diff -r cfc3d11884ad -r 27a0fdd7bca7 src/main/java/de/uapcore/lightpit/DatabaseFacade.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/java/de/uapcore/lightpit/DatabaseFacade.java	Sat May 09 14:26:31 2020 +0200
@@ -0,0 +1,178 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
+ * 
+ * Copyright 2018 Mike Becker. All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *   1. Redistributions of source code must retain the above copyright
+ *      notice, this list of conditions and the following disclaimer.
+ *
+ *   2. Redistributions in binary form must reproduce the above copyright
+ *      notice, this list of conditions and the following disclaimer in the
+ *      documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * 
+ */
+package de.uapcore.lightpit;
+
+import java.sql.Connection;
+import java.sql.DatabaseMetaData;
+import java.sql.SQLException;
+import java.util.Optional;
+import javax.naming.Context;
+import javax.naming.InitialContext;
+import javax.naming.NamingException;
+import javax.servlet.ServletContext;
+import javax.servlet.ServletContextEvent;
+import javax.servlet.ServletContextListener;
+import javax.servlet.annotation.WebListener;
+import javax.sql.DataSource;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Provides access to different privilege layers within the database.
+ */
+@WebListener
+public final class DatabaseFacade implements ServletContextListener {
+    
+    private static final Logger LOG = LoggerFactory.getLogger(DatabaseFacade.class);
+    
+    /**
+     * Timeout in seconds for the validation test.
+     */
+    private static final int DB_TEST_TIMEOUT = 10;
+    
+    public static enum Dialect {
+        Postgres;
+    }
+    
+    /**
+     * The database dialect to use.
+     * 
+     * May be override by context parameter.
+     * 
+     * @see Constants#CTX_ATTR_DB_DIALECT
+     */
+    private Dialect dialect = Dialect.Postgres;
+    
+    /**
+     * The default schema to test against when validating the connection.
+     * 
+     * May be overridden by context parameter.
+     * 
+     * @see Constants#CTX_ATTR_DB_SCHEMA
+     */
+    private static final String DB_DEFAULT_SCHEMA = "lightpit";
+    
+    /**
+     * The attribute name in the Servlet context under which an instance of this class can be found.
+     */
+    public static final String SC_ATTR_NAME = DatabaseFacade.class.getName();
+    private ServletContext sc;
+    
+    private static final String DS_JNDI_NAME = "jdbc/lightpit/app";
+    private Optional<DataSource> dataSource;
+    
+    /**
+     * Returns the data source.
+     * 
+     * The Optional returned should never be empty. However, if something goes
+     * wrong during initialization, the data source might be absent.
+     * Hence, users of this data source are forced to check the existence.
+     * 
+     * @return a data source
+     */
+    public Optional<DataSource> getDataSource() {
+        return dataSource;
+    }
+    
+    public Dialect getSQLDialect() {
+        return dialect;
+    }
+    
+    private static void checkConnection(DataSource ds, String testSchema, String errMsg) {
+        try (Connection conn = ds.getConnection()) {
+            if (!conn.isValid(DB_TEST_TIMEOUT)) {
+                throw new SQLException("Validation check failed.");
+            }
+            if (conn.isReadOnly()) {
+                throw new SQLException("Connection is read-only and thus unusable.");
+            }
+            if (!conn.getSchema().equals(testSchema)) {
+                throw new SQLException(String.format("Connection is not configured to use the schema %s.", testSchema));
+            }
+            DatabaseMetaData metaData = conn.getMetaData();
+            LOG.info("Connections as {} to {}/{} ready to go.", metaData.getUserName(), metaData.getURL(), conn.getSchema());
+        } catch (SQLException ex) {
+            LOG.error(errMsg, ex);
+        }
+    }
+    
+    private static Optional<DataSource> retrieveDataSource(Context ctx) {
+        DataSource ret = null;
+        try {
+            ret = (DataSource)ctx.lookup(DS_JNDI_NAME);
+            LOG.info("Data source retrieved.");
+        } catch (NamingException ex) {
+            LOG.error("Data source {} not available.", DS_JNDI_NAME);
+            LOG.error("Reason for the missing data source: ", ex);
+        }
+        return Optional.ofNullable(ret);
+    }
+
+    @Override
+    public void contextInitialized(ServletContextEvent sce) {
+        sc = sce.getServletContext();
+        
+        dataSource = null;
+        
+        final String contextName = Optional
+                .ofNullable(sc.getInitParameter(Constants.CTX_ATTR_JNDI_CONTEXT))
+                .orElse("java:comp/env");
+        final String dbSchema = Optional
+                .ofNullable(sc.getInitParameter(Constants.CTX_ATTR_DB_SCHEMA))
+                .orElse(DB_DEFAULT_SCHEMA);
+        final String dbDialect = sc.getInitParameter(Constants.CTX_ATTR_DB_DIALECT);
+        if (dbDialect != null) {
+            try {
+                dialect = Dialect.valueOf(dbDialect);
+            } catch (IllegalArgumentException ex) {
+                LOG.error("Unknown or unsupported database dialect {}. Defaulting to {}.", dbDialect, dialect);
+            }
+        }
+
+        try {
+            LOG.debug("Trying to access JNDI context {}...", contextName);
+            Context initialCtx = new InitialContext();
+            Context ctx = (Context) initialCtx.lookup(contextName);
+            
+            dataSource = retrieveDataSource(ctx);
+            
+            dataSource.ifPresent((ds) -> checkConnection(ds, dbSchema, "Checking database connection failed"));
+        } catch (NamingException | ClassCastException ex) {
+            LOG.error("Cannot access JNDI resources.", ex);
+        }
+        
+        sc.setAttribute(SC_ATTR_NAME, this);
+        LOG.info("Database facade injected into ServletContext.");
+    }
+
+    @Override
+    public void contextDestroyed(ServletContextEvent sce) {
+        dataSource = null;
+    }    
+}