Synchronization During Migration: Strategies for Continuous Data Consistency

Maintaining data synchronization during server migration is critical for achieving zero-downtime transitions and ensuring data integrity. Unlike one-time data transfers, synchronization establishes continuous data flow between source and destination systems, allowing both environments to operate in parallel while changes propagate seamlessly. This comprehensive guide covers synchronization strategies, tools, and best practices for various migration scenarios.

Understanding Migration Synchronization

Synchronization during migration involves maintaining data consistency across source and destination systems while services remain operational. The challenge lies in capturing and replicating changes that occur during the migration window without impacting user experience or causing data loss.

Key Synchronization Concepts

Bidirectional vs Unidirectional Sync: Whether data flows one way or both ways
Real-time vs Batch Sync: Continuous replication versus periodic updates
Eventual Consistency: Accepting temporary divergence with guaranteed convergence
Conflict Resolution: Handling simultaneous updates to the same data
Incremental Updates: Transferring only changed data
Checkpointing: Maintaining recovery points for interrupted syncs
Verification: Ensuring synchronized data matches source

Synchronization Challenges

Performance Impact: Sync processes consuming system resources
Network Reliability: Handling connection interruptions
Data Consistency: Maintaining referential integrity
Time Synchronization: Coordinating timestamps across systems
Bandwidth Constraints: Managing large data transfers
Application Impact: Minimizing disruption to running services
Conflict Resolution: Managing concurrent modifications
Monitoring Lag: Tracking synchronization delay

Pre-Synchronization Planning

Assess Synchronization Requirements

# Analyze data change rate
# Monitor write operations over time
vmstat 1 60  # Track I/O for 60 seconds
iostat -x 5 12  # Detailed I/O statistics

# Database write rate
mysql -e "SHOW GLOBAL STATUS LIKE 'Com_insert';"
mysql -e "SHOW GLOBAL STATUS LIKE 'Com_update';"
mysql -e "SHOW GLOBAL STATUS LIKE 'Com_delete';"

# File system change rate
inotifywatch -t 300 -r /var/www/html

Synchronization Planning Checklist

File System Synchronization Strategies

Method 1: Continuous rsync Synchronization

Real-time file synchronization using rsync in a loop:

# Create continuous sync script
cat > /root/continuous-sync.sh << 'EOF'
#!/bin/bash

SOURCE="/var/www/html/"
DEST="user@new-server:/var/www/html/"
LOG="/var/log/continuous-sync.log"
SYNC_INTERVAL=300  # 5 minutes
EXCLUDE_FILE="/root/sync-exclude.txt"

# Logging function
log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" | tee -a $LOG
}

log "Starting continuous synchronization"

while true; do
  START=$(date +%s)

  log "Sync cycle started"

  rsync -avz --delete \
    --exclude-from=$EXCLUDE_FILE \
    --timeout=300 \
    --partial \
    --log-file=$LOG \
    $SOURCE $DEST

  EXIT_CODE=$?

  if [ $EXIT_CODE -eq 0 ]; then
    log "Sync completed successfully"
  else
    log "Sync failed with exit code $EXIT_CODE"
  fi

  END=$(date +%s)
  DURATION=$((END - START))
  log "Sync took $DURATION seconds"

  # Calculate sleep time
  SLEEP_TIME=$((SYNC_INTERVAL - DURATION))
  if [ $SLEEP_TIME -gt 0 ]; then
    log "Sleeping for $SLEEP_TIME seconds"
    sleep $SLEEP_TIME
  else
    log "Sync took longer than interval, starting immediately"
  fi
done
EOF

chmod +x /root/continuous-sync.sh

# Run in background with nohup
nohup /root/continuous-sync.sh &

# Or use systemd service
cat > /etc/systemd/system/continuous-sync.service << 'EOF'
[Unit]
Description=Continuous File Synchronization
After=network.target

[Service]
Type=simple
User=root
ExecStart=/root/continuous-sync.sh
Restart=always
RestartSec=30

[Install]
WantedBy=multi-user.target
EOF

systemctl daemon-reload
systemctl enable continuous-sync
systemctl start continuous-sync

