Drools
What it is
Drools is a free, open-source rule engine for building expert systems with forward- and backward-chaining inference. Its decision model and notation (DMN) engine supports Conformance level 3, which covers the full standard, and statically checks decision tables for gaps, overlaps, and rule practices. The event-processing engine evaluates streams with temporal operators, sliding windows, and event expiration. Developers can define rules in DRL text files alongside Java code and version them, or use Excel decision tables, rule templates, and a Java executable model API. Rule units organize rules around typed input-data containers; Kogito can generate REST endpoints from them. Drools rules can also run from a BPMN business rule task in jBPM within the same service. The project reports a 99.9% pass rate on DMN TCK tests as of Apache KIE 10.2.0. Drools is available at no cost for API, Linux, and self-hosted use. Getting started requires an IDE, JDK 21 or 17, and Maven 3.9 or later. Apache KIE is in incubation and has not yet been fully endorsed by the ASF.
For and against
For
- Free Apache software with API, Linux, and self-hosted platforms.
- DMN engine supports Conformance level 3.
- Multiple options for authoring rules.
- DMN engine statically checks decision tables.
- Rule units can generate REST endpoints through Kogito.
Against
- Requires an IDE, JDK 21 or 17, and Maven 3.9+.
- Apache KIE has not yet been fully endorsed by the ASF.
- No free trial is listed.
The verdict
Drools combines rule authoring, DMN decision modeling, event processing, and jBPM integration in a free package. Consider the Java and Maven prerequisites and Apache KIE's incubation status before adopting it.
Who it suits
Drools suits developers building rule-driven expert systems, decision models, or event-stream processing. It is a fit for teams working with Java, DMN, or jBPM and able to meet the documented setup prerequisites.
Plans and prices
Apache Drools (free)
Apache software download and use at no cost · Java based
Read 5 Oct 2026 · apache.org
On the record
- Decision modeling
- Its DMN engine provides runtime support at Conformance level 3, covering every feature in the standard.kie.apache.org · read 5 Oct 2026
- Developer prerequisites
- The getting started guide lists an IDE, JDK 21 or 17, and Maven 3.9 or later as prerequisites.kie.apache.org · read 5 Oct 2026
- DMN test results
- The project states that as of Apache KIE 10.2.0, its DMN engine passes 99.9% of the DMN TCK tests.kie.apache.org · read 5 Oct 2026
- DMN validation
- The DMN engine statically analyzes decision tables to detect gaps and overlaps and check that table rules follow industry best practices.kie.apache.org · read 5 Oct 2026
- Event processing
- Its Complex Event Processing engine reasons over event streams using temporal operators, sliding windows, and event expiration.kie.apache.org · read 5 Oct 2026
- Integrations
- Drools rules can be called directly by a BPMN business rule task in jBPM within the same service.kie.apache.org · read 5 Oct 2026
- Project status
- Apache KIE is undergoing incubation, and the site says incubation means it has yet to be fully endorsed by the ASF.kie.apache.org · read 5 Oct 2026
- Purpose
- Drools is a forward-chaining and backward-chaining inference-based rule engine used to build expert systems.kie.apache.org · read 5 Oct 2026
- Rule authoring
- Rules can also be expressed through Excel decision tables, rule templates, or a Java executable model API.kie.apache.org · read 5 Oct 2026
- Rule language
- DRL lets developers define rules in plain text files alongside Java code and under version control.kie.apache.org · read 5 Oct 2026
- Rule units
- Rule units organize rules with typed containers for their input data, and Kogito can generate REST endpoints from them.kie.apache.org · read 5 Oct 2026
- Security
- The Apache Security Team coordinates handling of Apache project vulnerabilities and encourages private reports before public disclosure.apache.org · read 5 Oct 2026
- Support
- The project lists a users mailing list and Zulip chat as community contact channels.kie.apache.org · read 5 Oct 2026
