Akavache: An Asynchronous Key-Value Store for Native Applications

Akavache is an asynchronous, persistent (i.e. writes to disk) key-value store created for writing desktop and mobile applications in C#, based on SQLite3. Akavache is great for both storing important data (i.e. user settings) as well as cached local data that expires.

Where can I use it?

Akavache is currently compatible with:

• Xamarin.iOS / Xamarin.Mac
• Xamarin.Android
• .NET 4.5 Desktop (WPF)
• Windows Phone 8.1 Universal Apps
• Windows 10 (Universal Windows Platform)

What does that mean?

Downloading and storing remote data from the internet while still keeping the UI responsive is a task that nearly every modern application needs to do. However, many applications that don't take the consideration of caching into the design from the start often end up with inconsistent, duplicated code for caching different types of objects.

Akavache is a library that makes common app patterns easy, and unifies caching of different object types (i.e. HTTP responses vs. JSON objects vs. images).

It's built on a core key-value byte array store (conceptually similar to a Dictionary<string, byte[]>), and on top of that store, extensions are added to support:

• Arbitrary objects via JSON.NET
• Fetching and loading Images and URLs from the Internet
• Storing and automatically encrypting User Credentials

Platform-specific notes

• Xamarin.iOS / Xamarin.Mac - No issues.

• Xamarin.Android - No issues.

• .NET 4.5 Desktop (WPF) - No issues

• Windows Phone 8.1 Universal Apps - You must mark your application as x86 or ARM, or else you will get a strange runtime error about SQLitePCL_Raw not loading correctly. You must also ensure that the Microsoft Visual C++ runtime is added to your project.

• Windows 10 (Universal Windows Platform) - You must mark your application as x86 or ARM, or else you will get a strange runtime error about SQLitePCL_Raw not loading correctly. You must also ensure that the Microsoft Visual C++ runtime is added to your project.

Getting Started

Interacting with Akavache is primarily done through an object called BlobCache. At App startup, you must first set your app's name via BlobCache.ApplicationName. After setting your app's name, you're ready to save some data.

Choose a location

There are four build-in locations, that have some magic applied on some systems:

• BlobCache.LocalMachine - Cached data. This data may get deleted without notification.
• BlobCache.UserAccount - User settings. Some systems backup this data to the cloud.
• BlobCache.Secure - For saving sensitive data - like credentials.
• BlobCache.InMemory - A database, kept in memory. The data is stored for the lifetime of the app.

The magic

• Xamarin.iOS may remove data, stored in BlobCache.LocalMachine, to free up disk space (only if your app is not running). The locations BlobCache.UserAccount and BlobCache.Secure will be backed up to iCloud and iTunes. (https://developer.apple.com/library/content/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html#//apple_ref/doc/uid/TP40010672-CH2-SW1)
• Xamarin.Android may also start deleting data, stored in BlobCache.LocalMachine, if the system runs out of disk space. It isn't clearly specified if your app could be running while the system is cleaning this up. (https://developer.android.com/reference/android/content/Context.html#getCacheDir%28%29)
• Windows 10 (UWP) will replicate BlobCache.UserAccount and BlobCache.Secure to the cloud and synchronize it to all user devices on which the app is installed (https://msdn.microsoft.com/en-us/library/windows/apps/hh465094.aspx)

Let's start off

The most straightforward way to use Akavache is via the object extensions:

using System.Reactive.Linq;   // IMPORTANT - this makes await work!

// Make sure you set the application name before doing any inserts or gets
BlobCache.ApplicationName = "AkavacheExperiment";

var myToaster = new Toaster();
await BlobCache.UserAccount.InsertObject("toaster", myToaster);

//
// ...later, in another part of town...
//

// Using async/await
var toaster = await BlobCache.UserAccount.GetObject<Toaster>("toaster");

// or without async/await
Toaster toaster;

BlobCache.UserAccount.GetObject<Toaster>("toaster")
    .Subscribe(x => toaster = x, ex => Console.WriteLine("No Key!"));

Handling Xamarin Linker

Add the following class anywhere in your project to make sure Akavache.Sqlite3 will not be linked out by Xamarin

public static class LinkerPreserve
{
  static LinkerPreserve()
  {
    var persistentName = typeof(SQLitePersistentBlobCache).FullName;
    var encryptedName = typeof(SQLiteEncryptedBlobCache).FullName;
  }
}

Handling Errors

When a key is not present in the cache, GetObject throws a KeyNotFoundException (or more correctly, OnError's the IObservable). Often, you would want to return a default value instead of failing:

Toaster toaster;

try {
    toaster = await BlobCache.UserAccount.GetObject("toaster");
} catch (KeyNotFoundException ex) {
    toaster = new Toaster();
}

// Or without async/await:
toaster = await BlobCache.UserAccount.GetObject<Toaster>("toaster")
    .Catch(Observable.Return(new Toaster()));

Shutting Down

Critical to the integrity of your Akavache cache is the BlobCache.Shutdown() method. You must call this when your application shuts down. Moreover, be sure to wait for the result:

BlobCache.Shutdown().Wait();

Failure to do this may mean that queued items are not flushed to the cache.

Examining Akavache caches

Using Akavache Explorer, you can dig into Akavache repos for debugging purposes to see what has been stored.

