Preventing dup backend names with dynamic backends in VMODs

[Geoff Simmons]

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On 10/27/2015 01:25 PM, Poul-Henning Kamp wrote:
> 
> Yes, uniqueness of backend names affect only CLI and VSM.
> 
> The former only if people get confused, the latter always because 
> the VSM counter segment is named based on the backend name, so the
> two will clash.

Something else I thought of since yesterday -- the backend names used
in VSL for entries like Backend, BackendOpen, BackendClose and so forth.

If you're using the log to diagnose a problem, doesn't help much to
have to wonder which backend it's talking about.

> My current thinking is that we'll name the backend whatever the 
> user/vcl/vmod writer likes (ie:  Backend name), and deal with the 
> fall-out.

At the very least, VMODs should have a way to discover if a backend
name is already in use by a VCL -- the VCL_HasBackend(vcl,name) idea.
Then a VMOD can choose to raise an error for duplicate names.

Maybe add a sentence or two to the "Writing a Director" doc about
backend names, what they're used for, and why this could be a problem.

> That really only leaves one question:  What do we do when some
> code tries to add a backend with a name which already exists.
> 
> One option is to fail the backend creation, but since it is only 
> CLI/VSM that can get confused, that seems too draconian to me.
> 
> So I'm tempted to simply add a ".%d" suffix for ever increasing 
> %d's until the name becomes unique.
> 
> Would that work for people ?

If you thought that you called your backend "be", and then see "be.1"
and "be.2" in backend.list, VSL, VBE.* stats and so forth, is it
really any less confusing? The order would be probably result from
which backend was created first, which may be difficult or impossible
to determine after the fact.

That would have the advantage that one set of VSC stats doesn't get
lost altogether.

So I guess the choices are: be draconian, or permit a situation that
will probably create confusion, however much we try to minimize it.


Best,
Geoff
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