Tags and object IDs

[John Meinel]

On Sat, Jan 23, 2016 at 1:28 AM, William Reade <william.reade at canonical.com>
wrote:

> On Fri, Jan 22, 2016 at 9:53 PM, Nate Finch <nate.finch at canonical.com>
> wrote:
>
>> Working in the model layer on the server between the API and the DB.
>> Specifically in my instance, an API call comes in from a unit, requesting
>> the bytes for a resource.  We want to record that this unit is now using
>> the bytes from that specific revision of the resource.  I have a pointer to
>> a state.Unit, and a function that takes a Resource metadata object and some
>> reference to the unit, and does the actual transaction to the DB to store
>> the unit's ID and the resource information.
>>
>
> I'm a bit surprised that we'd be transferring those bytes over an API call
> in the first place (is json-over-websocket really a great way to send
> potential gigabytes? shouldn't we be getting URL+SHA256 from the apiserver
> as we do for charms, and downloading separately? and do we really want to
> enforce charmstore == apiserver?); and I'd point out that merely having
> agreed to deliver some bytes to a client is no indication that the client
> will actually be using those bytes for anything; but we should probably
> chat about those elsewhere, I'm evidently missing some context.
>

So I would have expected that we'd rather use a similar raw
HTTP-to-get-content instead of a JSON request (given the intent of
resources is that they may be GB in size), but regardless it is the intent
that you download the bytes from the charm rather from the store directly.
Similar to how we currently fetch the charm archive content itself.
As for "will you be using it", the specific request from the charm is when
it calls "resource-get" which is very specifically the time when the charm
wants to go do something with those bytes.

John
=:->


> But whenever we do record the unit-X-uses-resource-Y info I assume we'll
> have much the same stuff available in the apiserver, in which case I think
> you just want to pass the *Unit back into state; without it, you just need
> to read the doc from the DB all over again to make appropriate
> liveness/existence checks [0], and why bother unless you've already hit an
> assertion failure in your first txn attempt?
>
> Cheers
> William
>
> [0] I imagine you're not just dumping (unit, resource) pairs into the DB
> without checking that they're sane? that's really not safe
>
>
>> On Fri, Jan 22, 2016 at 3:34 PM William Reade <
>> william.reade at canonical.com> wrote:
>>
>>> Need a bit more context here. What layer are you working in?
>>>
>>> In general terms, entity references in the API *must* use tags; entity
>>> references that leak out to users *must not* use tags; otherwise it's a
>>> matter of judgment and convenience. In state code, it's annoying to use
>>> tags because we've already got the globalKey convention; in worker code
>>> it's often justifiable if not exactly awesome. See
>>> https://github.com/juju/juju/wiki/Managing-complexity#workers
>>>
>>> Cheers
>>> William
>>>
>>> On Fri, Jan 22, 2016 at 6:02 PM, Nate Finch <nate.finch at canonical.com>
>>> wrote:
>>>
>>>> I have a function that is recording which unit is using a specific
>>>> resource.  I wrote the function to take a UnitTag, because that's the
>>>> closest thing we have to an ID type. However, I and others seem to remember
>>>> hearing that Tags are really only supposed to be used for the API. That
>>>> leaves me with a problem - what can I pass to this function to indicate
>>>> which unit I'm talking about?  I'd be fine passing a pointer to the unit
>>>> object itself, but we're trying to avoid direct dependencies on state.
>>>> People have suggested just passing a string (presumably
>>>> unit.Tag().String()), but then my API is too lenient - it appears to say
>>>> "give me any string you want for an id", but what it really means is "give
>>>> me a serialized UnitTag".
>>>>
>>>> I think most places in the code just use a string for an ID, but this
>>>> opens up the code to abuses and developer errors.
>>>>
>>>> Can someone explain why tags should only be used in the API? It seems
>>>> like the perfect type to pass around to indicate the ID of a specific
>>>> object.
>>>>
>>>> -Nate
>>>>
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>
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