What is the difference between Amazon SNS and Amazon SQS?

Question

When would I use SNS versus SQS, and why are they always coupled together?

Solution 1

SNS is a distributed publish-subscribe system. Messages are pushed to subscribers as and when they are sent by publishers to SNS.

SQS is distributed queuing system. Messages are not pushed to receivers. Receivers have to poll or pull messages from SQS. Messages can't be received by multiple receivers at the same time. Any one receiver can receive a message, process and delete it. Other receivers do not receive the same message later. Polling inherently introduces some latency in message delivery in SQS unlike SNS where messages are immediately pushed to subscribers. SNS supports several end points such as email, SMS, HTTP end point and SQS. If you want unknown number and type of subscribers to receive messages, you need SNS.

You don't have to couple SNS and SQS always. You can have SNS send messages to email, SMS or HTTP end point apart from SQS. There are advantages to coupling SNS with SQS. You may not want an external service to make connections to your hosts (a firewall may block all incoming connections to your host from outside).

Your end point may just die because of heavy volume of messages. Email and SMS maybe not your choice of processing messages quickly. By coupling SNS with SQS, you can receive messages at your pace. It allows clients to be offline, tolerant to network and host failures. You also achieve guaranteed delivery. If you configure SNS to send messages to an HTTP end point or email or SMS, several failures to send message may result in messages being dropped.

SQS is mainly used to decouple applications or integrate applications. Messages can be stored in SQS for a short duration of time (maximum 14 days). SNS distributes several copies of messages to several subscribers. For example, let’s say you want to replicate data generated by an application to several storage systems. You could use SNS and send this data to multiple subscribers, each replicating the messages it receives to different storage systems (S3, hard disk on your host, database, etc.).

Solution 2

Here's a comparison of the two:

Entity Type

• SQS: Queue (Similar to JMS)
• SNS: Topic (Pub/Sub system)

Message consumption

• SQS: Pull Mechanism - Consumers poll and pull messages from SQS
• SNS: Push Mechanism - SNS Pushes messages to consumers

Use Case

• SQS: Decoupling two applications and allowing parallel asynchronous processing
• SNS: Fanout - Processing the same message in multiple ways

Persistence

• SQS: Messages are persisted for some (configurable) duration if no consumer is available (maximum two weeks), so the consumer does not have to be up when messages are added to queue.
• SNS: No persistence. Whichever consumer is present at the time of message arrival gets the message and the message is deleted. If no consumers are available then the message is lost after a few retries.

Consumer Type

• SQS: All the consumers are typically identical and hence process the messages in the exact same way (each message is processed once by one consumer, though in rare cases messages may be resent)
• SNS: The consumers might process the messages in different ways

Sample applications

• SQS: Jobs framework: The Jobs are submitted to SQS and the consumers at the other end can process the jobs asynchronously. If the job frequency increases, the number of consumers can simply be increased to achieve better throughput.
• SNS: Image processing. If someone uploads an image to S3 then watermark that image, create a thumbnail and also send a Thank You email. In that case S3 can publish notifications to an SNS topic with three consumers listening to it. The first one watermarks the image, the second one creates a thumbnail and the third one sends a Thank You email. All of them receive the same message (image URL) and do their processing in parallel.

Solution 3

You can see SNS as a traditional topic which you can have multiple Subscribers. You can have heterogeneous subscribers for one given SNS topic, including Lambda and SQS, for example. You can also send SMS messages or even e-mails out of the box using SNS. One thing to consider in SNS is only one message (notification) is received at once, so you cannot take advantage from batching.

SQS, on the other hand, is nothing but a queue, where you store messages and subscribe one consumer (yes, you can have N consumers to one SQS queue, but it would get messy very quickly and way harder to manage considering all consumers would need to read the message at least once, so one is better off with SNS combined with SQS for this use case, where SNS would push notifications to N SQS queues and every queue would have one subscriber, only) to process these messages. As of Jun 28, 2018, AWS Supports Lambda Triggers for SQS, meaning you don't have to poll for messages any more.

Furthermore, you can configure a DLQ on your source SQS queue to send messages to in case of failure. In case of success, messages are automatically deleted (this is another great improvement), so you don't have to worry about the already processed messages being read again in case you forgot to delete them manually. I suggest taking a look at Lambda Retry Behaviour to better understand how it works.

