Transaction

Learn transactions with Begin, Commit and Rollback.

Transactions

Use a transaction when several operations must either all happen or none. db.Begin() gives a *sql.Tx; operations go through tx and end with Commit or Rollback.

tx, err := db.Begin()
if err != nil { return err }
defer tx.Rollback() // no effect after Commit
if _, err := tx.Exec("UPDATE hesap SET bakiye = bakiye - 100 WHERE id = 1"); err != nil { return err }
if _, err := tx.Exec("UPDATE hesap SET bakiye = bakiye + 100 WHERE id = 2"); err != nil { return err }
return tx.Commit()

The defer tx.Rollback pattern

defer tx.Rollback() rolls the transaction back on early returns and errors. After a successful Commit, Rollback returns sql.ErrTxDone and does nothing, so the pattern is safe. To work with a context, use db.BeginTx(ctx, nil); if the context is cancelled the transaction is rolled back automatically.

Exercise Transaction flow

Model the flow without a real database: write transfer(fail bool) string. Return "rollback" if fail is true, otherwise "commit". Print transfer(false) and transfer(true) on one line.

Show example solution

package main

import "fmt"

func transfer(fail bool) string {
    if fail {
        return "rollback"
    }
    return "commit"
}

func main() {
    fmt.Println(transfer(false), transfer(true))
}

Test yourself

Inside a transaction, why run the query with <code>tx.Exec</code> instead of <code>db.Exec</code>?
- There is no difference
- db.Exec takes another pool connection and stays outside the transaction
- tx.Exec is faster
- db.Exec closes the transaction
Show answer

db.Exec takes another pool connection and stays outside the transaction — A transaction is bound to one connection; operations through db don't use it and aren't rolled back.

Commit

if err := step(tx); err != nil {
    return err
}
return tx.____()

Show answer

if err := step(tx); err != nil {
    return err
}
return tx.Commit()

Transaction steps

db.Begin

Starts it
tx.Exec

Runs inside it
tx.Commit

Makes it permanent
tx.Rollback

Undoes it

Next lesson: Summary - database/sql

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