Fire and forget goroutine golang - go

I have written an API that makes DB calls and does some business logic. I am invoking a goroutine that must perform some operation in the background.
Since the API call should not wait for this background task to finish, I am returning 200 OK immediately after calling the goroutine (let us assume the background task will never give any error.)
I read that goroutine will be terminated once the goroutine has completed its task.
Is this fire and forget way safe from a goroutine leak?
Are goroutines terminated and cleaned up once they perform the job?
func DefaultHandler(w http.ResponseWriter, r *http.Request) {
// Some DB calls
// Some business logics
go func() {
// some Task taking 5 sec
}()
w.WriteHeader(http.StatusOK)
}

I would recommend always having your goroutines under control to avoid memory and system exhaustion.
If you are receiving a spike of requests and you start spawning goroutines without control, probably the system will go down soon or later.
In those cases where you need to return an immediate 200Ok the best approach is to create a message queue, so the server only needs to create a job in the queue and return the ok and forget. The rest will be handled by a consumer asynchronously.
Producer (HTTP server) >>> Queue >>> Consumer
Normally, the queue is an external resource (RabbitMQ, AWS SQS...) but for teaching purposes, you can achieve the same effect using a channel as a message queue.
In the example you'll see how we create a channel to communicate 2 processes.
Then we start the worker process that will read from the channel and later the server with a handler that will write to the channel.
Try to play with the buffer size and job time while sending curl requests.
package main
import (
"fmt"
"log"
"net/http"
"time"
)
/*
$ go run .
curl "http://localhost:8080?user_id=1"
curl "http://localhost:8080?user_id=2"
curl "http://localhost:8080?user_id=3"
curl "http://localhost:8080?user_id=....."
*/
func main() {
queueSize := 10
// This is our queue, a channel to communicate processes. Queue size is the number of items that can be stored in the channel
myJobQueue := make(chan string, queueSize) // Search for 'buffered channels'
// Starts a worker that will read continuously from our queue
go myBackgroundWorker(myJobQueue)
// We start our server with a handler that is receiving the queue to write to it
if err := http.ListenAndServe("localhost:8080", myAsyncHandler(myJobQueue)); err != nil {
panic(err)
}
}
func myAsyncHandler(myJobQueue chan<- string) http.HandlerFunc {
return func(rw http.ResponseWriter, r *http.Request) {
// We check that in the query string we have a 'user_id' query param
if userID := r.URL.Query().Get("user_id"); userID != "" {
select {
case myJobQueue <- userID: // We try to put the item into the queue ...
rw.WriteHeader(http.StatusOK)
rw.Write([]byte(fmt.Sprintf("queuing user process: %s", userID)))
default: // If we cannot write to the queue it's because is full!
rw.WriteHeader(http.StatusInternalServerError)
rw.Write([]byte(`our internal queue is full, try it later`))
}
return
}
rw.WriteHeader(http.StatusBadRequest)
rw.Write([]byte(`missing 'user_id' in query params`))
}
}
func myBackgroundWorker(myJobQueue <-chan string) {
const (
jobDuration = 10 * time.Second // simulation of a heavy background process
)
// We continuosly read from our queue and process the queue 1 by 1.
// In this loop we could spawn more goroutines in a controlled way to paralelize work and increase the read throughput, but i don't want to overcomplicate the example.
for userID := range myJobQueue {
// rate limiter here ...
// go func(u string){
log.Printf("processing user: %s, started", userID)
time.Sleep(jobDuration)
log.Printf("processing user: %s, finisehd", userID)
// }(userID)
}
}

There is no "goroutine cleaning" you have to handle, you just launch goroutines and they'll be cleaned when the function launched as a goroutine returns. Quoting from Spec: Go statements:
When the function terminates, its goroutine also terminates. If the function has any return values, they are discarded when the function completes.
So what you do is fine. Note however that your launched goroutine cannot use or assume anything about the request (r) and response writer (w), you may only use them before you return from the handler.
Also note that you don't have to write http.StatusOK, if you return from the handler without writing anything, that's assumed to be a success and HTTP 200 OK will be sent back automatically.
See related / possible duplicate: Webhook process run on another goroutine

