New: [AEA-4490] - Add an alarm on Dynamo capacity (#939)

## Summary

- ✨ New Feature

### Details

[Ticket](https://nhsd-jira.digital.nhs.uk/browse/AEA-4490)

We want to alert when the database consumption rate approaches the
provisioned capacity. This adds an alarm to the existing `alarms.yml` to
do so. The alarm is placed in the PSU stack so as to also apply itself
to alternate stack names (e.g. PR stacks)

SAMtemplates/alarms/main.yaml

@@ -22,6 +22,11 @@ Parameters:
   EnableAlerts:
     Type: String
 
+  DynamoDBUtilizationPercentageThreshold:
+    Type: Number
+    Description: When the database usage crosses this percentage of provisioned capacity, trigger an alarm
+    Default: 90
+
 Resources:
   GetStatusUpdatesErrorsLogsMetricFilter:
     Type: AWS::Logs::MetricFilter
@@ -220,6 +225,49 @@ Resources:
       OKActions:
         - !ImportValue lambda-resources:SlackAlertsSnsTopicArn
 
+  DynamoDBWriteConsumptionAlarm:
+    Type: AWS::CloudWatch::Alarm
+    Properties:
+      AlarmName: !Sub "${AWS::StackName}_DynamoDB_ConsumedWriteCapacity"
+      ActionsEnabled: !Ref EnableAlerts
+      AlarmActions:
+        - !ImportValue lambda-resources:SlackAlertsSnsTopicArn
+      InsufficientDataActions:
+        - !ImportValue lambda-resources:SlackAlertsSnsTopicArn
+      OKActions:
+        - !ImportValue lambda-resources:SlackAlertsSnsTopicArn
+      EvaluationPeriods: 1
+      Threshold: !Ref DynamoDBUtilizationPercentageThreshold
+      ComparisonOperator: GreaterThanOrEqualToThreshold
+      TreatMissingData: notBreaching
+      Metrics:
+        - Id: e1
+          Label: UtilizationPercentage
+          ReturnData: true
+          Expression: "(m1 / (m2 * 60)) * 100"
+        - Id: m1
+          ReturnData: false
+          MetricStat:
+            Metric:
+              Namespace: AWS/DynamoDB
+              MetricName: ConsumedWriteCapacityUnits
+              Dimensions:
+                - Name: TableName
+                  Value: !Ref PrescriptionStatusUpdatesTableName
+            Period: 60
+            Stat: Sum
+        - Id: m2
+          ReturnData: false
+          MetricStat:
+            Metric:
+              Namespace: AWS/DynamoDB
+              MetricName: ProvisionedWriteCapacityUnits
+              Dimensions:
+                - Name: TableName
+                  Value: !Ref PrescriptionStatusUpdatesTableName
+            Period: 60
+            Stat: Maximum
+
   CPSUFhirConverterErrorLogsMetricFilter:
     Type: AWS::Logs::MetricFilter
     Properties:

SAMtemplates/main_template.yaml

@@ -81,6 +81,12 @@ Parameters:
       - true
       - false
 
+  DynamoDBUtilizationPercentageThreshold:
+    Type: Number
+    Description: When the database usage crosses this percentage of provisioned capacity, trigger an alarm
+    Default: 90
+
+
 Resources:
   Tables:
     Type: AWS::Serverless::Application
@@ -150,4 +156,5 @@ Resources:
         UpdatePrescriptionStatusFunctionName: !GetAtt Functions.Outputs.UpdatePrescriptionStatusFunctionName
         PrescriptionStatusUpdatesTableName: !GetAtt Tables.Outputs.PrescriptionStatusUpdatesTableName
         ConvertRequestToFhirFormatFunctionName: !GetAtt Functions.Outputs.ConvertRequestToFhirFormatFunctionName
+        DynamoDBUtilizationPercentageThreshold: !Ref DynamoDBUtilizationPercentageThreshold
         EnableAlerts: !Ref EnableAlerts