Recently, we selected an agricultural forecasting application (AgForecast), developed by the Data Science Institute at UW–Madison, to port to OpenLambda: UW-Madison-DSI/ag_forecasting_api. AgForecast is an interesting case study, because it implements its REST API using FastAPI, which in turn uses ASGI, the so-called “spiritual successor” to WSGI, which we recently started supporting in OpenLambda. WSGI is the basis for popular Python web-programming packages such as Django and Flask; new ASGI support opens the door to an even broader range of applications.
In this post, we describe the challenges of porting AgForecast to OL, and four new features we added to OL to make deployment of similar applications in the future simpler. The features are built-in ASGI support, direct GitHub-to-OL deployments, OL function environment variables, and OL-based pip compilation.
+
+Background: Agricultural Forecasting API
+Motivation. The agricultural forecasting app is an open-source tool designed to address farmers’ needs in a customizable way. We first built a backend infrastructure using FastAPI to serve crop disease forecasting models for corn and soybean, focusing on integration with Wisconet weather stations and on-demand model serving. We then developed a custom R Shiny front-end interface for farmer use.
+Fit with OpenLambda. We began exploring OpenLambda to leverage the benefits of serverless technology. Hosting our tool on this platform offers significant value not only to the developer community, but also to plant pathology practitioners and scientists who want to build on robust, nationwide infrastructure.
+Technical overview. The Ag Forecasting API is a FastAPI-based backend that serves crop disease forecasting models for corn and soybean across Wisconsin. It exposes two parallel data pipelines through a unified interface: one sourced from Wisconet, the public mesonet of weather stations across the state, and another from IBM Environmental Intelligence for point-location queries by latitude and longitude. The core logic lives in the ag_models_wrappers module, which dynamically pulls the daily and hourly weather variables each disease model requires for a given forecasting date, runs the risk calculations, and returns localized predictions. Currently supported models include Sporecaster (white mold in soybean, with dry and irrigated row-spacing variants), Tarspotter (tar spot of corn), gray leaf spot, and frogeye leaf spot — all based on peer-reviewed plant pathology research from UW–Madison.
+Infrastructure and integration. The project ships with a Dockerfile and docker-compose.yml for containerized deployment, and includes a Starlette WSGIMiddleware wrapper so the FastAPI app can also be served behind WSGI servers for legacy or mixed environments. A companion sub-package, pywisconet, provides a thin REST wrapper over the Wisconet v1 API with endpoints for active station discovery (/wisconet/active_stations/), station field metadata (/station_fields/{station_id}), and bulk measurement retrieval (/bulk_measures/{station_id}) at configurable frequencies (5-minute, hourly, or daily) for variables like air temperature, dew point, wind speed, and relative humidity. IBM credentials are handled via environment variables (IBM_API_KEY, TENANT_ID, ORG_ID) and validated against query parameters at request time. The API is MIT-licensed, fully open source, and currently deployed at connect.doit.wisc.edu/ag_forecasting_api.
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+
+Porting to OpenLambda
+When porting AgForecast to OL, we encountered 5 challenges related to: expectations about writable directories, package version selection, deployment from GitHub, asynchronous execution, and parallel pool execution. To overcome these challenges, we introduced four new features to OL and made minor changes to AgForecast itself.
+
+Challenge 1: File Management
+AgForecast is semi-stateless: data files describing stations and measurements are used across requests, but if these files are deleted, it can generate them on-the-fly from upstream data sources, such as the IBM Weather API or Wisconet. This is a good match for FaaS platforms such as OL, where lambda instances frequently persist (along with their state) across multiple invocations, even if an instance can silently be terminated at any time to reclaim memory.
+However, most directory locations are read-only for an OL function; the one exception is a single “scratch directory” and various other locations that are symbolic links to the scratch directory (such as /tmp). Full-featured sandboxes such as Docker containers use union file systems to make many directories editable, on a copy-on-write basis. OL’s limitation is due to its use of bind mounts, a leaner, but less flexible mechanism. AgForecast wasn’t originally built for OL, so we modified the code to make the directory location for station and measurement data configurable via environment variables (otherwise AgForecast attempted to write to read-only locations). We added support to OL function configuration files to support the specification of environment variables, like this:
+triggers:
+ http:
+ - method: "*"
+environment:
+ MEASUREMENTS_CACHE_DIR: /host/tmp/cache
+ STATIONS_CACHE_FILE: /host/tmp/cache/wisconsin_stations_cache.csv
+ ...
+
+
Challenge 2: Package Management
+Like many Python projects, AgForecast specifies PyPI package requirements in a requirements.txt file. Also like most projects, not all version requirements are exact. Here are 3 of the 21 lines in AgForecast’s requirements.txt:
+...
+matplotlib==3.9.1
+fastapi>=0.95.0
+pydantic
+...
+
+
Note the different levels of specificity: matplotlib must be a specific version, whereas pip can select any version for pydantic (probably the latest, barring version conflicts based on other dependencies); fastapi specifies a range. Note that some of these might also have indirect dependencies on other packages not explicitly listed.
+OL requires exact versions for all packages, direct or indirect. For this purpose, we recommend the use of pip-compile to translate a partially specified requirements.txt file to a fully specified one, based on the latest packages at the time of compilation.
