doc.go

Documentation: github.com/lib/pq/example/listen

     1  /*
     2  
     3  Package listen is a self-contained Go program which uses the LISTEN / NOTIFY
     4  mechanism to avoid polling the database while waiting for more work to arrive.
     5  
     6      //
     7      // You can see the program in action by defining a function similar to
     8      // the following:
     9      //
    10      // CREATE OR REPLACE FUNCTION public.get_work()
    11      //   RETURNS bigint
    12      //   LANGUAGE sql
    13      //   AS $$
    14      //     SELECT CASE WHEN random() >= 0.2 THEN int8 '1' END
    15      //   $$
    16      // ;
    17  
    18      package main
    19  
    20      import (
    21          "database/sql"
    22          "fmt"
    23          "time"
    24  
    25          "github.com/lib/pq"
    26      )
    27  
    28      func doWork(db *sql.DB, work int64) {
    29          // work here
    30      }
    31  
    32      func getWork(db *sql.DB) {
    33          for {
    34              // get work from the database here
    35              var work sql.NullInt64
    36              err := db.QueryRow("SELECT get_work()").Scan(&work)
    37              if err != nil {
    38                  fmt.Println("call to get_work() failed: ", err)
    39                  time.Sleep(10 * time.Second)
    40                  continue
    41              }
    42              if !work.Valid {
    43                  // no more work to do
    44                  fmt.Println("ran out of work")
    45                  return
    46              }
    47  
    48              fmt.Println("starting work on ", work.Int64)
    49              go doWork(db, work.Int64)
    50          }
    51      }
    52  
    53      func waitForNotification(l *pq.Listener) {
    54          select {
    55              case <-l.Notify:
    56                  fmt.Println("received notification, new work available")
    57              case <-time.After(90 * time.Second):
    58                  go l.Ping()
    59                  // Check if there's more work available, just in case it takes
    60                  // a while for the Listener to notice connection loss and
    61                  // reconnect.
    62                  fmt.Println("received no work for 90 seconds, checking for new work")
    63          }
    64      }
    65  
    66      func main() {
    67          var conninfo string = ""
    68  
    69          db, err := sql.Open("postgres", conninfo)
    70          if err != nil {
    71              panic(err)
    72          }
    73  
    74          reportProblem := func(ev pq.ListenerEventType, err error) {
    75              if err != nil {
    76                  fmt.Println(err.Error())
    77              }
    78          }
    79  
    80          minReconn := 10 * time.Second
    81          maxReconn := time.Minute
    82          listener := pq.NewListener(conninfo, minReconn, maxReconn, reportProblem)
    83          err = listener.Listen("getwork")
    84          if err != nil {
    85              panic(err)
    86          }
    87  
    88          fmt.Println("entering main loop")
    89          for {
    90              // process all available work before waiting for notifications
    91              getWork(db)
    92              waitForNotification(listener)
    93          }
    94      }
    95  
    96  
    97  */
    98  package listen
    99
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