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Service Broker 101 Lesson 1: What is Service Broker?

May 19, 2020

If you don’t really know what Service Broker is, you’re not alone. I had probably heard the term a couple of times in my 14 years as a SQL developer, but had never come across anyone using it until I started my latest job. Even then, I only discovered it when I imported a database into an SSDT database project, and saw a Queue object appear.

I did a little investigation after that; and it seemed an interesting, if little used, piece of functionality. I didn’t really think anything more of it, but filed it away in the bit of my brain that stores marked “investigate someday” (that part of my brain gets pretty cluttered and seldom cleared out).

Then, recently, I had an issue where Service Broker seemed the perfect solution, so spent a weekend experimenting, coded the fix using Service Broker, and that release is making it’s way through UAT at the moment.

But what is Service Broker?

I can hear you thinking “yeah, yeah, get to the point already”, so I will.

Service Broker is a queueing system within SQL Server. It allows you to send a message from one queue to another, and handle what happens when that message arrives. That’s pretty much it.

So, why would I want that?

Well, that’s where the story I told you at the start comes in. The issue we had was that out client wanted to run a process during the day that usually gets run at night. This process is pretty long, and locks up some tables for a minute or more, including some tables it locks through an indexed view (that’s a whole other issue that I’ll maybe blog about some other day). At the same time, users are logging onto the application to do various things, including downloading one-use vouchers. The stored proc that sits behind that API reads the data ok, but wants to write the record that these vouchers have been viewed to one of the locked tables.

What I’ve done is shift the write to table from the stored procedure to a queue. Now when a user requests their vouchers the system selects them from the right table, and fires off a message to the queue with all the details of the vouchers they just viewed, and the queue adds them to the table whenever it has a chance.

So, that’s scenario #1, when you have a process that needs to do something but doesn’t need to do it immediately, you can split that part of the process out and use a queue to have it happen as soon as possible, allowing your main process to complete quicker. Typically this will be logging that something has happened, where you need to return the result of the thing immediately but the logging table might be blocked by either a big read/write, or lots of small reads/writes.

Scenario #2 is when you are running something like the ETL systems I’ve seen and heard getting built more and more. These systems work off queues in an case, typically built as tables, where you have an overarching system that dynamically decides what happens and in what order.

As an example, you start by importing 3 files, so you add those imports to the queue and they all start executing. Once the first one is finished, you add a validation for that to the queue and that starts processing. File 2 finishes importing but that’s flagged as not needing validation so you check what data loads you can run with just that file and add them to the queue. File 3 takes a long time to load, and File 1 finishes validation in that time but there’s nothing that can be populated from File 1 until File 3 is finished loading so nothing gets added to the queue at that point.

If you have a system that wants to work like that, where you are executing a set of modules in a dynamic order, then Service Broker may be useful as a way of managing that order.

I was going to post a bit more here about the different components that make up Service Broker, but I’ve gone on for longer than I expected just on this, so I think I’ll leave that for the next post.

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