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Recently, I have been helping a client implement an event-sourced system. In the process, I put together a very simple demo app to illustrate how one could build such a system using Lambda and DynamoDB. The source code is available on GitHub here.
Before you go ahead and read all about the demo app, I want to give the client in question, InDebted, a quick shout out. They are disrupting the debt collection industry which has been riddled with malpractices and horror stories, and looking to protect the most vulnerable of us in society. They are also doing it by leveraging modern technologies and building with a serverless-first mentality. I have been working with the team for about 4 months and I have nothing but good things to say about them. If you’re looking for opportunities in the Sydney area, or are looking to relocate there, then please get in touch with Wagner. They’re looking for good people.
This demo app uses the banking example where a user can:
- create an account
- check his/her balance
- withdraw money
- credit the account
DynamoDB is the datastore.
Every time the account holder withdraws from or credits the account, I will record an event.
It means that when I need to work out the current balance of the account I will have to build up its current state from these events.
A common question people ask about event-sourced systems is “how do you avoiding reading lots of data on every request?”
The solution is to create snapshots from time to time. In this demo app, I ensure that there are regular snapshots of the current state. One snapshot for every 10 rows in the table, to be precise.
These snapshots allow me to limit the number of rows I need to fetch on every request. In this case, I have a constant cost of fetching 10 items every time.
Rebuilding the current state
To rebuild the current state, I find the most recent snapshot and apply the events since the snapshot was taken.
For example, given the below:
The most recent snapshot is
Version 22, with a
Balance of 60. There have been 3 events since then. So the current balance is 60–10–10+10 = 50.
Here’s what it looks like in code:
To protect against concurrent updates to the account, the
Version attribute is configured as the
RANGE key. Whenever I add an event to the DynamoDB table, I will check that the version doesn’t exist already.
To bring down the cold start as well as warmed performance of the endpoints. I applied a number of basic optimization:
- enable HTTP keep-alive for the AWS SDK
- don’t reference the full AWS SDK
- use webpack to bundle the functions
Streaming events to other consumers
It wasn’t included in the demo app, but you can also stream these events to other systems by:
a) letting other services subscribe to the DynamoDB table’s stream
b) create another Kinesis stream, and convert these DynamoDB
INSERT events into domain events such as
My personal preference would be option b. It lets other consumers work with domain events and decouples them from implementation details in your service.
From here, you can also connect the Kinesis stream to Kinesis Firehose to persist the data to S3 as the data lake. You can then use Athena to run complex, ad-hoc queries over ALL the historical data, or to generate daily reports, or to feed a BI dashboard hosted in QuickSight.
If you want to learn more about event-sourcing in the real-world (and at scale!), I recommend following this series by Rob Gruhl. Part 2 has some delightful patterns that you can use. You should also check out their Hello-Retail demo app.
Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.
Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.
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My 4-week Production-Ready Serverless online workshop is back!
This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.
We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.
If you enrol now you can also get 15% OFF with the promo code “yanprs15”.
Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.
Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!
Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!
Here is a complete list of all my posts on serverless and AWS Lambda. In the meantime, here are a few of my most popular blog posts.
- All you need to know about caching for serverless applications
- Lambda optimization tip – enable HTTP keep-alive
- You are wrong about serverless and vendor lock-in
- You are thinking about serverless costs all wrong
- Just how expensive is the full AWS SDK?
- Check-list for going live with API Gateway and Lambda
- How to choose the right API Gateway auth method
- CloudFormation protip: use !Sub instead of !Join
- AWS Lambda – should you have few monolithic functions or many single-purposed functions?
- Guys, we’re doing pagination wrong
- Top 10 Serverless framework best practices
- How to break the “senior engineer” career ceiling
- My advice to junior developers