Tags and object IDs

[Nate Finch]

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.

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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