Hello,

> Thanks for sharing your work. Indeed, nginx currently lacks a good solution
> for serving a file that's being downloaded from upstream. We tried to address
> this issue a few years ago. Our solution was similar to yours, but instead
> of sharing the temp file between workers, we moved the temp file to its
> destination right after writing the header. A new bit was added to the header
> signalling that this file is being updated.
>
> The biggest issue with this kind of solutions is how we wait for updates in
> a file. We believe that polling a file with a given time interval is not a
> perfect approach, even though nginx does that for cache locks.

polling is done only on the ngx_http_file_cache_tf_node_t struct in the
shared memory (see patch 09 of 15, where c->length is updated from
c->tf_node->length and then this length is compared with
c->body_sent_bytes), not on the file itself. Length in the tf_node is
updated with each write from the primary request (see patch 05 of 15).

It is better than polling individual files, but I agree it is still
polling, which isn't great.

> [...]
> Another approach would be to create an
> inter-worker messaging system for signalling file updates.

We were thinking about creating something like that but we buried this
idea because it seems quite complex to do it right and reliable. And
polling the tf_node in the shared memory (with very low
proxy_cache_tempfile_loop) works sufficiently good.


> It's good to know the solution works for you. Please keep us posted about
> future improvements especially the ones which would avoid polling and decrease
> complexity.

We would be happy to get this patch to the mainline nginx in the future,
so that all nginx users could benefit from it.

We will be thinking about avoiding polling and implementing some
inter-worker messaging, but it may be some time, because it seems quite
complex. Could you share some hints about how do you thing it would be
best to implement it in the worker's event loop?

Jiří Setnička
CDN77

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