I have this project that tries to run unlimited bigqueries at the same time in Go. The parent project is all Python. I need to be able to keep track of the query results, like in a map.
Input:
{
'reports_portal': 'select * from reports_portal',
'billing_portal': 'select * from billing_portal',
}
output:
{
'reports_portal': [23, 123, 5234, 632],
'billing_portal': [23, 123, 5234, 632],
}
and so on
these bigqueries need to be run asynchronously as they're very slow (from a UI perspective, an SRE waiting 15-30 seconds for results.
I first try to asynchronously write items to a map:
package main
import (
"fmt"
)
func add_to_map(m map[string] string, word string) map[string]string {
added_word := word + " plus more letters"
m[word] = added_word
return m
}
func main() {
words_map := make(map[string]string)
words := []string{"giraffe", "cat", "dog", "turtle"}
for _, this_word := range words {
go add_to_map(words_map, this_word)
}
fmt.Println(words_map)
}
blows up like:
$ go run try_asynchronous.go
fatal error: concurrent map writes
goroutine 7 [running]:
runtime.throw(0x10b3b96, 0x15)
/usr/local/Cellar/go/1.8.1/libexec/src/runtime/panic.go:596 +0x95 fp=0xc420032eb8 sp=0xc420032e98
runtime.mapassign(0x109ad20, 0xc420016270, 0xc420032fa0, 0x10b3268)
/usr/local/Cellar/go/1.8.1/libexec/src/runtime/hashmap.go:499 +0x667 fp=0xc420032f58 sp=0xc420032eb8
main.add_to_map(0xc420016270, 0x10b1ba0, 0x3, 0x0)
/tmp/golang-w-python/try_asynchronous.go:10 +0xa3 fp=0xc420032fc0 sp=0xc420032f58
runtime.goexit()
/usr/local/Cellar/go/1.8.1/libexec/src/runtime/asm_amd64.s:2197 +0x1 fp=0xc420032fc8 sp=0xc420032fc0
created by main.main
/tmp/golang-w-python/try_asynchronous.go:19 +0xc8
goroutine 1 [runnable]:
fmt.(*pp).fmtString(0xc42001e0c0, 0x10b1f52, 0x7, 0x76)
/usr/local/Cellar/go/1.8.1/libexec/src/fmt/print.go:424 +0x1a2
fmt.(*pp).printValue(0xc42001e0c0, 0x10953c0, 0xc42000e260, 0x98, 0x76, 0x1)
/usr/local/Cellar/go/1.8.1/libexec/src/fmt/print.go:729 +0x27aa
fmt.(*pp).printValue(0xc42001e0c0, 0x109ad20, 0xc420016270, 0x15, 0x76, 0x0)
/usr/local/Cellar/go/1.8.1/libexec/src/fmt/print.go:750 +0x103d
fmt.(*pp).printArg(0xc42001e0c0, 0x109ad20, 0xc420016270, 0x76)
/usr/local/Cellar/go/1.8.1/libexec/src/fmt/print.go:682 +0x217
fmt.(*pp).doPrintln(0xc42001e0c0, 0xc420045f28, 0x1, 0x1)
/usr/local/Cellar/go/1.8.1/libexec/src/fmt/print.go:1138 +0xa1
fmt.Fprintln(0x1108140, 0xc42000c018, 0xc420045f28, 0x1, 0x1, 0xc420045ef0, 0xc420045ee0, 0x1087218)
/usr/local/Cellar/go/1.8.1/libexec/src/fmt/print.go:247 +0x5c
fmt.Println(0xc420045f28, 0x1, 0x1, 0x10b1e6f, 0x6, 0x0)
/usr/local/Cellar/go/1.8.1/libexec/src/fmt/print.go:257 +0x57
main.main()
/tmp/golang-w-python/try_asynchronous.go:21 +0x132
exit status 2
based on needing to run many queries at once and trying to keep track of the results by their name, I expected to write to a map during asynchronous. But fatal error: concurrent map writes says you can't.
I don't understand
EDIT:
The closest thing I have, that returns results, is not asynchronous:
package main
import (
"fmt"
"math/rand"
"sync"
"time"
)
var mutex sync.Mutex
var wg sync.WaitGroup
func random_sleep() {
r := rand.Intn(3000)
time.Sleep(time.Duration(r) * time.Millisecond)
}
func add_to_map(m map[string] string, word string) {
defer wg.Done()
added_word := word + " plus more letters"
mutex.Lock()
defer mutex.Unlock()
fmt.Println("Before sleep")
random_sleep()
m[word] = added_word
fmt.Println("Added word %v", word)
}
func main() {
words_map := make(map[string]string)
words := []string{"giraffe", "cat", "dog", "turtle"}
for _, this_word := range words {
wg.Add(1)
go add_to_map(words_map, this_word)
}
wg.Wait()
fmt.Println(words_map)
}
Results are wrong:
cchilders:~/work_projects/metricsportal/golang_integration (feature/golang-query)
$ go run try_async.go
Before sleep
Added word %v turtle
Before sleep
Added word %v cat
Before sleep
Added word %v giraffe
Before sleep
Added word %v dog
map[dog:dog plus more letters turtle:turtle plus more letters cat:cat plus more letters giraffe:giraffe plus more letters]
cchilders:~/work_projects/metricsportal/golang_integration (feature/golang-query)
$ go run try_async.go
Before sleep
Added word %v turtle
Before sleep
Added word %v cat
Before sleep
Added word %v giraffe
Before sleep
Added word %v dog
map[dog:dog plus more letters turtle:turtle plus more letters cat:cat plus more letters giraffe:giraffe plus more letters]
Results should be very fast, no longer than 3 seconds (the max of random I think):
Expectation -
Before sleep
Before sleep
Before sleep
Before sleep
Added word %v cat
Added word %v giraffe
Added word %v turtle
Added word %v dog
OK let me clarify some things and help you.
You don't need to return a modified map from here because your function gets a reference to map not copy of it. (Let's ignore the fact that you are completely ignoring return value)
func add_to_map(m map[string] string, word string) map[string]string {
added_word := word + " plus more letters"
m[word] = added_word
return m
}
Next thing is that you need to synchronize access to map. You can use mutex for this.
import "sync"
var mutex sync.Mutex //glabal variable but can be created as local also
func add_to_map(m map[string] string, word string) {
added_word := word + " plus more letters"
// here you can do long to compute task and calculate result
// calc here
mutex.Lock() //result ready lock mutex
defer mutex.Unlock() // unlock mutex when we return from function
m[word] = added_word // result write to shared map
}
Note that in Go 1.9 there will be a Concurrent Map type.
Edit:
You need to wait for all go-routines to finish because your main() now finishes before them. You can do this by using WaitGroup
package main
import (
"fmt"
"sync"
)
var mutex sync.Mutex
var wg sync.WaitGroup
func add_to_map(m map[string] string, word string) {
defer wg.Done()
added_word := word + " plus more letters"
// do heavy work here
//
mutex.Lock()
defer mutex.Unlock()
m[word] = added_word
}
func main() {
words_map := make(map[string]string)
words := []string{"giraffe", "cat", "dog", "turtle"}
for _, this_word := range words {
wg.Add(1)
go add_to_map(words_map, this_word)
}
wg.Wait()
fmt.Println(words_map)
}