Custom Message Headers: amqp vs. paho

Question

Is the difference between the data structures used for custom message headers in mqttv5 paho and amqp due to the protocol, or simply an implementation decision?

Background: I noticed that the most recent paho library for mqttv5 replaced the type for Publish.PublishProperties.User from map[string]string with []UserProperty. I'm assuming the reason for the change is the prevalence of use cases that require multiple values sharing a single key, but wouldn't it be more efficient to utilize the amqp style customer message header? amqp uses amqp.Table where Table is map[string]interface{}, which seems to serve the same purpose (just use []string as your interface implementation).

Retrieving a slice of values that share a custom message header in amqp requires one line of code:

slice := message.Headers["key"]

Retrieving a slice of values that share a custom message header in paho requires a loop of string comparisons:

// GetAll returns a slice of all entries in the UserProperties
// that match key, or a nil slice if none were found.
func (u UserProperties) GetAll(key string) []string {
    var ret []string
    for _, v := range u {
        if v.Key == key {
            ret = append(ret, v.Value)
        }
    }

    return ret
}

Is there a reason for this choice of implementation?

Answer 1

As per the contributors, the performance of returning UserProperty in that format is much lower.

See the github discussion here: https://github.com/eclipse/paho.golang/issues/47

From that discussion, with the current implementation being the second performance run:

With 10 k/v in the user properties and 5 being the same key

pkg: github.com/eclipse/paho.golang/paho
BenchmarkUserProperties-8         703861              1476 ns/op             784 B/op         15 allocs/op
BenchmarkUserProperty-8          6933013               158 ns/op             320 B/op          1 allocs/op