docs: add info on fastapi async/sync tasks

docs/dev-guide/web-frameworks.md

@@ -2,4 +2,183 @@
 
 ## Python
 
+Frameworks are generally divided into synchronous and asynchronous.
+
+Async is a newer paradigm in Python, often slightly more complex to code,
+but should be faster and more suited to a web API.
+
+Synchronous frameworks include **flask**, **Django**, etc.
+
+The asynchronous framework we recommend at HOT, as of 2024, is **FastAPI**.
+It's what we use for most projects.
+
+There is a great [comparison](https://fastapi.tiangolo.com/alternatives/)
+with other frameworks in the ecosystem available.
+
+Another contender would be [LiteStar](https://github.com/litestar-org/litestar),
+a project spawned from some frustrations with the governance of FastAPI.
+
+### FastAPI
+
+These docs provide some helpful info for FastAPI best practices.
+
+#### Async Programming
+
+Asynchronous programming can be a learning curve for Python developers.
+
+- FastAPI is an asynchronous web framework that is built to use async code.
+- Using async (`async def`) function with await is more scalable than
+  using synchronous code `def`, so this is always the preferred default
+  approach.
+- Using synchronous code is possible, but devs should be aware of the pitfalls:
+  if the code runs for a long time, it will block the async event loop
+  (i.e. block the thread until the process completes).
+  - Bear in mind that 'synchronous' code could be from what you write
+    in the crud functions, OR could be from a library that you use
+    (e.g. osm-fieldwork is synchronous for the most part).
+
+#### Workers & Thread Blocking
+
+- We run FastAPI (uvicorn) with a number of workers defined. This is the
+  number of threads available to run processes.
+- If a process blocks a thread (as described above), then the remaining threads
+  are available to take new requests.
+- If all of the workers/threads are blocked by tasks, the server will hang / be unresponsive!
+
+##### Using Synchronous Code
+
+It is of course possible to use synchronous code, but if necessary, be
+sure to run this in another thread.
+
+To do this you have several options.
+
+#### Options
+
+##### 1) Using sync code within an `async def` function
+
+- Use the BackgroundTasks implementation we have, with polling for the
+  task completion.
+- The task should be written as a standard `def`. FastAPI will handle
+  this automatically and ensure it runs in a separate thread.
+- Alternatively, if you wish to run the task in the foreground and return
+  the response, use the FastAPI helper `run_in_threadpool`:
+
+```python
+from fastapi.concurrency import run_in_threadpool
+
+def long_running_sync_task(time_to_sleep):
+    sleep(time_to_sleep)
+
+async def some_func():
+    data = await run_in_threadpool(lambda: long_running_sync_task(time_to_sleep))
+```
+
+##### 2) Running multiple standard `def` from within an `async def` function
+
+- Sometimes you need to run multiple `def` functions in parallel.
+- To do this, you can use ThreadPoolExecutor:
+
+```python
+from concurrent.futures import ThreadPoolExecutor, wait
+
+def a_synchronous_function(db):
+    # Run with expensive task via threadpool
+    def wrap_generate_task_files(task):
+        """Func to wrap and return errors from thread.
+
+        Also passes it's own database session for thread safety.
+        If we pass a single db session to multiple threads,
+        there may be inconsistencies or errors.
+        """
+        try:
+            generate_task_files(
+                next(get_db()),
+                project_id,
+                task,
+                xlsform,
+                form_type,
+                odk_credentials,
+            )
+        except Exception as e:
+            log.exception(str(e))
+
+    # Use a ThreadPoolExecutor to run the synchronous code in threads
+    with ThreadPoolExecutor() as executor:
+        # Submit tasks to the thread pool
+        futures = [
+            executor.submit(wrap_generate_task_files, task)
+            for task in tasks_list
+        ]
+        # Wait for all tasks to complete
+        wait(futures)
+```
+
+Note that in the above example, we cannot pass the db object from the parent
+function into the functions spawned in threads. A single database
+connection should not be written to by multiple processes at the same time,
+as you may get data inconsistencies. To solve this we generate a new
+db connection within the pool for each separate task we run in a thread.
+
+> To avoid issues, look into limiting the thread usage via:
+> <https://stackoverflow.com/questions/73195338/how-to-avoid-database-connection-pool-from-being-exhausted-when-using-fastapi-in>
+
+##### 3) Running an `async def` within a sync `def`
+
+- As we try to write most functions async for FastAPI, sometime we need to
+  run some `async def` logic within a sync `def`. This is not possible normally.
+- To avoid having to write a duplicated `def` equivalent of the `async def`
+  code, we can use the package `asgiref`:
+
+```python
+from asgiref.sync import async_to_sync
+
+async def get_project(db, project_id):
+    return something
+
+def a_sync_function():
+     get_project_sync = async_to_sync(get_project)
+     project = get_project_sync(db, project_id)
+     return project
+```
+
+##### 4) Efficiency running batch async tasks
+
+- Sometime you may have a very efficient async task you need to call
+  within a for loop.
+- Instead of that, you can use `asyncio.gather` to much more efficiently
+  collect and return the async data (e.g. async web requests, or async
+  file requests, or async db requests):
+
+```python
+from asyncio import gather
+
+async def parent_func(db, project_id, data, no_of_buildings, has_data_extracts):
+    ... some other code
+
+    async def split_multi_geom_into_tasks():
+        # Use asyncio.gather to concurrently process the async generator
+        split_poly = [
+            split_polygon_into_tasks(
+                db, project_id, data, no_of_buildings, has_data_extracts
+            )
+            for data in boundary_geoms
+        ]
+
+        # Use asyncio.gather with list to collect results from the
+        # async generator
+        return (
+            item for sublist in await gather(*split_poly)
+            for item in sublist if sublist
+        )
+
+    geoms = await split_multi_geom_into_tasks()
+```
+
+#### Note
+
+- If you regularly find you are running out of workers/threads and the
+  server is overloaded, it may be time to add a task queuing system to your stack.
+- Celery is made for just this - defer tasks to a queue, and run gradually
+  to reduce the immediate load.
+
 ## JavaScript