Recipe for "full" build caching in CI

[BeyondEvil]

I've tried to find a recipe for a full cache of a dotnet build, but my research hasn't revealed much.

I'm using .NET 8 and Github Actions.

If I run dotnet build for my project (ProjectA), it takes me about 2 minutes. If I rerun it, it takes a couple of seconds. So something is obviously being cached.

I've tried to solve this in CI using the actions/cache GH action, and I do get a cache hit, but the build still takes the full 2 minutes.

So obviously I'm missing something and/or doing something wrong.

Here's my latest attempt:

name: Test on Ubuntu

on:
  pull_request:

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

permissions:
  contents: read
  id-token: write

jobs:
  test:
    name: Test on Ubuntu
    runs-on: ubuntu-22.04
    defaults:
      run:
        shell: bash

    steps:
      - name: Check out the repository
        uses: actions/checkout@v4

      - name: Install dotnet
        uses: actions/setup-dotnet@v4
        with:
          global-json-file: global.json

      - name: Restore build artifacts
        id: restore-build
        uses: actions/cache/restore@v4
        with:
          path: |
            ProjectA/build
          key: ${{ runner.os }}-dotnet-build-${{ hashFiles('ProjectA/packages.lock.json') }}
          restore-keys: |
            ${{ runner.os }}-dotnet-build-

      - name: Build
        run: |
          dotnet build --artifacts-path build
        working-directory: ProjectA

      - name: Cache build artifacts
        if: steps.restore-build.outputs.cache-hit != 'true'
        uses: actions/cache/save@v4
        with:
          path: |
            ProjectA/build
          key: ${{ runner.os }}-dotnet-build-${{ hashFiles('ProjectA/packages.lock.json') }}

I have more projects in the same repository, some are dependencies to ProjectA.

I've tried using the "built-in" caching that setup-node provides, but that seems to only have cached packages.

Now, the above CI workflow works, but it doesn't use the cache, as it takes 2 minutes to run dotnet build every time.

Run actions/cache/restore@v4
Received 192937984 of 230484525 (83.7%), 184.0 MBs/sec
Cache Size: ~220 MB (230484525 B)
/usr/bin/tar -xf /home/runner/work/_temp/51aa203a-8ad4-4a5f-ad82-33c3559cce54/cache.tzst -P -C /home/runner/work/myproject/myproject --use-compress-program unzstd
Received 230484525 of 230484525 (100.0%), 109.7 MBs/sec
Cache restored successfully
Cache restored from key: Linux-dotnet-build-e207bfd1abb820e61911a2fb949eaf5de507bcaf6fb[40](https://github.com/mycompany/myproject/actions/runs/11329397027/job/31504855445#step:4:41)aaf8ac4947036a8ff6e

Here's an anonymized version of my .csproj file:

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
    <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>
  </PropertyGroup>
  <ItemGroup>
    <Content Remove="log4net.config" />
  </ItemGroup>
  <ItemGroup>
    <None Include="log4net.config">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </None>
  </ItemGroup>
  <ItemGroup>
    <PackageReference Include="AutoMapper" Version="13.0.1" />
    <PackageReference Include="ClosedXML" Version="0.102.3" />
    <PackageReference Include="EasyNetQ" Version="7.8.0" />
    <PackageReference Include="HtmlAgilityPack.NetCore" Version="1.5.0.1" />
    <PackageReference Include="ITfoxtec.Identity.Saml2.MvcCore" Version="4.12.6" />
    <PackageReference Include="log4net" Version="2.0.17" />
    <PackageReference Include="log4net.Ext.Json" Version="2.0.10.1" />
    <PackageReference Include="Magick.NET-Q16-AnyCPU" Version="13.10.0" />
    <PackageReference Include="Microsoft.AspNetCore.DataProtection.EntityFrameworkCore" Version="8.0.8" />
    <PackageReference Include="Microsoft.AspNetCore.SignalR.Common" Version="8.0.8" />
    <PackageReference Include="Microsoft.Extensions.Caching.StackExchangeRedis" Version="8.0.8" />
    <PackageReference Include="Microsoft.Extensions.Configuration.UserSecrets" Version="8.0.0" />
    <PackageReference Include="Microsoft.Extensions.Logging.Log4Net.AspNetCore" Version="8.0.0" />
    <PackageReference Include="Pomelo.EntityFrameworkCore.MySql" Version="8.0.2" />
    <PackageReference Include="Swashbuckle.AspNetCore" Version="6.7.3" />
    <PackageReference Include="Swashbuckle.AspNetCore.Newtonsoft" Version="6.7.3" />
    <PackageReference Include="Microsoft.AspNetCore.Identity.EntityFrameworkCore" Version="8.0.8" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="8.0.8" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="8.0.8" />
    <PackageReference Include="Microsoft.AspNetCore.Owin" Version="8.0.8" />
    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="8.0.8" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="8.0.8" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.Proxies" Version="8.0.8" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="8.0.8" />
    <PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="8.0.4" />
  </ItemGroup>
  <ItemGroup>
    <ProjectReference Include="..\ProjectB\ProjectB.csproj" />
  </ItemGroup>
</Project>

[AustinWise]

Perhaps cacheing the ~/.nuget/packages folder would help? This is where Nuget packages are restored to by default. See the example here:

https://github.com/actions/cache/blob/main/examples.md#c---nuget

[BeyondEvil]

Yeah, I've tried that both with actions/setup-dotnet and actions/cache but it only seems to cache NuGet packages and not anything that is locally built.

So I'm looking to cache the result of dotnet build essentially.

I've tried caching all obj and bin, I've tried using the --artifacts-path (which only creates the path for the main project, not its local dependencies), but nothing works.

But I'm missing something, something is messing up the dotnet internals so that it thinks everything needs to be rebuilt.

[mjburch]

I was trying to do a similar thing: cache test results keyed on the hash of the built test projects.

I tried obj and bin, as well as using --output to redirect the build ouput to a different directory and hash that.

It's been a while, so I don't have the commands handy, but I performed a local experiment that went something like:

• Built the project originally
  • Hashed build output = hashA
• Modified a bit of code, built again
  • Hash build output = hashB
• Undid the change I previously made, built again
  • Hashed build output = hashA again. So far, so good.
• Commit the current code.
  • Hashed build output = hashC (!)

I think maybe the build command is pulling git commit info into the assembly?

[BeyondEvil]

Yeah, I did a bunch of experiments as well. Can't remember any details tho.

But it became very clear that build is looking at different timestamps for folders and files.

That became apparent when I renamed the output folder and then renamed again to it's original name - which invalidated the cache.

It's amazing to me that they can't provide a way to create deterministic builds. 🤷