+One challenge is that pip and pip-compile sometimes select packages based on the host environment. For example, in certain cases, the suitable Python package version for Ubuntu 24.04 might be different than for Ubuntu 26.04. Initially, we encountered this when deploying AgForecast on OL. The requirements.txt created by doing pip-compile on the host machine was not quite compatible with the environment inside the lambda function. To address this, we created a new OL function that does pip-compile inside the OL environment. It works like this:
+curl -X POST -d '<some URL>' http://localhost:5000/run/pip-compile/url > requirements.txt
+
+
Challenge 3: Deployment
+AgForecast lives on a public GitHub repo: UW-Madison-DSI/ag_forecasting_api. We wanted to make it as easy as possible to deploy directly from GitHub to an OL function. Thus, we added a new ol admin install option to point directly to a repo:
+./ol admin install -c ol.yaml -r requirements.txt https://github.com/<org>/<repo>.git
+
+
Challenge 4: Asynchronous Execution
+AgForecast is built on FastAPI, which in turn is based on ASGI. ASGI is an asynchronous alternative to WSGI (Web Server Gateway Interface). The idea of WSGI is to let you mix and match servers (for example, Gunicorn, uWSGI) with application frameworks (for example, Flask, Django). The server/framework interface is minimalist, a single function signature that the server calls for each incoming HTTP request (GET/POST/etc). The framework implements the function; a common framework pattern is to route the call to a user-written handler function. For example, consider these two functions:
+app = Flask(__name__)
+
+@app.route("/")
+def home():
+ return "Home page"
+
+@app.route("/about")
+def about():
+ return "About page"
+
+
The app object is a Python callable (meaning it is an object that acts like a function); app implements the WSGI interface. So when a server sends a request to app, app in turn calls the correct user function (home or about) to execute and obtain a result.
+ASGI was introduced as an alternative to WSGI to provide more options for handling concurrent calls. Consider how (in the above Flask/WSGI example) two different users may want to visit the home and about pages at the same time. Can we handle the requests concurrently?
+There are a few ways to do this: multiple processes, multiple threads in a process, or Python’s async functionality introduced in Python 3.5 (2015). If we want all execution in a single process (useful when there is shared state), we can either use threading (with WSGI) or async (with ASGI).
+Threads are a non-cooperative form of scheduling, meaning that a scheduler can switch from running one thread on a CPU to another thread at any time (perhaps a very inconvenient time!). Writing multi-threaded programs is notoriously difficult, as one must identify shared state, introduce locks to protect that state, and acquire/release locks at the right points. In contrast, async offers a form of cooperative scheduling, where switches can only occur at well-defined points (e.g., an await). Thus, programming is simpler (no need for locks).
+Normally, concurrent programs allow (a) parallel execution on multiple CPU cores at the same time and (b) execution of code at the same time that input/output occurs, say to the disk or network. In many languages, threads offer both benefits and cooperative scheduling only provides the I/O benefit. However, Python threads only offer the I/O benefit due to the GIL (Global Interpreter Lock), though this may be changing. Thus, async is especially appealing in Python since it (in theory) matches threading in terms of performance benefits, and surpasses threads in terms of ease-of-programming.
+AgForecast implements REST calls in FastAPI, which is based on ASGI, which is the async-based alternative to WSGI. To support AgForecast, we implemented ASGI server functionality in OpenLambda. This makes OpenLambda an ASGI server implementation (in the same role as Uvicorn, Gunicorn, Daphne, etc). When a request arrives, OpenLambda uses asyncio.run(...) to invoke the user-provided entry point, with async send/receive callbacks. The user-provided entry-point can then be a full application, written in any ASGI-compatible framework (FastAPI, Starlette, Django/Channels, etc).
+A user can indicate their lambda function is an ASGI application entry point by configuring their ol.yaml as follows:
+environment:
+ OL_ENTRY_FILE: app.py
+ OL_ASGI_ENTRY: app
+
+
Challenge 5: Worker Pools
+AgForecast indirectly uses /dev/shm, an in-memory file system called tmpfs. Docker containers have a /dev/shm mount by default; as it is frequently used for inter-process communication, its behavior is configurable via the --ipc flag. In contrast, OL functions do not have any /dev/shm mount.
+Why does AgForecast need /dev/shm? AgForecast uses a concurrent.futures.ProcessPoolExecutor to run a Python compute-heavy function called compute_risks in parallel over different chunks of data. ProcessPoolExecutor creates different Python processes for different chunks of work, which is desirable as a way to get around Python’s per-process GIL (Global Interpreter Lock), which is held whenever regular Python code is being executed. With multiple processes (created by the pool), each process will have its own GIL that it holds during execution; thus, multiple processes can hold their own locks at the same time and execute on multiple CPU cores in parallel.
+As a fix to get AgForecast working as an OL function, we replaced ProcessPoolExecutor with a ThreadPoolExecutor; this avoids inter-process communication via /dev/shm because coordination occurs within a process, between threads. Unfortunately, all the threads share a GIL, so the performance benefits of using a pool for parallelism is lost in this case (the only value would be if compute_risks were I/O heavy, which it is not).
+Using a ThreadPoolExecutor works as a short term fix, but this experience suggests that eventually adding /dev/shm availability (and thus ProcessPoolExecutor functionality) would be a useful future feature for OL.
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