SNS vs SQS: What are the Key Differences?

AWS offers two powerful messaging services: Simple Notification Service (SNS) and Simple Queue Service (SQS). SNS excels at broadcasting messages to multiple subscribers, making it ideal for real-time notifications and fan-out scenarios. On the other hand, SQS is designed for processing messages sequentially or in parallel, making it perfect for decoupling systems, handling message queues, and asynchronous processing.

This blog post will explore the key differences between SNS vs SQS and help you determine which one best suits your specific needs.

Introduction to SNS

What it is: SNS , is a fully managed pub-sub messaging service. It allows you to send messages to a large number of subscribers efficiently. Think of it like a broadcast system.

Key Features

• Integration with various endpoints: Supports a wide range of endpoints for message delivery.
• Fan-out messaging: Delivers the same message to multiple subscribers simultaneously.
• Real-time notifications: Enables rapid delivery of messages to various endpoints (e.g., mobile devices, email addresses, other AWS services).

Introduction to SQS

What it is: Simple Queue Service (SQS) is a fully managed message queuing service. Imagine it like a virtual mailbox where messages are stored temporarily before being processed by one or more consumers.

Key Features

• Decoupling: Enables different parts of an application to operate independently, improving system flexibility.
• Asynchronous processing: Allows messages to be processed at a later time, improving system responsiveness.
• Reliable message delivery: Ensures messages are not lost, even if the receiver is temporarily unavailable.
• Scalability: Handles fluctuating message volumes without performance degradation.

Understanding Critical Differences in SNS vs SQS

In this section, you will discuss some of the critical primary differences of SNS vs SQS. Moreover, you will explore:

• Primary Differences
• Tabular Comparison
• Use Cases
• Sample Cooperative Scenarios

Primary Differences

There are some primary differences between SQS and SNS, owing to the way they are designed and the purpose they serve. 

•  SQS does not push messages to the consumers, instead, consumers have to poll the queue, and as soon as one of them receives a message, the message is out of the queue and no other consumer can access it. This polling inevitably introduces a certain latent delay in message delivery. 
• SNS pushes the messages to all its subscribers as soon as it receives it, hence there is no latency and you can easily add subscribers down the line. 
•  SQS is mainly used to decouple applications or integrate applications.
• SNS is used to broadcast messages and it’s up to the receivers how they interpret and process those messages.

Tabular Comparison 

	SQS	SNS
Paradigm	Client Pull	Server Push
Entity Type	Queue, FIFO	Topic, Broadcast
Persistence	Configurable persistence duration	No persistence
Consumers	Identical consumers	Multiple types

Use Cases

Lets now have a look at some sample application scenarios:

1. SQS can be used in scenarios where the order of events ( and the order of processing them) are important. Also, in cases where multiple similar processors (actors) are present to get hold of a message and process it, at a given point in time any one of these processors gets to receive and process the message.

E.g. Processing multiple commands in the right order. 

A user can fire successive commands on a system and there are many threads that can process them. An SQS can ensure that user-entered commands are executed in the right order. Moreover, it can ensure that the output of the previous command can be used as input for the next. 

2. SNS can be used in scenarios where multiple subscribers receive the same message and process it according to their responsibilities/capabilities simultaneously. 

E.g. A food ordering App can generate order confirmations message(s) which can then be distributed via SNS to the pickup and delivery boy (on his mobile), the Kitchen manager (on his Tab), to the Bookkeeper(on his Desktop) and to the BI and Analytics tool (via API or database call). These subscribers can then move ahead and fulfil their duties to achieve the final goal. 

Sample Cooperative Scenarios 

SQS and SNS can be coupled to work together, making this combination very useful in certain scenarios. A typical example would be a Fanout scenario which requires asynchronous processing of messages. When a message is published to SNS, it can distribute it to multiple SQS queues in parallel. 

E.g. A system that takes bulk orders from its clients and distributes them for further action. 

A large manufacturing company can have multiple SQS queues, one for each department, subscribed to a single SNS topic. A bulk order of 10,000 units is received and it is sent to the SNS to be broadcasted. All departments will receive identical notifications for the new order, via their respective SQS. 

• Next, the procurement department can proceed to ensure that the raw materials are available at the right time. 
• The personnel department receives the same order, fetches the information relevant to it and ensures that the worker schedules are adjusted to making the labour and machines available for manufacturing. 
• The manufacturing department can make the detailed schedules and plans. 
• The Finance department can make sure that the flow of liquidity is maintained to smoothly finish the above processes. 

When to Use AWS SNS vs SQS?

AWS SNS (Simple Notification Service) and SQS (Simple Queue Service) are both messaging services offered by Amazon Web Services, but they serve different purposes. Here’s a breakdown of when to use each:

Use AWS SNS when:

• Broadcasting messages: You need to send the same message to multiple subscribers (e.g., mobile devices, email addresses, other AWS services).
• Real-time notifications: You require immediate delivery of messages to subscribers.
• Fan-out scenarios: You need to distribute messages to a large number of recipients efficiently.

Use AWS SQS when:

• Asynchronous processing: You want to defer the processing of messages to improve system responsiveness and scalability.
• Processing messages sequentially or in parallel: You need to ensure that messages are processed in a specific order or by multiple consumers independently.
• Decoupling systems: You want to decouple different parts of your application, allowing them to operate independently.
• Handling message queues: You need to store and process messages reliably, even during periods of high traffic.

Conclusion

To summarize, we have discussed the differences between SNS and SQS and elaborated on the scenarios where they can be put to good use, individually and together. 

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