Jon, I think you're misunderstanding the point of the "No Deployment on Friday" policy.

Let's look from far, Why is a work day a no-deploy day? Why are we limiting ourselves to a four-day week while we can use our full potential? The reason is that if we deploy something to production on Friday and if it breaks production, then there is no one around to fix the issue over the weekend. So far it's rather obvious.

In other words, the requirement for having a time to deploy changes is to have some buffer until the weekend *to fix issues caused by changes deployed*. That buffer is Friday. It's made a no deployment day so you could push urgent fixes (including train blockers). Of course, the fix should not be too big to cause issues later.

Fridays are not "no deployment day" in the same of sense Sunday is a "no deployment day". It's the buffer to fix UBN issues, not a long weekend. If you're fixing an UBN issue, then please go ahead and deploy on Friday. The ultimate goal of RelEng policies is to have major issues live in production for the shortest possible time. Refusing to deploy a major fix on a Friday does the exact opposite of that.

Regarding Catalan and Hebrew Wikipedia, the other Amir said it well, I don't think I have much to add beside the fact that I have personally seen them finding major issues before they hit all Wikipedia languages many times, more than I can count.

HTH

On Tue, Jun 22, 2021 at 11:11 PM Scott Bassett <sbassett@wikimedia.org> wrote:
On Tue, Jun 22, 2021 at 3:03 PM Jon Robson <jrobson@wikimedia.org> wrote:
A few questions to provoke discussion/share knowledge better:
* Why does the train run Tue,Wed, Thur rather than Mon,Tue,Wed

I'd note here that the standard security deployment window is Monday between 21:00 and 23:00 UTC.  That date and time is not a hard requirement by any means, but having such a window exist early in the week, prior to the start of the train, has worked out well for a few reasons.  It's both convenient and less risky to only  deploy security patches to a single wmf production branch, which is the case most Mondays.  It's also less risky having the space to monitor patches and roll them back or re-patch during the week, as opposed to say, on a Friday, with substantially reduced coverage going into most weekends.  Of course there are times when critical security issues need to be dealt with on a Friday or even over the weekend, but in general, the Security Team likes to avoid this.  Moving the train to a Mon, Tue, Wed cadence would imply the security window be moved to the previous Friday or possibly Thursday, which is doable, but not desired for the aforementioned reasons.

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