Fix issue #179: Leader Followers Pattern (#1189)

* add leader followers pattern

* use var and streams instead in App::execute

* use logger instead of printing to system output stream

leader-followers/README.md

@@ -0,0 +1,32 @@
+---
+layout: pattern
+title: Leader/Followers
+folder: leader-followers
+permalink: /patterns/leader-followers/
+categories: Concurrency
+tags:
+ - Performance
+---
+
+## Intent
+The Leader/Followers pattern provides a concurrency model where multiple 
+threads can efficiently de-multiplex events and dispatch event handlers 
+that process I/O handles shared by the threads.
+
+## Class diagram
+![Leader/Followers class diagram](./etc/leader-followers.png)
+
+## Applicability
+Use Leader-Followers pattern when
+
+* multiple threads take turns sharing a set of event sources in order to detect, de-multiplex, dispatch and process service requests that occur on the event sources.
+
+## Real world examples
+
+* [ACE Thread Pool Reactor framework](https://www.dre.vanderbilt.edu/~schmidt/PDF/HPL.pdf)
+* [JAWS](http://www.dre.vanderbilt.edu/~schmidt/PDF/PDCP.pdf)
+* [Real-time CORBA](http://www.dre.vanderbilt.edu/~schmidt/PDF/RTS.pdf)
+
+## Credits
+
+* [Douglas C. Schmidt and Carlos O’Ryan - Leader/Followers](http://www.kircher-schwanninger.de/michael/publications/lf.pdf)