Is there any system to Cathay Pacific’s service order in economy?

Is it Front to Back, or Back to Front?

After a jaunt from Australia back to the UK on A330s and A340s, I ended up both times in row 54 (the emergency exit row).

On one flight service started at the back of each cabin and worked its way towards the front. No choice for me!

Another flight... This time, service front to back, but starting at row 55... so when the forward cabin trolley finally got to me... again, no choice!

Separately, is there a rule about when the in flight economy class snackbar appears? It wasn’t there from SYD to HKG, but from HKG to LHR there was one on a trolley laid out with sandwiches, nuts, cans, Timtams, a whole host of snacks... is it only on longer flights? (plus, of course, I can ask for the Noddles, but I like walking up the aisle and grabbing Timtam every now and then in the flight if I'm awake).

I couldn’t make head or tail of it, so to speak, and wondered if there is a rule?