[openstack-dev] [nova] [placement] [api] cache headers in placement service

Chris Dent cdent+os at anticdent.org
Fri Aug 18 17:23:20 UTC 2017

(I put [api] in the subject tags because this might be of interest
to a wider audience that cares about APIs.)

Long ago and far away I made this bug:


"placement api responses should not be cacehable"

Today I've pushed up a WIP that demonstrates a way to address this:


Before I get too far along with it, I'd like us to decide whether we
think it is worth doing and consider some of the questions it will

I think it is worth doing not just because it would be correct but
because without it, we cannot be assured that proxies or user agents
will not cache resource providers and other entities, and that would
lead to bizarre results.

At the same time, any entity you put on the web, according to the
RFCs[1], should have a last-modified header if it "can be reasonably
and consistently determined".

So my change above adds 'last-modified' and 'cache-control:
no-cache' to GET of /resource_providers and
/resource_providers/{uuid} and proposes we do it for everything

Should we?

If we do, some things to think about:

* The related OVO will need the updated_at and created_at
   fields exposed. This is pretty easy to do with the
   NovaTimestampObject mixin. This doesn't need to require a object
   version bump because we don't do RPC with them.

* Adding a response header violates interop guidelines, so this
   would mean a microversion bump that impacts all GET requests. In
   systems that currently use placement (the report client in nova,
   mostly) no attention is being paid to either of the headers being
   added, so there would be no need for motion on that side.

* The current implementation of getting the last modified time for a
   collection of resources is intentionally naive and decoupled from
   other stuff. For very large result sets[3] this might be annoying,
   but since we are already doing plenty of traversing of long lists,
   it may not be a big deal. If it is we can incorporate getting the
   last modified time in the loop that serializes objects to JSON

I think we should. Generally speaking I think it is good form to
fulfil the expectations of HTTP. It helps make sure the HTTP APIs
work with the unknown client. Working with the unknown client is one
of the best reasons to have an HTTP API.k

[1] https://tools.ietf.org/html/rfc7232#section-2.2

[2] An argument could be made that this change is fixing a protocol
level bug, but I reckon that argument wouldn't fly with most people.

Chris Dent                      (⊙_⊙')         https://anticdent.org/
freenode: cdent                                         tw: @anticdent

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