5 ms·

What is the benefit of using a remote cache instead of a local ~/.cache directory? Is it only for sharing build results among team members? How do you make sure

by xjia 3y ago

What is the benefit of using a remote cache instead of a local ~/.cache directory? Is it only for sharing build results among team members? How do you make sure the build results are not spoofed?

sgammon 3y ago
Sharing with team members, sharing with CI, and the ability to pull from more than just what's on your machine (i.e. a larger addressable cache than you are willing to keep on disk). Cache objects also compound across projects, so it's nice to ship them up somewhere and have them nearby when you need them. Re/spoofing, obviously it's all protected with API keys and tokens, and we're working on mechanisms to perform end-to-end encryption. In general, build cache objects are usually addressed by a content-addressable-hash, so that also helps because your build typically knows the content it's looking for and can verify. That isn't true for all tools, though, so we're working to understand where the gaps are and fix them.
- sgammon 3y ago
  (Fwiw, group conversation encryption tech like MLS is somewhat applicable, and that's the sort of pattern we're looking at, but it would be cool to know if that's moving to you on the problem of safety w.r.t. builds.)
- Thorrez 3y ago
  >In general, build cache objects are usually addressed by a content-addressable-hash How does that work? I would think the simplest case of a build object that needs to be cached is a .o file created from a .c file. The compiler sees the .c file and can determine its hash, but how can the compiler determine the hash of the .o file to know what to look up in the cache? I think the compiler would need to perform the lookup using the hash of the .c file, which isn't a hash of the data in the cache.
  - sgammon 3y ago
    In Bazel’s case and other cases, build cache objects are held in CAS and then referenced from other keys. I believe BuildXL from Microsoft also works this way. Of course one other advantage to build caches is they are verifiable: the intent is to produce the exact same output as a normal call, and that’s easily checked on the client side. No question that build caching poses inherent supply chain risks though and that’s part of what we want to solve. I think people are hesitant to trust build caching for good reason until there are safer mechanisms and better cryptographic patterns applied.
  - krupan 3y ago
    When a .o is stored in the cache it is associated with the hash of the .c file
  - aseipp 3y ago
    In the case of the Remote Execution/Cache API used by Bazel among others[1] at least, it's a bit more detailed. There's an "ActionCache" and an actual content-addressed cache that just stores blobs ("ContentAddressableStorage"). When you run a `gcc -O2 foo.c -o foo.o` command (locally or remotely; doesn't matter), you upload an "Action" into the action cache, which basically said "This command was run. As a result it had this stderr, stdout, error code, and these input files read and output files written." The input and output files are referenced by the hash of their contents, in this case, and they get uploaded into the CAS system. Most importantly you can look up an action in the ActionCache without actually running it, provided you have the inputs at hand. So now when another person comes by and runs the same build command, they say "Has this Action, with these inputs, been run before?" and the server can say "Yes, and the output is a file identified by hash XYZ" where XYZ is the hash of foo.o, so you can just instantly download it from the CAS. So there are a few more moving parts to make it all work. But the system really is ultimately content-addressed, for the most part. [1] https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/remote_execution.proto#L555 https://github.com/bazelbuild/remote-apis/blob/main/build/ba...
    - bgmeister 3y ago
      If you're only using remote caching (ie no remote execution) then all cache clients need to trust each other, because a malicious client can upload any result it wants to a given ActionCache key, and there's no way to verify the ActionCache entries are correct unless the actions are reproducible. (And verifying ActionCache entries by rerunning the actions kind of defeats the purpose of using a build cache.) If you don't want clients to have to trust each other, then you can block ActionCache write access to the clients and add remote execution. In this setup clients upload an action to the CAS, remote executors run the action and then upload the result to the ActionCache, using the hash of the action as the key. This way malicious clients can't spoof cache results for other clients, because other clients won't ever look for the malicious action's key in the ActionCache.
  - sgammon 3y ago
    Yep, aseipp, and we support the full gRPC interface for remote caching offered by Bazel, including the newer APIs. Explained better than I could for sure. I find it very interesting how BuildXL and Bazel ended up at similar models for this problem. I don’t yet know the history of which informed which. (As compared to, say, Gradle, which works based on input hashes instead.)
- xjia 3y ago
  IIUC the actual computation (e.g. compiling, linking, ...) happens on client (CI or developer) machines and the results are written to the server-side cache. By spoofing I meant to say that an authenticated but malicious client (intentionally or not, e.g. a clueless intern) may be able to write malicious contents to the cache. For example, their build toolchain could be contaminated and the resulting build outputs are contaminated. The "action" per se and its hash is still legit, but the hash is only used as the lookup key -- their corresponding value is "spoofed." The only safe way I can imagine to use such a remote cache is for CI to publish its build results so that they could be reused by developers. The direction from developers to developers or even to CI seems difficult to handle and has less value. But I might be missing some important insights here so my conclusion could be wrong. But if that's the case, is the most valuable use case to just configure the CI to read from / write to the remote cache, and developers to only read from the remote cache? And given such an assumption, is it much easier to design/implememt a remote cache product?
