Go Concurrency Patterns

Go Concurrency Patterns
By Vitalii Perehonchuk, Software Developer

www.eliftech.com
Let’s lets get through...
1. Projects not using concurrency
2. Atomic operations
3. Timeouts and tickers
4. Exit control
5. Event handling
6. Error handling
7. Tests and benchmarks
8. Something really interesting

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No concurrency in
▪ GoCV (computer vision)
▪ upper/db (drivers for MySQL,
PostgreSQL, SQLite, MongoDB,
etc)
▪ goquery (jQuery port)
▪ authboss (Auth library)
▪ go-jose (JS-way data signing and
encryption library)
▪ go-oauth2-server
▪ goth (Multi-provider auth library)
▪ oauth2 (OAuth2 library)
▪ docopt-go (Toolset for pretty CLI)
▪ go-flags (CLI options parser)
▪ kingpin (CLI library)
▪ urfave/cli (CLI library)
▪ asciigraph (lightweight ASCII line
graphs in CLI)
▪ go-prompt (CLI library)
▪ godotenv (.env library)
▪ ini (.ini library)
▪ gods (Go data structures)
▪ ...

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Atomic operations

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Cockroach - scalable & transactional datastore
Uuid generator
// Returns the epoch and clock sequence.
func (g *Gen) getClockSequence() (uint64, uint16, error) {
g.clockSequenceOnce.Do(func() {
buf := make([]byte, 2)
io.ReadFull(g.rand, buf)
g.clockSequence = binary.BigEndian.Uint16(buf)
})
...

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goleveldb - fast key-value storage library
type memdbReleaser struct {
once sync.Once
m *memDB
}
func (mr *memdbReleaser) Release() {
mr.once.Do(func() {
mr.m.decref()
})
}

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Groupcache - cache library
// An AtomicInt is an int64 to be accessed atomically.
type AtomicInt int64
func (i *AtomicInt) Add(n int64) {
atomic.AddInt64((*int64)(i), n)
}
func (i *AtomicInt) Get() int64 {
return atomic.LoadInt64((*int64)(i))
}
func (i *AtomicInt) String() string {
return strconv.FormatInt(i.Get(), 10)
}

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Tickers and timeouts

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tebeka/selenium - WebDriver client for Go
go func() {
bufr := bufio.NewReader(r)
s, err :=
bufr.ReadString('n')
ch <- resp{s, err}
}()
var display string
select {
case resp := <-ch:
if resp.err != nil {
return nil,
resp.err
}
display =
strings.TrimSpace(resp.display)
case <-time.After(3 * time.Second):
return nil,
errors.New("timeout waiting for
Xvfb")
}

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minimp3 - lightweight MP3 decoder library
// Started check the record mp3 stream started or not.
func (dec *Decoder) Started() (channel chan bool) {
channel = make(chan bool)
go func() {
for {
select {
case <-dec.context.Done():
channel <- false
default:
}
...

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...
if len(dec.decodedData) != 0 {
channel <- true
} else {
<-time.After(time.Millisecond *
100)
}
}
}()
return channel
}

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casbin - auth library with access control models support
func (e *SyncedEnforcer) StartAutoLoadPolicy(d time.Duration) {
e.autoLoad = true
go func() {
n := 1
for {
if !e.autoLoad { break }
e.LoadPolicy()
n++
time.Sleep(d)
}
}()
}

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ticker := time.NewTicker(time.Second)
defer ticker.Stop()
for range ticker.C {
if atomic.LoadInt32(&stopAll) > 0 {
return
}
if time.Now().After(endTs) {
return
}
}

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rqlite - lightweight relational database with SQLite storage
// checkConnections periodically checks which connections should
close due to timeouts.
func (s *Store) checkConnections() {
s.wg.Add(1)
ticker := time.NewTicker(s.connPollPeriod)
go func() {
defer s.wg.Done()
defer ticker.Stop()
for {
select {
case <-s.done: return
case <-ticker.C:
...

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BigCache - efficient key-value store for gigabytes
ticker := time.NewTicker(config.CleanWindow)
defer ticker.Stop()
for {
select {
case t := <-ticker.C:
cache.cleanUp(uint64(t.Unix()))
case <-cache.close:
return
}
}

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telegram-bot-api - Telegram bot client
// GetUpdatesChan starts and returns a channel for getting updates.
func (bot *BotAPI) GetUpdatesChan(config UpdateConfig)
(UpdatesChannel, error) {
ch := make(chan Update, bot.Buffer)
go func() {
for {
select {
case <-bot.shutdownChannel: return
default:
}
updates, err := bot.GetUpdates(config)
...

