shellac
shellac
is a suite of software which aims to provides OS-level
process control for the Elixir language. The software is composed of two
sub-projects which coordinate to accomplish this goal:
lacca
: an Elixir process control library, implemented as an OTP application which can be included in amix
project.resin
: a Rust binary which acts as a companion to thelacca
OTP application. This binary provides process control and supervision, it also multiplexes the process's open file descriptors onto the single BEAM port.
Prerequisites
You will need a Rust toolchain, which includes the build tool cargo
, in
order to sucesfully build this package. Please visit the Rust website for
instructions on how to install these tools. This library
can be built with the "stable channel" of the Rust compiler.
Getting Started
- Add
{:lacca, "~> 0.1"}
to yourmix.exs
file's dependencies. - Run
mix deps.get
to download the dependency. - Run
mix deps.compile
to verify that the package compiles sucessfully. - Use the library in your program, for instance ...
{:ok, pid} = Lacca.start "echo", ["hello, world."]
Lacca.read_stdout pid
# {:ok, "hello, world.\n"}
Lacca.stop pid
Note on Native Code
This library builds a native executable which is bundled into the priv/
directory of this OTP application during the mix compile
phase. To do
this you must have a working Rust toolchain installed on any machine that
will be compiling a project that depends on lacca
. You do not need the
toolchain installed on deployment targets, however when building a release,
e.g: with the mix release
command, you will need to ensure that the resulting
binaries can be executed on the target system.
Some common gotchas include:
- Building on Mac OS (mach) & deploying on Linux (ELF), etc.
- Building on an x64 (64-bit) architecture and deploying on i686 (32-bit).
- On Windows: Rust has two toolchains, GNU and MSVC, so care should be taken to choose the correct one for your environment.
Protocol Versioning
The lacca
and resin
programs do not follow standard semantic versioning
guideliens. Please read the following to understand how the project's versions
are controlled. The developer's suggest pinning both a major AND minor version
when using build tools which expect semantic versioning. i.e: pin to
"~> 1.3.0" rather than "~> 1.3" in your build tool. This would allow the patch
version to change, but not the major or minor version.
The version number consits of protocol
, library
, and patch
components.
Their significance to the project is as follows:
protocol
will whenever the protocol betweenlacca
andresin
are changed in non-backwards compatible ways. Newer major versions ofresin
will work with older versions oflacca
, but not vice versa.minor
will be changed to reflect breaking changes to the program's API.Such changes for
lacca
might include:- Removed functions
- Change of non-optional arguments to a function
- Change in return value from a function
Such changes to
resin
might include:- Command line options renamed or removed
- Change in interpretation of command line arguments
patch
will be changed when backwards-compatible changes are introduced to either the protocol, library, or daemon.
NOTE: v0
of the protocol is considered unstable. For best results
you should always build and deploy the v0
programs together.
Protocol Format
shellac
v0 packets are sent on the wire as follows:
- (n) two bytes identifying packet length
- (data) packet specific data (n bytes, CBOR encoded)
Packet Types
Packet types are as follows:
TBD
- Start Process (exec: string, args: [string]])
- Stop Process ()
- DataOut (ty: (Stdout | Stderr), buf: [u8])
- DataIn (buf: [u8])
Future Additions
- POSIX signal support?
- windows support?
- Standard stream redirection?
- Buffered input mode?
Lacca Design
When the lacca
application starts it will kick off a supervision
process. To start an OS-level process lacca
will perform the
following initialization sequence:
Start a child BEAM process w/ the given command line.
Open a port to a
resin
daemon in:binary
mode.Handshake w/ the
resin
daemon to verify matching protocol versions.Pass the initialization arguments to the
resin
daemon.Enter a message loop awaiting messages from the port, as well as messages from other peers in the Erlang VM.
TODO
resin: general clean up of error handling
- what to do with errors if stdout goes away?
- logging facilities?
resin: signal handling architecture
- need more generalized support for sending processes different kinds of
signals, not just calling
Child::kill()
- wtf do we do on Windows land?
- need more generalized support for sending processes different kinds of
signals, not just calling
resin: restructure as state machine?
- there are bits of the server process state which don't exist until the inferior process is running. it would be nice if this was encoded in the type system as some kind of FSM so I can stop unwrapping Option<..> everywhere.
resin: multi proc support
Should one daemon be able to handle multiple inferior processes? I think this is what
erlexec
does but I'm not sure.Not sure this is actually desirable; reduces fault isolation, removes 1:1 mapping of erlang process -> os process which makes it harder to structure OTP applications. there are arguments to be made about resource efficiency though (less vm ports/memory/CPU/file descriptors used, et al.)
lacca: explore stdout/stderr API design space
- with streams how do we signal EOF? (separate packet type?)
- what if user wants to stream by line?
- should we send messages to interested processes instead of a
buffer that is read-once? (
flush()
destroys the contents of the buffer...)
shellac: lightweight protocol
once the protocol is relatively stable I'd like to design a custom wire format. we have pretty straightforward types (integer sizes, byte lists) and CBOR seems to have a fair bit of overhead since rust enums get encoded as dictionaries.
this will be a major breaking change so probably do it before 1.0
shellac: error reporting
internally resin has multiple threads coordinating to manage the process. this means that currently a request, and errors resulting from that request, happen asynchronously.
- for e.g: resin accepts input, passes input to the child, and then encounters an error. -- the client process has already moved on, since it sucesfully wrote the data to the port.
only way I can see to fix this is to either enforce synchrony, or have the client provide a coorleation ID for errors. (basically tag each requests with a unique ID)
that raises the question of what do we use for request IDs, how do we serialize it on the wire, is it part of packet header or packet itself? etc...