# Concurrency interview

A little while back I had an interview where they posed a problem, in summary:

1. Watch a certain directory, and process incoming JSON files
2. These JSON files have various "Type" fields
3. Every second report out how many files of each "Type" were processed, and the average processing time (from added to disk, to added to sums) for all.

The solution was supposed to optimize for a minimal "processing time" which is why I chose a concurrent approach.

After I submitted the solution, I didn't hear back for about a month, got rejected and had absolutely no feedback on my solution. Since I've never done anything like this before, I expected it has problems and I greatly appreciate any feedback.

Here's how I represent an Event:

type Event struct {
Type string
CreatedAt time.Time
}


Here's my main loop:

func main() {
config := Config{}

fmt.Println("Monitoring...")
fmt.Println("Press Enter to Exit")
fmt.Println("--------------------\n")

foundEvents := make(chan Event)
normalizedEvents := make(chan Event)
ticker := time.NewTicker(1 * time.Second)

go discoverNewEvents(config, foundEvents)
go normalizeEvents(foundEvents, normalizedEvents)
go outputOnTick(normalizedEvents, ticker)

}


Here's discoverNewEvents which watches the directory and deserializes the json

func discoverNewEvents(appConfig Config, out chan Event) {
for _ = range time.Tick(1 * time.Microsecond) {

for _, fileInfo := range filesInfo {
inputPath := path.Join(appConfig.InputDirectory, fileInfo.Name())
processedPath := path.Join(appConfig.ProcessedDirectory, fileInfo.Name())

event := Event{}
_ = json.Unmarshal(rawJson, &event)

event.CreatedAt = fileInfo.ModTime()

os.Rename(inputPath, processedPath)
out <- event
}
}
}


Here's normalizeEvents which does a slight bit of data normalization (making the "Type" the same case):

func normalizeEvents(in chan Event, out chan Event) {
for event := range in {
normalized := Event{
Type: strings.ToLower(event.Type),
CreatedAt: event.CreatedAt }

out <- normalized
}
}


And here's outputOnTick which selects on the 1 second tick and aggregates results:

func outputOnTick(in chan Event, ticker *time.Ticker) {
results := newAggregator()

for {
select {
case event := <-in:
results.Consider(event)
case <-ticker.C:
fmt.Println(results.String())
results.Init()
}
}
}


Here's the result aggregator, which serves as a databag for the events and processing information. It also lazily calculates the results string (at the end of processsing):

type ResultAggregator struct {
processingTimeSum time.Duration
processingTimeCount int64
normalizedTypeToCount map[string]int64
mutex sync.Mutex
}

func newAggregator() ResultAggregator {
aggregator := ResultAggregator{}
aggregator.Init()
return aggregator
}

func (self *ResultAggregator) Init() {
self.mutex.Lock()
defer self.mutex.Unlock()

self.processingTimeSum = 0
self.processingTimeCount = 0
self.normalizedTypeToCount = map[string]int64{}
}

func (self *ResultAggregator) Consider(event Event) {
self.mutex.Lock()
defer self.mutex.Unlock()

self.normalizedTypeToCount[event.Type] += 1
self.processingTimeSum += time.Since(event.CreatedAt)
self.processingTimeCount += 1
}

func (self *ResultAggregator) String() string {
self.mutex.Lock()
defer self.mutex.Unlock()

return fmt.Sprintf(
"DoorCnt:%d, ImgCnt:%d, AlarmCnt:%d, avgProcessingTime: %dms",
self.normalizedTypeToCount["door"],
self.normalizedTypeToCount["img"],
self.normalizedTypeToCount["alarm"],
self.averageProcessingTime())
}

func (self *ResultAggregator) averageProcessingTime() int64 {
if self.processingTimeCount == 0 {
return 0
}

return self.processingTimeSum.Nanoseconds() / (1000000 * self.processingTimeCount)
}


Any feedback is appreciated, but there are a few specific things I'd like to know:

1. Correctness -- did I fail to solve the problem (I can give more details on the exact problem, if needed)
2. Idiom -- is the code idiomatic go? I have a main.go containing everything but the result aggregator which is in its own file
3. Concurrency -- Did I use Mutexes correctly? Is there an issue with the goroutines? Is there something else here that's wrong (e.g. unexpected race condition)
• Did they ask you to write a solution in Go or did they leave the language up to you? – ErikR Aug 28 '15 at 11:37
• @ErikR they left the solution up to me, however, I knew the company used go – mjgpy3 Aug 28 '15 at 13:38
• Please say that you omitted error checking just while cut-n-pasting your code here. If the code you sent them had no error handling what-so-ever … – Dave C Aug 28 '15 at 16:37

The main issue I see is that it appears you are busy waiting on the directory, i.e. you are reading its contents at regular intervals to see if there are new files.

Note - I have not written a single golang program, so correct me if I'm wrong, but are you trying to poll the directory every microsecond?

The right approach is to use file system notifications from the operating system. For instance, check out this library:

https://github.com/go-fsnotify/fsnotify

Using file system notifications your program can remain idle until something interesting happens at which point you receive an event on a channel.

This is an interesting architecture problem and I might have more to say about the rest of your program later.

A few other minor things...

The variable filesInfo should be named fileInfos - i.e. the 's' should be on the end. filesInfo conveys the idea of a singular piece of information about a bunch of files, whereas fileInfos is more clearly a plurality of file information.

Also, "info" is a very generic term. Everything is information. It appears that fileInfo is really a "directory entry". You'll see the term "dirent" commonly used in the POSIX world to describe a directory entry, e.g. this man page for the readdir() system call: