Currently, Atlas App Services (formerly Stitch) builds a library libmongo.so (for our linux envs) and assisted_agg (mainly exists for intel macOS) from the 4.4 server branch to accomplish to limit stages and apply rule expressions and filters to Atlas App Services Function Users.

The API we utilize is captured in stub.cpp

We extend the MongoProcessInterface found in baas_mongo_process_interface.h and utilize some utils.

Additional files include:
aggregate.cpp
aggregate.h
document_source_stitch.cpp
document_source_stitch.h
main.cpp
utils.h

In order to build the binaries we append code to the Sconscript in src/mongo/SConscript. Then call the build script while including all the cpp files we have on our side.

In order to maintain a healthier build process as this is in frequently updated or touched, this API should be exposed to the existing stitch_support library (reducing the number of needed binaries from 2 down to 1), with server producing it for Atlas App Services.