103 lines
2.7 KiB
Go
103 lines
2.7 KiB
Go
/*
|
|
|
|
Below you will find a self-contained Go program which uses the LISTEN / NOTIFY
|
|
mechanism to avoid polling the database while waiting for more work to arrive.
|
|
|
|
//
|
|
// You can see the program in action by defining a function similar to
|
|
// the following:
|
|
//
|
|
// CREATE OR REPLACE FUNCTION public.get_work()
|
|
// RETURNS bigint
|
|
// LANGUAGE sql
|
|
// AS $$
|
|
// SELECT CASE WHEN random() >= 0.2 THEN int8 '1' END
|
|
// $$
|
|
// ;
|
|
|
|
package main
|
|
|
|
import (
|
|
"database/sql"
|
|
"fmt"
|
|
"time"
|
|
|
|
"github.com/lib/pq"
|
|
)
|
|
|
|
func doWork(db *sql.DB, work int64) {
|
|
// work here
|
|
}
|
|
|
|
func getWork(db *sql.DB) {
|
|
for {
|
|
// get work from the database here
|
|
var work sql.NullInt64
|
|
err := db.QueryRow("SELECT get_work()").Scan(&work)
|
|
if err != nil {
|
|
fmt.Println("call to get_work() failed: ", err)
|
|
time.Sleep(10 * time.Second)
|
|
continue
|
|
}
|
|
if !work.Valid {
|
|
// no more work to do
|
|
fmt.Println("ran out of work")
|
|
return
|
|
}
|
|
|
|
fmt.Println("starting work on ", work.Int64)
|
|
go doWork(db, work.Int64)
|
|
}
|
|
}
|
|
|
|
func waitForNotification(l *pq.Listener) {
|
|
for {
|
|
select {
|
|
case <-l.Notify:
|
|
fmt.Println("received notification, new work available")
|
|
return
|
|
case <-time.After(90 * time.Second):
|
|
go func() {
|
|
l.Ping()
|
|
}()
|
|
// Check if there's more work available, just in case it takes
|
|
// a while for the Listener to notice connection loss and
|
|
// reconnect.
|
|
fmt.Println("received no work for 90 seconds, checking for new work")
|
|
return
|
|
}
|
|
}
|
|
}
|
|
|
|
func main() {
|
|
var conninfo string = ""
|
|
|
|
db, err := sql.Open("postgres", conninfo)
|
|
if err != nil {
|
|
panic(err)
|
|
}
|
|
|
|
reportProblem := func(ev pq.ListenerEventType, err error) {
|
|
if err != nil {
|
|
fmt.Println(err.Error())
|
|
}
|
|
}
|
|
|
|
listener := pq.NewListener(conninfo, 10 * time.Second, time.Minute, reportProblem)
|
|
err = listener.Listen("getwork")
|
|
if err != nil {
|
|
panic(err)
|
|
}
|
|
|
|
fmt.Println("entering main loop")
|
|
for {
|
|
// process all available work before waiting for notifications
|
|
getWork(db)
|
|
waitForNotification(listener)
|
|
}
|
|
}
|
|
|
|
|
|
*/
|
|
package listen_example
|