Documentation: improve instructions for scaling configuration

docs/src/main/resources/templates/scaling.html

@@ -11,9 +11,12 @@ <h4 id="scaling_solus">
   <a class="headerlink" href="#scaling_solus" title="Permalink to this headline">¶</a>
 </h4>
 <p>
-  To store the state we use a PostgreSQL database, the database connection should be configured with the
-  following Java system properties:
+  MapFish-Print's horizontal scaling solution is based on storing states in a PostgreSQL database. To enable
+  this mode, the file <code>mapfish-spring-application-context-override-db.xml</code> must be copied to
+  <code>mapfish-spring-application-context-override.xml</code> and database connection parameters must be
+  configured (see below).
 </p>
+<p>The database connection must be configured with the following Java system properties:</p>
 
 <ul>
   <li><code>db.host</code>: The database server host name</li>
@@ -25,12 +28,11 @@ <h4 id="scaling_solus">
 </ul>
 
 <p>
-  The schema should exist, and the necessary tables are created automatically (<code>print_accountings</code>,
-  <code>print_job_results</code>, <code>print_job_statuses</code>). In this mode, the container will wait for
-  the database to be reachable before actually starting the tomcat server.
+  The schema must exist, and the necessary tables are created automatically (<code>print_accountings</code>,
+  <code>print_job_results</code>, <code>print_job_statuses</code>)
 </p>
 
-<p>The DB polling can be tuned with these two environment variables:</p>
+<p>Database polling can be tuned with these two environment variables:</p>
 <ul>
   <li>
     <code>PRINT_CANCEL_OLD_POLL_INTERVAL</code>: How often in seconds the DB is polled for jobs to be canceled
@@ -43,6 +45,18 @@ <h4 id="scaling_solus">
   </li>
 </ul>
 
+<h5 ud="docker_image">
+  Existing MapFish-Print Packages
+  <a class="headerlink" href="#docker_image" title="Permalink to this headline">¶</a>
+</h5>
+<p>
+  Existing MapFish-Print distribution packages may already contain functionality to ease the setup of
+  horizontal scaling. For example, if you use the Docker image
+  https://hub.docker.com/r/camptocamp/mapfish_print), you do not need to manually rename the file mentioned
+  above, and a startup script contains functionality to verify database availability before the application
+  server is started.
+</p>
+
 <h5 id="scaling_docker">
   Docker
   <a class="headerlink" href="#scaling_docker" title="Permalink to this headline">¶</a>