Pattern for updating slave SQL Server 2008 databases from a master whilst minimising disruption

Question

We have an ASP.NET web application hosted by a web farm of many instances using SQL Server 2008 in which we do aggregation and pre-processing of data from multiple sources into a format optimised for fast end user query performance (producing 5-10 million rows in some tables). The aggregation and optimisation is done by a service on a back end server which we then want to distribute to multiple read only front end copies used by the web application instances to facilitate maximum scalability.

My question is about the best way to get this data from a back end database out to the read only front end copies in such a way that does not kill their performance during the process. The front end web application instances will be under constant high load and need to have good responsiveness at all times.

The backend database is constantly being updated so I suspect that transactional replication will not be the best approach, as the constant stream of updates to the copies will hurt their performance.

Staleness of data is not a huge issue so snapshot replication might be the way to go, but this will result in poor performance during the periods of replication.

Doing a drop and bulk insert will result in periods with no data for user queries.

I don't really want to get into writing a complex cluster approach where we drop copies out of the cluster during updating - is there something along these lines that we can do without too much effort, or is there a better alternative?

Answer 1

I've never had to deal with this scenario before but did come up with a possible solution for this. Basically, it would require a change in your main database structure. Instead of storing the data, you would keep records of modifications of this data. Thus, if a record is added, you store "Table X, inserted new record with these values: ..." With modifications, just store the table, field and changed value. With deletions, just store which record is deleted. Every modification will be stored with a timestamp.

Your client systems would keep their local copies of the database and will regularly ask for all database modifications after a certain date/time. You then execute those modifications on the local database and it will be up-to-date again.

And the back-end? Well, it would just keep a list of modifications and perhaps a table with the base data. Keeping just the modifications also means you're keeping track of history, allowing you to ask the system what it looked like a year ago.

How well this would perform depends on the number of modifications on the back-end database. But if you request the changes every 15 minutes, it shouldn't be that much data every time.

But again, I never had the chance to work this out in a real application so it's still a theoretic principle for me. It seems fast but a lot of work will be required.

Answer 2

Option 1: Write an app to transfer the data using row level transactions. It might take longer but would result in no interruption of the site using the data because the rows are there before and after the read occurs, just with new data. This processing would happen on a separate server to minimize load.

In sql server 2008 you can set READ_COMMITTED_SNAPSHOT to ON to ensure that the row being updated is not causing blocking.

But basically all this app does is read the new data as it is available out from one database and into the other.

Option 2: Move the data (tables or entire database) from the aggregation server to the front-end server. Automate this if possible. Then switch your web application to point to the new database or tables for future requests. This works but requires control over the web app, which you may not have.

Option 3: If you were talking about a single table (or this could work with many) what you can do is a view swap. So you write your code against a sql view which points to table A. You do you work on Table B and when it's ready, you update the view to point to Table B. You can even write a function that determines the active table and automate the whole swap thing.

Option 4: You might be able to use something like byte-level replication of the server. That sounds scary though. Which is basically copying the server from point A to point B exactly down to the very bytes. It's mostly used in DR situations which this sounds like it could be a kinda/sorta DR situation, but not really.

Option 5: Give up and learn how to sell insurance. :)

Answer 3

There is actually a technology built into SQL Server 2005 (and 2008) that is designed to address this kind of issues. Service Broker (I'll refer further as SSB). The problem is that it has a very steep learning curve.

I know MySpace went public how uses SSB to manage their park of SQL Servers: MySpace Uses SQL Server Service Broker to Protect Integrity of 1 Petabyte of Data. I know of several more (major) sites that use similar patterns but unfortunately they have not gone public so I cannot refer names. I was personally involved with some projects around this technology (I am a former member of the SQL Server team).

Now bear in mind that SSB is not a dedicate data transfer technology like Replication. As such you will not find anyhting similar to the publishing wizards and simple deployment options of Replication (check a table and it gets transferred). SSB is a reliable messaging technology and as such its primitives stop at the level of message exchange, you would have to write the code that leverages the data change capture, packs it as messages and also the unpacking of message into relational tables at destination.

Why still some companies preffer SSB over Replication at a task like you describe is because SSB has a far better story when it comes to reliability and scalability. I know of projects that exchange data between 1500+ sites, far beyond the capabilities of Replication. SSB is also abstracted from the physical topology: you can move databases, rename machines, rebuild servers all without changing the application. Because data flow occurs over logical routes the application can addapt on-the-fly to new topologies. SSB is also resilient to long periods of disocnnect and downtime, being capable of resuming the data flow after hours, days and even months of disconnect. High troughput achieved by engine integration (SSB is part of the SQL engine itself, is not a collection of sattelite applications and processes like Replication) means that the backlog of changes can be processes on reasonable times (I know of sites that are going through half a million transactions per minute). SSB applications typically rely on internal Activation to process the incomming data. SSB also has some unique features like built-in load balancing (via routes) with sticky session semantics, support for deadlock free application specific correlated processing, priority data delivery, specific support for database mirroring, certificate based authentication for cross domain operations, built-in persisted timers and many more.

This is not a specific answer 'how to move data from table T on server A to server B'. Is more a generic technology on how to 'exhange data between server A and server B'.