A forum to MailChimp sync that stopped resending the whole list

A custom extension pushed every forum member's email address to MailChimp on every run. I changed it to send only the addresses added since the last push.

The objective

A developer had built an extension that sent the forum's user emails to MailChimp roughly every hour. It sent the entire user table each time rather than just the new addresses.

Besides being wasteful, that had a cost attached. Pushing the whole user base into a new list would have moved the client onto a higher MailChimp plan.

So the change had to be right the first time. There was no room to test it by running it against the live list and seeing what happened.

What I did

I added tracking of what had already been pushed, so each run only sends the addresses added since the previous one.

I left the other fields the extension recorded alone, as the client asked.

And I tested against a separate list, so the real list and their subscription tier were never at risk.

The result

The hourly job now sends only new addresses. The list stayed as it was and their plan didn't change.