The solution captures requests to that image and stores visitors asynchronously in the plain text log for further analysis. The solution consists of two services (projects): one collects the data, other handles writing to the file.

Should be ASP.NET Core 6+ Minimal API. This API has only one endpoint GET /track that returns a transparent 1-pixel image in GIF format and collects the following information:

1. Referrer Header
2. User-Agent Header
3. Visitor IP Address

After the information has been collected, it sends it to the Storage Service.

Receives event and stores it in the append-only file. The path to the file is defined via appsettings.json with the defaults to /tmp/visits.log The format of the file is “date-time of the visit in ISO 8601 format in UTC | referrer | user-agent | ip“. The IP address is the only mandatory value. The rest can be substituted with null when empty.

Example:

• 2022-12-19T14:16:49.9605280Z|https://google.com|SomeUserAgent 1.2.3|192.168.1.1
• 2022-12-19T14:17:49.9605280Z|https://bing.com|AnotherUserAgent 4.5.6|10.0.0.1
• 2022-12-19T14:16:49.9605280Z|null|null|8.8.8.8

1. There could be multiple instances of Pixel Service sending events simultaneously, but the Storage Service is always one. No need to handle any concurrency issues other than writing to the file. The order of writes does not matter.
2. Pixel Service runs without any HTTP balancers / WAFs in front of it. Thereʼs no need to handle proxied headers.
3. Chose the communication protocol between Pixel and Store services that you think makes sense for that task. Pixel and Store services might be running inside different networks without direct access to each other, but both have access to any required third-party services. The connection string or any appsettings.json or get overridden by the connection parameters will be passed via the environment variables.
4. Keep things as simply as possible. If something is not clear from the task description, make an assumption and leave a comment in your code.
5. Extra task: write unit tests for both services.
6. Extra task: write separate Dockerfile for two services with build and runtime split into two stages.

The solution is divided in two distinct services: PixelService(Minimum API) and StorageService(Worker Service).

The architectural design approach used is Microsservice, where Pixel Service and Storage Service are two distinct software running completely decoupled from each other.

They are working in a Publisher (Pixel Service) and Consumer (Storage Service) solution.

Apache Kafka was the choice of technology to enabled them to communicate since it fits perfectly in this scenario.

That solution matches the need for being able to run multiple instances of Pixel Service simultaneously and only one Storage Service (Of course it can use several as well, if needed).

Both services are designed with Clean Architecture, SOLID and TDD in mind.

On the containerization part, both are running in a Linux Container. As well as the Apache Kafka instance.

When everything is running in Docker, the address to communicate within the docker networks changes, so both solutions are using environment variable to override the BootstrapServer address.

Both services are being tested with xUnit and Moq.