#7 | ENTRYPOINT and CMD

29 February 2024

454 words

docker

When a docker container kicks off the ENTRYPOINT instruction is run, and the commands/arguments from the CMD instruction are passed to it as arguments.

Executable Pattern

When you want the docker container to act like an executable you can use CMD to pass arguments to the ENTRYPOINT. Below is an example that will fetch data from the Irish Single Electricty Market website.

Here’s the dockerfile called entrypoint_cmd.docker. Note that there is no CMD instruction because it will be specified when we run the container.

FROM python:3.11.8-bullseye

WORKDIR /usr/src/app
COPY ./requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

COPY ./main.py  .

ENTRYPOINT ["python", "main.py"]

And here is main.py.

import requests
import sys
import xml.etree.ElementTree as ET
from datetime import datetime, timedelta

SEMO_MINIMAL_COST_URL = "https://reports.sem-o.com/documents/PUB_30MinImbalCost_{period}.xml"
SEMO_PERIOD_FORMAT = "%Y%m%d%H%M"

def fetch_semo_xml(period: str):
    """
    Fetch semo data for a period. This won't be tested.

    Args:
    - period (str): The start time of the period to fetch .

    Returns:
     XML as a string or None if there is an error

    """
    semo_url = SEMO_MINIMAL_COST_URL.format(period=period)
    response = requests.get(semo_url)

    if response.status_code != 200:
        raise Exception(f"Semo data unavailable for period: {period}")

    return response.text

def parse_semo_xml(period: str, semo_xml: str):
    """
    Fetch semo data for a period.

    Args:
    - semo_xml (str): The xml to parse.

    Returns:
     A dictionary of attributes from the xml or None if there is an error parsing the xml.

    """
    try:
        xml_root = ET.fromstring(semo_xml)
        imbalance_xml = xml_root.find("PUB_30MinImbalCost")
        xml_as_dict =  {"period": period,
                        "imbalance_volume": float(imbalance_xml.attrib.get('ImbalanceVolume')),
                        "imbalance_price": float(imbalance_xml.attrib.get('ImbalancePrice')),
                        "imbalance_cost": float(imbalance_xml.attrib.get('ImbalanceCost'))}
    except:
        raise Exception(f"Unable to parse Semo xml.")
    
    return xml_as_dict

def main():

    if len(sys.argv)<2:
        current_time = datetime.now()
        new_time = current_time - timedelta(hours=2)
        period = new_time.strftime('%Y%m%d%H') + "00"
    else:
        period = sys.argv[1]
    
    semo_xml = fetch_semo_xml(period=period)
    semo_xml_as_dict = parse_semo_xml(period=period, semo_xml=semo_xml)
    print(semo_xml_as_dict)

if __name__ == "__main__":
    main()

First build the image using docker build -t semo_image -f "./entrypoint_cmd.docker" .. Then run the container to get data from a specific half hour with docker run -t --rm semo_image "202402241030". Any additional arguments provided overwrite the CMD instruction in the Dockerfile (if there are any), so in this case "202402241030" is passed to the ENTRYPOINT script. Here’s the output:

{'period': '202402241030', 'imbalance_volume': -2.923, 'imbalance_price': 101.15, 'imbalance_cost': -295.66145}

If no CMD argument is specified, i.e. docker run -t --rm semo_image, then this little example is setup to get the data from 2 hours ago.

Setup Pattern

Another typical pattern seems to be using the ENTRYPOINT script to carry out setup that is required before the CMD commands are run. For an example see this mysql Dockerfile where the ENTRYPOINT does lots of setup and then the last command is exec "$@" which will exec the CMD arguments that are passed to the ENTRYPOINT.

Documented

Unsurprisingly this is all very well documented but great to put together a little example nonetheless.