Exactly eighteen years ago today, on October 30 2006, we shipped curl 7.16.0 that among a whole slew of new features and set of bugfixes bumped the libcurl SONAME number from 3 to 4. ## ABI breakage This bump meant that libcurl 7.16.0 was not binary compatible with the previous releases. Users could not just easily and transparently bump up to this version from the previous, but they had to check their use of libcurl and in some cases adjust source code. This was not the first ABI breakage in the curl project, but at this time our use base was larger than at any of the previous bumps and this time people complained about the pains and agonies such a break brought them. ## We took away FTP features In the 7.16.0 release we removed a few FTP related features and their associated options. Before this release, you could use curl to do “third party” transfers over FTP, and in this release you could no longer do that. That is a feature when the client (curl) connects to server A and instructs that server to communicate with server B and do file transfers among themselves, without sending data to and from the client. This is an FTP feature that was not implemented well in curl and it was poorly tested. It was also a feature that barely no FTP server allowed and subsequently this was not used by many users. We ripped it out. ## A near pitchfork situation Because so few people used the removed features, barely anyone actually noticed the ABI breakage. It remained theoretical to most users and I believe that detail only made people more upset over the SONAME bump because they did not even see the necessity: we just made their lives more complicated for no benefit (to them). The Debian project even decided to override our decision _“no, that is not an ABI breakage”_ and added a local patch in their build that lowered the SONAME number back to 3 again in their builds. A patch they would stick to for many years to come. The obvious friction this bump caused, even when in reality it actually did not affect many users and the loud feedback we received, made a huge impact on me. It had not previously dawned on me exactly how important this was. I decided there and then to do the utmost to never go through this again. To put ABI compatibility at the top of the priority list. Make it one of the most fundamental key properties of libcurl. **Do. Not. Break. The. ABI** (we don’t break the API either) ## A never-breaking ABI The decision was initially made to avoid the negativity the bump brought, but I have since over time much more come to appreciate the upsides. _Application authors everywhere can always and without risk keep upgrading to the latest libcurl._ It sounds easy and simple, but the impact is huge. The examples, the documentation, the applications, everything can just always upgrade and continue. As libcurl over time has become even more popular and compared to 2006, used in many magnitudes more installations, it has grown into an even more important aspect of the curl life. Possibly _the_ single most important properly of curl. There is a small caveat here and that is that we occasionally of course have bugs and regressions, so when I say that users can always upgrade, that is true in the sense that we have not broken the ABI since. We have however had a few regressions that sometimes have triggered some users to downgrade again or wait a little longer for the next release that has the bug fixed. When we took that decision in 2006 we had less than 50,000 lines of product code. Today we are approaching 180,000 lines. ## Effects of never breaking ABI We know that once we adopt a change, we are stuck with it for decades to come. It makes us double-check every knot before we accept new changes. Once accepted and shipped, we keep supporting code and features that we otherwise could have reconsidered and perhaps removed. Sometimes we think of a better way to do something _after_ the initial merge, but by then it is too late to change. We can then always introduce new and better ways to do things, but we have to keep supporting the old way as well. A most fundamental effect is that we can never shrink the list of options we support. We can never actually rename something. Doing new things and features consistently over this long time is hard if not impossible, as we learn new things and paradigms vary through the decades. ## How The primary way we maintain this is by manual code view and code inspection of every change. Followed of course by a large range of tests that make sure that assumptions remain. Occasionally we have (long) discussions around subtle details when someone proposes a change that potentially might be considered an ABI break. Or not. What exactly is covered by _ABI compatibility_ is not always straight forward or easy to have carved in stone. In particular since the project can be built and run on such a wide range of systems and architectures. ## Deprecating We _can_ still remove functionality if the conditions are right. Some features and options are documented and work in a way so that something is _requested_ or _asked for_ and libcurl then tries to satisfy that ask. Like for example libcurl once supported HTTP/1 pipelining like that. libcurl still provides the option to enable pipelining and applications can still ask for it so it is still ABI and API compatible, but a modern libcurl simply will never do it because that functionality has been removed. Example two: we dropped support for NPN a few years back. NPN being a TLS extension called Next Protocol Negotiation that was used briefly in the early days of HTTP/2 development before ALPN was introduced and replaced NPN. Virtually nothing requires NPN anymore, and users can still set the option asking for it, but it will never actually happen over the wire. Furthermore, a typical libcurl build involves multiple third party libraries that provide features it needs. For things like TLS, SSH, compression and binary HTTP protocol management. Over the years, we have removed support for several such libraries and introduced support for new, in ways that was never visible in the API or ABI. Some users just had to switch to building curl with different helper libraries. In reality, libcurl is typically more stable than most existing servers and URLs. The libcurl examples you wrote in 2006 can still be built with the modern libcurl, but the servers and URLs you used back then most probably cannot be used anymore. ## If no one can spot it, it did not happen As blunt as it may sound, it has came down to this fundamental statement several times to judge if a change is an ABI breakage or not: _If no one can spot an ABI change, it is not an ABI change_ Of course what makes it harder than it sounds is that it is extremely difficult to actually know if someone will notice something ahead of time. libcurl is used in so ridiculously many installations and different setups, second-guessing whatever everyone does and wants is darned close to impossible. Adding to the challenge is the crazy long upgrade cycles some of our users seem to sport. It is not unusual to see questions appear on the mailing lists from users bumping from curl versions from eight or ten years ago. The fact that we have not heard users comment on a particular change might just mean that they are still stuck on ancient versions. Getting frustrated comments from users today about a change we landed five years ago is hard to handle. ## Forwards compatible I should emphasize that all this means that users can always upgrade to a _later_ release. It does not necessarily mean that they can switch back to an older version without problems. We do add new features over time and if you start using a new feature, the application of course will not work, or even still compile, if you would switch to a libcurl version from before that feature was added. ## How long is never What I have laid out here is our plan and ambition. We have managed to stick to this for eighteen years now and there is no particular known blockers in the known future either. I cannot rule out that we might at some point in the future run into an obstacle so huge or complicated that we will be forced to do the unthinkable. To break the ABI. But until we see absolutely no other way forward, it is not going to happen.

