I need a client side browser database. What are my options [closed]

Question

I'm creating a web site that I think must have a client side database. The other option would be to stick everything on the server at the expense of increased complexity and decreased scalability. What options do I have? Must I build a plugin? Must I wait until everybody's HTML5 compliant?

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Update There's been a lot of comments about why I would actually need this. Here are my thoughts. Tell me if I'm being silly:

• The clients will have a large and complex state that will require something like a database to provide the data interaction that I need. Therefore (I think) cookies are out of the picture.
• This data is transient, so the client won't care if it gets erased as soon as they close a session. However they will need to keep the data if they go to a different web page and then come back. Therefore (I think) somehow storing the data in some sort of a javascript SQL implementation will not work.
• I can certainly do everything that I want to do on the server, and servers can scale to manage the load (Facebook). But (I think) I'd rather build a plugin than pay for the infrastructure to support this load. This is for a bare bones startup. (The richer the startup is, the barer my bones will be.)

Answer 1

Indexed Database (Can I use)
Web SQL (Can I use)
localStorage