On Sun, Apr 25, 2010 at 4:58 PM, Magnus L. [email protected] wrote:
Say I have a location /url that I want to construct responses to based
on a sequence of communications with an upstream, host:port. To be
specific, the upstream would be a database, and the sequence of
communications would comprise commands and responses.
We’ve been doing this kind of things via ngx_echo’s echo_location
directive in our ngx_memc and ngx_drizzle’s tests suite. Here’s some
an example to do sequential memcached command execution against the
ngx_memc upstream module in a single location /main:
location /main {
echo_location '/memc?cmd=flush_all';
echo_location '/memc?key=foo&cmd=set&val=';
echo_location '/memc?key=foo&cmd=get';
}
location /memc {
set $memc_cmd $arg_cmd;
set $memc_key $arg_key;
set $memc_value $arg_val;
memc_pass 127.0.0.1:11211;
}
You can find more examples here:
http://github.com/agentzh/memc-nginx-module/blob/master/test/t/
And here’s an example for sequential SQL queries against a mysql
backend:
location /test {
echo_location /mysql "drop table if exists foo";
echo;
echo_location /mysql "create table foo (id serial, name
char(10), age integer);";
echo;
echo_location /mysql “insert into foo (name, age) values (‘’,
null);”;
echo;
echo_location /mysql “insert into foo (name, age) values (null,
0);”;
echo;
echo_location /mysql “select * from foo order by id”;
echo;
}
location /mysql {
drizzle_pass backend;
drizzle_module_header off;
drizzle_query $query_string;
rds_json on;
}
Responses look like this
{"errcode":0}
{"errcode":0}
{"errcode":0,"insert_id":1,"affected_rows":1}
{"errcode":0,"insert_id":2,"affected_rows":1}
[{"id":1,"name":"","age":null},{"id":2,"name":null,"age":0}]
More examples can be seen here:
http://github.com/agentzh/rds-json-nginx-module/blob/master/test/t/sanity.t
A simple example
of what I’d like to do would be:
- client communicates message to /url meaning “if key foo exists,
add 1 to bar”
- module handling /url sends first database command to check if foo
exists, if not reply “failed” to client, or it does and
- module sends database command to increment bar, replies “succeeded”.
It reminds me of the “safe incr” sample in my slides for my talk on
nginx.conf scripting:
http://agentzh.org/misc/slides/nginx-conf-scripting/nginx-conf-scripting.html
(Use the arrow keys on your keyboard to switch pages there.)
The details are arbitrary, but I would think there are myriad examples
where you’d want a module to construct a reply based on back and forth
communications.
Check out ngx_echo module to see if it fits your needs:
http://wiki.nginx.org/NginxHttpEchoModule
I’m hoping there’s some nice way of handling this case, but from I’ve
gathered looking at the code it seems oriented to “1 message to
module, filter as desired, send to upstream, get response, filter
again, then response back to client.”
IMHO, Evan M.'s Guide is very limited in expressing nginx’s full
power 
It’s just a good old introductory guide anyway 
Still many
thanks to Evan M. because the guide has helped so many people
including me
I must add that the echo_location and echo_subrequest thingies in
ngx_echo are still limited in power We’re currently releasing the
full power of nginx’s subrequests in our ngx_lua module. Soon we’ll be
able to do things like this on the Lua land:
res = ngx.location.capture('/sub1?id=32');
if (res ...) {
res2 = ngx.location.capture('/sub2?id=56');
} else {
res2 = ngx.subrequest.capture(ngx.HTTP_POST, '/sub2?id=56',
{ body: ‘hello’ });
}
And all the capture operations are transparent non-blocking I/O ones
that are based on nginx’s subrequests. Thanks to coco lua’s C-level
coroutine support!
There will also be equivalences to the echo_location and
echo_location_async directives on the Lua land, i.e.,
ngx.location.echo and ngx.location.echo_async
Stay tuned!
-agentzh
