Update getting started guide with retry-loop and short explanation (#5635)

Signed-off-by: Joffrey F <joffrey@docker.com>

compose/gettingstarted.md

@@ -26,21 +26,50 @@ Define the application dependencies.
 
 2.  Create a file called `app.py` in your project directory and paste this in:
 
+        import time
+
+        import redis
         from flask import Flask
-        from redis import Redis
+
 
         app = Flask(__name__)
-        redis = Redis(host='redis', port=6379)
+        cache = redis.Redis(host='redis', port=6379)
+
+
+        def get_hit_count():
+            retries = 5
+            while True:
+                try:
+                    return cache.incr('hits')
+                except redis.exceptions.ConnectionError as exc:
+                    if retries == 0:
+                        raise exc
+                    retries -= 1
+                    time.sleep(0.5)
+
 
         @app.route('/')
         def hello():
-            count = redis.incr('hits')
+            count = get_hit_count()
             return 'Hello World! I have been seen {} times.\n'.format(count)
 
         if __name__ == "__main__":
             app.run(host="0.0.0.0", debug=True)
 
-      In this example, `redis` is the hostname of the redis container on the application's network. We use the default port for Redis, `6379`.
+
+      In this example, `redis` is the hostname of the redis container on the
+      application's network. We use the default port for Redis, `6379`.
+
+      > Handling transient errors
+      >
+      > Note the way the `get_hit_count` function is written. This basic retry
+      > loop lets us attempt our request multiple times if the redis service is
+      > not available. This is useful at startup while the application comes
+      > online, but also makes our application more resilient if the Redis
+      > service has to be restarted anytime during the app's lifetime. In a
+      > cluster, this also helps handling momentary connection drops between
+      > nodes.
+
 
 3.  Create another file called `requirements.txt` in your project directory and
     paste this in:
@@ -87,8 +116,6 @@ the following:
         build: .
         ports:
          - "5000:5000"
-        links:
-         - redis
       redis:
         image: "redis:alpine"