vertx kafka client consume messages and do a REST api call sequentially

Question

    protected void consume(KafkaReadStream<String, JsonObject> consumer) {
        consumer.handler(record -> {
           // call a REST API to write a value to DB
           doRestApi(record);  // such as client.postAbs(...).send(...)
        });
    }

For example, records in topic like,

{"createtime":"2021-10-29 00:03:49.0"}
{"createtime":"2021-10-29 00:03:50.0"}
{"createtime":"2021-10-29 00:03:56.0"}
...
{"createtime":"2021-10-29 00:13:56.0"}

When the record {"createtime":"2021-10-29 00:03:49.0"} was coming, the handler did a REST api call to save the createtime=2021-10-29 00:03:49.0 to mongodb.
when the second record came, createtime=2021-10-29 00:03:50.0 would be saved.
Then the third record, createtime=2021-10-29 00:03:56.0 would be saved.
And so on. I hope these messages to be handled sequentially. As a result, the createtime in the last message 2021-10-29 00:13:56.0 would be the final value in DB.

When I ran my code, the result value was random and the former value(such as 2021-10-29 00:03:56.0) might be the final value. How could I make the handler to run sequentially?