Creating a Numerical Simulation Microservice in C++ with Docker

Question

Greetings Stackoverflow community!

I recently learned about the power of microservices and containers, and I decided to wrap some of my numerical simulations codes in C++ and make them available as an API. Here are some requirements/details of my applications:

1. My simulators are coded in C++ with a few dependencies that I link via dynamic or static libraries in windows (e.g. Hypre, for solution of linear systems). They also run in parallel with MPI/OpenMP (in the future I would like to implement CUDA support as well).

2. The input for a simulator is a simple configuration file with some keys (.json format) and a data file (ascii but could be binary as well) with millions of entries (these are fields with one value for each simulation cells, and my models can be as large as 500x500x500 (=125000000 cells).

3. A typical call to the simulator in Windows is: mpiexec -n 4 mysimulator.exe "C:\path\to\config.json". Inside my configuration file I have other absolute path to the ascii file with the cellwise values.

I would like to "containerize" this whole mess and create an api available through HTTP requests or any other protocol that would allow the code to be run from outside the container. While the simulation microservice is running on a remote machine, anyone should be able to send a configuration file and the big ascii or binary file to the container, which would receive the request, perfom the simulation and somehow send back the results (which can be files and/or numerical values).

After some research, I feel this could be achieved with the following approach.

1. Create a docker image with the C++ code. When the container is created using the image as a blueprint, we obtain a binary executable of the C++ simulator.
2. Implement a python interface that handles the incoming requests using flask or django. We listen to requests at a certain port and once we get a request, we call the binary executable using python's subprocess.
3. The simulator somehow needs to send a "simulation status" back since these simulations can take hours to finish.

I have a few questions:

1. Is python "subprocess" call to a binary executable with the C++ code the way to go? Or is it easier/more recommended to implement the treatment to the API calls inside the C++ code?
2. How do you typically send a big binary/ascii file through HTTP to the microservice running inside a docker container?
3. If I have a workstation with - let's say - 16 cores...and I want to allow each user to run at most 2 processors, I could have a max of 8 parallel instances. This way, would I need 8 containers running simultaneously in the computer?
4. Since the simulations take hours to finish, what's the best approach to interact with the client who's requesting the simulation results? Are events typically used in this context?

Thanks, Rafael.

Answer 1

Is python "subprocess" call to a binary executable with the C++ code the way to go? Or is it easier/more recommended to implement the treatment to the API calls inside the C++ code?

1. If you don't have performance concerns, use whatever faster to achieve and easier to scale according to your skills. Use the language that you're comfortable with. If performance is essential, then choose it wisely or refactor them later.

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How do you typically send a big binary/ascii file through HTTP to the microservice running inside a docker container?

2. Depends on the scenario. It's possible to send a data through end point or send them part by part. You may refer to this post for restful update.

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If I have a workstation with - let's say - 16 cores...and I want to allow each user to run at most 2 processors, I could have a max of 8 parallel instances. This way, would I need 8 containers running simultaneously in the computer?

3. Keep your service simple. If one service uses only 1 or 2 cores. Then run multiple instance. Since it's easy to scale rather than create a complex multithreading program.

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Since the simulations take hours to finish, what's the best approach to interact with the client who's requesting the simulation results? Are events typically used in this context?

4. Event would be good enough. Use polling if simulation status is important.

Note: This is more of opinion based post, but it has general scenarios worth answering.