Copy/Cache Azure database locally for offline use. - sql-server

Question

I have a piece of software that connects to an Azure database to gather formulas for several varieties of colors to allow the user to follow a recipe to create their own product.
Basically there is just one big database pull when the application launches, that pulls down all the formulas, and from there the user can simply use, modify, or even delete the formulas as they wish.
The problem is that where this software is used, there are seldom constant internet connections, and the application so far is simply designed to shutdown if there isn't one present.
I am looking for a solution for being able to allow the application to BOTH connect to the database on application startup(If a connection is present) and save a copy locally, or if no connection is present, check for a locally saved copy to work with.
I have looked everywhere, but have been unable to find any methods to "programmatically" retrieve the pertinent(or all, if necessary) data, and either export it to a local file, or cache it somehow for offline use.
Any suggestions?

Answer 1

You don't mention what kind of technology you're using on the client side, and if you require access to a database on the client or just the data - but regardless there are several ways to do this.
Azure Mobile apps have a quickstart which will implements an Azure SQL <-> SQL Lite (mobile) database sync framework (table controllers on the server side). This lets you use libraries on the mobile side to use the local db for getting its data, and when you know you're online you can sync to/from the server. This is quite sophisticated and probably gives you more than just the caching that you're looking for.
I've used two other strategies for caching - one for HTML/javascript based applications (phonegap/cordova) and the other for Xamarin c# apps on iOS and Android. I'm assuming if it's a standard windows desktop app you know how to persist data so you can use whatever kind of cache/file system/db you like.
JavaScript/html - use the html5 localStorage functions to store the JSON output of the web server calls you're making. This is really easy then to abstract, where your app before is making an ajax call to the server to get some data, instead move that to a "liveorcache" class, which can determine whether to go to the server or just use the local storage. Code snippet for saving/loading json in localstorage below:
$scope.saveFixtures = function () {
localStorage["fixtures"] = JSON.stringify($scope.fixtures);
};
$scope.loadFixtures = function () {
if (localStorage["fixtures"] != undefined) {
$scope.fixtures = JSON.parse(localStorage["fixtures"]);
}
};
If you're writing your app in Xamarin you can do the same kind of thing, but using a PCL library - I used "PCLStorage" which works on Android and iOS. Same strategy though, in my code I just write the JSON data to a file with an appropriate filename, but usually wrap the object in another object that contains the cache write date/time. You then serialise the object to the file - something like below.
public class CacheProvider
{
public static async Task<CacheModel> ReadCache<T>(string filename)
{
IFolder rootFolder = FileSystem.Current.LocalStorage;
IFolder cache = await rootFolder.CreateFolderAsync("sportenzaCache", CreationCollisionOption.OpenIfExists);
try
{
IFile file = await cache.GetFileAsync(filename);
var data = await file.ReadAllTextAsync();
return JsonConvert.DeserializeObject<T>(data) as CacheModel;
}
catch(FileNotFoundException ex)
{
return null;
}
}
public static async void WriteCache<T>(string filename, CacheModel data)
{
IFolder rootFolder = FileSystem.Current.LocalStorage;
IFolder cache = await rootFolder.CreateFolderAsync("sportenzaCache", CreationCollisionOption.OpenIfExists);
IFile file = await cache.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
if (file != null)
{
data.CacheCreated = DateTime.Now;
string json = JsonConvert.SerializeObject(data);
await file.WriteAllTextAsync(json);
}
}
public static async void DeleteCache(string filename)
{
IFolder rootFolder = FileSystem.Current.LocalStorage;
IFolder cache = await rootFolder.CreateFolderAsync("sportenzaCache", CreationCollisionOption.OpenIfExists);
IFile file = await cache.GetFileAsync(filename);
if (file != null)
await file.DeleteAsync();
}
}