farbodahm / dynamodb-optimistic-locking

A simple project to implement optimistic locking (using versioning) on DynamoDB.

Dynamodb Optimistic Locking

This is a simple project written in a weekend to implement optimistic locking (versioning model) on DynamoDB.

Problem statement

Assume we have a table called products on DynamoDB. One of the attributes is quantity showing the number of available products in the wharehouse.

Now assume different users order the same product concurrently, resulting in race condition. How would you fix this problem?

• Assume that we are not serializing the requests in one Goroutine (or in one thread, in generic); each request is handled by one Goroutine concurrently.
• Special edge case of this problem, will be scenario where we have only 1 product available in the wharehouse; but 2 users order that exactly at the same time.

Solution

By default, DynamoDB is not ACID complient. But with a correct combination of Transactional Write and Conditional Write we can achieve a similar behavour.

Architecture

One of the easiest ways to solve this problem is to use Optimistic Locking implemented with Conditional Write.

1. To achieve this, we can add a version attribute (with a default value of 1) to the table.
2. All of the requests, get the same item from DynamoDB.
3. Next to the change each request want to do with the item, they should also bump the version by +1. Ex: if the previous version was 1, it will become 2 locally.
4. All of the threads can now send the update request; with the following condition expression: version = oldVersion.
   • Only request for one of the threads will be accepted and every other request will be rejected as the version is updated and is not equal to the old value.

A picture is worth a thousand words:

How to run the project

Creating the table

Make sure you have Terraform installed. Then easily create the table using:

cd infra/
terraform init
terraform apply

Note: Make sure your IAM role has the correct permissions to create DynamoDB table and has read/write access on that table.

Running the application

First thing first, build the application:

go mod tidy
go build

Then to populate the table, you can easily provide the --populate-table so the script will populate the table with sample data for you:

./dynamodb-optimistic-locking --populate-table

Note: You can skip this part if you don't want to populate the table with sample data.

and to run the application:

./dynamodb-optimistic-locking --number-of-requests 100

This will simulate submitting 100 orders simultaneously, which if everything goes well, 99 of them should fail: