我怎么知道Azure VIP交换何时完成？

Question

I just did a VIP swap using the management REST API. It took 30s before the operation returned the status code "Succeeded", but it was another minute or so before requests to the service started returning new content. Because I need to manage slow-to-start processes, I need to notify my worker roles before and after a VIP swap. So the question is this: how can I be confident that a VIP swap has finished? If I start getting content back from the new deployment, how long should I wait before taking action? That is, do all web roles get swapped close-to-simultaneously? This thread reports old content being returned for up to 30 minutes, but I find that hard to believe. Perhaps they had caching or proxies in place.

Answer 1

The actual VIP swap should never take more than on the order of tens of seconds, so "just wait a minute" would work fine. That said, existing connections can persist for quite some time. If you're refreshing in the same browser over and over, you may manage to hold a single TCP socket open (due to HTTP keep-alive). Despite the VIP swap, the socket is still open and connected to the old deployment.

So how long it takes depends on what it is you're trying to measure. The process of reprogramming the load balancer to point to the new deployment is quite fast. The process of all users disconnecting and establishing a new connection (and their caches being flushed, etc.) could take longer.

Answer 2

I do not know the precise answer to your question with respect to the actual physical action of doing a VIP swap on the Azure side. However, when the swap occurs, DNS name change takes sometime to propagate. Even if your TTL (time to live) is relatively small. This is because most ISP's cache DNS resolution on their own servers as well and by the time a new IP is resolving to the old name, all sorts of DNS caching has to refresh