Storage Backend based on Websockets

So finally the questions:

• Have I missed an open source project which covers exactly my needs?No you've covered the open source projects. Open source only gets you about halfway there though. To implement a Global Realtime Network requires equal parts implementation and equal parts operations. You have to think about dropped messages, retries, what happens if a particular geography gets hot how do you scale your servers ...etc. I would argue that an open source solution won't achieve what you want unless you're willing to invest significant resources into operations. I would recommend a service like PubNub: http://pubnub.com

• If I would like to develop my own server with autobahn and a database, could you point me to right direction? Have lot of concerns and not enough depth understanding.. I know Autobahn gives you already a server, but this one is not very close to my final needs.. how to build a server efficient so that he can handle all connected sockets? How handle when a client needs server push? Are there schemas, models or concept how such a server should look like? A good database to back a realtime framework would be Cassandra because it supports high write volumes and handles time series data well: http://cassandra.apache.org/.

• Twisted is a very powerful python framework but not regarded as the most convenient for writing apps.. But I guess a Socket based storage server with db access should be possible? When I run twisted as a web ressource and develop server components with other python framework, would this compromise the latency/performance much? I would not use Twisted. I would use Gevent:http://www.gevent.org/. Its coroutine based so you don't get into callback hell. To support more connections you just increase your greenlet pool to listen on the socket.

• Is such a desired server backend with lot of data storage (JSON fields and also binary data such as documents, images) reasonable to build with Sockets by a single devoloper/small team or is this smth. which only bigger companies like Dropbox can do at the moment?Again I would not build this on your own. A service like PubNub: http://pubnub.com which takes care of all the operational issues for you and has a clean API would service your needs with minimal cost. PubNub takes care of the protocol for you so if your on a mobile device that doesn't support WebSockets it will use TCP, HTTP or whatever the best transport is for the device.