Method 2: inotify-based Real-Time Sync

Event-driven synchronization using inotify:

# Install inotify tools
sudo apt install inotify-tools -y

# Create real-time sync script
cat > /root/inotify-sync.sh << 'EOF'
#!/bin/bash

SOURCE="/var/www/html/"
DEST="user@new-server:/var/www/html/"
LOG="/var/log/inotify-sync.log"

log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" | tee -a $LOG
}

log "Starting inotify-based synchronization"

# Monitor file system events and sync
inotifywait -m -r -e modify,create,delete,move $SOURCE --format '%w%f' | while read FILE
do
  log "Change detected: $FILE"

  rsync -avz --delete \
    --timeout=60 \
    $SOURCE $DEST >> $LOG 2>&1

  if [ $? -eq 0 ]; then
    log "Sync completed for change: $FILE"
  else
    log "Sync failed for change: $FILE"
  fi
done
EOF

chmod +x /root/inotify-sync.sh
nohup /root/inotify-sync.sh &

Method 3: Lsyncd for Production Environments

Lsyncd combines inotify with rsync for efficient real-time sync:

# Install lsyncd
sudo apt install lsyncd -y

# Configure lsyncd
sudo nano /etc/lsyncd/lsyncd.conf.lua

-- Add configuration
settings {
  logfile = "/var/log/lsyncd/lsyncd.log",
  statusFile = "/var/log/lsyncd/lsyncd.status",
  statusInterval = 20,
  maxProcesses = 4,
  nodaemon = false,
}

sync {
  default.rsync,
  source = "/var/www/html",
  target = "user@new-server:/var/www/html",
  exclude = { '*.log', 'cache/', 'tmp/' },
  rsync = {
    archive = true,
    compress = true,
    verbose = true,
    _extra = {"--delete-after", "--partial", "--bwlimit=50000"}
  },
  delay = 5,
}

-- Save and start lsyncd
sudo systemctl enable lsyncd
sudo systemctl start lsyncd
sudo systemctl status lsyncd

# Monitor lsyncd status
sudo lsyncd -status /var/log/lsyncd/lsyncd.status

Database Synchronization Strategies

MySQL/MariaDB Replication Synchronization

Master-slave replication for continuous database sync:

# On source server (master): Configure for replication
sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf

# Add these settings:
# server-id = 1
# log_bin = /var/log/mysql/mysql-bin.log
# binlog_format = ROW
# binlog_do_db = your_database

sudo systemctl restart mysql

# Create replication user
mysql -u root -p << 'EOF'
CREATE USER 'replicator'@'%' IDENTIFIED BY 'strong_password';
GRANT REPLICATION SLAVE ON *.* TO 'replicator'@'%';
FLUSH PRIVILEGES;
FLUSH TABLES WITH READ LOCK;
SHOW MASTER STATUS;
EOF

# Record File and Position values

# Create initial dump
mysqldump -u root -p \
  --single-transaction \
  --master-data=2 \
  your_database > /tmp/initial_dump.sql

# Unlock tables
mysql -u root -p -e "UNLOCK TABLES;"

# Transfer dump to new server
scp /tmp/initial_dump.sql user@new-server:/tmp/

# On new server (slave): Configure replication
sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf

# server-id = 2
# relay_log = /var/log/mysql/mysql-relay-bin
# log_bin = /var/log/mysql/mysql-bin.log

sudo systemctl restart mysql

# Import dump
mysql -u root -p your_database < /tmp/initial_dump.sql

# Set up replication
mysql -u root -p << 'EOF'
CHANGE MASTER TO
  MASTER_HOST='source-server-ip',
  MASTER_USER='replicator',
  MASTER_PASSWORD='strong_password',
  MASTER_LOG_FILE='mysql-bin.000001',
  MASTER_LOG_POS=12345;

START SLAVE;
SHOW SLAVE STATUS\G
EOF

# Monitor replication continuously
cat > /root/monitor-replication.sh << 'EOF'
#!/bin/bash

while true; do
  echo "=== $(date) ==="

  mysql -u root -pYOUR_PASSWORD -e "SHOW SLAVE STATUS\G" | \
    grep -E "Slave_IO_Running|Slave_SQL_Running|Seconds_Behind_Master|Last_Error"

  sleep 10
done
EOF

chmod +x /root/monitor-replication.sh
nohup /root/monitor-replication.sh > /var/log/replication-monitor.log &

