Ideas for Combining Thousand Databases into One Database

Question

We have a SQL server that has a database for each client, and we have hundreds of clients. So imagine the following: database001, database002, database003, ..., database999. We want to combine all of these databases into one database.

Our thoughts are to add a siteId column, 001, 002, 003, ..., 999.

We are exploring options to make this transition as smoothly as possible. And we would LOVE to hear any ideas you have. It's proving to be a VERY challenging problem.

I've heard of a technique that would create a view that would match and then filter.

Any ideas guys?

Answer 1

Create a client database id for each of the client databases. You will use this id to keep the data logically separated. This is the "site id" concept, but you can use a derived key (identity field) instead of manually creating these numbers. Create a table that has database name and id, with any other metadata you need.

The next step would be to create an SSIS package that gets the ID for the database in question and adds it to the tables that have to have their data separated out logically. You then can run that same package over each database with the lookup for ID for the database in question.

After you have a unique id for the data that is unique, and have imported the data, you will have to alter your apps to fit the new schema (actually before, or you are pretty much screwed).

If you want to do this in steps, you can create views or functions in the different "databases" so the old client can still hit the client's data, even though it has been moved. This step may not be necessary if you deploy with some downtime.

The method I propose is fairly flexible and can be applied to one client at a time, depending on your client application deployment methodology.

Answer 2

If the data is large you could look at using a partioned view. This would simplify your access code as all you'd have to maintain is the view; however, if the data is not large, just add a column to identify the customer.

Answer 3

The "site-id" solution is what's done.

Another possibility that may not work out as well (but is still appealing) is multiple schemas within a single database. You can pull common tables into a "common" schema, and leave the customer-specific stuff in customer-specific schema. In some database products, however, the each schema is -- effectively -- a separate database. In other products (Oracle, DB2, for example) you can easily write queries that work in multiple schemas.

Also note that -- as an optimization -- you may not need to add siteId column to EVERY table.

Sometimes you have a "contains" relationship. It's a master-detail FK, often defined with a cascade delete so that detail cannot exist without the parent. In this case, the children don't need siteId because they don't have an independent existence.

Answer 4

Your first step will be to determine if these databases even have the same structure. Even if you think they do, you need to compare them to make sure they do. Chances are there will be some that are customized or missed an upgrade cycle or two.

Now depending on the number of clients and the number of records per client, your tables may get huge. Are you sure this will not create a performance problem? At any rate you may need to take a fresh look at indexing. You may need a much more powerful set of servers and may also need to partion by client anyway for performance.

Next, yes each table will need a site id of some sort. Further, depending on your design, you may have primary keys that are now no longer unique. You may need to redefine all primary keys to include the siteid. Always index this field when you add it.

Now all your queries, stored procs, views, udfs will need to be rewritten to ensure that the siteid is part of them. PAy particular attention to any dynamic SQL. Otherwise you could be showing client A's information to client B. Clients don't tend to like that. We brought a client from a separate database into the main application one time (when they decided they didn't still want to pay for a separate server). The developer missed just one place where client_id had to be added. Unfortunately, that sent emails to every client concerning this client's proprietary information and to make matters worse, it was a nightly process that ran in the middle of the night, so it wasn't known about until the next day. (the developer was very lucky not to get fired.) The point is be very very careful when you do this and test, test, test, and test some more. Make sure to test all automated behind the scenes stuff as well as the UI stuff.

Answer 5

Why do you want to do that?
You can read about Multi-Tenant Data Architecture and also listen to SO #19 (around 40-50 min) about this design.

Answer 6

Depending on what the data is and your security requirements the threat of cross contamination may be a show stopper.

Assuming you have considered this and deem it "safe enough". You may need/want to create VIEWS or impose some other access control to prevent customers from seeing each-other's data.

IIRC a product called "Trusted Oracle" had the ability to partition data based on such a key (about the time Oracle 7 or 8 was out). The idea was that any given query would automagically have "and sourceKey = @userSecurityKey" (or some such) appended. The feature may have been rolled into later versions of the popular commercial product.

Answer 7

Hey Chris,

what I was explaining in Florence towards the end of last year is if you had to keep the database names and the logical layer of the database the same for the application. In that case you'd do the following:

• Collapse all the data into consolidated tables into one master, consolidated database (hereafter referred to as the consolidated DB).
• Those tables would have to have an identifier like SiteID.
• Create the new databases with the existing names.
• Create views with the old table names which use row-level security to query the tables in the consolidated DB, but using the SiteID to filter.
• Set up the databases for cross-database ownership chaining so that the service accounts can't "accidentally" query the base tables in the consolidated DB. Access must happen through the views or through stored procedures and other constructs that will enforce row-level security. Now, if it's the same service account for all sites, you can avoid the cross DB ownership chaining and assign the rights on the objects in the consolidated DB.
• Rewrite the stored procedures to either handle the change (since they are now referring to views and they don't know to hit the base tables and include SiteID) or use InsteadOf Triggers on the views to intercept update requests and put the appropriate site specific information into the base tables.

Answer 8

To expand on Gregory's answer, you can also make a parent ssis that calls the package doing the actual moving within a foreach loop container.

The parent package queries a config table and puts this in an object variable. The foreach loop then uses this recordset to pass variables to the package, such as your database name and any other details the package might need.

You table could list all of your client databases and have a flag to mark when you are ready to move them. This way you are not sitting around running the ssis package on 32,767 databases. I'm hooked on the foreach loop in ssis.