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.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/core/ngx_resolver.h b/src/core/ngx_resolver.h
index 5aedabfaa..ab4ae74ee 100644
--- a/src/core/ngx_resolver.h
+++ b/src/core/ngx_resolver.h
@@ -93,6 +93,8 @@ typedef struct {
     time_t                    valid;
     uint32_t                  ttl;
 
+    ngx_uint_t                last_connection;
+
     ngx_resolver_ctx_t       *waiting;
 } ngx_resolver_node_t;