Swiped from my answer on StackOverflow, only to prevent others from spending effort on the same type of answer.
Personally I think this will work a lot better than transactional replication, though I haven't done any formal comparisons of the two in a true migration scenario. I know that with the amount of troubles folks have with transactional replication, compared to even standard mirroring (and this is an upgrade of that), you are certainly bound to have fewer problems.
The biggest boon is that the secondary can be marked as read only - so you can run all the reporting off of it that you want, and it won't affect the mirroring at all. You just need beefier tempdb (since it essentially uses rcsi to do this).
Of course you do need to be aware that both sides of the AG need to be fully licensed in order to use the replica for read-only operations. And both sides need to be running on specific versions of Windows (Enterprise or better on 2008 R2 and lower; Standard or better on 2012 and above) because they require failover clustering - the SQL instances in AGs don't need to be clustered, but they need to be sitting on top of that infrastructure in the OS.
Given the fact that you mentioned you have Master-Master replication mode, I would not recommend any automatic failover unless you properly account for Replication Lag. After all, MySQL Replication is asynchronous.
It is theoretically possible to have the following:
DBServer1 as Master to DBServer2DBServer2 as Master to DBServer1- DBVIP pointing at DBServer1
DBServer2 is 180 seconds behindDBServer1 goes down- Automatic Failover moves DBVIP to
DBServer2
With this scenario, DBServer2 could have auto increment keys that do not exist yet. Upon failover, the DBVIP will allow WebServers to connect to DBServer2 and ask for data that does not exist yet.
This would therefore require background processes running on each DBServer.
For the above scenario:
- DBVIP is on
DBServer1
DBServer1 runs HeartBeatDBServer2 runs HeartBeat- Background Process on
DBServer1 to monitor
- a) Data Mount Availability
- b) Data Mount Writeability
- c) MySQL Connectivity
- Once a,b, or c fail, kill HeartBeat
Background Process on DBServer2 to make sure DBVIP is pingable
What should killing HeartBeat do? Trigger the startup script defined for it.
What should the startup script on DBServer2 look for?
- Loop until DBVIP is unreachable via ping
- Connect to MySQL and
- Run
SHOW SLAVE STATUS\G in a Loop until Seconds_Behind_Master is NULL
- RUn
SHOW SLAVE STATUS\G in a Loop until `Exec_Master_Log_Pos stops changing
- Assign DBVIP to
DBServer2 via ip addr add
This is essentailly the algorithm for failing over safely to a Passive Master in a Master/Master Replication Cluster.
ALTERNATIVE
If ALL your data is InnoDB, I recommend something with less rigor. Perhaps you should look into using DRBD and HeartBeat. Here is why:
DRBD provides network RAID-1 for a Block Device on two servers.
You would essentially do this:
- Have
DBServer1's DRBD Block Device as Primary
- Have
DBServer2's DRBD Block Device as Secondary
- Mount
DBServer1's DRBD Device on /var/lib/mysql
- Startup MySQL on
DBServer1
- Have HeartBeat Monitor Ping Activity Between Servers
What would startup script look like in a DRBD scenario?
- Loop until DBVIP is unreachable via ping
- Kill HeartBeart
- Disconnect DRBD
- Promote DRBD to Primary
- Mount DRBD on /var/lib/mysql
- Start MySQL (InnoDB Crash Recovery Fills in Missing Data)
- Assign DBVIP via
ip addr add
This is a lot more straightforward because only one side is Active. The Passive side (DRBD Secondary) is a Synchronous Disk Copy of the Active Side (DRBD Primary).
CAVEAT
If all or most of the working set data is MyISAM, do not touch DRBD. Crash scnearios quickly result in MyISAM tables being marked crashed and need auto-repair (which can be paintfully slow to wait for).
UPDATE 2012-12-29 08:00 EDT
Here are my past posts on using DRBD with MySQL
Best Answer
These days, Data Guard is included with Enterprise Edition.If you have enough spare capacity on each of the servers, I believe they can work as a physical standby for each other.
Logical and possibly bi-directional replication is more complex, and you would have to consider the types of objects and replication in the solution. Most solutions does not easily support the replication of DML etc.
Note that Data Guard configured as physical standby might still consume a considerable amount of resources and depending on the configuration, may impact on production performance on both servers.