What is the difference between Amazon SNS and Amazon SQS? - amazon-web-services

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

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.).

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.

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

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.
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.
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.
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.

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

Following are the major differences between the main messaging technologies on AWS (SQS, SNS, +EventBridge). In order to choose a particular AWS service, we should know the functionalities a service provides as well as its comparison with other services.
The below diagram summarizes the main similarities as well as differences between this service.

In simple terms,
SNS - sends messages to the subscriber using push mechanism and no need of pull.
SQS - it 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.
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 Amazon SNS FAQs.

One reason for coupling SQS and SNS would be for data processing pipelines.
Let's say you are generating three kinds of product, and that products B & C are both derived from the same intermediate product A. For each kind of product (i.e., for each segment of the pipeline) you set up:
a compute resource (maybe a lambda function, or a cluster of virtual machines, or an autoscaling kubernetes job) to generate the product.
a queue (describing units of work that need to be performed) to partition the work across the compute resource (so that each unit of work is processed exactly once, but separate units of work can be processed separately in parallel and asynchronously with each other).
a news feed (announcing outputs that have been produced).
Then arrange so that the input queues for B & C are both subscribing to the output announcements of A.
This makes the pipeline modular on the level of infrastructure. Rather than having a monolithic server application that generates all three products together, different stages of the pipeline can utilise different hardware resources (for example, perhaps stage B is very memory intensive, but the two other stages can be performed with cheaper hardware/services). This also makes it easier to iterate on the development of one pipeline segment without disrupting delivery of the other products.

There are some key distinctions between SNS and SQS:
SNS supports A2A and A2P communication, while SQS supports only A2A
communication.
SNS is a pub/sub system, while SQS is a queuing system. You'd
typically use SNS to send the same message to multiple consumers via
topics. In comparison, in most scenarios, each message in an SQS
queue is processed by only one consumer. With SQS, messages are
delivered through a long polling (pull) mechanism, while SNS uses a
push mechanism to immediately deliver messages to subscribed
endpoints.
SNS is typically used for applications that need real time
notifications, while SQS is more suited for message processing use
cases.
SNS does not persist messages - it delivers them to subscribers that
are present, and then deletes them. In comparison, SQS can persist
messages (from 1 minute to 14 days).
Individually, Amazon SQS and SNS are used for different use cases. You can, however, use them together in some scenarios.

Related

How to send notification to multiple sns topics at once

I have 9000 AWS SNS topics with more than 1M subscribers in each topic. At the moment I am lopping to each topic to send a push message which is consuming lot of my system resources. Is there a way to send message to all the topics at once? what is the best approach to handle the scenario?
It is not possible to subscribe Amazon SNS queues to an Amazon SNS queue, so there is no out-of-the-box method for sending one message to multiple queues.
I would recommend creating an AWS Lambda function that will:
Retrieve a list of all relevant queues (based on tag?)
Loops through and sends a message to each queue
Thus, you would just trigger the Lambda function with one message and it would go to all other queues. It would not "consume system resources", but it is charged based upon run duration. Lambda functions can run for a maximum of 15 minutes, so as long as it sends 10+ messages per minute, it can send to 9000 topics.
Depending upon your use-case, you might also consider using Amazon Pinpoint:
Amazon Pinpoint is an AWS service that you can use to engage with your customers across multiple messaging channels. You can use Amazon Pinpoint to send push notifications, emails, SMS text messages, and voice messages.

Simulating message persistence in SNS using SQS

We are evaluating SNS for our messaging requirements to integrate multiple applications. we have a single producer that publishes messages to multiple topics on SNS. Each topic has 2-5 subscribers. In event of subscriber failures (down for maintenance) I have a few questions on the recommended strategy of using SQS queues per consumer
Is it possible to configure SNS to push to SQS only in event of failure in delivering the message to a subscriber? Dumping all the messages in SQS queue creates a problem for the consumer to analyze all messages in the queue when it restarts.
In event of subscriber failure, it can read messages from SQS queue on restart but how would it know that it missed messages from SNS when it was overloaded?
Any suggestions on handling subscriber failures are welcome.
Thanks!
No, it is not possible to "configure SNS to push to SQS only in event of failure".
Rather than trying to recover a message after a failure, you can configure the Amazon SNS retry policies.
From Setting Amazon SNS Delivery Retry Policies for HTTP/HTTPS Endpoints:
You can use delivery policies to control not only the total number of retries, but also the time delay between each retry. You can specify up to 100 total retries distributed among four discrete phases. The maximum lifetime of a message in the system is one hour. This one hour limit cannot be extended by a delivery policy.
So, you don't need to worry as long as the destination is back online within an hour.
If it is likely to be offline for more than an hour, you will need to find a way to store and "replay" the messages, possibly by inspecting CloudWatch Logs.
Or, here's another idea...
Push initially to SQS. Have an AWS Lambda function triggered by SQS. The Lambda function can do the 'push' that would normally be done by SNS. If it fails, then the standard SQS invisibility process will retry it later, eventually going to a Dead Letter Queue.

