Hacker News Slackbot

In this example, we use Modal to deploy a cron job that periodically queries Hacker News for new posts matching a given search term, and posts the results to Slack.

Import and define the app

Let’s start off with imports, and defining a Modal app.

import os
from datetime import datetime, timedelta

import modal

app = modal.App(
    "example-hn-bot"
)  # Note: prior to April 2024, "app" was called "stub"

Now, let’s define an image that has the slack-sdk package installed, in which we can run a function that posts a slack message.

slack_sdk_image = modal.Image.debian_slim().pip_install("slack-sdk")

Defining the function and importing the secret

Our Slack bot will need access to a bot token. We can use Modal’s Secrets interface to accomplish this. To quickly create a Slack bot secret, navigate to the create secret page, select the Slack secret template from the list options, and follow the instructions in the “Where to find the credentials?” panel. Name your secret hn-bot-slack, so that the code in this example still works.

Now, we define the function post_to_slack, which simply instantiates the Slack client using our token, and then uses it to post a message to a given channel name.

@app.function(
    image=slack_sdk_image, secrets=[modal.Secret.from_name("hn-bot-slack")]
)
async def post_to_slack(message: str):
    import slack_sdk

    client = slack_sdk.WebClient(token=os.environ["SLACK_BOT_TOKEN"])
    client.chat_postMessage(channel="hn-alerts", text=message)

Searching Hacker News

We are going to use Algolia’s Hacker News Search API to query for posts matching a given search term in the past X days. Let’s define our search term and query period.

QUERY = "serverless"
WINDOW_SIZE_DAYS = 1

Let’s also define an image that has the requests package installed, so we can query the API.

requests_image = modal.Image.debian_slim().pip_install("requests")

We can now define our main entrypoint, that queries Algolia for the term, and calls post_to_slack on all the results. We specify a schedule in the function decorator, which means that our function will run automatically at the given interval.

@app.function(image=requests_image)
def search_hackernews():
    import requests

    url = "http://hn.algolia.com/api/v1/search"

    threshold = datetime.utcnow() - timedelta(days=WINDOW_SIZE_DAYS)

    params = {
        "query": QUERY,
        "numericFilters": f"created_at_i>{threshold.timestamp()}",
    }

    response = requests.get(url, params, timeout=10).json()
    urls = [item["url"] for item in response["hits"] if item.get("url")]

    print(f"Query returned {len(urls)} items.")

    post_to_slack.for_each(urls)

Test running

We can now test run our scheduled function as follows: modal run hackernews_alerts.py::app.search_hackernews

Defining the schedule and deploying

Let’s define a function that will be called by Modal every day

@app.function(schedule=modal.Period(days=1))
def run_daily():
    search_hackernews.remote()

In order to deploy this as a persistent cron job, you can run modal deploy hackernews_alerts.py,