  - sgammon 3y ago
    All great points but in practice, tools like Bazel and sccache are incredibly conservative about hashes matching, to include file path on disk and even env var state. One goal of these tools is to guarantee that such misconfiguration results in a cache key mismatch, rather than a hit and a bug. There are tons of challenges designing a remote build cache product, like anything, but that one has turned out to be a reliable truth. Some other interesting insights: - transmitting large objects is often not profitable, so we found that setting reasonable caps on what’s shared with the cache can be really effective for keeping transmissions small and hits fast - deferring uploads is important because you can’t penalize individual devs for contributing to the cache, and not everybody has a fast upload link. making this part smooth is important so that everyone can benefit from every compile. - build caching is ancient, Make does its own simple form of build caching, but the protocols for it vary in robustness greatly, from WebDAV in ccache to Bazel’s gRPC interface - most GitHub Actions builds occur in a small physical area, so accelerating build artifacts is an easier problem than, say, full blown CDN serving The assumptions that definitely help: - it’s a cache, not a database; things can be missing, it doesn’t need strong consistency - replication lag is okay because a build cache entry is typically not requested multiple times in a short window of time; the client that created it has it locally - it’s much better to give a fast miss than a slow hit, since the compiler is quite fast - it’s much better to give a fast miss than an error. You can NEVER break a build; at worst it should just not be accelerated. It’s an interesting problem to work on for sure.
aseipp 3y ago
Not just team members; if you make your cache publicly readable, contributors to e.g. your GitHub/GitLab/Whatever project can also use them and get really fast builds, the first time they try to contribute. So a remote cache is nice to have, if it's seamless. Nix works this way by default (and much of the community operates caches like this) and it can be a massive, massive time saver. > How do you make sure the build results are not spoofed? What do you mean "spoofed?" As in, someone put an evil artifact in the cache? Or overwrote an existing artifact with a new one? Or someone just stole your developers access and started shoving shit in there? There's a whole bunch of small details here that really matter to understand what security/integrity properties you want the cache to uphold. FWIW, I've been looking into this in Buck2/Bazel land, and my understanding is that most large orgs just use some kind of terminating auth proxy that the underlying connection/flow/build artifacts can be correlated back to. So you know this cache artifact was first inserted by build B, done by user X, who authenticated with their key K, etc etc.
- sgammon 3y ago
  Exactly — just like Git, everything is ultimately identified with a key which can tie back to a stable identity thru OIDC or similar mechanisms. At least that’s how we did it.
- yjftsjthsd-h 3y ago
  Nix only caches at the package level, doesn't it?
  - sgammon 3y ago
    Nix is different, yeah, and it won’t wire together a build cache for you. Nix is great for many things of course, it’s just not a replacement for sccache per se Nix + sccache would probably be pretty great for preserving paths and environment, which is really healthy for build caching in general.
    - mikepurvis 3y ago
      Properly handling cargo and bazel builds in nix is very much not a solved problem, and Nix's resistance to allowing ccache into the sandbox for purity reasons definitely exacerbates the problem.
mgaunard 3y ago
I built my own build system that does something similar. I've set it up at work with two S3 buckets: trusted and untrusted. CI/CD read/write from trusted only. Developers read/write from untrusted, and read-only from trusted.
- sgammon 3y ago
  We decided to back our main cache with in-memory storage for spicier performance. I’m curious how well S3 has worked for you here? Is it fast enough? Or, maybe the blobs you’re dealing with are on the bigger end? That would also make sense
  - mgaunard 3y ago
    Each object file (.o) has a unique hash and is stored as thehash.o. It's certainly much faster to download the .o than it is to build it. Once it's downloaded it stays on the local filesystem until it's garbage-collected.
    - sgammon 3y ago
      Hm, interesting. Our free tier is planned to be this plus R2, so I’m happy to hear S3-style data exchange is working for people. Thanks for sharing
      - mgaunard 3y ago
        The whole point of S3 is that it is inexpensive. You don't want to pay premium money for terabytes of data that are usually invalidated everyone someone makes a significant change.
        sgammon 3y ago
        Our blobs end up quite a bit smaller than that, and I can see using S3 for many build cache problems. In our case in-memory worked better with overflow to disk for large or infrequently used objects. R2 is S3 to me, btw, or at least it’s the same API. Object storage as a model is really what I’m asking about. I’m genuinely glad to hear S3 was enough to be impactful.
throwawaaarrgh 3y ago
It's for sharing and aggregating. Ccache is useful locally, but really shines when combined with Distcc, a distributed compiler. Every host contributes a cache object that other hosts can use, and every host can use the cache object contributed by other hosts. So you don't even have to built it once yourself to benefit from the cache of everyone else. It therefore speeds up multiple hosts/users builds, distributed builds and the dev experience of individuals.

