Lesson Learned #436: Implementing Retry Mechanism with sqlcmd in Bash (Linux)

Introduction:

Efficiently managing temporary failures and timeouts is crucial in production environments when connecting to databases. In this article, we’ll explore how to implement a retry mechanism with sqlcmd in a Bash script, dynamically increasing timeouts with each failed attempt.

Problem Statement:

Operations can fail due to network issues, overloaded servers, or other temporary problems when interacting with databases. Implementing a retry mechanism helps address these temporary issues without manual intervention.

Solution Overview:

Below is a Bash script utilizing sqlcmd to interact with an SQL Server database. The script attempts to execute an SQL command and, upon failure, retries the operation up to a maximum number of times, incrementing timeouts with each attempt.

#!/bin/bash

# Variable declaration

MAX_RETRIES=5

COUNT=0

SERVER="server.database.windows.net"

DATABASE="dbname"

USER="username"

PASSWORD="Password123!"

INPUT_FILE="/home/user/sql.sql"

LOG_FILE="/home/user/logfile.txt"

LOGIN_TIMEOUT=15 # Timeout for login, in seconds

QUERY_TIMEOUT=30 # Timeout for query, in seconds

# Main loop for retry mechanism

while [ $COUNT -lt $MAX_RETRIES ]; do

# Execute sqlcmd and capture the return value

sqlcmd -S $SERVER -d $DATABASE -U $USER -P $PASSWORD -i $INPUT_FILE -l $LOGIN_TIMEOUT -t $QUERY_TIMEOUT 2>>$LOG_FILE

RETVAL=$?

# Get current date and time

timestamp=$(date +"%Y-%m-%d %H:%M:%S")

# Check the return value

if [ $RETVAL -eq 0 ]; then

message="$timestamp - sqlcmd executed successfully with login timeout $LOGIN_TIMEOUT and query timeout $QUERY_TIMEOUT!"

echo $message

echo $message >> $LOG_FILE

break

else

message="$timestamp - Error with sqlcmd, retrying with increased timeouts..."

echo $message

echo $message >> $LOG_FILE

# Increase timeouts by fixed amounts

LOGIN_TIMEOUT=$((LOGIN_TIMEOUT + 15))

QUERY_TIMEOUT=$((QUERY_TIMEOUT + 30))

((COUNT++))

# Wait for 5 seconds before retrying; adjust as needed

sleep 5

fi

done

# Get current date and time

timestamp=$(date +"%Y-%m-%d %H:%M:%S")

# Check if the maximum number of attempts was reached

if [ $COUNT -eq $MAX_RETRIES ]; then

message="$timestamp - Failed after $MAX_RETRIES attempts with login timeout $LOGIN_TIMEOUT and query timeout $QUERY_TIMEOUT."

echo $message

echo $message >> $LOG_FILE

fi

Script Explanation:

• 
  Variable Declaration: The script begins by declaring variables like the maximum number of retries, counter, server, database, user, password, input file, and timeouts.
   
   
  
• 
  Main Loop: A while loop serves as the main retry mechanism. The script runs sqlcmd and captures its return value.
   
   
  
• 
  Return Value Check: If sqlcmd executes successfully (return value 0), the script prints a success message and terminates. Otherwise, it prints an error message, increments timeouts and the retry counter, and sleeps for a while before retrying.
   
   
  
• 
  Timeout Increment: On each failed retry, login and query timeouts are increased by 15 and 30 seconds respectively, providing more time for subsequent operations.
   
  

Conclusion:

This Bash script is a valuable tool for managing database operations that might face temporary issues. With a retry mechanism and dynamically adjusted timeouts, you can enhance the robustness and reliability of your database interactions in unstable or high-load environments. We hope this article provided insightful information on effectively implementing a retry mechanism with sqlcmd in Bash. For optimal results, adjust variable values and timeouts to cater to your specific needs and production environment.

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