Event driven programs use events to initiate succeeding steps in a process. For example, the completion of an upload job may then initiate an image processing job. This allows developers to create complex architectures by using the principle of decoupling. Decoupling is preferable for many workflows, as it allows each component to perform its tasks independently, which improves efficiency. Examples are ecommerce order processing, image processing, and other long running batch jobs.
Amazon Simple Storage Service (S3) is an object-based storage solution from Amazon Web Services (AWS) that allows you to store and retrieve any amount of data, at any scale. Amazon S3 Event Notifications provides users a mechanism for initiating events when certain actions take place inside an S3 bucket.
In this blog post, we will illustrate how you can use Amazon S3 Event Notifications in combination with a powerful suite of Amazon messaging services. This will allow you to implement an event driven architecture for a variety of common use cases.
Setting up Amazon S3 Event Notifications
We first must understand the types of events that can be initiated with Amazon S3 Event Notifications. Events can be initiated by uploading, modifying, deleting an object, or other actions. When an event is initiated, a payload is created containing the event metadata. This includes information about the object that initiated the event itself.
To enable notifications, you must first add a notification configuration that identifies the events you want Amazon S3 to publish. Specify the destinations where you want Amazon S3 to send the notifications. This configuration is stored in the notification subresource, which you can find under the Properties tab within your S3 bucket, see Figure 1.
An event notification can be initiated anytime an object is uploaded, modified, or deleted, depending on your configuration details. You can create multiple notification configurations for different scenarios, shown in Figure 2. For example, one configuration can handle new or modified objects, and another configuration can handle deletions. You can specify that events will only be initiated when objects contain a specific prefix, or following the restoration of an object. For a complete listing of all the configuration options and event types, read documentation on supported event types.
When all of the conditions in your configuration have been met, a new event will be initiated and sent to the destination you specify. An S3 event destination can be an AWS Lambda function, an Amazon Simple Queue Service (SQS) queue, or an Amazon Simple Notification Service (SNS) topic, see Figure 3.
Event driven design patterns
There are many common design patterns for building event driven programs with Amazon S3 Event Notifications. Once you have set up your notification configuration, the next step is to consume the event. The following describes a few typical architectures you might consider, depending on the needs of your application.
Synchronous and reliable point-to-point processing
One common use case for event driven processing, is when synchronous and reliable information is required. For example, a mobile application processes images uploaded by users and automatically tags the images with the detected objects using Artificial Intelligence/Machine Learning (AI/ML). From an architectural perspective (Figure 4), an image is uploaded to an S3 bucket, which generates an event notification. This initiates a Lambda function that sends the details of the uploaded image to Amazon Rekognition for tagging. Results from Amazon Rekognition could be further processed by the Lambda function and stored in a database like Amazon DynamoDB.
With this type of architecture, there is no contingency for dealing with multiple images arriving simultaneously in the S3 bucket. If this application sends too many requests to Lambda, events can start to pile up. This can cause a failure to process some of the images. To make our program more fault tolerant, adding an Amazon SQS queue would help, as shown in Figure 5.
Asynchronous and queued point-to-point processing
Architectures that require the processing of information in an asynchronous fashion can use this pattern. Building off the first example, a mobile application might provide a solution to allow end users to bulk upload thousands of images simultaneously. It can then use AWS Lambda to send the images to Amazon Rekognition for tagging.
By providing a queue-based asynchronous solution, the Lambda function can retrieve work from the SQS queue at its own pace. This allows it to control the processing flow by processing files sequentially without risk of being overloaded. This is especially useful if the application must handle incomplete or partial uploads when a connection is temporarily lost.
Currently, Amazon S3 Event Notifications only work with standard SQS queues, and first-in-first-out (FIFO) SQS queues are not supported. Read more about how to configure S3 event notification with an SQS queue as a destination. Your Lambda function in this architecture must be adjusted to handle the message payload arriving from SQS. This is because it will have a slightly different form than the original event notification body generated from S3.
Parallel processing with “Fan Out” architecture
To create a “fan out” style architecture where a single event is propagated to many destinations in parallel, SNS is combined with SQS. Configure your S3 event notification to use an SNS topic as its destination, as shown in Figure 6. You can then direct multiple subsequent processes to act on the same event. This is especially useful if you aim to do parallel processing on the same object in S3.
For example, if you wanted to process a source image into multiple target resolutions, you could create a Lambda function. The function will use the “fan-out” pattern to process all images at the same time, at each resolution. You could then subscribe an SQS queue to your SNS topics. This ensures that Event Notifications sent to SNS are verified as complete by SQS, once they’ve been processed by your Lambda function.
To extend the use case of image processing even further, you could create multiple SNS topics to handle different types of events from the same S3 bucket. As depicted in Figure 7, this architecture would allow your program to handle creations and updates differently than deletions. You could also process images differently based on their S3 prefix.
Adjust your Lambda code to handle messages making their way through SNS and SQS. Their payloads will be slightly different than the original S3 Event Notification payload.
In addition to application-to-application messaging, Amazon SNS provides application-to-person (A2P) communication (see Figure 8). Amazon SNS can send SMS text messages to mobile subscribers in over 100 countries. It can also send push notifications to Android and Apple devices and emails over SMTP. Using A2P, uploading an image to an Amazon S3 bucket can generate a notification to a group of users via their choice of Amazon SNS A2P platform.
In this blog post, we’ve shown you the basic design patterns for developing an event driven architecture using Amazon S3 Event Notifications. You can create many more complicated architecture patterns to suit your needs. By using Amazon SQS, Amazon SNS, and AWS Lambda, you can design an event driven program that is fault tolerant, scalable, and smartly decoupled. But don’t stop there! Consider expanding your program further by utilizing AWS Lambda destinations. Or combine parallel image processing with highly scalable A2P notifications, which will alert your users when a task is complete.
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