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Titus Winters leading the google abseil library eventually came to the conclusion that the only sane way to manage a large scale C++ system is to "live at head"

Titus Winters leading the google abseil library eventually came to the conclusion that the only sane way to manage a large scale C++ system is to "live at head" [1] -- that is, libraries should live at the head production version of their dependencies. This is patchworked around in more easygoing languages with dependency management systems, docker containers, etc. etc. but if you can enforce living at head from the start it makes everyone's life easier. https://abseil.io/about/philosophy#we-recommend-that-you-choose-to-live-at-head https://abseil.io/about/philosophy#we-recommend-that-you-cho...

fbdab103 4y ago
Doesn't that require you have a Google sized army of engineers and tooling to stay in sync?
- jcelerier 4y ago
  I do it pretty much on my own for https://ossia.io https://ossia.io ; it takes me roughly a day every other month to update my mac, linux and windows SDKs to the new LLVM / Qt / FFMPEG / {... other large dependency I use ...}. Definitely not the end of the world. Said SDKs & build scripts are available here: https://github.com/ossia/sdk https://github.com/ossia/sdk for anyone interested (I'll be honest though: the scripts are a mess!)
  - dataflow 4y ago
    1 day for... ~500 kLOC, it seems. Which is pretty good I think, but larger codebases would have a lot more to deal with -- not just more dependencies, but also more breakages per dependency.
- mastax 4y ago
  Or you just checkout the version that does what you need and leave it alone.
  - deleted 4y ago
    [deleted]
  - UncleMeat 4y ago
    google3 has a one version rule. You can’t just check out the version of a dependency that does what you want.
bluGill 4y ago
That works for google web where they can upgrade everything at will. When you work in embedded upgrades can cost millions of dollars since someone needs to go to remote locations to run the upgrade.
bb88 4y ago
That's fine as long as HEAD doesn't break things. I can no longer count the number of times we had an issue with a "supposedly" minor release that ended up breaking major things in our stack. Most of them were things that could have been detected using unit tests or some kind of basic regression testing. If you have a 1000 dependency packages, and at any point in time 0.1% of them are broken, then odds are you will always have something broken.
- lbrandy 4y ago
  We should be clear... in these large organizations... HEAD is always broken. But it has the advantage of being broken for everyone, tested by everyone, fixed by everyone, and thus fixed for everyone. And this usually makes it far better than the alternatives. Having 1000 dependencies with versions pinned means you are living alone and will run into fewer issues, but when they do come, they will be absolute nightmares that no one else is dealing with and no one can help with. And one day you'll have to do the game of begging someone else to upgrade their version of a downstream thing to fix the issue, and they won't, so you'll try to get the other group to backport the fix in their thing to the version you can't upgrade off. And they won't. etc. etc. Full versioning is the worst of all approaches, IMO, for large complex interconnected codebases (especially ones that are many-to-many from libraries to output binaries) but it absolutely is sometimes the only viable one (for example, the entire open-source-ecosystem is a giant(er) version of this problem, and in that space, versioning is the only thing that I can imagine working).
  - galangalalgol 4y ago
    Supply chain attacks are worsened if everyone lives at the head. Staying far enough behind that some brave (and hopefully small) project discovers the compromise of a repo for some dependency five layers deep before you re-pin to a new version is probably the best mitigation short of some permissions based model like Austral is working on.
    - VHRanger 4y ago
      Supply chain attacks only matter for libraries that can make their own network call, or libraries that directly touch unsanitized web input however?
leroy-is-here 4y ago
is "living at the head" constantly pulling each update or version-controlled source of every library?
dataflow 4y ago
Note that "large scale" is pulling a heavy weight here. Also, living at the HEAD of your language standard is quite a bit different from living at the HEAD of other dependencies.
- jcelerier 4y ago
  It's not just the language standard, they build and deploy the latest clang HEAD every week or so
  - dataflow 4y ago
    Yes I realize. I didn't claim otherwise. The original post was about dropping support for older C++ standards. I was pointing out that Abseil's proposition is quite a bit more heavy handed than Boost's.
pabs3 4y ago
Does that include the head of GCC/LLVM?
- nlewycky 4y ago
  It does. Google mirrors every commit to LLVM in their monorepo, builds and tests the whole monorepo with a fresh Clang nightly, and (ideally) one of those Clang nightlies is released as the new stable compiler for all users of the monorepo every week. This helps keep Google at HEAD and helps keeps LLVM upstream stable.
  - fooker 4y ago
    I thought llvm lived in /third_party, and internal projects usually target specific LLVM versions, not the current open source trunk.
    - nlewycky 4y ago
      As far as I know, there was only ever one version of LLVM at a time in the monorepo. It's possible things changed after I left (either the compiler team or Google). Each upstream commit didn't land directly into the monorepo, instead there was a long lived branch, and on the compiler team there was a buildcop rotation responsible for doing an integrate from that branch into //third_party/llvm. This included running the tests (and fixing any problems) for any other software that depends on LLVM as well as building an unstable crosstool and doing some basic smoke tests on that. Taking that crosstool through testing and to stable crosstool was the responsibility of a different buildcop rotation, using a special compiler team tool for testing the testing crosstool nightly, then to release to stable we used the ordinary presubmits, but for all projects at once, making its testing as similar as possible to any normal code change.
      - pabs3 4y ago
        How does Google deal with other projects' embedded code copies? ie some other project embeds a random old snapshot of LLVM, when you import that project into the monorepo do you end up with two copies of LLVM, or do you strip the old copy and port the codebase to the current LLVM in the monorepo?
noobermin 4y ago
Who is "everyone" here? It sounds like "everyone" is just the developers.
friendzis 4y ago
This somehow reminds me of the argument around git-flow. It's a decent reasoning, however, the whole idea is based on having a single, bleeding edge version. Basically a SaaS. A lot of companies/products do not work that way. Some have physical products out there that have to be updated, some have on-premises deployments, some sell user software of which there are multiple versions under support. Each of these live versions have to have their own source branches and dependency trees. A single `:latest` can render future bugfixes unbuildable.
saagarjha 4y ago
Google concludes the way they’ve always done things is the only way that scales, news at 11. Staying at HEAD can be quite nice in some respects, but the conclusion there only applies to Google, and only then the Google that was actively created to be amenable to the results you’re seeing.
Narann 4y ago
I'm not sure to understand: What "living at the head" means?
- humanrebar 4y ago
  There are various ways to implement this, but to simplify the explanation, assume that versioning always works via lockfiles [1]. Lockfiles record all of the versions of all of the projects that contributed to some experiment or release. They're common in various language-specific ecosystems like gem, pip, cargo, and so on. Assuming you have these lockfiles, you have the typical option, which would be to make a lockfile for each released entity, record it for later reproduction, and update it at least every release. The "live at head" approach would be to instead have a main shared lockfile for every project in the company in a recorded sequence. All projects pick a version of that lockfile to release from. Practically speaking, all projects probably just take the latest version (the head) of that lockfile and everyone works hard to make sure that lockfile always works for everyone. The main advantage here is pretty straightforward combinatorial math. Maintaining and validating unique combinations of dependencies for every release in a codebase is NP-silly, whereas sharing one set of dependencies across as many applications as possible isn't easy but it has a much nicer cost curve. In theory at least, but a lot of large organizations claim practice backs the theory up as well. [1] Versioning doesn't have to work this way. Putting all code into one big source repository (vendoring) has the same effect.

