ngx.ctx in subrequests

cefan.daniel.rubin · 2014-11-01T01:44:09+00:00

Hi Hamish,Thank you for your reply, it's working as expected now :) I'm using lua-resty-redis as you suggested.Regards,CarlosOn Friday, October 31, 2014 2:59...

ngx.ctx in subrequests

cefan.daniel.rubin

Hey all!

I would expect the ngx.ctx between subsequent content_by_lua_file calls in the same subrequest to be shared but it seems that this is not the case?

We recently came across this behaviour which seemed odd:

file1.lua

---

ngx.ctx.message = 'Present'

file2.lua

---

if ngx.ctx.message == 'Present' then

ngx.log( ngx.DEBUG, 'Yes' )

else

ngx.say( ngx.DEBUG, 'No )

end

file3.lua

---

local res = ngx.location.capture( '/test/endpoint1' )

nginx.conf

---

location = /test/endpoint1 {

...

content_by_lua_file 'file1.lua';

content_by_lua_file 'file2.lua';

...

}

location = /test/endpoint2 {

...

content_by_lua_file 'file3.lua';

...

}

Visiting /test/endpoint1 will echo "Yes" as ngx.ctx.message == 'Present' but visiting /test/endpoint2 will echo "No" as ngx.ctx == nil by that point in the execution of file2.lua

I did not expect this behaviour, is there a part of the docs that discuss it? Thanks!

- Daniel

agentzh

Hello!

On Fri, Oct 31, 2014 at 10:44 AM, Cefan Daniel Rubin wrote:
> I would expect the ngx.ctx between subsequent content_by_lua_file calls in
> the same subrequest to be shared but it seems that this is not the case?
>

ngx.ctx is a per request construct while a subrequest is considered a
different request than its parent, by definition.

So ngx.ctx is not shared by default with subrequests created by
ngx.location.capture. But you can explicitly pass your parent ctx
table to the subrequest via ngx.location.capture's "ctx" option.

> I did not expect this behaviour, is there a part of the docs that discuss
> it? Thanks!
>

https://github.com/openresty/lua-nginx-module#ngxlocationcapture

"
* ctx

specify a Lua table to be the ngx.ctx table for the subrequest. It can
be the current request'sngx.ctx table, which effectively makes the
parent and its subrequest to share exactly the same context table.
This option was first introduced in the v0.3.1rc25 release.
"

Regards,
-agentzh

cefan.daniel.rubin

Hey agentzh

I understand clearly (and the docs are clear on this too) that a subrequest does not by default have access to the context of its parent. This issue described here is that the context does not seem to be shared between two content_by_lua_file calls within the same subrequest.

In the example provided file1.lua sets the ctx field message'. file2.lua (whose content is included in the same location block using content_by_lua_file) then tries to access that field. Only in a subrequest the field 'message' is lost .

This seems to mean that if the chain:

content_by_lua_file 'file1.lua';

content_by_lua_file 'file2.lua';

occurs in a request then code in file2.lua has access to the same ctx as file1.lua.

However in a subrequest (where the parent request does not specify a ctx) we are seeing that the ctx = {} for file2.lua. Surely in this case a subrequest should work like the request did so that subsequent content_by_lua_file calls work with the same ctx?

Thanks as ever!

- Daniel

On Friday, October 31, 2014 1:44:08 PM UTC-4, Cefan Daniel Rubin wrote:

Hey all!

I would expect the ngx.ctx between subsequent content_by_lua_file calls in the same subrequest to be shared but it seems that this is not the case?

We recently came across this behaviour which seemed odd:

file1.lua
---
ngx.ctx.message = 'Present'

file2.lua
---
if ngx.ctx.message == 'Present' then
ngx.log( ngx.DEBUG, 'Yes' )
else
ngx.say( ngx.DEBUG, 'No )
end

file3.lua
---
local res = ngx.location.capture( '/test/endpoint1' )

nginx.conf
---
location = /test/endpoint1 {
...
content_by_lua_file 'file1.lua';
content_by_lua_file 'file2.lua';
...
}

location = /test/endpoint2 {
...
content_by_lua_file 'file3.lua';
...
}

Visiting /test/endpoint1 will echo "Yes" as ngx.ctx.message == 'Present' but visiting /test/endpoint2 will echo "No" as ngx.ctx == nil by that point in the execution of file2.lua

I did not expect this behaviour, is there a part of the docs that discuss it? Thanks!