PostgreSQL Streaming Replication

Real-time PostgreSQL synchronization:

# On primary server: Configure streaming replication
sudo nano /etc/postgresql/14/main/postgresql.conf

# listen_addresses = '*'
# wal_level = replica
# max_wal_senders = 10
# max_replication_slots = 10
# hot_standby = on
# archive_mode = on
# archive_command = 'test ! -f /var/lib/postgresql/14/archive/%f && cp %p /var/lib/postgresql/14/archive/%f'

# Configure authentication
sudo nano /etc/postgresql/14/main/pg_hba.conf

# Add:
# host replication replicator new-server-ip/32 md5

sudo systemctl restart postgresql

# Create replication user
sudo -u postgres psql << 'EOF'
CREATE USER replicator REPLICATION LOGIN ENCRYPTED PASSWORD 'password';
EOF

# On standby server: Set up streaming replication
sudo systemctl stop postgresql
sudo rm -rf /var/lib/postgresql/14/main/*

# Create base backup
sudo -u postgres pg_basebackup \
  -h primary-server-ip \
  -D /var/lib/postgresql/14/main \
  -U replicator \
  -P -v -R -X stream -C -S standby_slot

# Start PostgreSQL
sudo systemctl start postgresql

# Monitor replication lag
cat > /root/monitor-pg-replication.sh << 'EOF'
#!/bin/bash

while true; do
  echo "=== $(date) ==="

  # On primary
  ssh primary-server "sudo -u postgres psql -c 'SELECT * FROM pg_stat_replication;'"

  # On standby
  sudo -u postgres psql -c "SELECT * FROM pg_stat_wal_receiver;"

  sleep 10
done
EOF

chmod +x /root/monitor-pg-replication.sh

Logical Replication for Selective Sync

PostgreSQL logical replication for specific tables:

# On source: Create publication
sudo -u postgres psql your_database << 'EOF'
CREATE PUBLICATION migration_pub FOR TABLE table1, table2, table3;
-- Or for all tables:
-- CREATE PUBLICATION migration_pub FOR ALL TABLES;
EOF

# On destination: Create subscription
sudo -u postgres psql your_database << 'EOF'
CREATE SUBSCRIPTION migration_sub
  CONNECTION 'host=source-server port=5432 dbname=your_database user=replicator password=password'
  PUBLICATION migration_pub;
EOF

# Monitor subscription status
sudo -u postgres psql your_database << 'EOF'
SELECT * FROM pg_stat_subscription;
SELECT * FROM pg_subscription_rel;
EOF

Application-Level Synchronization

Session Synchronization

Shared session storage for seamless user experience:

# Install Redis for shared sessions
sudo apt install redis-server -y

# Configure Redis for replication
sudo nano /etc/redis/redis.conf

# On master:
# bind 0.0.0.0
# requirepass your_strong_password

# On replica:
# replicaof master-server-ip 6379
# masterauth your_strong_password

sudo systemctl restart redis-server

# Configure PHP to use Redis for sessions
sudo nano /etc/php/8.2/fpm/php.ini

# session.save_handler = redis
# session.save_path = "tcp://redis-server:6379?auth=your_strong_password"

sudo systemctl restart php8.2-fpm

# Monitor Redis replication
redis-cli -a your_strong_password INFO replication

Queue Synchronization

Ensure job queues remain synchronized:

# RabbitMQ mirrored queues
# Configure queue mirroring for high availability
rabbitmqctl set_policy ha-migrate "^migrate\." \
  '{"ha-mode":"all","ha-sync-mode":"automatic"}'

# Monitor queue synchronization
rabbitmqctl list_queues name messages consumers

# Redis queue replication
# Already handled by Redis replication configuration above

Cache Synchronization

Synchronized caching across environments:

# Memcached with mcrouter for distributed caching
# Install mcrouter
git clone https://github.com/facebook/mcrouter.git
cd mcrouter/mcrouter/scripts
sudo ./install_ubuntu_20.04.sh /usr/local

