System Design Example

A photo sharing application at the scale of Instagram is a useful example, because it has many services and two very different kinds of request.

The Gateway

Every request from every client passes through the gateway: phone apps, web browsers and partner integrations. It does six things.

• Routing: sends each request to the service that owns it.
• Aggregation: calls several services for one screen and combines the replies.
• Authentication and authorization: checks who the user is and what they are allowed to do.