fbdab103 4y ago

Doesn't that require you have a Google sized army of engineers and tooling to stay in sync?

jcelerier 4y ago
I do it pretty much on my own for https://ossia.io https://ossia.io ; it takes me roughly a day every other month to update my mac, linux and windows SDKs to the new LLVM / Qt / FFMPEG / {... other large dependency I use ...}. Definitely not the end of the world. Said SDKs & build scripts are available here: https://github.com/ossia/sdk https://github.com/ossia/sdk for anyone interested (I'll be honest though: the scripts are a mess!)
- dataflow 4y ago
  1 day for... ~500 kLOC, it seems. Which is pretty good I think, but larger codebases would have a lot more to deal with -- not just more dependencies, but also more breakages per dependency.
mastax 4y ago
Or you just checkout the version that does what you need and leave it alone.
- deleted 4y ago
  [deleted]
- UncleMeat 4y ago
  google3 has a one version rule. You can’t just check out the version of a dependency that does what you want.

bluGill 4y ago

That works for google web where they can upgrade everything at will. When you work in embedded upgrades can cost millions of dollars since someone needs to go to remote locations to run the upgrade.

bb88 4y ago

That's fine as long as HEAD doesn't break things. I can no longer count the number of times we had an issue with a "supposedly" minor release that ended up breaking major things in our stack. Most of them were things that could have been detected using unit tests or some kind of basic regression testing. If you have a 1000 dependency packages, and at any point in time 0.1% of them are broken, then odds are you will always have something broken.

