folder's name: .folder) and also a the Media Scanner "avoider" (.nomedia), because, as you pointed, you want the application to work in old devices, and precisely those devices don't have to much internal memory.

You can also add an option in your application to remove that folder, so in case the user wants to uninstall, he can do that before.

Answer 3

I would use internal storage for cache. This will take away the chance of a user being able to access the files used in your app. I would go for internal