Re: Asynchronous route-mapping and blue-green deployment

Shannon Coen

Hello Thomas,

Comments inline.

On Thu, Feb 2, 2017 at 10:40 PM, Anderer, Thomas <thomas.anderer(a)>

Hello everyone,

we already had an internal discussion on the topic and since we could not
quite come to a solution, I'd like to ask in this list.

We're using a self-made blue-green-deployment-script which pretty much
does what's explained on (https://docs.cloudfoundry.
org/devguide/deploy-apps/blue-green.html), and additionally checks
(HTTP-GET) the application after push, map-route and unmap-route. This
script has been working fine on an instance A of CF (DEA-based and
relatively small), but has issues on instance B (Diego-based, larger). The
aforementioned HTTP-GET sometimes fails with "404 Not Found: Requested
route does not exist" directly after push or map-route. We never
experienced this issue on instance A. This could of course mean that the
instance is small enough that all router operation are handled faster than
our script is able to react. On instance B it sometimes takes up to a 1
second or even longer until the route mapping is finally completed.
I am reviewing the logic for how routes are registered for apps on DEAs,
but I believe it is also asynchronous. It may be that registration of
routes takes slightly longer with Diego, or you're seeing another
difference between the environments.

In our internal discussion with our CF operators, we found a couple of
parts of documentation which at least hint to different, maybe even
inconsistent inner workings of the route mapping:
Step 3: Map-route - The CF Router immediately begins to load balance
traffic for between Blue and Green.
- In my opinion this implies or at least strongly hints that the
map-route call was supposed to be synchronous.
The routing table is indeed updated asynchronously, and we can update the
docs to clarify this; "immediately" may be a bit misleading.

2) PUT to "/v2/apps/#{app_guid}/routes/#{route_guid}" returns 201 CREATED
and not 202 ACCEPTED, which also implies that it is a synchronous
operation. It also does not return an event or operation which could be
pinged to wait for completion of route mapping.
3) Operations which show the state of the route, like for example 'cf
routes' already shows the route as being successfully mapped, although 404
is still returned.
Currently Cloud Controller has no way of knowing whether a route is ever
registered with a router, whether the app is on DEAs or Diego. We could
consider how to provide such a guarantee; e.g. CC could poll routes until
an app returns a 200 (several issues with this). Also, if we were to add
this feature we couldn't change the response in v2 from 201 to 202 as that
would not be backwards compatible; we could consider this for the v3 CC API.

4) See
domains.html#map-route: Applications running on the DEA architecture must
be restarted after routes for an app are mapped or unmapped. Applications
running on Diego do not need to be restarted.
- This is contrary to what I experienced, since route mapping on our
instance A always worked synchronously and without restart of the
application. Furthermore, it contradicts what's explained in 1) at least
for DEA.
This sentence in the docs is incorrect. We will update them. Restarting an
app on DEA is not required after mapping or unmapping routes.

Routing Translation Components: Route-Emitter
a) monitors DesiredLRP state and ActualLRP state via the BBS. When a
change is detected, the Route-Emitter emits route registration and
unregistration messages to the gorouter via the NATS message bus,
b) periodically emits the entire routing table to the router,
c) maintains a lock in consul to ensure only one route-emitter handles
route registration at a time.
- This hints that route mapping is an asynchronous task.

Some of the issues could be fixed by repeatedly pinging the application in
order to wait until the route has been mapped. But what about
blue-green-deployment, where I map the application to a route on which
there is already a running application. Here, I cannot find out in a
general way, if the route mapping is completed and when I can start
unmapping the route from the old application. If route-mapping is indeed an
asynchronous task, in my opinion the description on is
misleading at best. And all blue-green-deployment-scripts which I've seen
so far have this issue.

So, is the route mapping really supposed to be asynchronous? If so, is
there any general way to find out when the route-mapping process has
Route registration is an asynchronous operation. Other than polling the
application, CF does not expose a guaranteed way to discover that the route
has been registered with the routers themselves.

We recommend adding a short wait to your blue-green deploy script. The
blue-green-deploy CLI plugin effectively does this, but running a few other
commands (renaming apps) between mapping the route to one app and unmapping
it from another.

Thank you for your feedback! I'll submit a PR to update the docs now.

Thank you for your help and clarification,

Best regards,
Thomas Anderer
Agile Software Engineer
andrena objects ag
currently working at SAP

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