Understanding context in Golang through Postgres

I was trying to learn Go package context especially with respect to Postgres. On a very high level context provides context to the operation. Yeah, I agree, the previous statement doesn’t really add much value, but hold on I don’t really know how to explain it, rather let’s go over some code. Sometimes its easier to understand by seeing it in action :)

The complete program is here

• Time based context:

  There are two methods. context.WithTimeout() context.WithDeadline()

  package main
  
  import (
  	"context"
  	"database/sql"
  	"log"
  	"os"
  
  	_ "github.com/lib/pq"
  )
  
  func main() {
  	dsn := "user=postgres dbname=postgres sslmode=disable port=5433 password=replaceit"
  	db, err := sql.Open("postgres", dsn)
  	defer db.Close()
  	if err != nil {
  		log.Fatalf("error opening a connection to database %v", err)
  	}
  	err = db.PingContext(context.Background())
  	if err != nil {
  		log.Fatalf("error connecting to the database %v", err)
  	}
  	ctx := context.Background()
  	ctx, cancel := context.WithTimeout(ctx, 2000*time.Millisecond)
  	defer cancel()
  	_, err = db.QueryContext(ctx, "select * from generate_series(1, 5000000000)")
  	if err != nil {
  		log.Printf("error querying %v", err)
  	}
  	log.Println("done")
  

  ctx := context.Background() creates an empty context.

  Then ctx, cancel := context.WithTimeout(ctx, 2000*time.Millisecond) which creates a timed context for ctx. When we use that context in db.QueryContext the context gets cancelled at 2s. Meaning any query taking more than 2s will be stopped.

• Using WithCancel():

  package main
  
  import (
  	"context"
  	"database/sql"
  	"log"
  	"os"
  
  	_ "github.com/lib/pq"
  )
  
  func main() {
  	dsn := "user=postgres dbname=postgres sslmode=disable port=5433 password=replaceit"
  	db, err := sql.Open("postgres", dsn)
  	defer db.Close()
  	if err != nil {
  		log.Fatalf("error opening a connection to database %v", err)
  	}
  	err = db.PingContext(context.Background())
  	if err != nil {
  		log.Fatalf("error connecting to the database %v", err)
  	}
  	ctx := context.Background()
  	ctx, cancel := context.WithCancel(ctx)
  	go func() {
  	   	select {
  	   	case <-time.After(2000 * time.Millisecond):
  	   		fmt.Fprint(os.Stderr, "query timed out \n")
  	   		cancel()
  	   	}
  	   }()
  	_, err = db.QueryContext(ctx, "select * from generate_series(1, 50000000)")
  	cancel()
  

Achieving the same thing (time out queries which takes more than 2 seconds) but using cancel(). In a separate go routine we checking whether its been 2 seconds and by calling cancel() it cancels the context.

I don’t know is there any preferable method. Certainly for time based contexts, context.WithTimeout() or context.WithDeadline() seems more straightforward.

• go
• postgres