Sql-server – the difference between sys.dm_hadr_ tables and sys.availability_groups / sys.availability_replicas

Downsides of Transparent Data Encryption compared to Always Encrypted:

• Only protects data at rest - backups and data files are "safe" but data in motion or in memory is vulnerable
• Whole database only
• All data is encrypted the same way

• Backup compression can take longer and be counter-productive

  • Well, actually, there are some improvements here in SQL Server 2016 that defy what we've typically known about trying to compress encrypted data - it's much better than previous versions, but presumably still worse than only encrypting a handful of columns (untested)
• tempdb also inherits encryption – stays even after disabling TDE
• Requires Enterprise Edition
• Data always accessible to sysadmin

Always Encrypted addresses all of these issues in part or in full:

• Data is protected at rest, in motion, and in memory - much more control over certs, keys, and exactly who can decrypt data
• Can be just a single column
• Encryption type is a choice:
  • Can use deterministic encryption to support indexes and point lookups (say, SSN)
  • Can use random encryption for higher protection (say, credit card number)
• Since it's not database-wide, backup compression isn't necessarily affected - of course the more columns you encrypt, the worse luck you'll have
• tempdb is uninvolved
• As of SQL Server 2016 Service Pack 1, Always Encrypted now works in all editions
• Data can be protected from sysadmin (but not sysadmin AND Windows security/cert/key admins, in other words you can separate responsibility as long as those two groups don't collude)

There is a limitation, though, and that is that not all drivers and applications can deal with the encrypted data directly, so in some cases this will require updating/changing drivers and/or modifying code.

Transactional Replication uses the TDS protocol and applies changes using SQL. The connections are established from the Distributor for Push Subscriptions, and from the Subscriber for Pull Subscriptions.

AlwaysOn Availability groups use TCP/IP connection between the replicas using the Database Mirroring Endpoint. The connections are established from the instances hosting the Secondary replicas to the instance hosting the Primary replica. But as the Primary replica can move, every server hosting a replica needs to be able to connect to every other one.