# Configure mcrouter
cat > /etc/mcrouter/config.json << 'EOF'
{
  "pools": {
    "A": {
      "servers": [
        "old-server:11211",
        "new-server:11211"
      ]
    }
  },
  "route": "PoolRoute|A"
}
EOF

# Start mcrouter
mcrouter --config-file=/etc/mcrouter/config.json \
  --port=5000 \
  --num-proxies=4

Monitoring Synchronization Status

Comprehensive Monitoring Script

cat > /root/monitor-sync-status.sh << 'EOF'
#!/bin/bash

LOG="/var/log/sync-monitoring.log"
ALERT_EMAIL="[email protected]"

log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" | tee -a $LOG
}

alert() {
  log "ALERT: $1"
  echo "$1" | mail -s "Sync Alert" $ALERT_EMAIL
}

# Monitor file sync lag
check_file_sync() {
  SOURCE_FILES=$(find /var/www/html -type f | wc -l)
  DEST_FILES=$(ssh user@new-server "find /var/www/html -type f | wc -l")

  DIFF=$((SOURCE_FILES - DEST_FILES))

  log "File sync status: Source=$SOURCE_FILES, Dest=$DEST_FILES, Diff=$DIFF"

  if [ $DIFF -gt 100 ]; then
    alert "File sync lag detected: $DIFF files behind"
  fi
}

# Monitor database replication lag
check_db_replication() {
  LAG=$(mysql -u root -pPASSWORD -e "SHOW SLAVE STATUS\G" | \
    grep Seconds_Behind_Master | awk '{print $2}')

  log "Database replication lag: $LAG seconds"

  if [ "$LAG" != "0" ] && [ "$LAG" -gt 10 ]; then
    alert "Database replication lag: $LAG seconds"
  fi

  # Check replication status
  IO_RUNNING=$(mysql -u root -pPASSWORD -e "SHOW SLAVE STATUS\G" | \
    grep Slave_IO_Running | awk '{print $2}')
  SQL_RUNNING=$(mysql -u root -pPASSWORD -e "SHOW SLAVE STATUS\G" | \
    grep Slave_SQL_Running | awk '{print $2}')

  if [ "$IO_RUNNING" != "Yes" ] || [ "$SQL_RUNNING" != "Yes" ]; then
    alert "Database replication stopped! IO=$IO_RUNNING, SQL=$SQL_RUNNING"
  fi
}

# Monitor network connectivity
check_network() {
  if ! ping -c 3 new-server-ip > /dev/null 2>&1; then
    alert "Network connectivity lost to new server"
  fi
}

# Monitor disk space
check_disk_space() {
  SOURCE_USAGE=$(df -h /var/www/html | awk 'NR==2 {print $5}' | sed 's/%//')
  DEST_USAGE=$(ssh user@new-server "df -h /var/www/html | awk 'NR==2 {print \$5}' | sed 's/%//'")

  log "Disk usage: Source=$SOURCE_USAGE%, Dest=$DEST_USAGE%"

  if [ $DEST_USAGE -gt 85 ]; then
    alert "Destination disk usage critical: $DEST_USAGE%"
  fi
}

# Main monitoring loop
while true; do
  log "=== Starting sync status check ==="

  check_network
  check_file_sync
  check_db_replication
  check_disk_space

  log "=== Sync status check complete ==="

  sleep 300  # Check every 5 minutes
done
EOF

chmod +x /root/monitor-sync-status.sh
nohup /root/monitor-sync-status.sh &

Real-Time Sync Dashboard

# Create dashboard script
cat > /root/sync-dashboard.sh << 'EOF'
#!/bin/bash

while true; do
  clear
  echo "============================================"
  echo "   MIGRATION SYNCHRONIZATION DASHBOARD"
  echo "============================================"
  echo ""
  echo "Time: $(date)"
  echo ""

  # File sync status
  echo "--- FILE SYNC STATUS ---"
  SOURCE_SIZE=$(du -sh /var/www/html | awk '{print $1}')
  DEST_SIZE=$(ssh user@new-server "du -sh /var/www/html | awk '{print \$1}'")
  echo "Source size: $SOURCE_SIZE"
  echo "Destination size: $DEST_SIZE"