by xjia 3y ago

What is the benefit of using a remote cache instead of a local ~/.cache directory? Is it only for sharing build results among team members? How do you make sure the build results are not spoofed?

sgammon 3y ago

Sharing with team members, sharing with CI, and the ability to pull from more than just what's on your machine (i.e. a larger addressable cache than you are willing to keep on disk). Cache objects also compound across projects, so it's nice to ship them up somewhere and have them nearby when you need them. Re/spoofing, obviously it's all protected with API keys and tokens, and we're working on mechanisms to perform end-to-end encryption. In general, build cache objects are usually addressed by a content-addressable-hash, so that also helps because your build typically knows the content it's looking for and can verify. That isn't true for all tools, though, so we're working to understand where the gaps are and fix them.

sgammon 3y ago
(Fwiw, group conversation encryption tech like MLS is somewhat applicable, and that's the sort of pattern we're looking at, but it would be cool to know if that's moving to you on the problem of safety w.r.t. builds.)
Thorrez 3y ago
>In general, build cache objects are usually addressed by a content-addressable-hash How does that work? I would think the simplest case of a build object that needs to be cached is a .o file created from a .c file. The compiler sees the .c file and can determine its hash, but how can the compiler determine the hash of the .o file to know what to look up in the cache? I think the compiler would need to perform the lookup using the hash of the .c file, which isn't a hash of the data in the cache.
- sgammon 3y ago
  In Bazel’s case and other cases, build cache objects are held in CAS and then referenced from other keys. I believe BuildXL from Microsoft also works this way. Of course one other advantage to build caches is they are verifiable: the intent is to produce the exact same output as a normal call, and that’s easily checked on the client side. No question that build caching poses inherent supply chain risks though and that’s part of what we want to solve. I think people are hesitant to trust build caching for good reason until there are safer mechanisms and better cryptographic patterns applied.
- krupan 3y ago
  When a .o is stored in the cache it is associated with the hash of the .c file
- aseipp 3y ago
  In the case of the Remote Execution/Cache API used by Bazel among others[1] at least, it's a bit more detailed. There's an "ActionCache" and an actual content-addressed cache that just stores blobs ("ContentAddressableStorage"). When you run a `gcc -O2 foo.c -o foo.o` command (locally or remotely; doesn't matter), you upload an "Action" into the action cache, which basically said "This command was run. As a result it had this stderr, stdout, error code, and these input files read and output files written." The input and output files are referenced by the hash of their contents, in this case, and they get uploaded into the CAS system. Most importantly you can look up an action in the ActionCache without actually running it, provided you have the inputs at hand. So now when another person comes by and runs the same build command, they say "Has this Action, with these inputs, been run before?" and the server can say "Yes, and the output is a file identified by hash XYZ" where XYZ is the hash of foo.o, so you can just instantly download it from the CAS. So there are a few more moving parts to make it all work. But the system really is ultimately content-addressed, for the most part. [1] https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/remote_execution.proto#L555 https://github.com/bazelbuild/remote-apis/blob/main/build/ba...
  - bgmeister 3y ago