One great benefit of using SQS is that it enables batch processing. Each batch can contain up to 10 messages, so if 100 messages arrive at once in your SQS queue, then 10 Lambda functions will spin up (considering the default auto-scaling behaviour for Lambda) and they'll process these 100 messages (keep in mind this is the happy path as in practice, a few more Lambda functions could spin up reading less than the 10 messages in the batch, but you get the idea). If you posted these same 100 messages to SNS, however, 100 Lambda functions would spin up, unnecessarily increasing costs and using up your Lambda concurrency.

However, if you are still running traditional servers (like EC2 instances), you will still need to poll for messages and manage them manually.

You also have FIFO SQS queues, which guarantee the delivery order of the messages. SQS FIFO is also supported as an event source for Lambda as of November 2019

Even though there's some overlap in their use cases, both SQS and SNS have their own spotlight.

Use SNS if:

• multiple subscribers is a requirement
• sending SMS/E-mail out of the box is handy

Use SQS if:

• only one subscriber is needed
• batching is important

Solution 4

AWS SNS is a publisher subscriber network, where subscribers can subscribe to topics and will receive messages whenever a publisher publishes to that topic.

AWS SQS is a queue service, which stores messages in a queue. SQS cannot deliver any messages, where an external service (lambda, EC2, etc.) is needed to poll SQS and grab messages from SQS.

SNS and SQS can be used together for multiple reasons.

1. There may be different kinds of subscribers where some need the immediate delivery of messages, where some would require the message to persist, for later usage via polling. See this link.

2. The "Fanout Pattern." This is for the asynchronous processing of messages. When a message is published to SNS, it can distribute it to multiple SQS queues in parallel. This can be great when loading thumbnails in an application in parallel, when images are being published. See this link.

3. Persistent storage. When a service that is going to process a message is not reliable. In a case like this, if SNS pushes a notification to a Service, and that service is unavailable, then the notification will be lost. Therefore we can use SQS as a persistent storage and then process it afterwards.

Solution 5

From the AWS documentation:

Amazon SNS allows applications to send time-critical messages to multiple subscribers through a “push” mechanism, eliminating the need to periodically check or “poll” for updates.

Amazon SQS is a message queue service used by distributed applications to exchange messages through a polling model, and can be used to decouple sending and receiving components—without requiring each component to be concurrently available.

Fanout to Amazon SQS queues

View more solutions

Comments

[Nick Ginanto]

so basically, to implement something like push notification messages, it is recommended to use SNS and SQS so the pushes with sns will be queued until the user is only to retrieve them from the queue? Is it possible to create a queue per user?

[Srikanth]

Yeah. You can have as many subscribers as you want for SNS. You can have notifications sent to multiple queues.

[John]

Hi sorry, i see this question is old but i am wondering about SQS does it know and store offline messages? Because APNS do not store offline messages, only the newest message. Would it know when the IOS devices are offline and stores the offline messages straight away? And send it out later when the devices are back online?

[Srikanth]

@John SQS does store messages until someone takes them out of the queue (and delete of course) or the queue retention time expires. Queue retention time can be configured. There is a limit to it, which can be found in documentation. APNS has nothing to do with SQS. SQS is not for push notifications to mobile devices.

[Trenton]

@NickGinanto Queue per user probably isn't what you want. You'd probably want one queue for each service that then handles user-specific messages. This diagram may help: aws.amazon.com/blogs/aws/…

[Kamal]

So lets say I am publishing messages to an SNS, and I have some web service running on some host which is supposed to receive the service. If the service fails for whatever reason, when the SNS tries to push messages to the service, will they just be lost? So if I make the SNS push to an SQS, and the service polls the SQS, if the service goes down, then the messages from the SNS are still stored in the SQS, right?

[Srikanth]

@Kamal If SNS fails to deliver message, it will retry according to the retry policy configured on the topic. I would look in their documentation on how to setup retry policies. If you make SNS push to SQS, messages will stay in your queue until retention period expires. You can configure retention period on the queue as well. Check their documentation.

[Arpit Solanki]

if no consumers are available then there is a retry mechanism, even default value is 10 retries.

[Andy Dufresne]

Nice detailed post. We have different messages for different consumers - What should we do - use SNS and define different topics or use SQS and define different queues? A topic/queue could though have one or more consumers.