  # Check if lsyncd is running
  if pgrep -x lsyncd > /dev/null; then
    echo "Lsyncd: RUNNING"
  else
    echo "Lsyncd: STOPPED"
  fi

  echo ""

  # Database replication status
  echo "--- DATABASE REPLICATION ---"
  mysql -u root -pPASSWORD -e "SHOW SLAVE STATUS\G" 2>/dev/null | \
    grep -E "Slave_IO_Running|Slave_SQL_Running|Seconds_Behind_Master"

  echo ""

  # Network status
  echo "--- NETWORK STATUS ---"
  PING=$(ping -c 1 new-server-ip | grep time= | awk -F'time=' '{print $2}' | awk '{print $1}')
  echo "Latency to new server: $PING ms"

  echo ""

  # System resources
  echo "--- SYSTEM RESOURCES ---"
  echo "Source server:"
  ssh old-server "uptime"
  echo "Destination server:"
  ssh new-server "uptime"

  echo ""
  echo "Press Ctrl+C to exit"

  sleep 5
done
EOF

chmod +x /root/sync-dashboard.sh

Handling Synchronization Conflicts

Conflict Detection and Resolution

# Create conflict detection script
cat > /root/detect-conflicts.sh << 'EOF'
#!/bin/bash

SOURCE="/var/www/html"
DEST_USER="user@new-server"
DEST_PATH="/var/www/html"
CONFLICT_LOG="/var/log/sync-conflicts.log"

log_conflict() {
  echo "[$(date)] CONFLICT: $1" >> $CONFLICT_LOG
}

# Find files modified on both servers
find $SOURCE -type f -mtime -1 | while read SOURCE_FILE; do
  REL_PATH="${SOURCE_FILE#$SOURCE}"
  DEST_FILE="$DEST_PATH$REL_PATH"

  # Check if file exists on destination
  if ssh $DEST_USER "[ -f $DEST_FILE ]"; then
    SOURCE_MTIME=$(stat -c %Y "$SOURCE_FILE")
    DEST_MTIME=$(ssh $DEST_USER "stat -c %Y $DEST_FILE")

    # Both modified within last hour
    NOW=$(date +%s)
    if [ $((NOW - SOURCE_MTIME)) -lt 3600 ] && [ $((NOW - DEST_MTIME)) -lt 3600 ]; then
      # Check if content differs
      SOURCE_MD5=$(md5sum "$SOURCE_FILE" | awk '{print $1}')
      DEST_MD5=$(ssh $DEST_USER "md5sum $DEST_FILE | awk '{print \$1}'")

      if [ "$SOURCE_MD5" != "$DEST_MD5" ]; then
        log_conflict "File modified on both servers: $REL_PATH"
        echo "CONFLICT: $REL_PATH (source: $SOURCE_MD5, dest: $DEST_MD5)"

        # Resolution strategy: source wins
        rsync -avz "$SOURCE_FILE" "$DEST_USER:$DEST_FILE"
        log_conflict "Resolved by copying from source: $REL_PATH"
      fi
    fi
  fi
done
EOF

chmod +x /root/detect-conflicts.sh

Synchronization Performance Optimization

Optimize rsync Performance

# Parallel rsync for large directories
cat > /root/parallel-rsync.sh << 'EOF'
#!/bin/bash

SOURCE="/var/www/html"
DEST="user@new-server:/var/www/html"
PARALLEL_JOBS=4

export SOURCE DEST

# Find top-level directories
find $SOURCE -maxdepth 1 -type d | \
  parallel -j $PARALLEL_JOBS \
  rsync -avz --delete {} $DEST/
EOF

# Optimize rsync with custom settings
rsync -avz \
  --compress-level=6 \
  --block-size=131072 \
  --partial-dir=/tmp/rsync-partial \
  --timeout=300 \
  --bwlimit=0 \
  -e "ssh -c [email protected] -o Compression=no" \
  /source/ user@dest:/destination/