    If you're only using remote caching (ie no remote execution) then all cache clients need to trust each other, because a malicious client can upload any result it wants to a given ActionCache key, and there's no way to verify the ActionCache entries are correct unless the actions are reproducible. (And verifying ActionCache entries by rerunning the actions kind of defeats the purpose of using a build cache.) If you don't want clients to have to trust each other, then you can block ActionCache write access to the clients and add remote execution. In this setup clients upload an action to the CAS, remote executors run the action and then upload the result to the ActionCache, using the hash of the action as the key. This way malicious clients can't spoof cache results for other clients, because other clients won't ever look for the malicious action's key in the ActionCache.
- sgammon 3y ago
  Yep, aseipp, and we support the full gRPC interface for remote caching offered by Bazel, including the newer APIs. Explained better than I could for sure. I find it very interesting how BuildXL and Bazel ended up at similar models for this problem. I don’t yet know the history of which informed which. (As compared to, say, Gradle, which works based on input hashes instead.)
xjia 3y ago
IIUC the actual computation (e.g. compiling, linking, ...) happens on client (CI or developer) machines and the results are written to the server-side cache. By spoofing I meant to say that an authenticated but malicious client (intentionally or not, e.g. a clueless intern) may be able to write malicious contents to the cache. For example, their build toolchain could be contaminated and the resulting build outputs are contaminated. The "action" per se and its hash is still legit, but the hash is only used as the lookup key -- their corresponding value is "spoofed." The only safe way I can imagine to use such a remote cache is for CI to publish its build results so that they could be reused by developers. The direction from developers to developers or even to CI seems difficult to handle and has less value. But I might be missing some important insights here so my conclusion could be wrong. But if that's the case, is the most valuable use case to just configure the CI to read from / write to the remote cache, and developers to only read from the remote cache? And given such an assumption, is it much easier to design/implememt a remote cache product?
- sgammon 3y ago
  All great points but in practice, tools like Bazel and sccache are incredibly conservative about hashes matching, to include file path on disk and even env var state. One goal of these tools is to guarantee that such misconfiguration results in a cache key mismatch, rather than a hit and a bug. There are tons of challenges designing a remote build cache product, like anything, but that one has turned out to be a reliable truth. Some other interesting insights: - transmitting large objects is often not profitable, so we found that setting reasonable caps on what’s shared with the cache can be really effective for keeping transmissions small and hits fast - deferring uploads is important because you can’t penalize individual devs for contributing to the cache, and not everybody has a fast upload link. making this part smooth is important so that everyone can benefit from every compile. - build caching is ancient, Make does its own simple form of build caching, but the protocols for it vary in robustness greatly, from WebDAV in ccache to Bazel’s gRPC interface - most GitHub Actions builds occur in a small physical area, so accelerating build artifacts is an easier problem than, say, full blown CDN serving The assumptions that definitely help: - it’s a cache, not a database; things can be missing, it doesn’t need strong consistency - replication lag is okay because a build cache entry is typically not requested multiple times in a short window of time; the client that created it has it locally - it’s much better to give a fast miss than a slow hit, since the compiler is quite fast - it’s much better to give a fast miss than an error. You can NEVER break a build; at worst it should just not be accelerated. It’s an interesting problem to work on for sure.

