Sharing data when download the same object from upstream

[Alex Garzão]

Hello guys,

This is my first post to nginx-devel.

First of all, I would like to congratulate NGINX developers. NGINX is
an amazing project :-)

Well, I'm using NGINX as a proxy server, with cache enabled. I noted
that, when two (or more) users trying to download the same object, in
parallel, and the object isn't in the cache, NGINX download them from
the upstream. In this case, NGINX creates one connection to upstream
(per request) and download them to temp files. Ok, this works, but, in
some situations, in one server, we saw more than 70 parallel downloads
to the same object (in this case, an object with more than 200 MB).

If possible, I would like some insights about how can I avoid this
situation. I looked to see if it's just a configuration, but I didn't
find nothing.

IMHO, I think the best approach is share the temp file. If possible, I
would like to known your opinions about this approach.

I looked at the code in ngx_http_upstream.c and ngx_http_proxy.c, and
I'm trying to fix the code to share the temp. I think that I need to
do the following tasks:

1) Register the current downloads from upstreams. Probably I can
address this with a rbtree, where each node has the unique object id
and a list with downstreams (requests?) waiting for data from the
temp.

2) Disassociate the read from upstream from the write to downstream.
Today, in the ngx_event_pipe function, NGINX reads from upstream,
writes to temp, and writes to downstream. But, as I can have N
downstreams waiting data from the same upstream, probably I need to
move the write to downstream to another place. The only way I think is
implementing a polling event, but I know that this is incorrect
because NGINX is event based, and polling waste a lote of CPU.

3) When I know that there more data in temp to be sent, which function
I must use? ngx_http_output_filter?

Suggestions will welcome :-)

Thanks people!

--
Alex Garzão
Projetista de Software
Azion Technologies
alex.garzao (at) azion.com