Support providing MLB templates via environment variables rather than… (

#492)

* Support providing MLB templates via environment variables rather than template tgz files
* Add documentation
* Add unit test for environment variable HAPROXY templates

README.md

@@ -197,14 +197,28 @@ of labels.
 
 ### Templates
 
-Marathon-lb searches for configuration files in the `templates/`
-directory. The `templates/` directory is located in a relative
-path from where the script is run. Some templates can also be
-[overridden _per app service port_](#overridable-templates). You may add your
-own templates to the Docker image, or provide them at startup.
-
-See [the configuration doc for the full list](Longhelp.md#templates)
-of templates.
+Marathon-lb global templates (as listed in the [Longhelp](Longhelp.md#templates)) can be overwritten in two ways:
+-By creating an environment variable in the marathon-lb container
+-By placing configuration files in the `templates/` directory (relative to where the script is run from)
+
+For example, to replace `HAPROXY_HTTPS_FRONTEND_HEAD` with this content:
+
+```
+frontend new_frontend_label
+  bind *:443 ssl crt /etc/ssl/cert.pem
+  mode http
+```
+
+Then this environment variable could be added to the Marathon-LB configuration:
+```
+"HAPROXY_HTTPS_FRONTEND_HEAD": "\\nfrontend new_frontend_label\\n  bind *:443 ssl {sslCerts}\\n  mode http"
+```
+
+Alternately, a file called`HAPROXY_HTTPS_FRONTEND_HEAD` could be placed in `templates/` directory through the use of an artifact URI.
+
+Additionally, some templates can also be [overridden _per app service port_](#overridable-templates). You may add your own templates to the Docker image, or provide them at startup.
+
+See [the configuration doc for the full list](Longhelp.md#templates) of templates.
 
 #### Overridable Templates
 

config.py

@@ -770,17 +770,25 @@ def __init__(self, directory='templates'):
         self.__load_templates()
 
     def __load_templates(self):
-        '''Loads template files if they exist, othwerwise it sets defaults'''
+        '''Look in environment variables for templates.  If not set in env,
+        load template files if they exist. Othwerwise it sets defaults'''
 
         for template in self.t:
             name = self.t[template].full_name
-            try:
-                filename = os.path.join(self.__template_directory, name)
-                with open(filename) as f:
-                    logger.info('overriding %s from %s', name, filename)
-                    self.t[template].value = f.read()
-            except IOError:
-                logger.debug("setting default value for %s", name)
+            if os.environ.get(name):
+                logger.info('overriding %s from environment variable', name)
+                env_template_val = os.environ.get(name)
+
+                # Handle escaped endlines
+                self.t[template].value = env_template_val.replace("\\n", "\n")
+            else:
+                try:
+                    filename = os.path.join(self.__template_directory, name)
+                    with open(filename) as f:
+                        logger.info('overriding %s from %s', name, filename)
+                        self.t[template].value = f.read()
+                except IOError:
+                    logger.debug("setting default value for %s", name)
 
     def get_descriptions(self):
         descriptions = '''\

tests/test_marathon_lb.py

@@ -98,6 +98,38 @@ def test_config_no_apps(self):
   bind *:9091
   mode http
 
+frontend marathon_https_in
+  bind *:443 ssl crt /etc/ssl/cert.pem
+  mode http
+'''
+        print("actual config:\n")
+        print(config)
+        self.assertMultiLineEqual(config, expected)
+
+    def test_config_env_template(self):
+        apps = dict()
+        groups = ['external']
+        bind_http_https = True
+        ssl_certs = ""
+        os.environ["HAPROXY_HTTP_FRONTEND_HEAD"] = '''
+frontend changed_frontend
+  bind *:80
+  mode http
+'''
+        templater = marathon_lb.ConfigTemplater()
+        del os.environ["HAPROXY_HTTP_FRONTEND_HEAD"]
+
+        config = marathon_lb.config(apps, groups, bind_http_https,
+                                    ssl_certs, templater)
+        expected = self.base_config + '''
+frontend changed_frontend
+  bind *:80
+  mode http
+
+frontend marathon_http_appid_in
+  bind *:9091
+  mode http
+
 frontend marathon_https_in
   bind *:443 ssl crt /etc/ssl/cert.pem
   mode http