aseipp 3y ago

Not just team members; if you make your cache publicly readable, contributors to e.g. your GitHub/GitLab/Whatever project can also use them and get really fast builds, the first time they try to contribute. So a remote cache is nice to have, if it's seamless. Nix works this way by default (and much of the community operates caches like this) and it can be a massive, massive time saver. > How do you make sure the build results are not spoofed? What do you mean "spoofed?" As in, someone put an evil artifact in the cache? Or overwrote an existing artifact with a new one? Or someone just stole your developers access and started shoving shit in there? There's a whole bunch of small details here that really matter to understand what security/integrity properties you want the cache to uphold. FWIW, I've been looking into this in Buck2/Bazel land, and my understanding is that most large orgs just use some kind of terminating auth proxy that the underlying connection/flow/build artifacts can be correlated back to. So you know this cache artifact was first inserted by build B, done by user X, who authenticated with their key K, etc etc.

sgammon 3y ago
Exactly — just like Git, everything is ultimately identified with a key which can tie back to a stable identity thru OIDC or similar mechanisms. At least that’s how we did it.
yjftsjthsd-h 3y ago
Nix only caches at the package level, doesn't it?
- sgammon 3y ago
  Nix is different, yeah, and it won’t wire together a build cache for you. Nix is great for many things of course, it’s just not a replacement for sccache per se Nix + sccache would probably be pretty great for preserving paths and environment, which is really healthy for build caching in general.
  - mikepurvis 3y ago
    Properly handling cargo and bazel builds in nix is very much not a solved problem, and Nix's resistance to allowing ccache into the sandbox for purity reasons definitely exacerbates the problem.

mgaunard 3y ago

I built my own build system that does something similar. I've set it up at work with two S3 buckets: trusted and untrusted. CI/CD read/write from trusted only. Developers read/write from untrusted, and read-only from trusted.

sgammon 3y ago
We decided to back our main cache with in-memory storage for spicier performance. I’m curious how well S3 has worked for you here? Is it fast enough? Or, maybe the blobs you’re dealing with are on the bigger end? That would also make sense
- mgaunard 3y ago
  Each object file (.o) has a unique hash and is stored as thehash.o. It's certainly much faster to download the .o than it is to build it. Once it's downloaded it stays on the local filesystem until it's garbage-collected.
  - sgammon 3y ago
    Hm, interesting. Our free tier is planned to be this plus R2, so I’m happy to hear S3-style data exchange is working for people. Thanks for sharing
    - mgaunard 3y ago
      The whole point of S3 is that it is inexpensive. You don't want to pay premium money for terabytes of data that are usually invalidated everyone someone makes a significant change.
      - sgammon 3y ago
        Our blobs end up quite a bit smaller than that, and I can see using S3 for many build cache problems. In our case in-memory worked better with overflow to disk for large or infrequently used objects. R2 is S3 to me, btw, or at least it’s the same API. Object storage as a model is really what I’m asking about. I’m genuinely glad to hear S3 was enough to be impactful.

throwawaaarrgh 3y ago

It's for sharing and aggregating. Ccache is useful locally, but really shines when combined with Distcc, a distributed compiler. Every host contributes a cache object that other hosts can use, and every host can use the cache object contributed by other hosts. So you don't even have to built it once yourself to benefit from the cache of everyone else. It therefore speeds up multiple hosts/users builds, distributed builds and the dev experience of individuals.