llvm dev - Dec 2019 - Network RPCs in LLVM projects

Short version: clangd would like to be able to build a client+server that
can make RPCs across the internet. An RPC system isn't a trivial dependency
and rolling our own from scratch isn't appealing.
Have other projects had a need for this? Any advice on how to approach such
dependencies?

--

Longer: clangd (a language server, like an IDE backend) builds an index of
the project you're working on in order to answer queries (go to definition,
code completion...). This takes *lots* of CPU-time to build, and RAM to
serve.
For large codebases with many developers, sharing an index across users
<https://llvm.discourse.group/t/sharing-indexes-for-multiple-users/202> is
a better approach - you spend the CPU in one place, you spend the RAM in a
few places, and an RPC is fast enough even for code completion. We have
experience with this approach inside Google.

We'd like to build this index server upstream (just a shell around
clangd's
current index code) and put the client in clangd. For open-source projects,
I imagine the server being publicly accessible over the internet.
This means we care about
 - latency (this is interactive, every 10ms counts)
 - security
 - proxy traversal, probably
 - sensible behavior under load
 - auth is probably nice-to-have

I don't think this is something we want to build from scratch, I hear
portable networking is hard :-)
The most obvious thing is to depend on something like Thrift, grpc, etc,
but these aren't trivial dependencies to take on. They could probably be
structured as an optional CMake dependency, which we'd want to ask
distributors to enable.

Have other projects had anything like these requirements? Any solutions, or
desire to use such infrastructure? I saw some RPC layer in ORC, but it
seems mostly abstract/FD-based IPC.
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> On Dec 12, 2019, at 5:58 AM, Sam McCall via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Short version: clangd would like to be able to build a client+server that
can make RPCs across the internet. An RPC system isn't a trivial dependency
and rolling our own from scratch isn't appealing.
> Have other projects had a need for this? Any advice on how to approach such
dependencies?
> 
> --
> 
> Longer: clangd (a language server, like an IDE backend) builds an index of
the project you're working on in order to answer queries (go to definition,
code completion...). This takes *lots* of CPU-time to build, and RAM to serve.
> For large codebases with many developers, sharing an index across users
<https://llvm.discourse.group/t/sharing-indexes-for-multiple-users/202> is
a better approach - you spend the CPU in one place, you spend the RAM in a few
places, and an RPC is fast enough even for code completion. We have experience
with this approach inside Google.
> 
> We'd like to build this index server upstream (just a shell around
clangd's current index code) and put the client in clangd. For open-source
projects, I imagine the server being publicly accessible over the internet.
> This means we care about
>  - latency (this is interactive, every 10ms counts)
>  - security
>  - proxy traversal, probably
>  - sensible behavior under load
>  - auth is probably nice-to-have
> 
> I don't think this is something we want to build from scratch, I hear
portable networking is hard :-)
It really isn't that bad. Just as a note, LLDB does have portable socket
communication already, so it could be a refactor and reuse exercise rather than
building from scratch.
> The most obvious thing is to depend on something like Thrift, grpc, etc,
but these aren't trivial dependencies to take on. They could probably be
structured as an optional CMake dependency, which we'd want to ask
distributors to enable.
This is possible, but adding large and non-standard external dependencies have
significant drawbacks for distribution.
> 
> Have other projects had anything like these requirements? Any solutions, or
desire to use such infrastructure? I saw some RPC layer in ORC, but it seems
mostly abstract/FD-based IPC.
The ORC RPC layer in-tree runs over sockets, but I've implemented it to run
over XPC (a Darwin low-latency IPC mechanism). It is actually a really useful
abstraction over true remote procedure calls.

-Chris
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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On Fri, Dec 13, 2019 at 2:12 PM Chris Bieneman via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
>
> On Dec 12, 2019, at 5:58 AM, Sam McCall via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Short version: clangd would like to be able to build a client+server that
> can make RPCs across the internet. An RPC system isn't a trivial
dependency
> and rolling our own from scratch isn't appealing.
> Have other projects had a need for this? Any advice on how to approach
> such dependencies?
>
> --
>
> Longer: clangd (a language server, like an IDE backend) builds an index of
> the project you're working on in order to answer queries (go to
definition,
> code completion...). This takes *lots* of CPU-time to build, and RAM to
> serve.
> For large codebases with many developers, sharing an index across users
>
<https://llvm.discourse.group/t/sharing-indexes-for-multiple-users/202>
> is a better approach - you spend the CPU in one place, you spend the RAM in
> a few places, and an RPC is fast enough even for code completion. We have
> experience with this approach inside Google.
>
> We'd like to build this index server upstream (just a shell around
> clangd's current index code) and put the client in clangd. For
open-source
> projects, I imagine the server being publicly accessible over the internet.
> This means we care about
>  - latency (this is interactive, every 10ms counts)
>  - security
>  - proxy traversal, probably
>  - sensible behavior under load
>  - auth is probably nice-to-have
>
> I don't think this is something we want to build from scratch, I hear
> portable networking is hard :-)
>
>
> It really isn't that bad. Just as a note, LLDB does have portable
socket
> communication already, so it could be a refactor and reuse exercise rather
> than building from scratch.
>
It sounds as if the clangd index server is supposed to work across the open
internet, which effectively means it needs to speak HTTPS. That's not
really something that you can just write.
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