#icza is absolutely right there is no "goroutine cleaning" you can use a webhook or a background job like gocraft. The only way I can think of using your solution is to use the sync package for learning purposes.
func DefaultHandler(w http.ResponseWriter, r *http.Request) {
// Some DB calls
// Some business logics
var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.Done()
// some Task taking 5 sec
}()
w.WriteHeader(http.StatusOK)
wg.wait()
}

you can wait for a goroutine to finish using &sync.WaitGroup:
// BusyTask
func BusyTask(t interface{}) error {
var wg = &sync.WaitGroup{}
wg.Add(1)
go func() {
// busy doing stuff
time.Sleep(5 * time.Second)
wg.Done()
}()
wg.Wait() // wait for goroutine
return nil
}
// this will wait 5 second till goroutune finish
func main() {
fmt.Println("hello")
BusyTask("some task...")
fmt.Println("done")
}
Other way is to attach a context.Context to goroutine and time it out.
//
func BusyTaskContext(ctx context.Context, t string) error {
done := make(chan struct{}, 1)
//
go func() {
// time sleep 5 second
time.Sleep(5 * time.Second)
// do tasks and signle done
done <- struct{}{}
close(done)
}()
//
select {
case <-ctx.Done():
return errors.New("timeout")
case <-done:
return nil
}
}
//
func main() {
fmt.Println("hello")
ctx, cancel := context.WithTimeout(context.TODO(), 2*time.Second)
defer cancel()
if err := BusyTaskContext(ctx, "some task..."); err != nil {
fmt.Println(err)
return
}
fmt.Println("done")
}

Related

Golang Server Sent Events Per User

I've been working with Go for some time but never done SSE before. I'm having an issue, can someone PLEASE provide with a working example of server sent events that will only send to a specific user(connection).
I'm using a gorilla - sessions to authenticate and I would like to use UserID to separate connections.
Or should I use 5 second polling via Ajax?
Many thanks
Here is what i found and tried:
https://gist.github.com/ismasan/3fb75381cd2deb6bfa9c it doenst send to an individual user and the go func wont stop if the connection is closed
https://github.com/striversity/gotr/blob/master/010-server-sent-event-part-2/main.go this is kind of what i need but it doesnt track once the connection is removed. So now, once you close and open the browser in private window it's not working at all. Also, as above, the go routine keeps going.
Create a "broker" to distribute messages to connected users:
type Broker struct {
// users is a map where the key is the user id
// and the value is a slice of channels to connections
// for that user id
users map[string][]chan []byte
// actions is a channel of functions to call
// in the broker's goroutine. The broker executes
// everything in that single goroutine to avoid
// data races.
actions chan func()
}
// run executes in a goroutine. It simply gets and
// calls functions.
func (b *Broker) run() {
for a := range b.actions {
a()
}
}
func newBroker() *Broker {
b := &Broker{
users: make(map[string][]chan []byte),
actions: make(chan func()),
}
go b.run()
return b
}
// addUserChan adds a channel for user with given id.
func (b *Broker) addUserChan(id string, ch chan []byte) {
b.actions <- func() {
b.users[id] = append(b.users[id], ch)
}
}
// removeUserchan removes a channel for a user with the given id.
func (b *Broker) removeUserChan(id string, ch chan []byte) {
// The broker may be trying to send to
// ch, but nothing is receiving. Pump ch
// to prevent broker from getting stuck.
go func() { for range ch {} }()
b.actions <- func() {
chs := b.users[id]
i := 0
for _, c := range chs {
if c != ch {
chs[i] = c
i = i + 1
}
}
if i == 0 {
delete(b.users, id)
} else {
b.users[id] = chs[:i]
}
// Close channel to break loop at beginning
// of removeUserChan.
// This must be done in broker goroutine
// to ensure that broker does not send to
// closed goroutine.
close(ch)
}
}
// sendToUser sends a message to all channels for the given user id.
func (b *Broker) sendToUser(id string, data []byte) {
b.actions <- func() {
for _, ch := range b.users[id] {
ch <- data
}
}
}
Declare a variable with the broker at package-level:
var broker = newBroker()
Write the SSE endpoint using the broker:
func sseEndpoint(w http.ResponseWriter, r *http.Request) {
// I assume that user id is in query string for this example,
// You should use your authentication code to get the id.
id := r.FormValue("id")
// Do the usual SSE setup.
flusher := w.(http.Flusher)
w.Header().Set("Content-Type", "text/event-stream")
w.Header().Set("Cache-Control", "no-cache")
w.Header().Set("Connection", "keep-alive")
// Create channel to receive messages for this connection.
// Register that channel with the broker.
// On return from the function, remove the channel
// from the broker.
ch := make(chan []byte)
broker.addUserChan(id, ch)
defer broker.removeUserChan(id, ch)
for {
select {
case <-r.Context().Done():
// User closed the connection. We are out of here.
return
case m := <-ch:
// We got a message. Do the usual SSE stuff.
fmt.Fprintf(w, "data: %s\n\n", m)
flusher.Flush()
}
}
}
Add code to your application to call Broker.sendToUser.

