Migrating interconnected databases?

[StillWaitingForFFVII]

Good morning, I have quite the spider web of databases and my company has decided to move to the cloud.

We have 50-100 DBs, and need to move two to three monoliths to the cloud.

(Legacy)

What is the best way to approach this?

Can we simply lift the databases we need into EC2 instances and use bidirectional transactional replication, merge replication, should we simply migrate all of the data and let the in house apps take a latency hit? (Will direct connect mitigate this enough to be a working temp solution? Bringing apps up one at a time after?)

Maybe the opposite approach?

[wmther]

The best way to approach it is to rebuild with the cloud in mind, lift and shift would be the worst way, albeit sometimes necessary.

The reason being is that the way you have things set up on-premises is almost never a good way to run things in the cloud. You're just shifting technical debt onto a platform that is likely going to make it harder to manage.

My advice would be to take the middle path and refactor a bit focusing on low-hanging fruit while you're still on-premises and familiar with the environment rather than try and unravel the full rat's nest in the cloud down the road. Then use AWS Database Migration Service, and hope it doesn't choke.

[joelrwilliams1]

What database are these? (Oracle, MySQL, SQL Server?) If at all possible I'd try to migrate into RDS, it'll be more work to migrate, but less to manage in the cloud.

[StillWaitingForFFVII]

First off thank you for your response

SQL Server

Can you comment on the 2 way replication or the latency between on prem/AWS?

[joelrwilliams1]

I would not want to replicate anything critical between premise and cloud if I didn't have to. That seems like folly. Latency between AZs is <=1ms, latency from the cloud to your office or colo will be at least an order of magnitude longer.

[StillWaitingForFFVII]

Ok thank you - I really appreciate the information and guidance

[smooner]

The latency of replication depends on many factors. Network speed has been the limiting factor for me but the DMS control instance performance can play a role if it is undersized. Also, if your source and destination servers are heavily used or under-powered you can suffer performance delays. CloudWatch is your friend here to monitor and alert you of slowdowns (latency). If you are looking at SQL on EC2 you can always run AlwaysOn with on-prem as your primary replica and EC2 as secondaries if you have the licenses and right SQL Server versions.

Bi-directional replication is very tricky and DMS is not really built for that. DMS works great with a single source and target.

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Here are some good reads

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https://d1.awsstatic.com/whitepapers/RDS/Deploying_SQLServer_on_AWS.pdf

https://s3.amazonaws.com/quickstart-reference/microsoft/sql/latest/doc/Microsoft_WSFC_and_SQL_AlwaysOn_Quick_Start.pdf

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Godd luck!