How to design a cross-site shopping cart settlement system?

Hello, gods. Recently, the company wants to upgrade its business. The company has several e-commerce websites, each of which has its own set of shopping carts, but each site has its own set of user data, and only the shopping cart is blocked. Now I want to turn the shopping cart into a set of services that can access the shopping cart functions of all sub-stations.
above put forward this idea, let"s come up with a plan. I thought about it.
should be developed as an independent service. In a service-oriented way, the framework forms a micro-service and provides the corresponding interface to each station. But I don"t know how to implement it. The great gods guide the design ideas.

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Message : Disk full (/tmp/#sql-temptable-64f5-1e80862-468bd.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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