7.1 Subscription Migration

[aaronpk]

7.1: “Subscription Migration”

Unclear how this should work and the text suddenly uses the term "clients". How should a subscriber learn about this? It discovers once on the publisher and then only communicates with the hub.

In 8 you write "A hub MAY use discovery from time to time to detect changes in a topic's canonical URL", which probably should read "A Subscriber MAY". This information is fundamental in 7.1, which itself would be better suited as a section 8.2.

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From #127 by @mkovatsc

[aaronpk]

Unclear how this should work and the text suddenly uses the term "clients". How should a subscriber learn about this? It discovers once on the publisher and then only communicates with the hub.

I see how this is confusing, so I've made some updates to the text. The new text is below:

• The previous topic URL should send a redirect to the new topic URL. This will provide a seamless transition for any HTTP client that did not use WebSub but instead was polling the topic URL.
• When existing WebSub subscriptions expire, subscribers will attempt to renew the subscription. The first step of renewing a subscription is to fetch the topic URL, which means the subscriber will encounter the redirect and end up at the new topic URL.
• At the new topic URL, the subscriber will see the new rel=self URL and the new hub, and will subscribe to the new topic URL at the new hub.

Does this clear things up? The idea is the subscriber gets websub notifications for the duration of the lease, and when the lease is up, the subscriber will have to go fetch the topic URL again anyway, so at that point it can be redirected to the new location.