In January 2025, the PSF submitted a proposal to the US government National Science Foundation under the Safety, Security, and Privacy of Open Source Ecosystems program to address structural vulnerabilities in Python and PyPI. It was the PSF’s first time applying for government funding, and navigating the intensive process was a steep learning curve for our small team to climb. Seth Larson, PSF Security Developer in Residence, serving as Principal Investigator (PI) with Loren Crary, PSF Deputy Executive Director, as co-PI, led the multi-round proposal writing process as well as the months-long vetting process. We invested our time and effort because we felt the PSF’s work is a strong fit for the program and that the benefit to the community if our proposal were accepted was considerable. We were honored when, after many months of work, our proposal was recommended for funding, particularly as only 36% of new NSF grant applicants are successful on their first attempt. We became concerned, however, when we were presented with the terms and conditions we would be required to agree to if we accepted the grant. These terms included affirming the statement that we “do not, and will not during the term of this financial assistance award, operate any programs that advance or promote DEI, or discriminatory equity ideology in violation of Federal anti-discrimination laws.” This restriction would apply not only to the security work directly funded by the grant, **but to any and all activity of the PSF as a whole**. Further, violation of this term gave the NSF the right to “claw back” previously approved and transferred funds. This would create a situation where money we’d already spent could be taken back, which would be an enormous, open-ended financial risk. Diversity, equity, and inclusion are core to the PSF’s values, as committed to in our mission statement: > _The mission of the Python Software Foundation is to promote, protect, and advance the Python programming language, and to support and facilitate the growth of**a diverse and international community** of Python programmers._ Given the value of the grant to the community and the PSF, we did our utmost to get clarity on the terms and to find a way to move forward in concert with our values. We consulted our NSF contacts and reviewed decisions made by other organizations in similar circumstances, particularly The Carpentries. In the end, however, the PSF simply can’t agree to a statement that we won’t operate any programs that “advance or promote” diversity, equity, and inclusion, as it would be a betrayal of our mission and our community. We’re disappointed to have been put in the position where we had to make this decision, because we believe our proposed project would offer invaluable advances to the Python and greater open source community, protecting millions of PyPI users from attempted supply-chain attacks. The proposed project would create new tools for automated proactive review of all packages uploaded to PyPI, rather than the current process of reactive-only review. These novel tools would rely on capability analysis, designed based on a dataset of known malware. Beyond just protecting PyPI users, the outputs of this work could be transferable for all open source software package registries, such as NPM and Crates.io, improving security across multiple open source ecosystems. In addition to the security benefits, the grant funds would have made a big difference to the PSF’s budget. The PSF is a relatively small organization, operating with an annual budget of around $5 million per year, with a staff of just 14. $1.5 million over two years would have been quite a lot of money for us, and easily the largest grant we’d ever received. Ultimately, however, the value of the work and the size of the grant were not more important than practicing our values and retaining the freedom to support every part of our community. The PSF Board voted unanimously to withdraw our application. Giving up the NSF grant opportunity—along with inflation, lower sponsorship, economic pressure in the tech sector, and global/local uncertainty and conflict—means the PSF needs financial support now more than ever. We are incredibly grateful for any help you can offer. If you're already a PSF member or regular donor, you have our deep appreciation, and we urge you to share your story about why you support the PSF. Your stories make all the difference in spreading awareness about the mission and work of the PSF. How to support the PSF: * Become a Member: When you sign up as a Supporting Member of the PSF, you become a part of the PSF. You’re eligible to vote in PSF elections, using your voice to guide our future direction, and you help us sustain what we do with your annual support. * Donate: Your donation makes it possible to continue our work supporting Python and its community, year after year. * Sponsor: If your company uses Python and isn’t yet a sponsor, send them our sponsorship page or reach out to [email protected] today. The PSF is ever grateful for our sponsors, past and current, and we do everything we can to make their sponsorships beneficial and rewarding.