commit b6a8f8c184b3126779d30e762b4ece6a5817d56e
author Roman Arutyunyan <arut@nginx.com> Thu Jan 28 15:28:20 2016 +0300
committer Roman Arutyunyan <arut@nginx.com> Thu Jan 28 15:28:20 2016 +0300
tree 865930ecccbf3fdd22f9d698fbf579ea37096dbb
parent ec16a6013ee08805d031ad51d3220679705474f1

Resolver: per-request DNS server balancer.

Previously, a global server balancer was used to assign the next DNS server to
send a query to.  That could lead to a non-uniform distribution of servers per
request.  A request could be assigned to the same dead server several times in a
row and wait longer for a valid server or even time out without being processed.

Now each query is sent to all servers sequentially in a circle until a
response is received or timeout expires.  Initial server for each request is
still globally balanced.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index a918552..ac0d54d 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -685,6 +685,11 @@
         return NGX_OK;
     }
 
+    rn->last_connection = r->last_connection++;
+    if (r->last_connection == r->connections.nelts) {
+        r->last_connection = 0;
+    }
+
     rn->naddrs = (u_short) -1;
 #if (NGX_HAVE_INET6)
     rn->naddrs6 = r->ipv6 ? (u_short) -1 : 0;
@@ -897,6 +902,11 @@
         goto failed;
     }
 
+    rn->last_connection = r->last_connection++;
+    if (r->last_connection == r->connections.nelts) {
+        r->last_connection = 0;
+    }
+
     rn->naddrs = (u_short) -1;
 #if (NGX_HAVE_INET6)
     rn->naddrs6 = (u_short) -1;
@@ -1098,11 +1108,7 @@
     ngx_resolver_connection_t  *rec;
 
     rec = r->connections.elts;
-
-    rec = &rec[r->last_connection++];
-    if (r->last_connection == r->connections.nelts) {
-        r->last_connection = 0;
-    }
+    rec = &rec[rn->last_connection];
 
     if (rec->udp == NULL) {
 
@@ -1244,6 +1250,10 @@
 
         if (rn->waiting) {
 
+            if (++rn->last_connection == r->connections.nelts) {
+                rn->last_connection = 0;
+            }
+
             (void) ngx_resolver_send_query(r, rn);
 
             rn->expire = now + r->resend_timeout;