diff --git a/.gitignore b/.gitignore
index b6e4761..36e68da 100644
--- a/.gitignore
+++ b/.gitignore
@@ -127,3 +127,6 @@ dmypy.json
 
 # Pyre type checker
 .pyre/
+
+# Bazel local cache/files ignore.
+bazel-*
\ No newline at end of file
diff --git a/WORKSPACE b/WORKSPACE
index 5960a4c..9712e76 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,11 +1,12 @@
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+# Load python specific bazel rules.
 http_archive(
     name = "rules_python",
     url = "https://github.com/bazelbuild/rules_python/releases/download/0.1.0/rules_python-0.1.0.tar.gz",
     sha256 = "b6d46438523a3ec0f3cead544190ee13223a52f6a6765a29eae7b7cc24cc83a0",
 )
 
-
 load("@rules_python//python:pip.bzl", "pip_install")
 
 pip_install(
diff --git a/wgkex/worker/BUILD b/wgkex/worker/BUILD
index 49cd760..d62932b 100644
--- a/wgkex/worker/BUILD
+++ b/wgkex/worker/BUILD
@@ -45,3 +45,21 @@ py_test(
        requirement("mock"),
     ],
 )
+
+py_binary(
+    name = "app",
+    srcs = ["app.py"],
+    deps = [
+       ":mqtt",
+       "//wgkex/config:config",
+    ],
+)
+
+py_test(
+    name = "app_test",
+    srcs = ["app_test.py"],
+    deps = [
+       ":app",
+       requirement("mock"),
+    ],
+)
diff --git a/wgkex/worker/app.py b/wgkex/worker/app.py
index 3c826a0..7f91a2f 100644
--- a/wgkex/worker/app.py
+++ b/wgkex/worker/app.py
@@ -1,15 +1,27 @@
-#!/usr/bin/env python3
+"""Initialises the MQTT worker."""
 
+import wgkex.config.config as config
+from wgkex.worker import mqtt
 
-from wgkex.config import load_config
-from wgkex.worker.mqtt import connect as mqtt
 
-config = load_config()
+class Error(Exception):
+    """Base Exception handling class."""
 
 
-def main():
+class DomainsNotInConfig(Error):
+    """If no domains exist in configuration file."""
+
 
-    mqtt(config.get("domains"))
+def main():
+    """Starts MQTT listener.
+
+    Raises:
+        DomainsNotInConfig: If no domains were found in configuration file.
+    """
+    domains = config.load_config().get("domains")
+    if not domains:
+        raise DomainsNotInConfig("Could not locate domains in configuration.")
+    mqtt.connect(domains)
 
 
 if __name__ == "__main__":
diff --git a/wgkex/worker/app_test.py b/wgkex/worker/app_test.py
new file mode 100644
index 0000000..b4f7366
--- /dev/null
+++ b/wgkex/worker/app_test.py
@@ -0,0 +1,28 @@
+"""Unit tests for app.py"""
+import unittest
+import mock
+import app
+
+
+class AppTest(unittest.TestCase):
+    @mock.patch.object(app.config, "load_config")
+    @mock.patch.object(app.mqtt, "connect", autospec=True)
+    def test_main_success(self, connect_mock, config_mock):
+        """Ensure we can execute main."""
+        connect_mock.return_value = None
+        config_mock.return_value = dict(domains=["domain.one"])
+        app.main()
+        connect_mock.assert_called_with(["domain.one"])
+
+    @mock.patch.object(app.config, "load_config")
+    @mock.patch.object(app.mqtt, "connect", autospec=True)
+    def test_main_fails_no_domain(self, connect_mock, config_mock):
+        """Ensure we fail when domains are not configured."""
+        connect_mock.return_value = None
+        config_mock.return_value = dict(domains=None)
+        with self.assertRaises(app.DomainsNotInConfig):
+            app.main()
+
+
+if __name__ == "__main__":
+    unittest.main()