CDC

🟢 What is CDC?

CDC is a feature in databases that records every change—when a row is added, updated, or deleted.

Think of it like a CCTV camera for your database.
Whenever data changes, CDC quietly writes a note about it.

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💡 Why is it useful?

• You can send only new changes to another system (like a data warehouse) instead of copying everything.

• You can trigger actions when data changes (e.g., send an email when an order ships).

• It helps with auditing (who changed what and when).

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⚙️ How it works (step by step)

1. Turn it on

   • An admin tells the database: “Watch this table for changes.”

2. Database watches the transaction log

   • Every insert, update, or delete is already recorded in the database log.

   • CDC reads that log.

3. Change table created

   • CDC puts details of each change into a special table (e.g., cdc.Orders_CT).

   • This table shows:

     • What kind of change happened (insert/update/delete)

     • Old and new values

     • The time of the change.

4. Your app reads the changes

   • Your app or ETL job asks: “Give me all changes since last time.”

   • It processes those and remembers the last point it read.

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🧠 Simple Example

• You have an Orders table.

• You update order #5’s status to “Shipped.”

• CDC writes a row into its change table:
  “Order #5 changed from Pending → Shipped at 10:32 AM.”

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✅ Good Things

• No triggers needed: It uses the existing transaction log.

• Fast: Only new changes are read.

• Real-time: You see changes almost immediately.

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⚠️ Things to keep in mind

• Change tables can grow big—set cleanup rules.

• Only users with permission should read CDC data.

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🏁 Quick Summary

CDC = Automatic change tracker for your database.
It watches every insert, update, or delete and stores those details in a special table, so other apps can easily know exactly what changed without scanning everything.