sync 30seconds btw my vps database to local server of particular client

I have a complete restaurant order system already in my website. But, all transaction is done on website server. I am facing one problem. sometimes when it is slow on internet connection. So, i want to sycn the database of my vps server for a particular client to his own local server for 30 seconds per sycn. Meaning that, when there is a new order recorded into my vps server, the information will be sycn to my client database of his own local server for that particular new order. The client will then process the order in his local system. After the order is completed, he will update the information to his database. Then, the database will again sycn to my VPS server. Then, the user will be formed of the lastest status of his order. In this manner, when the order is processed within the local system, the process will not be affected by internet line, server breakdown. I only need all related monetary transactions are sycn and a simple chatroom is sycn. The community chatroom will not be required to sycn, as it is very complicated. So, you only need to sycn for the order.