Added SNS Section after Quip Review (#509)

README.md

@@ -65,6 +65,7 @@ Table of Contents
 | [OpsWorks](#opsworks) | [📗](#opsworks-basics) | [📘](#opsworks-tips) | [📙](#opsworks-gotchas-and-limitations) |
 | [Batch](#batch) | [📗](#batch-basics) | [📘](#batch-tips) |
 | [SQS](#sqs) | [📗](#sqs-basics) | [📘](#sqs-tips) | [📙](#sqs-gotchas-and-limitations) |
+| [SNS](#sns) | [📗](#sns-basics) | [📘](#sns-tips) | [📙](#sns-gotchas-and-limitations) |
 
 **Special Topics**
 
@@ -1938,6 +1939,7 @@ SQS
 
 - Alternatives to SQS include [Kafka](https://kafka.apache.org/), [RabbitMQ](https://www.rabbitmq.com/), [ActiveMQ](http://activemq.apache.org/) and others.
 - Google Cloud Platform has Pub/Sub, and Azure has Azure Queue Service.
+- [SQS vs SNS](#sns-alternatives-and-lock-in)
 
 ### SQS Tips
 
@@ -1955,6 +1957,44 @@ SQS
 - 🔸 SQS cannot be directly used as a trigger for Lambda functions.
 
 
+SNS
+---------------------
+
+### SNS Basics
+
+- 📒  [_Homepage_](https://aws.amazon.com/sns/) ∙ [_Documentation_](https://aws.amazon.com/documentation/sns/) ∙ [_FAQ_](https://aws.amazon.com/sns/faqs/) ∙ [_Pricing_](https://aws.amazon.com/sns/pricing/)
+- **Amazon SNS** (Simple Notification Service) is a pub/sub based, highly scalable, and fully managed messaging service that can also be used for mobile notifications.
+- SNS can push the messages down to the subscribers via [SMS](http://docs.aws.amazon.com/sns/latest/dg/SMSMessages.html), [Email](http://docs.aws.amazon.com/sns/latest/dg/SubscribeTopic.html), [SQS](http://docs.aws.amazon.com/sns/latest/dg/SendMessageToSQS.html), and [HTTP/S](http://docs.aws.amazon.com/sns/latest/dg/SendMessageToHttp.html) transport protocols.
+- Producers publish messages to a SNS Topics, which can have many subscribers.
+- Each subscription has an associated [protocol](http://docs.aws.amazon.com/sns/latest/api/API_Subscribe.html), which is used to notify the subscriber.
+- A copy of the message is sent to each subscriber using the associated protocol.
+- SNS can also [invoke lambda functions](http://docs.aws.amazon.com/sns/latest/dg/sns-lambda.html).
+
+### SNS Alternatives and Lock-In
+
+- Popular alternatives to SNS are [Kafka](https://kafka.apache.org/), [Notification Hubs](https://azure.microsoft.com/en-us/services/notification-hubs/) on Azure, and [Pub/Sub](https://cloud.google.com/pubsub/docs/overview) on Google Cloud.
+- **SNS vs SQS:**
+    - Both SNS and SQS are highly scalable, fully managed messaging services provided by AWS.
+    - SQS supports a *pull* model, while SNS supports a *push* model. Consumers have to pull messages from an SQS Queue, while they're pushed the message from an SNS Topic.
+    - An SQS message is intended to be processed by only one subscriber, while SNS topics can have many subscribers.
+    - After processing, the SQS message is deleted from the queue by the subscriber to avoid being re-processed.
+    - An SNS message is *pushed* to all subscribers of the topic at the same time, and is not available for deletion at the topic.
+    - SNS supports multiple transport protocols of delivery of the messages to the subscribers, while SQS subscribers have to pull the messages off the queue over HTTPS.
+    - SNS can be used to trigger lambda functions, while SQS cannot.
+
+### SNS Tips
+
+- [Fan-out](http://docs.aws.amazon.com/sns/latest/dg/SNS_Scenarios.html) architecture can be achieved by having multiple subscribers for a topic. This is particularly useful when events have to be fanned out to multiple, isolated systems.
+- SNS topics can be used to power [webhooks](https://en.wikipedia.org/wiki/Webhook) with [backoff support](http://docs.aws.amazon.com/sns/latest/dg/DeliveryPolicies.html) to subscribers over HTTP/S.
+- [SQS queues](http://docs.aws.amazon.com/sns/latest/dg/SendMessageToSQS.html) can subscribe to SNS topics.
+- SNS is used to manage notifications for other AWS services like [Autoscaling Groups](http://docs.aws.amazon.com/autoscaling/latest/userguide/ASGettingNotifications.html)' notifications, [CloudWatch Alarms](http://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/US_SetupSNS.html), etc.
+- SNS is frequently used as “glue” between disparate systems— such as GitHub and AWS services.
+
+### SNS Gotchas and Limitations
+
+- 🔸 HTTP/S subscribers of SNS topics need to have public endpoints, as SNS does not support calling private endpoints (like those in a private subnet within a VPC).
+
+
 High Availability
 -----------------