Exit a goroutine after a timeout

I'm trying to write a program in go that is similar to cron with the addition that jobs are given a max runtime and if a function exceeds this duration, the job should exit. Here is my my whole code:
package main
import (
"fmt"
"log"
"sync"
"time"
)
type Job struct {
ID string
MaxRuntime time.Duration
Frequency time.Duration
Function func()
}
func testFunc() {
log.Println("OPP11")
time.Sleep(7 * time.Second)
log.Println("OP222")
}
func New(ID, frequency, runtime string, implementation func()) Job {
r, err := time.ParseDuration(runtime)
if err != nil {
panic(err)
}
f, err := time.ParseDuration(frequency)
if err != nil {
panic(err)
}
j := Job{ID: ID, MaxRuntime: r, Frequency: f, Function: implementation}
log.Printf("Created job %#v with frequency %v and max runtime %v", ID, f, r)
return j
}
func (j Job) Run() {
for range time.Tick(j.Frequency) {
start := time.Now()
log.Printf("Job %#v executing...", j.ID)
done := make(chan int)
//quit := make(chan int)
//var wg sync.WaitGroup
//wg.Add(1)
go func() {
j.Function()
done <- 0
}()
select {
case <-done:
elapsed := time.Since(start)
log.Printf("Job %#v completed in %v \n", j.ID, elapsed)
case <-time.After(j.MaxRuntime):
log.Printf("Job %#v halted after %v", j.ID, j.MaxRuntime)
// here should exit the above goroutine
}
}
}
func main() {
// create a new job given its name, frequency, max runtime
// and the function it should run
testJob := New("my-first-job", "3s", "5s", func() {
testFunc()
})
testJob.Run()
}
What I'm trying to do is that in the second case in the select of the Run() function, it should exit the goroutine which is running the function. I tried to do this by wrapping the function in a for loop with a select statement which listens on a quit channel like this:
go func() {
for {
select {
case <-quit:
fmt.Println("quiting goroutine")
return
default:
j.Function()
done <- 0
}
}
}()
And then having quit <- 1 in the Run() function, but that doesnt seem to be doing anything. Is there a better of doing this?
As explained in the comments, the whole problem is that you want to cancel the execution of a function (j.Function) that isn't cancellable.
There's no way to "kill a goroutine". Goroutines work in a cooperative fashion. If you want to be able to "kill it", you need to ensure that the function running in that Goroutine has a mechanism for you to signal that it should stop what it's doing and return, letting the Goroutine that was running it finally terminate.
The standard way of indicating that a function is cancellable is by having it take a context.Context as its first param:
type Job struct {
// ...
Function func(context.Context)
}
Then you create the context and pass it to the j.Function. Since your cancellation logic is simply based on a timeout, there's no need to write all that select ... case <-time.After(...), as that is provided as built-in functionality with a context.Context:
func (j Job) Run() {
for range time.Tick(j.Frequency) {
go j.ExecuteOnce()
}
}
func (j Job) ExecuteOnce() {
log.Printf("Job %#v executing...", j.ID)
ctx, cancel := context.WithTimeout(context.Background(), j.MaxRuntime)
defer cancel()
j.Function(ctx)
}
Now, to finish, you have to rewrite the functions that you're going to be passing to your job scheduler so that they take context.Context and, very importantly, that they use it properly and cancel whatever they're doing when the context is cancelled.
This means that if you're writing the code for those funcs and they will somehow block, you'll be responsible for writing stuff like:
select {
case <-ctx.Done():
return ctx.Err()
case ...your blocking case...:
}
If your funcs are invoking 3rd party code, then that code needs to be aware of context and cancellation, and you'll need to pass down the ctx your funcs receive.