[Arafat Nalkhande]

If your requirement is that your tooic/queue should have more than one consumers I assume you are saying that the same message should be broadcasted to multiple consumers... And if this assumption of mine is correct then using SNS is the only option available for you

[Thales Minussi]

SQS cannot send a message to many systems as it does not fan out the messages. Yes, many pollers can pull messages from it, but if one the consumers deletes the message then other subscribers won't be able to consume the same message again. SNS is preferred over SQS if you want to achieve a fan out pattern. Also, if the visibilityTimeout is set, than no other systems will be able to consume the message once it's being processed by other system.

[Krunal Barot]

A common pattern is to use SNS to publish messages to Amazon SQS queues to reliably send messages to one or many system components asynchronously. Reference from aws.amazon.com/sns/faqs

[Thales Minussi]

If that's what you meant (SNS -> multiple SQS queues) then please edit your answer and I will happily remove my downvote. The way you put it, it sounds like SQS can fan out.

[Krunal Barot]

Yes.. thats where the confusion was. I have edited it .. Thanks :)

[cyberwombat]

It should probably be noted that as of mid 2018 SQS can trigger lambdas and therefore is more akin to a pubsub in that case.

[Vadorequest]

@cyberwombat Agree. See read.acloud.guru/… for reference

[Yajairo87]

This is by far, the best and simple explanation I came across about the two AWS services.

[nad]

I do not think "SQS : All the consumers are supposed to be identical and hence process the messages in exact same way" this is right. I have used SQS where two different AWS services are picking up from SQS queue and processing the message in their own way (different application logic in those different services). Am I missing something?

[Arafat Nalkhande]

@nad I will need to understand your use case but to me it makes no sense that 2 SQS consumers are processing messages in non-identical ways. That is a use case for SNS

[0xc0de]

What puts that restriction of being identical on SQS consumers? You can very well have different consumers for a single message. One consumer can process it (say email) one way and not remove it from the SQS, another consumer processes differently (store in DB), while another can do a third thing (confirm email was sent and the message is stored in DB). Just because SNS is documented to be more suitable for 'fan-out' types of tasks doesn't mean the SQS consumers would have that restriction.

[Arafat Nalkhande]

Yes, but then who deletes the message. Wouldnt the consumer 1 and connsumer 2 and consumer 3 all keep getting the same message again and again

[rogerdpack]

SQS can "poll waiting for a message to come in" (which is similar to a push). But it only processes that message once so it's not a broadcast push, FWIW :)

[Andriy]

Your comment is the only one clearly explaining how SNS and SQS can be combined. Thanks!

[Peter Mortensen]

This is a start, but the question was about the differences, etc.: "When would I use SNS versus SQS, and why are they always coupled together?" Perhaps address that more directly? You can edit your answer (without "Edit:", "Update:", or similar - the answer should appear as if it was written today).

[buzztr]

I am a newbie to AWS, but has 3. changed since the answer was written? [docs.aws.amazon.com/sns/latest/dg/… Delivery Retries) Amazon SNS defines a delivery policy for each delivery protocol. The delivery policy defines how Amazon SNS retries the delivery of messages when server-side errors occur (when the system that hosts the subscribed endpoint becomes unavailable). When the delivery policy is exhausted, Amazon SNS stops retrying the delivery and discards the message—unless a dead-letter queue is attached to the subscription.

[hyankov]

In SQS a message is Pushed into the queue, which is still a single point of failure. Change my mind.

[Hans Hermans]

@buzztr 3. hasn't changed; it refers to the consumer of the SQS becoming unavailable. That documentation you cite describes a new feature of SNS in which failure to deliver a message to the consumer(s) (in this case, the SQS) after exhausting your retries results in SNS putting that message put onto a dedicated SQS queue for to handle transmission error. This means instead of the message being lost entirely, you have it saved and can re-attempt transmission through the SNS topic at a later date.

[trademark]

medium.com/awesome-cloud/… This article seems to have similar content to this answer plus some additional details.

[Arafat Nalkhande]

Thanks for pointing that out... But that medium.com article was published on May 25 2019 and this answer was given almost a year earlier on June 14 2018

[benjimin]

Distributing tasks among many consumers is a primary intended use case for SQS. (Regardless of whether consumers retrieve 1 message or 10 at a time, it is up to the application to control how many consumers exist concurrently.) It differs from SNS because SNS tries to send every message to all subscribers, whereas SQS tries to partition the messages among the consumers.