Build environments based on community standards (pixi, mamba, uv)
	Run scripts in those environments as worker processes
	Share memory between processes to avoid copying data

Appose is a library for interprocess cooperation with shared memory. Build environments, run scripts, share data across Java, Python and more. The guiding principles are simplicity and efficiency.

Appose was written to enable easy execution of Python-based deep learning from Java without copying tensors, but its utility extends beyond that. The steps for using Appose are:

• Build an Environment with the dependencies you need.
• Create a Service linked to a worker, which runs in its own process.
• Execute scripts on the worker by launching Tasks.
• Receive status updates from the task asynchronously via callbacks.

Python, Java, and JavaScript, working in harmony, side by side. Separate processes, shared memory, minimal dependencies.

1. Construct an environment. E.g.:

   • Java with dependencies from Maven.
   • Python with dependencies from conda-forge.
   • JavaScript/Node.js with dependencies from NPM (planned).

2. Invoke routines in that environment:

   • Routines are run in a separate process.
   • The routine's inputs and outputs are passed via pipes (stdin/stdout).
   • NDArrays are passed as named shared memory buffers, for zero-copy access across processes.

Each supported language/platform has its own implementation of Appose. You can find examples for each language (calling out to other languages) in each implementation's README:

• Java
• Python

A worker is a separate process created by Appose to do asynchronous computation on behalf of the calling process. The calling process interacts with a worker via its associated service.

Appose comes with built-in support for two worker implementations: python_worker to run Python scripts, and GroovyWorker to run Groovy scripts. These workers can be created easily by invoking the environment object's python() and groovy() methods respectively.

But Appose is compatible with any program that abides by the Appose worker process contract:

1. The worker must accept requests in Appose's request format on its standard input (stdin) stream.
2. The worker must issue responses in Appose's response format on its standard output (stdout) stream.

A request is a single line of JSON sent to the worker process via its standard input stream. It has a task key taking the form of a UUID, and a requestType key with one of the following values:

Asynchronously execute a script within the worker process. E.g.:

{
   "task" : "87427f91-d193-4b25-8d35-e1292a34b5c4",
   "requestType" : "EXECUTE",
   "script" : "task.outputs[\"result\"] = computeResult(gamma)\n",
   "inputs" : {"gamma": 2.2}
}

Cancel a running script. E.g.:

{
   "task" : "87427f91-d193-4b25-8d35-e1292a34b5c4",
   "requestType" : "CANCEL"
}

A response is a single line of JSON with a task key taking the form of a UUID, and a responseType key with one of the following values:

A LAUNCH response is issued to confirm the success of an EXECUTE request.

{
   "task" : "87427f91-d193-4b25-8d35-e1292a34b5c4",
   "responseType" : "LAUNCH"
}

An UPDATE response is issued to convey that a task has somehow made progress. The UPDATE response typically comes bundled with a message string indicating what has changed, current and/or maximum progress indicators conveying the step the task has reached, or both.

{
   "task" : "87427f91-d193-4b25-8d35-e1292a34b5c4",
   "responseType" : "UPDATE",
   "message" : "Processing step 0 of 91",
   "current" : 0,
   "maximum" : 91
}

A COMPLETION response is issued to convey that a task has successfully completed execution, as well as report the values of any task outputs.

{
   "task" : "87427f91-d193-4b25-8d35-e1292a34b5c4",
   "responseType" : "COMPLETION",
   "outputs" : {"result" : 91}
}

A CANCELATION response is issued to confirm the success of a CANCEL request.

{
   "task" : "87427f91-d193-4b25-8d35-e1292a34b5c4",
   "responseType" : "CANCELATION"
}

A FAILURE response is issued to convey that a task did not completely and successfully execute, such as an exception being raised.

{
   "task" : "87427f91-d193-4b25-8d35-e1292a34b5c4",
   "responseType" : "FAILURE",
   "error", "Invalid gamma value"}
}

Q: How about abstracting the transport layer so protocols besides pipes+JSON can be used? Then Appose could work with pipes+pickle, and/or with Google Protocol Buffers, Apache Arrow, NATS.io, over HTTP local loopback, between machines on the cloud...

A: It is tempting. But simplicity is an important core design goal, and additional transport layer implementations would increase complexity. There are already a plethora of existing solutions for interprocess communication, RPC, and data sharing between processes on the same or different machines. The reason Appose exists is to be less complicated than those other solutions, while supporting dynamic construction of subprocess environments, and access to large data in shared memory.

Q: What about more data types for inputs and outputs? Appose could be plugin driven, with extension libraries registering additional externalization routines to convert their own favorite kinds of objects to and from JSON.

A: Again, tempting! But nailing down (via either invention or reuse) a plugin mechanism for each supported language would increase the size of the codebase, and the modularization would make it more complicated to depend on Appose. Have you included the right plugins in your dependency set? Do they all have the right versions? Where is the bill of materials keeping all of the dependencies in sync? Etc. For now, Appose strives to be self-contained with all supported types handled by one single library per target language.

• CuVec
• Apache Arrow
• NATS.io