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telegram-bot-api - Telegram bot client
...
if err != nil {
time.Sleep(time.Second * 3)
continue
}
for _, update := range updates {
if update.UpdateID >= config.Offset
{
config.Offset =
update.UpdateID + 1
ch <- update
}
}}}()
return ch, nil
}

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Drone - CI platform built on Docker
import "golang.org/x/sync/errgroup"
var g errgroup.Group
g.Go(func() error {
return s1.ListenAndServe()
})
g.Go(func() error {
return
s2.ListenAndServeTLS(
s.Cert,
s.Key,
)
})
g.Go(func() error {
select {
case <-ctx.Done():
s1.Shutdown(ctx)
s2.Shutdown(ctx)
return nil
}
})
g.Wait()

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Event handling

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rakyll/portmidi - Go bindings for PortMidi
// Listen input stream for MIDI events.
func (s *Stream) Listen() <-chan Event {
ch := make(chan Event)
go func(s *Stream, ch chan Event) {
for {
// sleep for a while before the new polling
tick,
// otherwise operation is too intensive and
blocking
time.Sleep(10 * time.Millisecond)
events, err := s.Read(1024)
// Note: It's not very reasonable to push sliced data
into
// a channel, several perf penalties there
are.
// This function is added as a handy utility.
if err != nil { continue }
for i := range events { ch <- events[i] }
}
}(s, ch)
return ch

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gocui - minimalist library for CLI
go func() {
g.tbEvents <- termbox.Event{
Type: termbox.EventResize,
}
}()
for {
select {
case ev := <-g.tbEvents:
g.handleEvent(&ev)
case ev := <-g.userEvents:
ev.f(g)
}
g.consumeevents()
g.flush()
}

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Error handling

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dgraph - graph database
...
che := make(chan error, 1)
chb := make(chan []byte, 1000)
go func() {
che <- writeToFile(output, chb)
}()
…
return che

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Tests and benchmarks

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`go test` command runs tests in goroutines,
thus - concurrently, but not parallelly by default.
Having run `t.Parallel()` within its code, a test is run parallelly with another parallel tests (but
never - with other runs of itself).

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Many libraries use goroutines only in tests and examples - to test itselves against race
conditions.

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goleveldb - fast key-value storage library
b.SetParallelism(10)
b.RunParallel(func(pb *testing.PB) {
…
})

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TiDB - distributed HTAP database
// batchRW makes sure conflict free.
func batchRW(value []byte) {
wg := sync.WaitGroup{}
wg.Add(*workerCnt)
for i := 0; i < *workerCnt; i++ {
go func(i int) {
defer wg.Done()
…
}(i)
}
wg.Wait()
}

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1 million requests per
minute?..

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Malwarebytes
type Dispatcher struct {
// A pool of workers channels that are registered with the
dispatcher
WorkerPool chan chan Job
}
func NewDispatcher(maxWorkers int) *Dispatcher {
pool := make(chan chan Job, maxWorkers)
return &Dispatcher{WorkerPool: pool}
}
...

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Malwarebytes
...
func (d *Dispatcher) Run() {
// starting n number of workers
for i := 0; i < d.maxWorkers; i++ {
worker := NewWorker(d.pool)
worker.Start()
}
go d.dispatch()
}
...

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Malwarebytes
...
func (d *Dispatcher) dispatch() {
for {
select {
case job := <-JobQueue:
// a job request has been received
go func(job Job) {
// try to obtain a worker job channel
that is available.
// this will block until a worker is
idle
jobChannel := <-d.WorkerPool
// dispatch the job to the worker job
channel
jobChannel <- job
}(job)
}
}
}

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Malwarebytes
// Start method starts the run loop for the worker, listening for a quit
channel in
// case we need to stop it
func (w Worker) Start() {
go func() {
for {
// register the current worker into the worker
queue.
w.WorkerPool <- w.JobChannel
select {
case job := <-w.JobChannel:
// we have received a work request.
job.Payload.UploadToS3()
case <-w.quit:
return // we have received a signal to
stop
}
}
}()
}

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Conclusion
Concurrency and parallelism are widely used by Open
Source projects in Go.
Concurrency and parallelism are NOT used by many Open
Source projects in Go.
Parallel testing and benchmarking is a common practice.

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Sources
1. github.com
2. gopkg.in
3. godoc.org
4. golang.org
5. “Handling 1 Million Requests per Minute with Go“ by Marcio Castilho

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