ngx.say/ngx.print in balancer_by_lua*?

chet.nichols · 2019-05-29T13:07:27+00:00

Hi Wasseem,I hope you are doing well.Thanks for you answer.Wasseem, actually I'm doing it. I have s specific cache path per vhost. Like:proxy_cache_path&...

ngx.say/ngx.print in balancer_by_lua*?

chet.nichols

Hey there!

I have some logic implemented where, when I return a 4xx/5xx status, I'll set an X header (lets call it X-Sub-Status) with a code more specific to why the error was returned. For instance, a 500 might have an X-Sub-Status of SET_PEER_FAILED, or X-Sub-Status of SOME_OTHER_FAIL. This makes it easier for us to troubleshoot and generally helps with transparency.

I now want to return custom error page HTML such that I include the X-Sub-Status code in the page itself (I'd still present a generally basic error page for a 40[134], 50[0234], etc, but add an <hr> and the short code).

In the balancer_by_lua* code I have, there are two places where I'll set an X-Sub-Status response header. However, since ngx.say and ngx.print are unavailable in that phase, it seems like there's no way for me to accomplish what I'm trying to do (at least for those two cases).

I've been trying to come up with creative ideas to make it work, and I'm curious if anyone has run into this, and what you did.

Thanks in advance for the help!

Chet

david.birdsong

I only worked briefly with balancer_by_lua* and it was a ~1.5 years ago, but try storing necessary info on ngx.ctx and detect appropriately in something ilke content_by_lua* and do your says and prints there.

On Tue, May 28, 2019 at 10:07 PM Chet Nichols III <chet....@gmail.com> wrote:

Hey there!

I have some logic implemented where, when I return a 4xx/5xx status, I'll set an X header (lets call it X-Sub-Status) with a code more specific to why the error was returned. For instance, a 500 might have an X-Sub-Status of SET_PEER_FAILED, or X-Sub-Status of SOME_OTHER_FAIL. This makes it easier for us to troubleshoot and generally helps with transparency.

I now want to return custom error page HTML such that I include the X-Sub-Status code in the page itself (I'd still present a generally basic error page for a 40[134], 50[0234], etc, but add an <hr> and the short code).

In the balancer_by_lua* code I have, there are two places where I'll set an X-Sub-Status response header. However, since ngx.say and ngx.print are unavailable in that phase, it seems like there's no way for me to accomplish what I'm trying to do (at least for those two cases).

I've been trying to come up with creative ideas to make it work, and I'm curious if anyone has run into this, and what you did.

Thanks in advance for the help!

Chet

.

chet.nichols

Hey David!

Nice, thanks for the reply. Yeah, I'm already putting stuff into ngx.ctx to be used for building the custom body (and to signal that one of the error scenarios has occurred), so it looks like we're on the same page with that part! But my understanding is you can't mix content_by_lua and balancer_by_lua, so I wasn't able to leverage content_by_lua like I had hoped.

That said, this morning, I realized I can leverage body_filter_by_lua (at least in my balancer_by_lua case where no peer is found/set,). I just let it fall through to body_filter_by_lua (instead of exiting early), and have code check ngx.ctx for certain attributes that instruct it to set the error page accordingly.

At first it wasn't working, but then I realized it was because I wasn't sending a trailing "\n" in the body I was assigning to ngx.arg[1] -- whoops. ..and now it seems to be working!

This might not be the *most* optimal way to do it, but the change to implement it was actually pretty small, and it seems to fit in with how requests/responses flow through the phases.

If you or anyone else has any other ideas, definitely chime in (because it's fun to see what other people are doing), but, I've got something going for now. :)

Thanks!

Chet

piotrprz

You can intercept error generated in your upstream handler and tell nginx to handle it in a custom way - here I'm intercepting status code 500 from balancer. Just remember that this way you lose access to any content returned by upstream with this status code.

location / {
proxy_pass ...;
proxy_intercept_errors on;
recursive_error_pages on;
error_page 500 @http_error_handler;
}

location @http_error_handler {
root /etc/nginx/html/error;
recursive_error_pages off;

# error handling logic - here I'm just returning contents of 500.html
rewrite_by_lua_block {
    -- override method to properly handle non-GET requests
    ngx.req.set_method(ngx.HTTP_GET)
    ngx.req.set_uri("/500.html")
}
}

david.birdsong

Also if IIRC, error_page is handled under the hood by rewrite which I found to be something I avoided at all costs when working in ngx-lua land. Rewrite run in a new context and so you lose any data stored on the context.

On Fri, May 31, 2019 at 5:07 AM Piotr Przybylski <pio...@gmail.com> wrote:

You can intercept error generated in your upstream handler and tell nginx to handle it in a custom way - here I'm intercepting status code 500 from balancer. Just remember that this way you lose access to any content returned by upstream with this status code.

location / {
proxy_pass ...;
proxy_intercept_errors on;
recursive_error_pages on;
error_page 500 @http_error_handler;
}

location @http_error_handler {
root /etc/nginx/html/error;
recursive_error_pages off;

# error handling logic - here I'm just returning contents of 500.html
rewrite_by_lua_block {
    -- override method to properly handle non-GET requests
    ngx.req.set_method(ngx.HTTP_GET)
    ngx.req.set_uri("/500.html")
}
}

.