What's this Global Variable nonsense? Why can't I use $FAVORITE_IOC_LIBRARY

You totally can. Just instantiate SQLitePersistentBlobCache or SQLiteEncryptedBlobCache instead - the static variables are there just to make it easier to get started.

DateTime/DateTimeOffset Considerations

By default JSON.NET's BSON implementation writes DateTime as UTC and reads it back in local time. To override the reader's behavior you can set BlobCache.ForcedDateTimeKind as in the following example:

// Sets the reader to return DateTime/DateTimeOffset in UTC.
BlobCache.ForcedDateTimeKind = DateTimeKind.Utc;

Basic Method Documentation

Every blob cache supports the basic raw operations given below (some of them are not implemented directly, but are added on via extension methods):

/*
 * Get items from the store
 */

// Get a single item
IObservable<byte[]> Get(string key);

// Get a list of items
IObservable<IDictionary<string, byte[]>> Get(IEnumerable<string> keys);

// Get an object serialized via InsertObject
IObservable<T> GetObject<T>(string key);

// Get all objects of type T
IObservable<IEnumerable<T>> GetAllObjects<T>();

// Get a list of objects given a list of keys
IObservable<IDictionary<string, T>> GetObjects<T>(IEnumerable<string> keys);

/*
 * Save items to the store
 */

// Insert a single item
IObservable<Unit> Insert(string key, byte[] data, DateTimeOffset? absoluteExpiration = null);

// Insert a set of items
IObservable<Unit> Insert(IDictionary<string, byte[]> keyValuePairs, DateTimeOffset? absoluteExpiration = null);

// Insert a single object
IObservable<Unit> InsertObject<T>(string key, T value, DateTimeOffset? absoluteExpiration = null);

// Insert a group of objects
IObservable<Unit> InsertObjects<T>(IDictionary<string, T> keyValuePairs, DateTimeOffset? absoluteExpiration = null);

/*
 * Remove items from the store
 */

// Delete a single item
IObservable<Unit> Invalidate(string key);

// Delete a list of items
IObservable<Unit> Invalidate(IEnumerable<string> keys);

// Delete a single object (do *not* use Invalidate for items inserted with InsertObject!)
IObservable<Unit> InvalidateObject<T>(string key);

// Deletes a list of objects
IObservable<Unit> InvalidateObjects<T>(IEnumerable<string> keys);

// Deletes all items (regardless if they are objects or not)
IObservable<Unit> InvalidateAll();

// Deletes all objects of type T
IObservable<Unit> InvalidateAllObjects<T>();

/*
 * Get Metadata about items
 */

// Return a list of all keys. Use for debugging purposes only.
IObservable<IEnumerable<string>> GetAllKeys();

// Return the time which an item was created
IObservable<DateTimeOffset?> GetCreatedAt(string key);

// Return the time which an object of type T was created
IObservable<DateTimeOffset?> GetObjectCreatedAt<T>(string key);

// Return the time which a list of keys were created
IObservable<IDictionary<string, DateTimeOffset?>> GetCreatedAt(IEnumerable<string> keys);

/*
 * Utility methods
 */

// Attempt to ensure all outstanding operations are written to disk
IObservable<Unit> Flush();

// Preemptively drop all expired keys and run SQLite's VACUUM method on the
// underlying database
IObservable<Unit> Vacuum();

Extension Method Documentation

On top of every IBlobCache object, there are extension methods that help with common application scenarios:

/*
 * Username / Login Methods (only available on ISecureBlobCache)
 */

// Save login information for the given host
IObservable<Unit> SaveLogin(string user, string password, string host = "default", DateTimeOffset? absoluteExpiration = null);

// Load information for the given host
IObservable<LoginInfo> GetLoginAsync(string host = "default");

// Erase information for the given host
IObservable<Unit> EraseLogin(string host = "default");

/*
 * Downloading and caching URLs and Images
 */

// Download a file as a byte array
IObservable<byte[]> DownloadUrl(string url,
    IDictionary<string, string> headers = null,
    bool fetchAlways = false,
    DateTimeOffset? absoluteExpiration = null);

// Load a given key as an image
IObservable<IBitmap> LoadImage(string key, float? desiredWidth = null, float? desiredHeight = null);

// Download an image from the network and load it
IObservable<IBitmap> LoadImageFromUrl(string url,
    bool fetchAlways = false,
    float? desiredWidth = null,
    float? desiredHeight = null,
    DateTimeOffset? absoluteExpiration = null);

/*
 * Composite operations
 */

// Attempt to return an object from the cache. If the item doesn't
// exist or returns an error, call a Func to return the latest
// version of an object and insert the result in the cache.
IObservable<T> GetOrFetchObject<T>(string key, Func<Task<T>> fetchFunc, DateTimeOffset? absoluteExpiration = null);

// Like GetOrFetchObject, but isn't async
IObservable<T> GetOrCreateObject<T>(string key, Func<T> fetchFunc, DateTimeOffset? absoluteExpiration = null);

// Immediately return a cached version of an object if available, but *always*
// also execute fetchFunc to retrieve the latest version of an object.
IObservable<T> GetAndFetchLatest<T>(string key,
    Func<Task<T>> fetchFunc,
    Func<DateTimeOffset, bool> fetchPredicate = null,
    DateTimeOffset? absoluteExpiration = null);