lbrandy 4y ago
We should be clear... in these large organizations... HEAD is always broken. But it has the advantage of being broken for everyone, tested by everyone, fixed by everyone, and thus fixed for everyone. And this usually makes it far better than the alternatives. Having 1000 dependencies with versions pinned means you are living alone and will run into fewer issues, but when they do come, they will be absolute nightmares that no one else is dealing with and no one can help with. And one day you'll have to do the game of begging someone else to upgrade their version of a downstream thing to fix the issue, and they won't, so you'll try to get the other group to backport the fix in their thing to the version you can't upgrade off. And they won't. etc. etc. Full versioning is the worst of all approaches, IMO, for large complex interconnected codebases (especially ones that are many-to-many from libraries to output binaries) but it absolutely is sometimes the only viable one (for example, the entire open-source-ecosystem is a giant(er) version of this problem, and in that space, versioning is the only thing that I can imagine working).
- galangalalgol 4y ago
  Supply chain attacks are worsened if everyone lives at the head. Staying far enough behind that some brave (and hopefully small) project discovers the compromise of a repo for some dependency five layers deep before you re-pin to a new version is probably the best mitigation short of some permissions based model like Austral is working on.
  - VHRanger 4y ago
    Supply chain attacks only matter for libraries that can make their own network call, or libraries that directly touch unsanitized web input however?

leroy-is-here 4y ago

is "living at the head" constantly pulling each update or version-controlled source of every library?

dataflow 4y ago

Note that "large scale" is pulling a heavy weight here. Also, living at the HEAD of your language standard is quite a bit different from living at the HEAD of other dependencies.

jcelerier 4y ago
It's not just the language standard, they build and deploy the latest clang HEAD every week or so
- dataflow 4y ago
  Yes I realize. I didn't claim otherwise. The original post was about dropping support for older C++ standards. I was pointing out that Abseil's proposition is quite a bit more heavy handed than Boost's.

pabs3 4y ago

Does that include the head of GCC/LLVM?

nlewycky 4y ago
It does. Google mirrors every commit to LLVM in their monorepo, builds and tests the whole monorepo with a fresh Clang nightly, and (ideally) one of those Clang nightlies is released as the new stable compiler for all users of the monorepo every week. This helps keep Google at HEAD and helps keeps LLVM upstream stable.
- fooker 4y ago
  I thought llvm lived in /third_party, and internal projects usually target specific LLVM versions, not the current open source trunk.
  - nlewycky 4y ago
    As far as I know, there was only ever one version of LLVM at a time in the monorepo. It's possible things changed after I left (either the compiler team or Google). Each upstream commit didn't land directly into the monorepo, instead there was a long lived branch, and on the compiler team there was a buildcop rotation responsible for doing an integrate from that branch into //third_party/llvm. This included running the tests (and fixing any problems) for any other software that depends on LLVM as well as building an unstable crosstool and doing some basic smoke tests on that. Taking that crosstool through testing and to stable crosstool was the responsibility of a different buildcop rotation, using a special compiler team tool for testing the testing crosstool nightly, then to release to stable we used the ordinary presubmits, but for all projects at once, making its testing as similar as possible to any normal code change.
    - pabs3 4y ago
      How does Google deal with other projects' embedded code copies? ie some other project embeds a random old snapshot of LLVM, when you import that project into the monorepo do you end up with two copies of LLVM, or do you strip the old copy and port the codebase to the current LLVM in the monorepo?

noobermin 4y ago

Who is "everyone" here? It sounds like "everyone" is just the developers.

friendzis 4y ago

This somehow reminds me of the argument around git-flow. It's a decent reasoning, however, the whole idea is based on having a single, bleeding edge version. Basically a SaaS. A lot of companies/products do not work that way. Some have physical products out there that have to be updated, some have on-premises deployments, some sell user software of which there are multiple versions under support. Each of these live versions have to have their own source branches and dependency trees. A single `:latest` can render future bugfixes unbuildable.

saagarjha 4y ago

Google concludes the way they’ve always done things is the only way that scales, news at 11. Staying at HEAD can be quite nice in some respects, but the conclusion there only applies to Google, and only then the Google that was actively created to be amenable to the results you’re seeing.

Narann 4y ago

I'm not sure to understand: What "living at the head" means?

humanrebar 4y ago
There are various ways to implement this, but to simplify the explanation, assume that versioning always works via lockfiles [1]. Lockfiles record all of the versions of all of the projects that contributed to some experiment or release. They're common in various language-specific ecosystems like gem, pip, cargo, and so on. Assuming you have these lockfiles, you have the typical option, which would be to make a lockfile for each released entity, record it for later reproduction, and update it at least every release. The "live at head" approach would be to instead have a main shared lockfile for every project in the company in a recorded sequence. All projects pick a version of that lockfile to release from. Practically speaking, all projects probably just take the latest version (the head) of that lockfile and everyone works hard to make sure that lockfile always works for everyone. The main advantage here is pretty straightforward combinatorial math. Maintaining and validating unique combinations of dependencies for every release in a codebase is NP-silly, whereas sharing one set of dependencies across as many applications as possible isn't easy but it has a much nicer cost curve. In theory at least, but a lot of large organizations claim practice backs the theory up as well. [1] Versioning doesn't have to work this way. Putting all code into one big source repository (vendoring) has the same effect.