I am working on a system where I have to limit concurrent connections from a particular IP. Once a connection is made, some steps are performed and then it is dropped so that other connections can be made. Below is the code where I am updating the value of a particular variable.
Link for code - http://tpcg.io/_SQZEEX
package main
import "fmt"
var addresses = make([]string, 0)
func count(rAddr string, slice *[]string) map[string]int {
dict := make(map[string]int)
*slice = append(*slice, rAddr)
for _, a := range *slice {
dict[a]
}
return dict
}
func listen() int {
var rAddr string
rAddr = "1.1.1.2"
number_of_addresses:=count(rAddr ,&addresses)
var v int
for k, v := range number_of_addresses {
fmt.Println(k, "value is", v)
if (v > 0) {
v--
fmt.Println(k, "decreased value is", v)
}
}
return v
}
func main() {
listen()
listen()
}
Upon calling the function again, the values are not refreshing for the "v" variable and the output is as below
1.1.1.2 value is 1
1.1.1.2 decreased value is 0
1.1.1.2 value is 2
1.1.1.2 decreased value is 1
Expected output:
1.1.1.2 value is 1
1.1.1.2 decreased value is 0
1.1.1.2 value is 1
1.1.1.2 decreased value is 0
What could I do so that the value is updated each time it is decreased. I understand that the value is refreshed but not sent back to the count function. How can this be achieved
CodePudding user response：
You are modifying the copy of the value you get from the map, not the value you stored in the map.
if (v > 0) {
v--
number_of_addresses[k]=v
}
However, you mentioned this is related some rate-limiting feature. If that is really the case, then instead of using a []string to store addresses and map[string]int to count, you should consider using a map[string]*atomic.Int64 coupled with a mutex to protect concurrent access to the map.
CodePudding user response：
Per my comments, refactor to track IP addresses via a map i.e. a map[string]int to track usage.
A basic implementation to get you started:
// track unique addresses via a map:
// - key: string of the IP address
// - value: usage count
var (
usageMap = make(map[string]int) //
l sync.Mutex // ensure concurrent access to map is synchronized
)
func modifyAddr(addr string, delta int) int {
l.Lock()
defer l.Unlock()
usageMap[addr] = delta
return usageMap[addr]
}
func incAddr(addr string) int { return modifyAddr(addr, 1) }
func decAddr(addr string) int { return modifyAddr(addr, -1) }
and to use:
const maxConcurrentUsage = 20
func listen() {
rAddr := "1.1.1.2"
usage := incAddr(rAddr)
defer decAddr(rAddr)
if usage > maxConcurrentUsage {
log.Printf("maximum concurrent usage exceeded (%d) for address: %q", maxConcurrentUsage, rAddr)
return
}
// handler logic goes here
}