In relevant requests to the placement API add last-modified
and cache-control headers.
According the HTTP 1.1 RFC headers last-modified headers SHOULD always
be sent and should have a tie to the real last modified time. If we do
send them, we need Cache-Control headers to prevent inadvertent caching
of resources.
This change adds a microversion 1.15 which adds the headers to GET
requests and some PUT or POST requests.
Despite what it says 'no-cache' means "check to see if the version you
have is still valid as far as the server is concerned". Since our server
doesn't currently validate conditional requests and will always return an
entity, it ends up meaning "don't cache" (which is what we want).
The main steps in the patch are:
* To both the get single entity and get collection handlers add
response.cache_control = 'no-cache'
* For single entity add response.last_modified = obj.updated_at or
obj.created_at
* For collections, discover the max modified time when traversing the
list of objects to create the serialized JSON output. In most of
those loops an optimization is done where we only check for
last-modified information if we have a high enough microversion such
that the information will be used. This is not done when listing
inventories because the expectation is that no single resource
provider will ever have a huge number of inventory records.
* Both of the prior steps are assisted by a new util method:
pick_last_modfied.
Where a time cannot be determined the current time is used.
In typical placement framework fashion this has been done in a very
explicit way, as it makes what the handler is doing very visible, even
though it results in a bit of boilerplate.
For those requests that are created from multiple objects or by doing
calculations, such as usages and aggregate associations, the current time
is used.
The handler for PUT /traits is modified a bit more extensively than some
of the others: This is because the method can either create or validate
the existence of the trait. In the case where the trait already exists,
we need to get it from the DB to get its created_at time. We only do
this if the microversion is high enough (at least 1.15) to warrant
needing the info.
Because these changes add new headers (even though they don't do
anything) a new microversion, 1.15, is added.
Partial-Bug: #1632852
Partially-Implements: bp placement-cache-headers
Change-Id: I727d4c77aaa31f0ef31c8af22c2d46cad8ab8b8e
83030804cc