- Daniel

cefan.daniel.rubin

Had the example I gave been more clear I may have seen the error meself: Turns out that the chain is:

access_by_lua_file 'file1.lua';

content_by_lua_file 'file2.lua';

and since access_by_lua_file is not called in subrequests the ctx is not going to be changed by file1.lua.

Thank you agentzh as ever for giving us openresty - it is a great place to work!

- Daniel

On Saturday, November 1, 2014 7:50:41 AM UTC-4, Cefan Daniel Rubin wrote:

Hey agentzh

I understand clearly (and the docs are clear on this too) that a subrequest does not by default have access to the context of its parent. This issue described here is that the context does not seem to be shared between two content_by_lua_file calls within the same subrequest.

In the example provided file1.lua sets the ctx field message'. file2.lua (whose content is included in the same location block using content_by_lua_file) then tries to access that field. Only in a subrequest the field 'message' is lost .

This seems to mean that if the chain:

content_by_lua_file 'file1.lua';
content_by_lua_file 'file2.lua';

occurs in a request then code in file2.lua has access to the same ctx as file1.lua.

However in a subrequest (where the parent request does not specify a ctx) we are seeing that the ctx = {} for file2.lua. Surely in this case a subrequest should work like the request did so that subsequent content_by_lua_file calls work with the same ctx?

Thanks as ever!

- Daniel

On Friday, October 31, 2014 1:44:08 PM UTC-4, Cefan Daniel Rubin wrote:

Hey all!

I would expect the ngx.ctx between subsequent content_by_lua_file calls in the same subrequest to be shared but it seems that this is not the case?

We recently came across this behaviour which seemed odd:

file1.lua
---
ngx.ctx.message = 'Present'

file2.lua
---
if ngx.ctx.message == 'Present' then
ngx.log( ngx.DEBUG, 'Yes' )
else
ngx.say( ngx.DEBUG, 'No )
end

file3.lua
---
local res = ngx.location.capture( '/test/endpoint1' )

nginx.conf
---
location = /test/endpoint1 {
...
content_by_lua_file 'file1.lua';
content_by_lua_file 'file2.lua';
...
}

location = /test/endpoint2 {
...
content_by_lua_file 'file3.lua';
...
}

Visiting /test/endpoint1 will echo "Yes" as ngx.ctx.message == 'Present' but visiting /test/endpoint2 will echo "No" as ngx.ctx == nil by that point in the execution of file2.lua

I did not expect this behaviour, is there a part of the docs that discuss it? Thanks!

- Daniel

agentzh

Hello!

On Sat, Nov 1, 2014 at 4:50 PM, Cefan Daniel Rubin wrote:
>
> Had the example I gave been more clear I may have seen the error meself:
> Turns out that the chain is:
>
>   access_by_lua_file 'file1.lua';
>   content_by_lua_file 'file2.lua';
>
> and since access_by_lua_file is not called in subrequests the ctx is not
> going to be changed by file1.lua.
>

Well, access_by_lua is not run at all in subrequests. This is documented:

https://github.com/openresty/lua-nginx-module#access_by_lua

"As with other access phase handlers, access_by_lua will not run in
subrequests."

According to the nginx's design, subrequests skip all the access phase
handlers, which is not specific to ngx_lua.

Maybe you should use rewrite_by_lua here instead?

BTW, given that you're already using content_by_lua in the same
location, it is recommended to merge all your Lua logic into one
handler, e.g., in your content_by_lua, because multiple *_by_lua
directive handlers add extra overhead as compared to a single *_by_lua
directive, for obvious reasons.

Regards,
-agentzh