api - Parallelize the dashboard queries

[mohamedmansour]

Right now we are sequentially calling each query in the dashboard call. Would be nice to take go to its advantage and run all queries concurrently since they are independent from each other.

[mohamedmansour]

I tried taking a stab at this, by using channels and waitgroups, don't know if that is right. But I regressed:

Before:

[Wed Jul 21 2021 21:22:34 GMT-0700 (Pacific Daylight Time)] INFO Requests: 1341, requests per second: 269, mean latency: 33 ms
[Wed Jul 21 2021 21:22:39 GMT-0700 (Pacific Daylight Time)] INFO Requests: 2839, requests per second: 299, mean latency: 33 ms
[Wed Jul 21 2021 21:22:44 GMT-0700 (Pacific Daylight Time)] INFO Requests: 4343, requests per second: 301, mean latency: 34 ms

After:

[Wed Jul 21 2021 21:24:11 GMT-0700 (Pacific Daylight Time)] INFO Requests: 1345, requests per second: 269, mean latency: 16.4 ms
[Wed Jul 21 2021 21:24:16 GMT-0700 (Pacific Daylight Time)] INFO Requests: 2844, requests per second: 300, mean latency: 16.6 ms
[Wed Jul 21 2021 21:24:21 GMT-0700 (Pacific Daylight Time)] INFO Requests: 4343, requests per second: 300, mean latency: 16.8 ms

The way I did it was, maybe the speed latency increase was due to channel creation and close are expensive:

	clients := make(chan []client)
	languages := make(chan []client)
	operatingSystems := make(chan []client)
	versions := make(chan []client)

	var waitGroup sync.WaitGroup
	waitGroup.Add(4)

	go func() {
		data, err := clientQuery(a.db, topClientsQuery, whereArgs...)
		if err != nil {
			fmt.Println(err)
		}
		clients <- data
		defer waitGroup.Done()
	}()
	
	go func() {
		data, err := clientQuery(a.db, topLanguageQuery, whereArgs...)
		if err != nil {
			fmt.Println(err)
		}
		languages <- data
		defer waitGroup.Done()
	}()

	go func() {
		data, err := clientQuery(a.db, topOsQuery, whereArgs...)
		if err != nil {
			fmt.Println(err)
		}
		operatingSystems <- data
		defer waitGroup.Done()
	}()

	go func() {
		// When the filter has a name, we don't want to include that in the json response.
		if nameCountInQuery == 1 {
			data, err := clientQuery(a.db, topVersionQuery, whereArgs...)
			if err != nil {
				fmt.Println(err)
			}
			versions <- data
		}
		defer waitGroup.Done()
	}()

	go func() {
		waitGroup.Wait()
		close(clients)
		close(languages)
		close(operatingSystems)
		close(versions)
	}()