AWS SQS dump/restore

Is it possible to dump a SQS queue to open space for "urgent" messages and then restore the dump to keep SQS queue on track?
I am not talking about aws cli commands but any possibility of doing it.
Of course I could open a new SQS and change the application to look after that new queue, but it would have some implications.
No it's not possible. The design pattern I've seen AWS recommend when you want to have "high priority" messages is this:
Create 2 queues, one for high-priority messages and one for regular-priority messages.
Have your application always scan the high-priority queue first to check for new messages.
If you don't receive any messages from the high-priority queue, scan the regular-priority queue for messages.
AWS SQS does not provide a priority based queue at the moment. But you can do certain implementations and build a priority queue for your application (consumer). Following are some implementations you can use.
1) As #Markb mentioned, you can create two SQSs, where one is for high priority messages and other is for regular messages. Make sure application polls the high-priority SQS first and then move on to the regular SQS.
2) If using a single SQS, have a few worker threads on the application side, that will collect all the messages from SQS, and process them to see which ones have a higher priority. Take them and process first.
3) Use a combination of SQS and SNS. Send all the regular messages into the SQS. If there are high priority messages, send them to SNS to direct them to a specific endpoint in your application. From you application side (consumer side), have and endpoint that listens to high priority messages coming from SNS, and then have aprocess that polls the SQS to retrieve the regular messages.

