OSI-licensed open source
An enterprise service bus routes and transforms messages between applications so each does not need point-to-point integrations. Classic ESBs have largely given way to lighter integration frameworks and brokers, but the patterns (routing, mediation, protocol bridging) are the same.
The open-source ESB landscape has also shrunk: Apache ServiceMix was archived on GitHub in July 2026 and the Mule runtime repository in September 2026, so neither is listed. What remains active are Camel, WSO2 Micro Integrator, Spring Integration, Karaf, ActiveMQ Artemis and NiFi, each OSI-licensed, checked on 2026-11-07. Because few pure ESBs remain, this list is broadened to integration frameworks and runtimes.
Tools, licences and pricing checked against vendor and repository pages on November 7, 2026.
| Tool | Best for | Pricing | Licence |
|---|---|---|---|
| Apache Camel | Developer-driven integration with many connectors | Free, open source | Apache-2.0 |
| WSO2 Micro Integrator | Low-code ESB deployments | Free, open source (unsupported community distribution); production support via paid WSO2 Subscription, see vendor pricing. | Apache-2.0 |
| Spring Integration | Spring teams adding messaging and adapters | Free, open source | Apache-2.0 |
| Apache Karaf | Hosting integration apps in a managed runtime | Free, open source | Apache-2.0 |
| Apache ActiveMQ Artemis | Messaging backbone for an integration layer | Free, open source | Apache-2.0 |
| Apache NiFi | Visual dataflow and file/event routing | Free, open source | Apache-2.0 |
Best for: Developer-driven integration with many connectors · Free, open source
Apache Camel is an open-source integration framework supporting the Enterprise Integration Patterns, with 350+ components. Routes can be written in Java, YAML or XML and run on Spring Boot, Quarkus or standalone.
Best for: Low-code ESB deployments · Free, open source (unsupported community distribution); production support via paid WSO2 Subscription, see vendor pricing.
WSO2 Micro Integrator is an integration engine with a low-code graphical designer that can be deployed as microservices or a centralised ESB, handling routing, transformation and orchestration.
Best for: Spring teams adding messaging and adapters · Free, open source
Spring Integration applies the Enterprise Integration Patterns to Spring applications, with channels, adapters for REST, FTP, JMS, RabbitMQ and more, and a Java DSL.
Best for: Hosting integration apps in a managed runtime · Free, open source
Apache Karaf is an enterprise modulith runtime with a shell, hot deployment, dynamic configuration and remote management over SSH and JMX. Karaf Cellar clusters instances.
Best for: Messaging backbone for an integration layer · Free, open source
Apache Artemis is a multi-protocol message broker supporting AMQP 1.0, MQTT 3.1.1, MQTT 5 and STOMP.
Best for: Visual dataflow and file/event routing · Free, open source
Apache NiFi is a data processing and distribution system with a browser-based UI, data provenance tracking and back pressure control.
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| If you need… | Start with |
|---|---|
| Code-first routing with many connectors | Apache Camel |
| Low-code ESB with a designer | WSO2 Micro Integrator |
| Integration inside Spring apps | Spring Integration |
| A managed runtime with shell and clustering | Apache Karaf |
| Reliable multi-protocol messaging | Apache ActiveMQ Artemis |
| Visual dataflows with lineage | Apache NiFi |
Quick answers to common questions
An enterprise service bus is middleware that routes, transforms and mediates messages between applications.
The mulesoft/mule GitHub repository was archived in September 2026 and its licence was not shown on the repository page we read, so we do not list it.
Its GitHub repository was archived by the owner on 30 July 2026, so we left it out.
Many teams now use frameworks like Camel with a message broker instead of a central bus; the patterns still apply.
List it on TechLogHub and we'll review it for this guide.