How to run handler functions sequentially

I have a handler function for an endpoint. The handler takes a very long time to return a response, consists a lot of processing. I do not want other incoming requests to run concurrently but instead wait for the previous one to finish! Tried implementing waitGroups, check the code! Every time for a new request a new instance of wait group is created and it starts running concurrently instead of waiting for the older one to complete. Is my wait group approach incorrect?
var wg sync.WaitGroup
func Handler(c *gin.Context) {
// some stuff that takes ~10-15 seconds, can't be run concurrently
// If a second request comes put it in a queue and execute it only once this is done
wg.Add(1)
go func() {
defer wg.Done()
//some processing happens
time.Sleep(10 * time.Second)
}()
wg.Wait()
c.JSON(http.StatusOK, gin.H{"message": "Hello!"})
}
router.POST("/doSomething", Handler)
As already mentioned in the comments, this looks like a broken requirement. However if you really want to have one instance of the function running, you can use a mutex:
var lock sync.Mutex
func Handler(c *gin.Context) {
lock.Lock()
defer lock.Unlock()
// Process
}

Is there a non-blocking http client? [closed]

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Improve this question
Many languages have their own high-level non-blocking HTTP client, for example, python's aiohttp. Namely, they send out HTTP requests; do not wait for response; When response arrives they make some kind of callbacks.
My questions are
is there a Go package for that?
or we just create a goroutine in which we use normal HTTP clients?
which way is better?
Other languages have such features because when they block waiting for request they block the thread they are using. This is the case for Java, Python or NodeJS. Therefore to make them useful, the developers needed to implement such long-standing blocking operations with callbacks. The root cause of that is the usage of the C library beneath that blocks threads on input-output operations.
Go does not use C library (only in some cases, but it can be turned off) and makes system calls by itself. While doing this the thread that executes current goroutine parks it and executes another goroutine. Therefore you can have enormous number of blocked goroutines without running out of threads. Goroutines are cheap with regard to memory, threads are operating system entities.
In Go using goroutines is better. There is no need for creating asynchronous client because of the above.
For comparison in Java you would quickly end up with multiple threads. The next step would be pooling them as they are costly. Pooling means limiting the concurrency.
As others have stated, goroutines are the way to go (pun intended).
Minimal Example:
type nonBlocking struct {
Response *http.Response
Error error
}
const numRequests = 2
func main() {
nb := make(chan nonBlocking, numRequests)
wg := &sync.WaitGroup{}
for i := 0; i < numRequests; i++ {
wg.Add(1)
go Request(nb)
}
go HandleResponse(nb, wg)
wg.Wait()
}
func Request(nb chan nonBlocking) {
resp, err := http.Get("http://example.com")
nb <- nonBlocking{
Response: resp,
Error: err,
}
}
func HandleResponse(nb chan nonBlocking, wg *sync.WaitGroup) {
for get := range nb {
if get.Error != nil {
log.Println(get.Error)
} else {
log.Println(get.Response.Status)
}
wg.Done()
}
}
Yip, built into the standard library, just not usable by a simple function call out of the box.
Take this example
package main
