MongoDB, FerretDB clash over open source definition
Is the SSPL an open source license? Opinions differ.
MongoDB has sent a cease-and-desist letter giving notice of potential legal action to startup FerretDB, claiming it infringes MongoDB’s licensing terms, misuses its trademarks, and uses its intellectual property unlawfully.
Although sent in 2023 its contents have only just become available and provide an insight into the still unresolved tensions between open-source fundamentalists wanting unrestricted code availability and open source business realists. They claim large scale open source code use by hyperscale cloud service providers distorts the open source community as the CSPs take the code but give nothing back. The fundamentalists say the business realists are really only concerned about getting support revenue. The realists say they need the CSPs to add value by contributing code back to the community, and the support revenues so they can develop added value extensions and services to the core code.
MongoDB produces one of the top five NoSQL databases and has a massive user community. It is a publicly owned business that earned revenues of $2 billion in fiscal 2025. The database code was developed as open source under an Apache license for its drivers and then the AGPL (Affero General Public License) was applied to all versions released prior to October 16, 2018.
At the same time it started using the SSPL (Server-Side Public License), also known as a copyleft license, for all versions released after October 16, 2018, including the MongoDB API and WiredTiger storage engine code, and went proprietary at that point, according to the fundamentalists. This license requires that a user incorporating MongoDB code in a software product makes the whole stack of that product available as open source code at no charge, unless it signs a commercial licensing deal with MongoDB. This was seen as a departure from traditional open source licensing norms and aimed, we understand, at cloud service providers, such as AWS, which used MongoDB in its DBaaS (Database-as-a-Service) offerings and did not contribute anything back to the open source community.
The Open Source Initiative (OSI) declared the SSPL was not an open source license in 2021. Rather it was a fauxpen license. MongoDB had submitted its SSPL to OSI for validation but later withdrew it "when it became clear that the license would not be approved." The OSI board said: "It’s deception, plain and simple, to claim that the software has all the benefits and promises of open source when it does not."
FerretDB was founded in 2021 by three ex-Percona people: CEO Peter Farkas, CTO Alexey Palazhchenko, and former CEO Peter Zaitsev. They were reacting to MongoDB’s adoption of SSPL because, they claim, it encourages vendor lock-in, kills innovation, and is no longer fully open source. For example, companies like Red Hat and Debian dropped MongoDB from their distributions due to concerns over SSPL.
It is a tiny startup with just eight fully remote staff and $2.8 million initial funding. Farkas told an IT Press Tour briefing that FerretDB’s Apache 2-licensed open source software, available on-premises or in the public cloud, provides a MongoDB compatibility layer and a stateless proxy written in Go, which translates MongoDB wire protocol queries (version 5.0 and above) into SQL and sends them to a PostgreSQL back end database engine. It uses MQL (MongoDB Query Language) so that existing applications using MongoDB can continue using that interface but have requests processed by a fully open source back end and not the SSPL-using MongoDB.
v1.0 gained general availability in April 2023, and v2.0, released in January 2025, was over 20x faster due to enhanced integration with Microsoft’s DocumentDB extension for PostgreSQL. FerretDB offers additional services like FerretDB Cloud, a managed DBaaS available on AWS and GCP (with Azure support planned), as well as subscription and consulting options for enterprises.
It has become prominent enough for MongoDB to react with a cease-and-desist letter [PDF].
MongoDB’s letter, sent in November 2023 by General Counsel Andrew Stephens, asserts: "FerretDB infringes MongoDB’s copyrights and trademarks and violates MongoDB’s licensing scheme. It also relies on unfair and misleading advertising to promote itself. And by supplying the FerretDB proxy layer to partners – including SAP, Vultr, Scaleway, and others – with instructions for those vendors to use it to add MongoDB compatibility to their own database or service offerings, FerretDB is contributing to infringement of MongoDB’s patents."
It describes the various infringements, unfair practices, and so on, saying: "FerretDB must either cease and desist offering FerretDB, or work with us to obtain a valid license to use our specifications, documentation and other intellectual property," and puts it on "notice of potential legal action."
FerretDB disputes the claims in the Stephens' letter, saying that since FerretDB does not use any of MongoDB’s code in its software product, by definition the development of that software cannot implicate, let alone violate, any of the licenses that Stephens raised in his letter.
There was a Zoom call between Mongo’s VP for partnerships and Peter Farkas to try and sort out the differences between the two points of view but did not resolve anything. Farkas said he was likened to a thief in the call. The dispute is ongoing and unresolved, with Farkas telling us: "There has been no legal action or further correspondence since we responded to MongoDB's cease and desist letter. Instead, MongoDB proceeded to send a similar letter to our partners and users, claiming that the usage of FerretDB is illegal."
But no supplier has gone to court to get a legal judgment on the disputed claims and counter-claims about SSPL.
This dispute mirrors ones in the Elasticsearch and Redis open source communities. In 2021 Elastic changed the licenses for parts of Elasticsearch and Kibana from ASLv2 to SSPL or a new Elastic License 2.0 (ELv2). This followed what Elastic described as bad behaviourv by AWS over a period of years. ELv2 prevents Elastic code being used in a hosted or managed service giving third parties access to its functionality. Amazon then created the OpenSearch fork of Elasticsearch. There is more information about all this here.
Redis is an an open-source, in-memory, key-value database. In 2024 Redis the company moved the Redis project from its original BSD 3-Clause license to a dual-licensed model; the Redis Source Available License v2 (RSALv2) and the Server Side Public License v1 (SSPLv1). As with MongoDB it was caused by cloud service provider use of the SW with no code contributed back to the Redis community. CEO Rowan Trollope blogged: "Under the new license, cloud service providers hosting Redis offerings will no longer be permitted to use the source code of Redis free of charge."
This was a controversial change, with some community members viewing it as abandoning open-source principles, as with MongoDB. It led to a community-driven fork of Redis, called Valkey, under the Linux Foundation, designed as a drop-in replacement for Redis.
Open-source object storage supplier MinIO has also had licensing arguments with storage suppliers such as Nutanix and WEKA using its code in, it thinks, improper ways.
There is no clear and generally understood view about whether the SSPL is a true open source license or not. Irrespective of this, Farkas believes that there needs to be an open standard for MQL, as there is for SQL and also PDF. This, as it has done with both of these standards, will enlarge the market to the benefit of all its participants.