phase4

Overview

phase4 is a Java library for integrating AS4 message exchange into an existing application. It is best suited to developers building Peppol or eDelivery workflows who want an open-source component rather than a packaged service. Its flexibility comes with real engineering work, and missing AS4 capabilities may rule it out for some deployments.

Key features

Built-in Peppol and CEF eDelivery support gives teams a foundation for those protocols, while the project documentation covers additional profiles including BDEW, DBNAlliance, eDelivery2, EESPA, ENTSOG, EUCTP, and HR eDelivery. That breadth is useful when an integration must accommodate different profile requirements, but teams still need to incorporate the library into their own system.

Configuration can be supplied through application.properties, system properties, or environment variables. Keystores and truststores support JKS and PKCS12 formats, providing established options for certificate configuration. Java 17 is the minimum; Java 21 or later is recommended for performance, so older Java environments are not a fit.

The project includes automated unit and integration tests for many configurations, but that does not replace validation against a live remote endpoint. For Peppol testing with an existing service provider, the project guide requires an official Peppol AP certificate to test sending and receiving. Teams should account for that credential requirement in their end-to-end validation plans.

AS4 multi-hop and AS4 split and join are not implemented. Implementations that depend on either capability should choose a different approach; projects that do not need them can evaluate phase4 against their remaining protocol and integration requirements.

Pricing

The Open-source library plan costs nothing and is Apache 2.0 licensed. It includes Peppol access, API access, UBL structured formats, and on-premise deployment, but no tax validation. There is no software subscription charge, though teams must budget for the work of integrating and operating the library. The Apache 2 License permits use under its terms, and the maintainer can be contacted by email for commercial support.

Platforms

phase4 works on Windows, Linux, Mac, and AWS, with self-hosted deployment. It is a library rather than a standalone project, so teams must embed it in an existing system; the platform range does not make it a ready-to-run messaging service.

Who it's for

phase4 fits development teams that can build and maintain a Java integration, need AS4 messaging for Peppol or eDelivery profiles, and want to deploy on-premise. It is a weaker choice for organizations seeking a complete invoicing application, or teams unable to meet the Java 17 minimum and certificate requirements for Peppol endpoint testing.

Pros and cons

Pros

• Apache 2.0 licensing and a free plan avoid software fees.
• Built-in Peppol and CEF eDelivery support, alongside documented additional profiles, gives implementers options across several protocol contexts.
• Configuration through properties, system variables, or environment variables and JKS or PKCS12 stores supports common deployment setups.
• Automated tests cover many configurations, providing a basis for validation before remote-system testing.

Cons

• Manual integration means it takes development work to turn the library into a usable part of a system.
• Java 17 is required, with Java 21 or later recommended for performance.
• AS4 multi-hop and split and join are unsupported, which excludes projects that require those capabilities.
• Testing with an existing Peppol provider requires an official AP certificate.
• Tax validation is not included in the plan.

Alternatives

For a broader comparison, see Electronic Invoicing Software.

Choose Billit instead if you want a freemium service with Android, iOS, API, and web access; its free license covers Peppol e-invoices for sales invoices only and has limited functionality, while its listed paid plan is €690 a month.

Holodeck B2B is another free open-source Java option, under GNU GPL 3 and requiring Java 11 or later; consider it if that license and runtime better suit your project.

Choose B2Brouter if a web or API invoicing service with a free Basic plan suits you better than embedding a library; Basic is limited to 10 contacts, 24 transactions per year, and one user.

BillingEngine has a free API and web plan capped at three invoices, three quotes, and three receipts per month for one user; it may suit those seeking a limited hosted invoicing option.

Domibus is a free, self-hosted option for Linux with API access and a downloadable sample implementation under the European Union Public Licence.

PeppolSoft offers a free Buyer plan with receiving, a web portal, the PeppolSoft app, and ERP integration upon request and subject to budget approval.

ONESOURCE Pagero is another freemium option with API and web access.

Tradeshift has API and web access and a free tier covering up to 30 invoices per quarter.

Verdict

Choose phase4 if your team wants a no-fee, Apache-licensed Java component for embedding AS4 messaging and can handle integration, certificate setup, and deployment itself. Its protocol-profile range is a strong starting point, but the unsupported multi-hop and split-and-join features are the clearest reasons to look elsewhere.