On Mon, Oct 5, 2009 at 7:08 PM, dennis cao <dennis__cao@hotmail.com> wrote:
> When I use this variable in log_format directive ,it can print the request_body in the log file
>
> but somewhere else it can't!
>

The $request_body variable only has values when the underlying content
handler or upstream handler or anything else has actually *read* the
request body from the tcp connection and stored it into
r->request_body slot.

The variable itself merely reads that buffered data and will never
read from socket itself. See the ngx_http_variable_request_body
function in src/http/ngx_http_variables.c in the Nginx source tree.

It can be easily demonstrated by means of my echo module [1]:

location /echo_body {
echo [$request_body];
}

Accessing /echo from the client side gives

$ echo "my body" | lwp-request -m POST 'http://localhost:8080/echo_body'
[]

As we can see, it's empty, because the "echo" module does NOT read the
client request body [2]. Now let's use something that actually reads
the body, like the standard proxy module:

location /echo_body2 {
proxy_pass $scheme://127.0.0.1:$server_port/dummy;
echo_after_body [$request_body];
}
location /dummy { echo_duplicate 0 ''; }

Accessing /echo_body2 gives the *desired* results:

$ echo "my body" | lwp-request -m POST 'http://localhost:8080/echo_body2'
[my body
]

Hope this helps :)

Cheers,
-agentzh

References
[1] echo-nginx-module wiki page: http://wiki.nginx.org/NginxHttpEchoModule
[2] On the C level, it's usually the ngx_http_read_client_request_body
function that does the body reading work, but can be also done by any
other code (as in the "upload" module).