Ask HN: Offline mode for SaaS

im planning to build a SaaS, my potential clients will generate transactions every 15 minutes or so, they also have very weak internet links cause they dont live in the city, so it could be the case that internet is down for more than 30 minute. I guess the solution would be to build an "offline" mode, but im not quite sure how can i do that or where should i even start, any pointers in the right direction would be appreciated.

thanks, B.