How do you compact a Realm DB on iOS?

Question

I'd like to compact a Realm instance on iOS periodically to recover space. I think the process is to copy the db to a temporary location, then copy it back and use the new default.realm file.

My problem is Realm() acts like a singleton and recycles objects so I can't really close it and tell it to open the new default.realm file.

The docs here (https://realm.io/docs/objc/latest/api/Classes/RLMRealm.html) suggest I wrap all the Realm() calls in autorelease { } but it can't be this complicated.

Answer 1

The answer given by @marius has an issue: the open Realm might still reference the deleted file. This means some writes might end up in the old (deleted) file, causing the app to lose data.

The correct implementation of compactRealm method looks like this (swift 3):

func compactRealm() {
    let defaultURL = Realm.Configuration.defaultConfiguration.fileURL!
    let defaultParentURL = defaultURL.deletingLastPathComponent()
    let compactedURL = defaultParentURL.appendingPathComponent("default-compact.realm")

    autoreleasepool {
        let realm = try! Realm()
        try! realm.writeCopy(toFile: compactedURL)
    }
    try! FileManager.default.removeItem(at: defaultURL)
    try! FileManager.default.moveItem(at: compactedURL, to: defaultURL)
}

This issue has been driving me crazy until I found an answer here

Answer 2

It can be indeed tricky to completely tear down all retrieved model accessors, but there is unfortunately no other way to close a Realm.

As you wrote "periodically" every app launch might be often enough, depending on your use case.

On the launch of your application, it should be still relatively easy to open Realm in a dedicated autoreleasepool, write a compacted copy to a different path and replace your default.realm file with it.

Swift 2.1

func compactRealm() {
    let defaultURL = Realm.Configuration.defaultConfiguration.fileURL!
    let defaultParentURL = defaultURL.URLByDeletingLastPathComponent!
    let compactedURL = defaultParentURL.URLByAppendingPathComponent("default-compact.realm")

    autoreleasepool {
        let realm = try! Realm()
        realm.writeCopyToPath(compactedURL)
    }
    try! NSFileManager.defaultManager().removeItemAtURL(defaultURL)
    try! NSFileManager.defaultManager().moveItemAtURL(compactedURL, toURL: defaultURL)
}

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    compactRealm()

    // further setup …

    return true
}

Swift 3.0

func compactRealm() {
    let defaultURL = Realm.Configuration.defaultConfiguration.fileURL!
    let defaultParentURL = defaultURL.deletingLastPathComponent()
    let compactedURL = defaultParentURL.appendingPathComponent("default-compact.realm")

    autoreleasepool {
        let realm = try! Realm()
        try! realm.writeCopy(toFile: compactedURL)
    }
    try! FileManager.default.removeItem(at: defaultURL)
    try! FileManager.default.moveItem(at: compactedURL, to: defaultURL)
}

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    compactRealm()

    // further setup …

    return true
}