It would be nice that defaultUriVariables() could accept Supplier/Function as value of Map.

MDC.put("partition", "p0"); // value is variable per request

Map<String, ?> defaultUriVariables = Map.of(
//      "partition", (Supplier<String>) () -> MDC.get("partition"),
        "partition", (Function<String, String>) MDC::get
);
RestClient restClient = RestClient.builder().baseUrl("http://{partition}.example.com").defaultUriVariables(defaultUriVariables).build();

Comment From: snicoll

Duplicate of #34190