Database Sync Optimization

# Optimize MySQL replication
sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf

# Add optimizations:
# binlog_format = ROW  # More efficient for replication
# sync_binlog = 0  # Less disk I/O (less safe, acceptable for migration)
# innodb_flush_log_at_trx_commit = 2  # Better performance during sync

# On slave:
# slave_parallel_workers = 4  # Parallel replication
# slave_parallel_type = LOGICAL_CLOCK

sudo systemctl restart mysql

Final Synchronization and Cutover

Pre-Cutover Final Sync

cat > /root/final-sync.sh << 'EOF'
#!/bin/bash

set -e

LOG="/var/log/final-sync.log"

log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" | tee -a $LOG
}

log "=== Starting final synchronization ==="

# Stop continuous sync processes
log "Stopping continuous sync processes"
pkill -f continuous-sync.sh
pkill -f inotify-sync.sh
systemctl stop lsyncd

# Wait for processes to finish
sleep 5

# Final file sync
log "Performing final file synchronization"
rsync -avz --delete \
  --checksum \
  /var/www/html/ \
  user@new-server:/var/www/html/ | tee -a $LOG

# Verify database replication is caught up
log "Checking database replication status"
LAG=$(mysql -u root -pPASSWORD -e "SHOW SLAVE STATUS\G" | \
  grep Seconds_Behind_Master | awk '{print $2}')

log "Current replication lag: $LAG seconds"

while [ "$LAG" != "0" ] && [ "$LAG" != "NULL" ]; do
  log "Waiting for replication to catch up... ($LAG seconds behind)"
  sleep 5
  LAG=$(mysql -u root -pPASSWORD -e "SHOW SLAVE STATUS\G" | \
    grep Seconds_Behind_Master | awk '{print $2}')
done

log "Database fully synchronized"

# Stop writes to source database
log "Setting source database to read-only"
mysql -u root -pPASSWORD << 'MYSQL'
SET GLOBAL read_only = ON;
FLUSH TABLES WITH READ LOCK;
MYSQL

# Wait for final replication
sleep 10

# Stop replication on new server
log "Stopping replication on new server"
ssh user@new-server "mysql -u root -pPASSWORD << 'MYSQL'
STOP SLAVE;
RESET SLAVE ALL;
SET GLOBAL read_only = OFF;
MYSQL"

log "=== Final synchronization complete ==="
log "Ready for cutover to new server"
EOF

chmod +x /root/final-sync.sh

Verification After Synchronization

Data Integrity Verification

cat > /root/verify-sync.sh << 'EOF'
#!/bin/bash

REPORT="/var/log/sync-verification-$(date +%F-%H%M%S).txt"

echo "=== Synchronization Verification Report ===" > $REPORT
echo "Generated: $(date)" >> $REPORT
echo "" >> $REPORT

# File count comparison
echo "--- File Count Comparison ---" >> $REPORT
SOURCE_COUNT=$(find /var/www/html -type f | wc -l)
DEST_COUNT=$(ssh user@new-server "find /var/www/html -type f | wc -l")
echo "Source files: $SOURCE_COUNT" >> $REPORT
echo "Destination files: $DEST_COUNT" >> $REPORT

if [ $SOURCE_COUNT -eq $DEST_COUNT ]; then
  echo "Status: PASS" >> $REPORT
else
  echo "Status: FAIL - File count mismatch" >> $REPORT
fi

echo "" >> $REPORT

# Size comparison
echo "--- Size Comparison ---" >> $REPORT
SOURCE_SIZE=$(du -sb /var/www/html | awk '{print $1}')
DEST_SIZE=$(ssh user@new-server "du -sb /var/www/html | awk '{print \$1}'")
echo "Source size: $SOURCE_SIZE bytes" >> $REPORT
echo "Destination size: $DEST_SIZE bytes" >> $REPORT

if [ $SOURCE_SIZE -eq $DEST_SIZE ]; then
  echo "Status: PASS" >> $REPORT
else
  echo "Status: WARNING - Size mismatch" >> $REPORT
fi

