How to deploy Streamlit apps to Google App Engine

Things to know
There are some things to know when you deploy Streamlit apps to App Engine.
The flexible environment is mandatory
You have to choose the flexible environment because it supports WebSockets and the standard environment does not.
Streamlit heavily relies on WebSockets for the communication between the server and the client.
This is a screenshot of https://cloud.google.com/appengine/docs/the-appengine-environments on 2021/12/12. The left is about the standard env and the right is the flexible env.
Custom runtime is not necessary
You do not have to use a custom runtime (an original Docker image).
The flexible environment offers an official Python runtime as https://cloud.google.com/appengine/docs/flexible/python/runtime and you can use it.
You can also use a custom runtime. For example, it is a nice option when you want to use a different Python version from the one provided as the official runtime or when you already have a working Docker image.
NOTE:
As of 2021/12/12, the document says the Python version on the built-in Python3 runtime is 3.7.2, but it is actually 3.6.10 known from sys.version at least in the asia-northeast1 zone (Tokyo).
And Streamlit officially supports only Python>=3.7 while it is technically possible to be installed with Python 3.6, so maybe you should set up a custom runtime with Python>=3.7 following the section below.
The number of instances should be 1 if st.file_uploader or st.download_button is used
If your app contains st.file_uploader or st.download_button, you should set the maximum number of instances to 1. For that configuration, see https://cloud.google.com/appengine/docs/flexible/python/reference/app-yaml#services.
App Engine typically distributes the requests evenly among available instances so the file upload/download requests sometimes reach the instance different from the one where the session exists when there are multiple instances.
Errors as below appear in such cases.
This problem occurs with the file uploader/downloader components because they use normal stateless HTTP POST/GET requests while other components work on top of WebSocket connections consistent over sessions.
While I know this problem occurs at least with these components, I'm not sure whether there is another component which have this problem. I have not checked all.
NOTE:
App Engine offers the session affinity setting, but it does not help in this case because it is only for HTTP long polling like socket.io as documented though this problem is due to a different reason.
Recipes
I will show some examples deploying Streamlit apps to App Engine in different situations.
Each of them can be deployed with the command below.
$ gcloud app deploy
All sample resources are available at https://github.com/whitphx/streamlit-appengine-samples .
Basic setup
This is the simplest one.
  • No custom runtime
  • No config on the number of instances (scaling)
    • This app is not using file uploader or downloader.
  • File list
    .
    ├── app.yaml
    ├── requirements.txt
    └── streamlit-app.py
    app.yaml
    runtime: python
    env: flex
    
    runtime_config:
      python_version: 3
    
    entrypoint: streamlit run streamlit-app.py --server.port $PORT
  • The flexible environment is selected.
  • The built-in Python3 runtime is used.
  • entrypoint is configured to run the Streamlit process with the specified port number via the $PORT environment variable.
  • requirements.txt
    streamlit~=1.2.0
    streamlit-app.py
    import streamlit as st
    
    st.title("App Engine sample app")
    
    name = st.text_input("Your name?")
    
    st.write(f"Hello, {name or 'world'}!")
    File uploader and downloader
    This is a sample with a file uploader and a downloader.
    File list
    .
    ├── app.yaml
    ├── requirements.txt
    └── streamlit-app.py
    app.yaml
    runtime: python
    env: flex
    
    runtime_config:
      python_version: 3
    
    entrypoint: streamlit run streamlit-app.py --server.port $PORT
    
    automatic_scaling:
      max_num_instances: 1
    
    # Or manual scaling as below:
    # manual_scaling:
    #   instances: 1
    In addition to the basic set-up, automatic_scaling.max_num_instances is set to 1.
    If you want to use the manual scaling, use manual_scaling.instances instead.
    requirements.txt
    streamlit~=1.2.0
    streamlit-app.py
    import streamlit as st
    
    st.title("App Engine sample app")
    
    uploaded_file = st.file_uploader("Upload some file")
    if uploaded_file:
        st.write(f"{uploaded_file.name} was uploaded.")
    
        st.download_button(f"Download {uploaded_file.name}", data=uploaded_file, file_name=uploaded_file.name)
    Custom runtime
    This sample uses a custom runtime with Dockerfile.
    .
    ├── Dockerfile
    ├── app.yaml
    ├── requirements.txt
    └── streamlit-app.py
    app.yaml
    runtime: custom
    env: flex
  • Set runtime: custom to use a custom runtime.
  • entrypoint is not needed as it is defined in Dockerfile.
  • Dockerfile
    FROM gcr.io/google-appengine/python
    
    # Ref:
    # * https://github.com/GoogleCloudPlatform/python-runtime/blob/8cdc91a88cd67501ee5190c934c786a7e91e13f1/README.md#kubernetes-engine--other-docker-hosts
    # * https://github.com/GoogleCloudPlatform/python-runtime/blob/8cdc91a88cd67501ee5190c934c786a7e91e13f1/scripts/testdata/hello_world_golden/Dockerfile
    RUN virtualenv /env -p python3.7
    
    ENV VIRTUAL_ENV /env
    ENV PATH /env/bin:$PATH
    
    ADD requirements.txt /app/
    RUN pip install -r requirements.txt
    
    ADD . /app
    ENTRYPOINT [ "streamlit", "run", "streamlit-app.py", "--server.port", "8080" ]
  • Use the base image officially provided by Google for App Engine, gcr.io/google-appengine/python.
  • Specify the Python version at virtualenv creation.
  • Some Dockerfiles of the test data in that repository can be references
  • The main process has to listen to the port 8080 in a custom runtime.
  • requirements.txt
    streamlit~=1.2.0
    streamlit-app.py
    import sys
    
    import streamlit as st
    
    
    st.write(sys.version)

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    How to deploy Streamlit apps to Google App Engine