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• IndexedDB has a 250MB limit per app and an overall system limit of 375MB on drives smaller
than 32GB, or 4% (to a maximum 20GB) for drives over 32GB. So it could be true that your app
might not have much room to work with anyway, in which case you need to make sure you have
a fallback mechanism. (When the limit is exceeded the APIs will throw a Quota Exceeded
exception.)
• IndexedDB on Windows 8 has no complex key paths—that is, it does not presently support
multiple values for a key or index (multientry).
• By default, access to IndexedDB is given only to HTML pages that are part of the app package
and those declared as content URIs. (See the “Local and Web Contexts within the App Host”
section at the beginning of Chapter 3.) Random web pages you might host in an iframe will not
be given access, primarily to preserve space within the 250MB limit for those pages you really
care about in your app. However, you can grant access to arbitrary pages by including the
following tag in your home page and not setting the iframe src attribute until the
DOMContentLoaded or activated event has fired:
Beyond IndexedDB there are a few other database options for Store apps. For a local relational
database, try SQLite. This is an API that’s suited well for apps written in a language like C#, as described
in Tim Heuer’s blog on the subject, but fortunately, there is a version called SQL.js, which is SQLite
compiled to JavaScript via Emscripten. Very cool! There might also be other JavaScript solutions
available in the community.
If the storage limits for IndexedDB are a concern, you might use the Win32 “Jet” or Extensible
Storage Engine (ESE) APIs (on which the IndexedDB implementation is built). For this you’ll need to write
a WinRT Component wrapper in C# or C++ (the general process for which is in Chapter 16, “WinRT
Components”), since JavaScript cannot get to those APIs directly.
The same is true for other third-party database APIs. So long as that engine uses only the Win32 APIs
allowable for Store apps (listed on the Win32 and COM for Windows Store apps page), they’ll work just
fine.
It’s also worth noting that the OData Library for JavaScript also works great for Store apps to access
online SQL Servers, because the OData protocol itself just works via REST.
Finally, another option for searchable file-backed data is to use the system index by creating a folder
named “indexed” in your local AppData folder. The contents of the files in this folder will be indexed by
the system indexer and can be queried using Advanced Query Syntax (AQS) with the APIs explained
later in “Rich Enumeration with File Queries.” You can also do property-based searched for Windows
properties, making this approach a simple alternative to database solutions.
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