Hub.Topic shall be part of URL to post requests to

[m-arpad]

According to the Feb2020Ballot, the URL to post any request (subscription, context change, unsubscription) to is always the hub.url. We propose to change this to 'hub.url / hub.topic', e.g. 'https://hub.example.com/topicABC' mainly for making implementations more straightforward and in line with industry best practices.

Reasons:

• in cloud / high-availability deployments this allows scale out and redundancy by placing the hub behind a load balancing reverse proxy or API gateway
• the actual message body does not need to be deserialized/interpreted in order to identify the topic
• industry standard web frameworks typically support cross-cutting concerns such as logging, tracing, performance counters, authentication, authorization, security aspects based on URLs better than based on message content
• allows for clients to dynamically specify topic schemes with URL placeholders, such as "https://hub.example.com/%CASE_ID%" or "https://hub.example.com/%REPORT_ID%" or "https://hub.example.com/%IMAGE_ID%" or similar