echo "" >> $REPORT

# Database comparison
echo "--- Database Comparison ---" >> $REPORT

# Compare table counts
mysql -u root -pPASSWORD -e "
  SELECT
    TABLE_SCHEMA,
    COUNT(*) as table_count
  FROM information_schema.TABLES
  WHERE TABLE_SCHEMA NOT IN ('information_schema', 'mysql', 'performance_schema')
  GROUP BY TABLE_SCHEMA
" > /tmp/source-tables.txt

ssh user@new-server "mysql -u root -pPASSWORD -e \"
  SELECT
    TABLE_SCHEMA,
    COUNT(*) as table_count
  FROM information_schema.TABLES
  WHERE TABLE_SCHEMA NOT IN ('information_schema', 'mysql', 'performance_schema')
  GROUP BY TABLE_SCHEMA
\"" > /tmp/dest-tables.txt

echo "Table counts:" >> $REPORT
cat /tmp/source-tables.txt >> $REPORT
echo "" >> $REPORT
cat /tmp/dest-tables.txt >> $REPORT

# Compare row counts for critical tables
mysql -u root -pPASSWORD your_database -e "
  SELECT
    TABLE_NAME,
    TABLE_ROWS
  FROM information_schema.TABLES
  WHERE TABLE_SCHEMA = 'your_database'
  ORDER BY TABLE_NAME
" >> $REPORT

echo "" >> $REPORT
echo "=== Verification Complete ===" >> $REPORT

cat $REPORT
EOF

chmod +x /root/verify-sync.sh

Best Practices for Synchronization

1. Start Early

Begin synchronization well before cutover
Perform multiple sync passes to reduce final sync time
Allow time to identify and resolve issues

2. Monitor Continuously

Track synchronization lag in real-time
Set up alerts for sync failures
Monitor resource usage on both servers

3. Test Thoroughly

Verify synchronization with checksums
Test application functionality during sync
Validate data integrity regularly

4. Plan for Failures

Implement automatic retry logic
Maintain detailed logs of sync operations
Have rollback procedures ready

5. Optimize Performance

Use appropriate sync intervals
Implement bandwidth limiting during peak hours
Leverage compression for slow networks

6. Maintain Communication

Keep stakeholders informed of sync status
Document any issues and resolutions
Provide regular progress updates

Conclusion

Effective synchronization during migration is the cornerstone of zero-downtime server transitions. Key takeaways:

Choose the right tools: rsync for files, replication for databases
Monitor continuously: Track lag and detect issues immediately
Optimize for your scenario: Balance between performance and safety
Verify thoroughly: Always confirm data integrity after sync
Plan the final sync: Minimize the gap between last sync and cutover
Document everything: Maintain detailed logs and reports
Test extensively: Validate sync procedures before production use

Whether synchronizing terabytes of files or real-time database transactions, these strategies ensure data consistency and minimize risk during server migrations. Master these techniques, and you'll confidently execute even the most complex migration scenarios with minimal downtime and zero data loss.

Remember: Synchronization isn't just about copying data—it's about maintaining operational continuity while transforming your infrastructure.

Synchronization During Migration: Strategies for Continuous Data Consistency

Synchronization During Migration: Strategies for Continuous Data Consistency

Understanding Migration Synchronization

Key Synchronization Concepts

Synchronization Challenges

Pre-Synchronization Planning

Assess Synchronization Requirements

Synchronization Planning Checklist

File System Synchronization Strategies

Method 1: Continuous rsync Synchronization

Method 2: inotify-based Real-Time Sync

Method 3: Lsyncd for Production Environments

Database Synchronization Strategies

MySQL/MariaDB Replication Synchronization

PostgreSQL Streaming Replication

Logical Replication for Selective Sync

Application-Level Synchronization

Session Synchronization

Queue Synchronization

Cache Synchronization

Monitoring Synchronization Status

Comprehensive Monitoring Script

Real-Time Sync Dashboard

Handling Synchronization Conflicts

Conflict Detection and Resolution

Synchronization Performance Optimization

Optimize rsync Performance

Database Sync Optimization

Final Synchronization and Cutover

Pre-Cutover Final Sync

Verification After Synchronization

Data Integrity Verification

Best Practices for Synchronization

1. Start Early

2. Monitor Continuously

3. Test Thoroughly

4. Plan for Failures

5. Optimize Performance

6. Maintain Communication

Conclusion

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