Challenges of External Systems

In this lesson, we’ll review the driving force behind the hexagonal architecture approach—the difficulty of working with external systems. Dependencies on external systems cause problems in development. The solution leads to a nice design approach.

Handling external systems

Let’s look at a simple way of handling external systems. The task of our user is to pull a report of this month’s sales from a database. We’ll write one piece of code that does exactly that. The software design looks like this:

In this design, we have sales data stored in a database in the usual way. We write some code to pull the report on behalf of our user. It’s a single piece of code that does the whole job as a single step. It will connect to the database, send a query, receive the results, do some processing, and format the results ready for the user to read.

On the plus side, we know this style of coding works. It will achieve its aim of providing that sales report to the user. On the downside, the code combines three different responsibilities, accessing a database, performing logic, and formatting a report. It might mix up SQL statements to the database with html5 tags to make a formatted report. As we saw in a previous section, this can make future code changes in one area ripple out and impact the other areas. Ideally, that should not happen. But the real challenge is writing a test for this one piece of code. We’ll need to parse and understand whatever format we send the report to the user in. We’ll also need to work directly with that database.

Let’s review some ...