VIP23 (VSL refactoring) design sketch

Dridi Boukelmoune dridi at
Wed Apr 10 07:43:05 UTC 2019

On Wed, Apr 10, 2019 at 8:08 AM Nils Goroll <nils.goroll at> wrote:
> Hi,
> I am +1 (or more, if I got more votes) overall, in particular, the incremental
> approach seems to make a lot of sense to me. Depending on the state of the rest
> of varnish-cache core, I would try get this into production ASAP as I have been
> for the last couple of years.
> Some additional points:
> - It would make a lot of sense to include the field definitions for VSL clients
> in a VSM segment such that log clients do not need to be recompiled when we
> change fields.

I think the definitions should be the first step. The second one
should be changing the VSL file format magic number to include a
version to accommodate future changes, so that varnishlog -w can also
write the field definitions for vut -r to pick it up offline too.

> - This would also pave the way for possible future dynamic VSL records (added
> while varnishd runs, referenced as "VMOD log meta-data" in VIP23). At some
> point, phk mentioned he would see them as a requirement for H3, but last I know
> this is not the case, so we can put this to the end of the list. Yet we should
> head into the right direction to eventually get there.
> - We should make it clear that the VSL format does not constitute an API, so log
> access is only supported by using the vsl client in libvarnishapi
> - We might want to consider making the "file" format written by varnishlog -w an
> API, but I guess the advantage over libvarnishapi as the API is pretty minor.
> - Geoff, You mentioned once over a burger that the actual binary representation
> of doubles is not standardized. By the same argument you give for the byte
> order, I do not think that this bears any practical relevance for now, but we
> might want to at least mention this and, for future cross-platform support,
> might have some kind of "this log uses that representation" info similar to the BOM.
> Nils
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