diff --git a/actix-server/src/builder.rs b/actix-server/src/builder.rs
index c16de554..0e533528 100644
--- a/actix-server/src/builder.rs
+++ b/actix-server/src/builder.rs
@@ -64,7 +64,7 @@ impl ServerBuilder {
 
     /// Set number of workers to start.
     ///
-    /// By default server uses number of available logical cpu as threads
+    /// By default server uses number of available logical cpu as workers
     /// count.
     pub fn workers(mut self, num: usize) -> Self {
         self.threads = num;
@@ -108,8 +108,8 @@ impl ServerBuilder {
         self
     }
 
-    /// Run external configuration as part of the server building
-    /// process
+    /// Execute external configuration as part of the server building
+    /// process.
     ///
     /// This function is useful for moving parts of configuration to a
     /// different module or even library.
@@ -117,17 +117,20 @@ impl ServerBuilder {
     where
         F: Fn(&mut ServiceConfig) -> io::Result<()>,
     {
-        let mut cfg = ServiceConfig::new();
+        let mut cfg = ServiceConfig::new(self.threads);
 
         f(&mut cfg)?;
 
-        let mut srv = ConfiguredService::new(cfg.rt);
-        for (name, lst) in cfg.services {
-            let token = self.token.next();
-            srv.stream(token, name);
-            self.sockets.push((token, lst));
+        if let Some(apply) = cfg.apply {
+            let mut srv = ConfiguredService::new(apply);
+            for (name, lst) in cfg.services {
+                let token = self.token.next();
+                srv.stream(token, name);
+                self.sockets.push((token, lst));
+            }
+            self.services.push(Box::new(srv));
         }
-        self.services.push(Box::new(srv));
+        self.threads = cfg.threads;
 
         Ok(self)
     }
diff --git a/actix-server/src/config.rs b/actix-server/src/config.rs
index 85480978..603dadff 100644
--- a/actix-server/src/config.rs
+++ b/actix-server/src/config.rs
@@ -15,18 +15,28 @@ use super::services::{
 use super::Token;
 
 pub struct ServiceConfig {
-    pub(super) services: Vec<(String, net::TcpListener)>,
-    pub(super) rt: Box<ServiceRuntimeConfiguration>,
+    pub(crate) services: Vec<(String, net::TcpListener)>,
+    pub(crate) apply: Option<Box<ServiceRuntimeConfiguration>>,
+    pub(crate) threads: usize,
 }
 
 impl ServiceConfig {
-    pub(super) fn new() -> ServiceConfig {
+    pub(super) fn new(threads: usize) -> ServiceConfig {
         ServiceConfig {
+            threads,
             services: Vec::new(),
-            rt: Box::new(not_configured),
+            apply: None,
         }
     }
 
+    /// Set number of workers to start.
+    ///
+    /// By default server uses number of available logical cpu as workers
+    /// count.
+    pub fn workers(&mut self, num: usize) {
+        self.threads = num;
+    }
+
     /// Add new service to server
     pub fn bind<U, N: AsRef<str>>(&mut self, name: N, addr: U) -> io::Result<&mut Self>
     where
@@ -43,16 +53,20 @@ impl ServiceConfig {
 
     /// Add new service to server
     pub fn listen<N: AsRef<str>>(&mut self, name: N, lst: net::TcpListener) -> &mut Self {
+        if self.apply.is_none() {
+            self.apply = Some(Box::new(not_configured));
+        }
         self.services.push((name.as_ref().to_string(), lst));
         self
     }
 
-    /// Register service configuration function
-    pub fn rt<F>(&mut self, f: F) -> io::Result<()>
+    /// Register service configuration function. This function get called
+    /// during worker runtime configuration. It get executed in worker thread.
+    pub fn apply<F>(&mut self, f: F) -> io::Result<()>
     where
         F: Fn(&mut ServiceRuntime) + Send + Clone + 'static,
     {
-        self.rt = Box::new(f);
+        self.apply = Some(Box::new(f));
         Ok(())
     }
 }