Add health / loadbalancer endpoints for Mozilla Dockerflow

[spwoodcock]

Is your feature request related to a problem? Please describe.

• There isn't much standardisation across containerisation of web APIs.
• There is a good attempt taken by Mozilla: Dockerflow spec
• We should add these endpoints to help DevOps.

Describe the solution you'd like

• Always place app files under /app dir inside container.
• A file /app/version.json should be generated within the CI workflow and baked into the built image:

  {
    "source" : "https://github.com/hotosm/fmtm", 
    "version": "release tag or string for humans. can be blank for non-release", 
    "commit" : "<git hash>",
    "build"  : "uri to CI build job"
  }

• Respond to /__version__ with the contents of /app/version.json.
• Respond to /__heartbeat__ with a HTTP 200 or 5xx on error. This should check backing services like a database for connectivity and may respond with the status of backing services and application components as a JSON payload.
• Respond to /__lbheartbeat__ with an HTTP 200. This is for load balancer checks and should not check backing services.
• Always log to terminal (STDOUT), never to a file.
• Backend serves its own static, frontend serves it's own static. I.e. do not rely on public/static directory of frontend from backend.

Describe alternatives you've considered

I don't know any other good specs for this.

[spwoodcock]

Example implementation:

from pathlib import Path
from fastapi import Depends
from fastapi.responses import JSONResponse, Response
from loguru import logger as log
from sqlalchemy import text
from sqlalchemy.orm import Session

from app.__version__ import __version__
from app.db.database import get_db
from app.models.enums import HTTPStatus


@api.get("/__version__")
async def deployment_details():
    """Mozilla Dockerflow Spec: source, version, commit, and link to CI build."""
    details = {}

    version_path = Path("/opt/version.json")
    if version_path.exists():
        with open(version_path, "r") as version_file:
            details = json.load(version_file)
    commit = details.get("commit", "commit key was not found in file!")
    build = details.get("build", "build key was not found in file!")

    return JSONResponse(status_code=HTTPStatus.OK, content={
        "source" : "https://github.com/hotosm/fmtm", 
        "version": __version__,
        "commit" : commit or "/app/version.json not found",
        "build"  : build or "/app/version.json not found"
    })


@api.get("/__heartbeat__")
async def heartbeat_plus_db(db: Session = Depends(get_db)):
    """Heartbeat that checks that API and DB are both up and running."""
    try:
        db.execute(text("SELECT 1"))
        return Response(status_code=HTTPStatus.OK)
    except Exception as e:
        log.warning(e)
        log.warning("Server failed __heartbeat__ database connection check")
        return JSONResponse(
            status_code=HTTPStatus.INTERNAL_SERVER_ERROR, content={"error": str(e)}
        )


@api.get("/__lbheartbeat__")
async def simple_heartbeat():
    """Simple ping/pong API response."""
    return Response(status_code=HTTPStatus.OK)