issue-34: add default parameter for empty Cloudwatch metrics

Author: Sebastian Martinka

.gitignore

@@ -97,6 +97,10 @@ ENV/
 # mkdocs documentation
 /site
 
+# eclipse project settings
+.project
+.pydevproject
+
 # mypy
 .mypy_cache/
 

README.md

@@ -309,6 +309,23 @@ metric_sources:
           alias: cpu_usage
 ```
 
+Cloudwatch doesn't return values for some resource metrics if the resource was not used e.g. TargetResponseTime for AWS/ApplicationELB.
+You can add `default: <value>` so that the value is replaced with `<value>`.
+```
+metric_sources:
+  cloudwatch:
+    - namespace: AWS/ApplicationELB
+      metric_name: TargetResponseTime
+      dimensions:
+        - name: TargetGroup
+          value: targetgroup/ac-re-Targe-13P06UAEX2W9Y/f9002d01ab9053e0
+      period: 60
+      statistics:
+        - name: Average
+          alias: alb_response_time
+      default: 0
+```
+
 One thing to watch out for is how you define the `statistics` field above.
 The `name` part has to match exactly with a statistic used by CloudWatch,
 and the `alias` part is an arbitrary name you use to reference this metric when

lambda/ecsautoscale/metric_sources/cloudwatch.py

@@ -26,7 +26,8 @@ def get_data(metric_name: str = "MemoryUtilization",
              dimensions: List[dict] = None,
              statistics: List[dict] = None,
              namespace: str = "AWS/ECS",
-             period: int = 300) -> dict:
+             period: int = 300,
+             defaultValue: int = None) -> dict:
     """
     Retreive metrics from AWS CloudWatch.
 
@@ -72,8 +73,11 @@ def get_data(metric_name: str = "MemoryUtilization",
     )
     datapoints = res["Datapoints"]
     if not datapoints:
-        raise CloudWatchError(
-            namespace, metric_name, dimensions_, period, statistics_)
+        if defaultValue:
+            datapoints.append({statistics_[0] : defaultValue})
+        else:
+            raise CloudWatchError(
+                namespace, metric_name, dimensions_, period, statistics_)
 
     log_messages = ["Retreived the following statistics from CloudWatch:"]
     for stat in statistics: