Alert and Automate

Now you know that uploading a bad image causes an issue on your site and correlates with a log entry containing "ActiveStorage::InvariableError", you can now use this information to create a metric from the logs, alert on that metric and respond automatically to resolve the issue in seconds.

  1. Navigate to CloudWatch Logs

    • Click here or:
      • Navigate to the AWS Console
      • Start typing CloudWatch in the AWS Services search box
      • Select CloudWatch
      • Select Log groups from the navigation menu
  2. Create Metric

    • Select the application.log log group
    • Click on the Create Metric Filter under Actions dropdown
    • Enter "ActiveStorage::InvariableError" (with the quotation marks) for Filter Pattern
    • Click on the Assign Metric button
    • Enter ImageError as the Metric Name
    • Click on the Create Filter button
  3. Create Alarm

    • Click on Create Alarm
    • Change Period to 10 seconds
    • Select Static for Threshold type
    • Select Greater for Whenever ImageError is…
    • Select 0 under than… for the threshold value
    • Expand Additional Configuration
    • Select Treat missing data as good (not breaching threshold) for Missing data treatment
    • Click on the Next button
    • Select Select an existing SNS topic for Select an SNS topic
    • Select Lab-Monitoring… for Send a notification to…

    You will notice that there is a pre-existing endpoint (arn:aws:lambda:eu-west-1…) defined under Email (endpoints). This is because the lab setup created an SNS topic linked to a Lambda function that automatically resolve the issue.

    • Click on the Next button
    • Enter ImageErrorAlarm for Define a unique name
    • Click on the Next button
    • Click on the Create alarm button
    • The alarm will temporarily be in INSUFFICIENT status
    • The alarm will transition to OK automatically
  4. Navigate to Lambda

    • Click here or:
      • Navigate to the AWS Console
      • Start typing Lambda in the AWS Services search box
      • Select Lambda
  5. View the automation

    • Click on the Lab-Monitoring-… under Function name

    You can see that the function is triggered by the SNS topic that you selected when you created your alarm.


    You can also see the code that deletes the offending image in the database.

    var mysql = require('mysql');
    var connection = mysql.createConnection({
        host     : process.env.endpoint,
        user     : process.env.user,
        password : process.env.password,
        database : process.env.database
    exports.handler = function(event, context) {
        var message = event.Records[0].Sns.Message;
        console.log('Message received from SNS:', message); 
        var sql = "DELETE images, active_storage_blobs FROM images,active_storage_blobs WHERE = AND active_storage_blobs.content_type NOT LIKE 'image/%'";
        connection.query(sql, function(err, results) { 
            if (err){
            console.log("Rows Deleted:", results.affectedRows);

You have now created an alarm and linked it to a Lambda function via an SNS Topic to automate the response to an event that causes downtime. You should now be able to recover automatically from this outage with seconds of downtime.