GLOSSARY / DEFINITION
What is an open source fork
An open source fork is a new project built from a copy of an existing project's code, then developed on its own. For enterprises living through the relicensing wave, the fork is often the open path that stays open, and knowing what one is helps you judge whether it belongs in your stack.
Definition
An open source fork is a new project created by copying the source code of an existing project and continuing its development independently, under separate governance and sometimes a different name. Because open source licenses grant the right to copy, modify, and redistribute the code, anyone is free to fork a project, and forks have always been part of how open source evolves. At the moment of the fork, the new project is an exact copy of the original, which is why a fork starts out highly compatible. From there the two projects develop on their own, and how far they diverge depends on the choices each community makes.
Forks created by the relicensing wave
Most forks are routine, but a distinct category emerged from the recent relicensing wave: forks created specifically to keep a project open after its original moved to source available terms. When HashiCorp moved Terraform, Vault, Consul, Nomad, and Packer to the Business Source License as of August 2023, the community forked Terraform as OpenTofu. When Redis moved to a model combining the Redis Source Available License and the Server Side Public License as of March 2024, the response was Valkey. When Elastic moved Elasticsearch and Kibana to the Server Side Public License and the Elastic License in 2021, Amazon Web Services led the fork that became OpenSearch. Each of these forks took the last open source release and continued it under an open source license, often held by a foundation rather than a single company.
What it means for license risk
For a buyer, a credible fork is significant because it offers an open license alternative to a relicensed project. Adopting the fork removes the competitive use restriction that the source available terms introduced and reduces dependence on a single vendor whose terms can change again. A fork also shifts negotiating leverage, because a buyer who can move to an open alternative is not locked into the commercial offering and can weigh a vendor quote against a real exit. The tradeoffs are that a fork is a newer project whose roadmap can diverge from the original over time, and that migrating to it is still a project. Compatibility is strong at the fork point but should be tested against your actual workload rather than assumed permanent. Whether a given fork's license suits your use is a question for your own counsel.
Related reading
For the Redis fork specifically, see our definition of Valkey, and for a side by side view of the original and the fork, read Redis versus Valkey compared. For the wider pattern of license changes that produced these forks, see our relicensing pillar. For the rest of the terms, see our open source license risk glossary.
CONTAINMENT
Decide whether a fork belongs in your stack
An open source license risk assessment maps where a relicensed component runs and weighs the fork against holding and paying. Independent, buyer side, paid only by you.
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Start a risk assessmentCOMMON QUESTIONS
Questions buyers ask.
What is an open source fork?
An open source fork is a new project created from a copy of an existing project's source code, then developed independently under its own governance. Forks happen for many reasons, but the recent relicensing wave produced forks specifically to continue a project under an open source license after the original moved to source available terms.
What are examples of forks created after a relicense?
OpenTofu was forked from Terraform after the move to the Business Source License. Valkey was forked from Redis after the move to source available terms in 2024. OpenSearch was forked from Elasticsearch after the 2021 move to the Server Side Public License. Each continues under an open source license held by a foundation or community.
Is a fork a drop in replacement for the original?
A fork starts as an exact copy, so it is highly compatible at the fork point. Over time the two projects can diverge in features, clients, and behavior, so compatibility should be tested against your actual workload rather than assumed permanent.
Why do forks matter for license risk?
A credible fork gives a buyer an open license alternative to a relicensed project, which removes the competitive use restriction and reduces dependence on a single vendor's future terms. It also changes negotiating leverage, because the buyer is no longer locked to the commercial offering.
Is this legal advice?
No. This is commercial and licensing risk advisory, not legal advice. For interpretation of any license, including a fork's license, engage your own counsel.