DAG Server

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🖥️ What is a DAG Server in Exchange?

DAG stands for Database Availability Group.
A DAG server is a member server of that group, running Microsoft Exchange Server.

💡 Simple Definition:

A DAG server is part of a high-availability group of Exchange servers that replicate mailbox databases to each other, so if one fails, another takes over automatically.

🔁 What Does a DAG Do?

  • Provides high availability and disaster recovery for mailbox databases.

  • Automatically replicates mailbox data across multiple Exchange servers.

  • If one server goes down, another server can activate the database copy and users stay connected (no data loss).

🔧 How It Works:

  1. You create a Database Availability Group.

  2. Add 2 or more Exchange servers to it — these are your DAG members (DAG servers).

  3. Exchange replicates mailbox databases from one DAG server to the others.

  4. If a DAG server (or database) fails, Exchange fails over to another healthy copy.

🧱 Key Components:

ComponentDescription
Mailbox serversExchange servers that host copies of mailbox databases.
Witness serverA separate server (non-Exchange) used to maintain quorum in a DAG with an even number of members.
ReplicationLogs and mailbox data are continuously copied between servers.
FailoverAutomatic switch to a healthy database copy if the active one fails.

🛠️ Example:

You have 3 Exchange servers: EX01, EX02, EX03 in a DAG.

  • EX01 hosts the active copy of DB1.

  • EX02 and EX03 have passive copies of DB1.

  • If EX01 crashes, the DAG promotes the passive copy on EX02 or EX03 to active—users keep working.

✅ Benefits of DAG:

  • No single point of failure for mailbox databases.

  • Built-in automatic failover and load balancing.

  • Part of Microsoft’s native high availability solution—no need for third-party tools.

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