This is a demo application for a library book-borrowing application using Drools. This application is intended to show how you might go about having rules and a database interact without actually calling the database from the rules directly.
This is a Spring Boot application, with a REST HTTP interface and a h2 database. It's clearly not production ready, because H2 is not something you'd want to ever stick in production, but it's sufficient for the purpose of this demonstration.
This application uses Maven for dependency management. If you don't have Maven installed, we provide a local maven
wrapper instance (mvnw) that you can use instead.
This project uses Java 17. It uses Java records, which were added in JDK 14 as an experiment; you can down-rev this project to any version of the JDK that supports Records. If you wish to down-rev further (eg. to Java 11 or 8), you'll need to rewrite the records to be simple "POJO"-type classes with getters and setters.
The code in this repository is discussed at length in my blog post "Drools & Databases", where I talk about why it's a bad idea to interact with a database directly from inside of the rules.
To build and run this application from the commandline:
./mvnw spring-boot:run
The REST api will be available at http://localhost:8080/ .
To build the app and run the tests:
./mvnw package
If the app is already built, the tests can be run standalone:
./mvnw test
This is a tiny application which models a library book checkout/return. There are ten books in the system, and three registered users.
Our library patrons are:
| ID | Name | Description / Use Case |
|---|---|---|
| 1 | Jane Q. Public | Regular user, no books checked out. |
| 2 | Oliver Overdue | 1 book checked out, very overdue. |
| 3 | Ryan Reader | 3 books checked out, none overdue. |
With these patrons, we can try to do a variety of things with regards to our books. The following endpoints are available:
| Verb | Endpoint | Description |
|---|---|---|
| GET | /books |
Get all books owned by the library, along with checked in/checked out indicator. |
| GET | /books/{id} |
Get a specific book's details. Shows some basic info about the book (title, author, etc.) |
| POST | /books/{id} |
Query Params: action=BORROW or action=RETURN and patronId={userId}. Either check out or return a book. |
| GET | /users/{id} |
Get some information about the specified patron, including what books they've checked out. |
To borrow a book:
POST /books/{id}?patronId=1&action=BORROW
To return the book:
POST /books/{id}?patronId=1&action=RETURN
There is an in-memory H2 database instance which is tracking books and users.
We've modelled the following use cases in the rules:
- If a user has 3 books already checked out, no more books are allowed to be checked out. Any attempts to check out additional books should be denied.
- If a user has a book checked out for more than 7 days, that book is overdue. Any attempts to check out additional books should be denied.
- If a user has no overdue books, and fewer than 3 books checked out, they should be allowed to check out additional books.
- If an unrecognized user attempts to check out a book, they should be denied. Only the currently registered patrons are allowed to check out books.
You can find the rules at src/main/resources/rules/library-books.drl.
There is minimal error handling for complex use cases -- for example, if user 1 has checked out a book, nothing is stopping user 2 from checking out the same book. On the one hand, it's technically a logic flaw ... but at the same time it's technically modelling a use case where a book somehow made its way back onto the shelf without being properly scanned and accounted for.
To see where we invoke the rules, refer to the LibraryBookService class.
- Maven 3+
- Spring-Boot BOM 2.5.5 (Data-JPA, Web, Actuator)
- Drools 7.59.0.Final (Classic, with mvel)
- JDK 17
This project was written, built, and tested with OpenJDK. No guarantees are made about compatibility with Oracle's pay-to-play version of the JDK, but there's no reason it shouldn't work there as well.