Is there a Swift server specification similar to WSGI? I am specifically looking for a standardised way for server middleware.
Currently there is not.
In the long term this might be a fruitful idea, but unlike in the case of the specification of WSGI we do not currently have a proliferation of servers and frameworks that would like to interoperate. We currently have a small number of frameworks that all tightly integrate their server and framework code.
A reasonable long-term goal for the SSWG would be to define such an abstraction.
Adding @tanner0101 who is also interested in this topic I think.
As Cory says, I personally also think that this is a good long-term goal. At the current stage however I think we should use our energy on areas where there's a more immediate need, for example the areas laid out as the focus areas for the SSWG. Why? IMHO Swift on Server is in a good shape regarding web servers already but in other areas we're still lacking in other areas.
Needless to say I don't to stop anybody to work on something like WSGI, nor do I know what will is actually the most important thing when we look back in two years time or so. Therefore please take this as my personal opinion and no more .