Logic Apps are so expensive!!

I started writing EnfieldTownBot using Azure Logic Apps. It’s pretty easy, but i soon hit a challenge – it’s so expensive! My app was pretty simple – a trigger, a “for…each”, a condition and a http callout to my Twitter Poster Function App:


So – if there are no delays, that’s (recurrence + httprequest + foreach + 3 x (condition)) = 6 actions. There could be up to 9 if the postToTwitter action also triggers. I want to run this function 2x (once for trains FROM Enfield Town, once for trains TO Enfield Town), so that’s 12-18 actions per request. And I want to run it every 15-30 seconds to get the latest information published ASAP. So that’s 48-72 actions per minute. Over a day, that’s 69,120 to 103,680 actions. Over a (31 day) month, that’s 2,142,720 to 3,214,080 actions. Taking a mean of these (2,678,400), and looking at the current pricing, it would cost me £450 a month to run this app. Wow. I don’t care about late trains THAT much…

So – I rewrote the whole lot as a function app (actually 2 or 3 function apps, as some of the items, such as parsing the XML from National Rail to JSON are reusable elsewhere). Each execution takes about 300ms, and it executes (24 hours x 60 minutes per hour x 4 times per minute) = 5,760 per day. Over a month that’s 178,560 executions. That’s well within the “permanent free grant” provided for Functions. In fact, I’d have to run it something like 30,000,000 times per month, or about 600 times per second to even hit a cost of £1.