> What is the $key?
set $redis_key "$http_host$document_uri$query_string";

> Provide more information about redisbackend
> description.

upstream redisbackend {
server 127.0.0.1:6379;
keepalive 256 single;
}

Additional information : I've removed the line
static ngx_str_t ngx_http_redis_db = ngx_string("redis_db");

Because of error in compilation.
./configure ... --add-module=../ngx_http_redis-0.3.2 --add-module=../ngx_http_upstream_keepalive-c6396fef9295
make && make install

Thanks you