How can I keep Google Cloud Functions warm?

With all "serverless" compute providers, there is always going to be some form of cold start cost that you can't eliminate. Even if you are able to keep a single instance alive by pinging it, the system may spin up any number of other instances to handle current load. Those new instances will have a cold start cost. Then, when load decreases, the unnecessary instances will be shut down.

There are ways to minimize your cold start costs, as you have discovered, but the costs can't be eliminated.

As of Sept 2021, you can now specify a minimum number of instances to keep active. This can help reduce (but not eliminate) cold starts. Read the Google Cloud blog and the documentation. For Firebase, read its documentation. Note that setting min instances incurs extra billing - keeping computing resources active is not a free service.

If you absolutely demand hot servers to handle requests 24/7, then you need to manage your own servers that run 24/7 (and pay the cost of those servers running 24/7). As you can see, the benefit of serverless is that you don't manage or scale your own servers, and you only pay for what you use, but you have unpredictable cold start costs associated with your project. That's the tradeoff.

You're not the first to ask ;-)

The answer is to configure a remote service to periodically call your function so that the single|only instance remains alive.

It's unclear from your question but I assume your Function provides an HTTP endpoint. In that case, find a healthcheck or cron service that can be configured to make an HTTP call every x seconds|minutes and point it at your Function.

You may have to juggle the timings to find the Goldilocks period -- not too often that that you're wasting effort, not too infrequently that it dies -- but this is what others have done.