Manual thread storage, may stick with this pattern.

gistfile1.swift

extension RLMRealm {
    static var threadInstance: RLMRealm! {
        let threadDictionary = NSThread.currentThread().threadDictionary
        if let inst = threadDictionary["SharedRealmInst"] as! RLMRealm? {
            return inst
        } else {
            let inst = defaultRealmWithErrorHandling()
            threadDictionary["SharedRealmInst"] = inst
            return inst
        }
    }
    
    static var secureRealmPath: String {
        return RLMRealm.defaultRealmPath().stringByDeletingLastPathComponent.stringByAppendingPathComponent("secure.realm")
    }
    
    static func defaultRealmWithErrorHandling() -> RLMRealm? {
        var error: NSError?
        let realmPath = RLMRealm.secureRealmPath
        var realm = RLMRealm(path: realmPath, readOnly: false, error: &error)
        
        if (error != nil) {
            log.debug("realm error: \(error?.localizedDescription). Will remove local database and retry.")
            NSFileManager.defaultManager().removeItemAtPath(realmPath, error: nil)
            
            var error: NSError?
            realm = RLMRealm(path: secureRealmPath, readOnly: false, error: &error)
            if (error != nil) {
                log.error("unable to resolve realm error. \(error?.localizedDescription)")
                // TODO: analytics/log this to server
            } else {
                log.debug("ok")
            }
        }
        
        // configure realm to use iOS file system encryption
        // this is in addition to realm encryption
        let attr = [NSFileProtectionKey: NSFileProtectionComplete]
        var fileProtectionError: NSError?
        if !NSFileManager.defaultManager().setAttributes(attr, ofItemAtPath: realmPath, error: &fileProtectionError) {
            log.error("error setting \(attr) on realm db: \(fileProtectionError?.localizedDescription)")
        }
        
        return realm
    }
}

Realm does its own RLMRealm thread caching, so you can remove the threadInstance var and use defaultRealmWithErrorHandling() directly.