Read SQS queue from AWS Lambda

I have the following infrastructure:
I have an EC2 instance with a NodeJS+Express process listening on a port for messages (process 1). Every time the process receives a message it sends it to an SQS queue. Then I have another process in the same machine reading the queue using long polling (process 2). When it finds a message in the queue it inserts the data in a MariaDB database sitting on an RDS instance.
(Just to clarify, messages are generated by users, they send a chunk of data which can contain arbitrary information to the endpoint where the process 1 is listening)
Now I want to put the process that reads the SQS (process 2) in a Lambda function so that the process that writes to the queue and the one that reads from the queue are completely independent. The problem is that I don't know if this is possible.
I know that Lambda function are invoked in response to an event, and the events supported at the moment are S3, SNS, SES, DynamoDB, Kinesis, Cognito, CloudWatch and Cloudformation but NOT SQS.
I was thinking in using SNS notifications to invoke the Lambda function so that every time a message is pushed to the queue, an SNS notification is fired and invokes the Lambda function but after playing a bit with it I've realised that is not possible to create an SNS notification from SQS, it's only possible to write SNS notifications to the queue.
Right now I'm a bit stuck because I don't know how to continue. I have the feeling that is not possible to create this infrastructure due to the current limitations in the AWS services. Is there another way to do what I want or am I in a dead-end?
Just to extend my question with some research I've made, this github repo shows how to read an SQS queu from a Lambda function but the lambda function works only if is fired from the command line:
https://github.com/robinjmurphy/sqs-to-lambda
In the readme, the author mentions the following:
Update: Lambda now supports SNS notifications as an event source,
which makes this hack entirely unneccessary for SNS notifcations. You
might still find it useful if you like the idea of using a Lambda
function to process jobs on an SQS queue.
But I think this doesn't solve my problem, an SNS notification can invoke the Lambda function but I don't see how I can create a notification when a message is received in the SQS queue.
Thanks
There are couple of Strategies which can be used to connect the dots, (A)Synchronously or Run-Sleep-Run to keep the data process flow between SNS, SQS, Lambda.
Strategy 1 : Have a Lambda function listen to SNS and process it in real time [Please note that an SQS Queue can subscribe to an SNS Topic - which would may be helpful for logging / auditing / retry handling]
Strategy 2 : Given that you are getting data sourced to SQS Queue. You can try with 2 Lambda Functions [Feeder & Worker].
Feeder would be scheduled lambda function whose job is to take items
from SQS (if any) and push it as an SNS topic (and continue doing it forever)
Worker would be linked to listen the SNS topic which would do the actual data processing
We can now use SQS messages to trigger AWS Lambda Functions. Moreover, no longer required to run a message polling service or create an SQS to SNS mapping.
Further details:
https://aws.amazon.com/blogs/aws/aws-lambda-adds-amazon-simple-queue-service-to-supported-event-sources/
AWS SQS is one of the oldest products of Amazon, which only supported polling (long and short) up until June 2018. As mentioned in this answer, AWS SQS now supports the feature of triggering lambda functions on new message arrival in SQS. A complete tutorial for this is provided in this document.
I used to tackle this problem using different mechanisms, and given below are some approaches you can use.
You can develop a simple polling application in Lambda, and use AWS CloudWatch to invoke it every 5 mins or so. You can make this near real-time by using CloudWatch events to invoke lambda with short downtimes. Use this tutorial or this tutorial for this purpose. (This could cost more on Lambdas)
You can consider that SQS is redundant if you don't need to persist the messages nor guarantee the order of delivery. You can use AWS SNS (Simple Notification Service) to directly invoke a lambda function and do whatever the processing required. Use this tutorial for this purpose. This will happen in real-time. But the main drawback is the number of lambdas that can be initiated per region at a given time. Please read this and understand the limitation before following this approach. Nevertheless AWS SNS Guarantees the order of delivery. Also SNS can directly call an HTTP endpoint and store the message in your DB.
I had a similar situation (and now have a working solution deploed). I have addressed it in a following manner:
i.e. publishing events to SNS; which then get fanned-out to Lambda and SQS.
NOTE: This is not applicable to the events that have to be processed in a certain order.
That there are some gotchas (w/ possible solutions) such as:
racing condition: lambda might get invoked before messages is deposited into the queue
distributed nature of SQS queue may lead to returning no messages even though there is a message note1.
The solution to both cases would be to do long-polling of SQS queue; but this does make your lambda bill more expensive.
note1
Short poll is the default behavior where a weighted random set of machines is sampled on a ReceiveMessage call. This means only the messages on the sampled machines are returned. If the number of messages in the queue is small (less than 1000), it is likely you will get fewer messages than you requested per ReceiveMessage call. If the number of messages in the queue is extremely small, you might not receive any messages in a particular ReceiveMessage response; in which case you should repeat the request.
http://docs.aws.amazon.com/AWSSimpleQueueService/latest/APIReference/API_ReceiveMessage.html
We had some similar requirements so we ended up building a library and open sourcing it to help with SQS to Lambda async. I'm not sure if this fills your particular set of requirements, but thought it might be worth a look: https://read.iopipe.com/sqs-lambda-teaming-up-92c4096be49c

using AWS SNS and Lambda - what's the right use case for an activity feed

I want to use an AWS lambda function to fan out and insert activity stream info to a firebase endpoint for every user.
Should I be using Kinesis, SQS or SNS to trigger the lambda function for this use case? The updates to the activity stream can be triggered from the server and clients should receive the update near real time (within 60 seconds or so).
I think I have a pretty good idea on what SQS is, and have used Kinesis in the past but not quite sure about SNS.
If we created an SNS topic for each user and then each follower subscribes to these topics with an AWS lambda function - would that work?
Does it make sense to programmatically create topics and subscriptions for every user and follow relationship respectively?
As usual, answer to such a question is mostly, 'it depends on your use-case'.
Kinesis vs SQS:
If your clients care about relative (timestamp-based, for e.g.) ordering between events, you'll almost certainly have to go with Kinesis. SQS is a best-effort FIFO queue, meaning events can arrive out of order and it would up to your client to manage relative ordering.
As far as latencies are concerned, I have seen that data ingested into Kinesis can become visible to its consumer in as less as 300 ms.
When can SNS be interesting to you?
(Even with SNS, you'd have to use SQS). If you use SNS, it will be easy to add a new application that can process your events. For example, if in future you decide to ingest all events into, say, an Elasticsearch to provide real-time analytics, all you'd have to do is add another SQS queue to your existing topic(s) and write a consumer.