SNS (Simple Notification Service) example with ELB and CloudWatch

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What is SNS (Simple Notification Service)?

SNS is broken up into three sections:

Topics: Communication channels to send messages and subscribe to notifications.
Applications: Mobile devices that we can push notifications to.
Subscriptions: Subscribe an endpoint to a topic to receive messages published to that topic.

In the subsequent sections, we'll do the following:

Create a Topic
Subscribe to a Topic
Publish to a Topic
Clean Up

Create a Topic

A topic provides an access point for publishers and subscribers to communicate with each other.

Create a topic with a name "ELB-Health":

Select the new topic and then click the topic ARN. Then the Topic Details page appears:

Copy the topic ARN (arn:aws:sns:us-west-1:526262051452:ELB-Topic) for the next task.

To receive messages published to a topic, we have to subscribe an endpoint to that topic. An endpoint is a mobile app, web server, email address, or an Amazon SQS queue that can receive notification messages from Amazon SNS.

Once we subscribe an endpoint to a topic and the subscription is confirmed, the endpoint will receive all messages published to that topic.

Let's subscribe an endpoint to the topic we just created in the previous section. We configure the subscription to send the topic messages to our email account.

Click "Create subscription" on the Amazon SNS console at https://console.aws.amazon.com/sns/v2/home.

In the Topic ARN field, paste the topic ARN we created in the previous task, In the Protocol drop-down box, select Email, and in the Endpoint box, type an email address we can use to receive the notification:

Click "Create subscription".

As we can see, the email is in pending state, so we need to go to our email application and open the message from AWS Notifications, and then click the link to confirm our subscription:

Then, our web browser displays a confirmation response from Amazon SNS:

Publish to a Topic

Publishers send messages to topics. Once a new message is published, Amazon SNS attempts to deliver that message to every endpoint that is subscribed to the topic. In this section we publish a message to the email address we defined in the previous section.

In the left navigation pane on the Amazon SNS console at https://console.aws.amazon.com/sns/v2/home, click Topics and then select the topic we want to publish to.

Then, if we click the "Publish to topic" button, the "Publish a Message" page appears. In the Subject box, type a subject line for message, for the Message box, type a brief message.

Click "Publish Message", then in our mail, we can check if we received the message:

ELB health check

We used the default setting for instance health check:

Ping Protocol : HTTP
Ping Port : 80
Ping Path : /index.html

We need to install Apache server on the two instances that are connected to ELB.

To check the response, we can use the "curl" command via private ip address of an the instance from the other instance:

# curl -I 172.31.9.10:80/index.html
HTTP/1.1 200 OK
Date: Tue, 07 Feb 2017 08:39:18 GMT
Server: Apache/2.4.18 (Ubuntu)
Last-Modified: Tue, 07 Feb 2017 08:37:38 GMT
ETag: "2c39-547eca9199554"
Accept-Ranges: bytes
Content-Length: 11321
Vary: Accept-Encoding
Content-Type: text/html

With Nginx server, however, the default conf may respond with '404'. So, the ELB thinks it has an unhealthy instance even though we get a proper index page on the browser.

ELB and Alarm

We used the default setting for instance health check: