Update Interchange screenshots and add note on implementation

projects/interchange/index.md

@@ -14,3 +14,21 @@ GraphQL, with Redis as a primary storage mechanism.
 [Join the TestFlight](https://testflight.apple.com/join/HbuHfUgW){: .secondary-action }
 [Website](https://interchange.bchen.dev/){: .secondary-action }
 {: .horizontal-wrapper }
+
+## Design and implementation
+
+The server is built using Node.js. I designed a basic GraphQL schema and
+used [Apollo's GraphQL implementation](https://www.apollographql.com/docs/apollo-server/getting-started)
+to bring it to life.
+
+In GraphQL, data is represented using a schema, which is then fulfilled with
+resolvers defined in code. Each resolver function gets some information like the
+parent object, arguments, and a server-defined shared context. Seeing the dependency injection
+that you could do with this context, I decided on an object-oriented approach consisting
+of **loaders**, **repositories**, and **systems**. Loaders load data from an external source,
+repositories store data, and systems are composed from a set of both loaders and repositories.
+
+I then applied an event-driven pattern to implement the iOS notification sender, listening
+to updates on the shuttle repository. I would eventually reuse this pattern to implement
+**a custom ETA system** that could run independently of an external data source.
+