import (
"flag"
"log"
"net/http"
"sync"
"time"
)
var url string
var timeout time.Duration
func init() {
flag.StringVar(&url, "url", "http://www.stackoverflow.com", "url to GET")
flag.DurationVar(&timeout, "timeout", 5*time.Second, "timeout for the GET operation")
}
func main() {
flag.Parse()
// We use the channel as our means to
// hand the response over
rc := make(chan *http.Response)
// We need a waitgroup because all goroutines exit when main exits
var wg sync.WaitGroup
// We are spinning up an async request
// Increment the counter for our WaitGroup.
// What we are basically doing here is to tell the WaitGroup
// "Hey, there is one more task you have to wait for!"
wg.Add(1)
go func() {
// Notify the WaitGroup that one task is done as soon
// as we exit the goroutine.
defer wg.Done()
log.Printf("Doing GET request on \"%s\"", url)
resp, err := http.Get(url)
if err != nil {
log.Printf("GET for %s: %s", url, err)
}
// We send the reponse downstream
rc <- resp
// Now, the goroutine exits, the defered call to wg.Done()
// is executed.
}()
// And here we do our async processing.
// Note that you could have done the processing in the first goroutine
// as well, since http.Get would be a blocking operation and any subsequent
// code in the goroutine would have been excuted only after the Get returned.
// However, I put te processing into its own goroutine for demonstration purposes.
wg.Add(1)
go func() {
// As above
defer wg.Done()
log.Println("Doing something else")
// Setting up a timer for a timeout.
// Note that this could be done using a request with a context, as well.
to := time.NewTimer(timeout).C
select {
case <-to:
log.Println("Timeout reached")
// Exiting the goroutine, the deferred call to wg.Done is executed
return
case r := <-rc:
if r == nil {
log.Printf("Got no useful response from GETting \"%s\"", url)
// Exiting the goroutine, the deferred call to wg.Done is executed
return
}
log.Printf("Got response with status code %d (%s)", r.StatusCode, r.Status)
log.Printf("Now I can do something useful with the response")
}
}()
// Now we have set up all of our tasks,
// we are waiting until all of them are done...
wg.Wait()
log.Println("All tasks done, exiting")
}
If you look at this closely, we have all building blocks to make GETting an URL and processing the response async. We can start to abstract this a bit:
package main
import (
"flag"
"log"
"net/http"
"time"
)
var url string
var timeout time.Duration
func init() {
flag.StringVar(&url, "url", "http://www.stackoverflow.com", "url to GET")
flag.DurationVar(&timeout, "timeout", 5*time.Second, "timeout for the GET operation")
}
type callbackFunc func(*http.Response, error) error
func getWithCallBack(u string, callback callbackFunc) chan error {
// We create a channel which we can use to notify the caller of the
// result of the callback.
c := make(chan error)
go func() {
c <- callback(http.Get(u))
}()
return c
}
func main() {
flag.Parse()
c := getWithCallBack(url, func(resp *http.Response, err error) error {
if err != nil {
// Doing something useful with the err.
// Add additional cases as needed.
switch err {
case http.ErrNotSupported:
log.Printf("GET not supported for \"%s\"", url)
}
return err
}
log.Printf("GETting \"%s\": Got response with status code %d (%s)", url, resp.StatusCode, resp.Status)
return nil
})
if err := <-c; err != nil {
log.Printf("Error GETting \"%s\": %s", url, err)
}
log.Println("All tasks done, exiting")
}
And there you Go (pun intended): Async processing of GET requests.

