conditional rewrite or try_files with NGINX? - nginx

Question

I'm having trouble setting up a conditional rewrite, and I've been trying to use the if directive (despite all sources indicating it's "evil") with the -f switch to check for the presence of a file, but it's not working. I believe the issue/case is best explained by example, so here goes:
Directory structure
workspace/
myapp/
webroot/
index.php
assets/
baz.js
hello/
foo.js
modules/
hello/
assets/
foo.js
bar.js
Expected results
/ => /workspace/myapp/webroot/index.php
/assets/hello/foo.js => /workspace/myapp/webroot/assets/hello/foo.js
/assets/hello/bar.js => /workspace/myapp/modules/hello/assets/foo.js
/assets/baz.js => /workspace/myapp/webroot/assets/baz.js
In summary:
foo.js is only present in the modules/hello/assets folder and gets delivered from there.
bar.js is present both in webroot/assets/hello and modules/hello/assets and gets delivered from webroot.
(it hides/overrides the file in modules)
baz.js is only present in webroot/assets and gets delivered from there.
The part that doesn't work right now, is this:
location /assets/ {
if (-f $uri) {
break;
}
root /workspace/myapp/modules;
rewrite ^/assets/([^/]+)/(.*)$ /$1/assets/$2 break;
}
Namely the if directive, doesn't seem to have any affect - the bar.js file gets delivered from modules rather than webroot.
Should I be using if or not?
Is there any way I can solve this problem with try_files instead? I can't seem to grasp how this would work together with rewrite which I can't seem to get around.
Please do not suggest reorganizing the assets using a deploy script or something - it's not an option, for various other reasons.
I have used this pattern with Apache before, and NGINX seems more capable in most respects, so I'm sure this must be possible?
One requirement that isn't absolute, is I don't have to be able to override modules/hello/assets/foo.js with webroot/assets/hello/foo.js - serving scripts from webroot/assets/* is however a requirement.

Answer 1

The answer is divided into two parts: the first part explains why your configuration does not work and the second one provides examples of how to solve your problem. If you are only interested in the solution, go straight to the second part.
The problem
First of all, note that the positon of the root directive in a location block is not important. It does not matter if you put it at the very top or at the bottom of a location, it will affect the whole location anyway. Also, keep in mind that break in the end of the rewrite line tells Nginx to stay within the current location even if the URI has been successfully rewrited.
Having said that, let's take a look at your configuration and see how every request from the Expected results is processed and why nothing works as expected.
Let's presume that there is no other suitable location with a higher priority in your configuration. Since every request from Expected results starts with /assets, all of them will be handled according to the rules presented in your location. So:
/assets/hello/foo.js
The root is set to /workspace/myapp/modules. The if directive will be evaluated to false, because /assets/hello/foo.js does not exist and so break will not be executed. Finally, the last rewrite will change the requested URI from /assets/hello/foo.js to /hello/assets/foo.js and the following break will tell Nginx to stay within the current location. As a consequence /workspace/myapp/modules/hello/assets/foo.js will be served.
/assets/hello/bar.js
This request is processed exactly the same way as the previous one, so /workspace/myapp/modules/hello/assets/bar.js will be served.
/assets/baz.js
Yet again the root is set to /workspace/myapp/modules and the if is evaluated to false. But this time the final rewrite will not change the URI, because the request does not match the regular expression. As a consequence Nginx will try to serve /workspace/myapp/modules/assets/baz.js and since there is no such file exists, will return 404.
As you can see your configuration cannot possibly work as you want it to for several reasons:
if is always evaluated to false, because you try to check URIs and not files;
the request stays within the location because you tell it to stay there with break in the rewrite line;
root is always set to /workspace/myapp/modules in this location so no file can be served from anywhere else.
The solutions
The easiest solution would be to use try_files:
root /workspace/myapp/webroot;
location /assets/ {
try_files $uri #modules;
}
location #modules {
root /workspace/myapp/modules;
rewrite ^/assets/([^/]+)/(.*)$ /$1/assets/$2 break;
}
This configuration tells Nginx to look for a file in the webroot folder first and if nothing is found then go to the modules folder in another location. This approach is considered most preferable.
On the other hand, using if would allow you to solve the problem within one location:
location /assets/ {
root /workspace/myapp; # The parent folder
if (-f $document_root/webroot/$uri) {
rewrite ^(.*)$ /webroot/$1 break;
}
rewrite ^/assets/([^/]+)/(.*)$ /modules/$1/assets/$2 break;
}
However, this approach is considered outdated is not recommended for use.