Lessons from a Broken Replication Chain: A Troubleshooting Journey

Our data pipeline seemed flawless—until it wasn’t.

Here’s the setup: We use an Azure Managed Instance for our production database, transactional replication to sync data to an Azure SQL Database for reporting, and Change Data Capture (CDC) with Fivetran to move data into Snowflake for advanced analytics. When it works, reporting data is updated within minutes of hitting production. But when it breaks, the lessons come hard and fast.

One morning, users complained that reports were out of date. I hadn’t received any alerts for replication latency or failures, so I assumed the issue was elsewhere. But when I checked the replication monitor, I saw the publisher failing with an "Access Denied" error when connecting to the storage account.

The culprit? Our infrastructure team had switched the storage account to a private endpoint for security, breaking the replication connection. Since the replication agent kept retrying without hard failure, no alerts were triggered.

To fix it, we updated our Bicep deployment scripts to create a new storage account and configure the required private endpoint. After pointing the distributor to the new storage account, I thought replication would recover. Instead, I discovered all subscriptions had expired, and reinitialization was required.

This is where the real challenge began.

Reinitializing replication meant dropping and recreating tables on the reporting database. However, dependencies from views and functions caused failures. I had to drop all views, functions, and dependent objects before reinitializing the subscriber. Once the replication was re-synced, I recreated everything—views, functions, and indexes.

I thought the hard part was over—until I tried to re-enable CDC. Running sys.sp_cdc_enable_table gave me an error: "CDC already enabled for this table." Despite is_cdc_enabled showing 0 in sys.tables. CDC tables and functions left behind when I replication had dropped the main tables causing this issue.

After multiple attempts to resolve the issue manually by dropping cdc tables and functions, I disabled and re-enabled CDC for the entire database. Finally, I was able to enable CDC on all necessary tables and restore the pipeline.

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### Key Lessons Learned

1. If you need to reinitialize replication:

- Disable CDC on all tables.

- Drop all views and functions.

- Reinitialize replication.

- Recreate views, functions, and indexes.

- Enable CDC again for the required tables.

2. Proactive monitoring is critical: Set up latency alerts for replication so issues can be caught before users report them.

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This experience taught me that even the most “typical” setups can become unpredictable when things go wrong. Have you faced similar challenges with replication or CDC? I’d love to hear your stories and learn from your insights.