How to close all grpc server streams using gracefulStop?

I'm trying to stop all clients connected to a stream server from server side.
Actually I'm using GracefulStop method to handle it gracefully.
I am waiting for os.Interrupt signal on a channel to perform a graceful stop for gRPC. but it gets stuck on server.GracefulStop() when the client is connected.
func (s *Service) Subscribe(_ *empty.Empty, srv clientapi.ClientApi_SubscribeServer) error {
ctx := srv.Context()
updateCh := make(chan *clientapi.Update, 100)
stopCh := make(chan bool)
defer func() {
stopCh<-true
close(updateCh)
}
go func() {
ticker := time.NewTicker(1 * time.Second)
defer func() {
ticker.Stop()
close(stopCh)
}
for {
select {
case <-stopCh:
return
case <-ticker.C:
updateCh<- &clientapi.Update{Name: "notification": Payload: "sample notification every 1 second"}
}
}
}()
for {
select {
case <-ctx.Done():
return ctx.Err()
case notif := <-updateCh:
err := srv.Send(notif)
if err == io.EOF {
return nil
}
if err != nil {
s.logger.Named("Subscribe").Error("error", zap.Error(err))
continue
}
}
}
}
I expected the context in method ctx.Done() could handle it and break the for loop.
How to close all response streams like this one?
Create a global context for your gRPC service. So walking through the various pieces:
Each gRPC service request would use this context (along with the client context) to fulfill that request
os.Interrupt handler would cancel the global context; thus canceling any currently running requests
finally issue server.GracefulStop() - which should wait for all the active gRPC calls to finish up (if they haven't see the cancelation immediately)
So for example, when setting up the gRPC service:
pctx := context.Background()
globalCtx, globalCancel := context.WithCancel(pctx)
mysrv := MyService{
gctx: globalCtx
}
s := grpc.NewServer()
pb.RegisterMyService(s, mysrv)
os.Interrupt handler initiates and waits for shutdown:
globalCancel()
server.GracefulStop()
gRPC methods:
func(s *MyService) SomeRpcMethod(ctx context.Context, req *pb.Request) error {
// merge client and server contexts into one `mctx`
// (client context will cancel if client disconnects)
// (server context will cancel if service Ctrl-C'ed)
mctx, mcancel := mergeContext(ctx, s.gctx)
defer mcancel() // so we don't leak, if neither client or server context cancels
// RPC WORK GOES HERE
// RPC WORK GOES HERE
// RPC WORK GOES HERE
// pass mctx to any blocking calls:
// - http REST calls
// - SQL queries etc.
// - or if running a long loop; status check the context occasionally like so:
// Example long request (10s)
for i:=0; i<10*1000; i++ {
time.Sleep(1*time.Milliscond)
// poll merged context
select {
case <-mctx.Done():
return fmt.Errorf("request canceled: %s", mctx.Err())
default:
}
}
}
And:
func mergeContext(a, b context.Context) (context.Context, context.CancelFunc) {
mctx, mcancel := context.WithCancel(a) // will cancel if `a` cancels
go func() {
select {
case <-mctx.Done(): // don't leak go-routine on clean gRPC run
case <-b.Done():
mcancel() // b canceled, so cancel mctx
}
}()
return mctx, mcancel
}
Typically clients need to assume that RPCs can terminate (e.g. due to connection errors or server power failure) at any moment. So what we do is GracefulStop, sleep for a short time period to allow in-flight RPCs an opportunity to complete naturally, then hard-Stop the server. If you do need to use this termination signal to end your RPCs, then the answer by #colminator is probably the best choice. But this situation should be unusual, and you may want to spend some time analyzing your design if you do find it is necessary to manually end streaming RPCs at server shutdown.

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