Service Broker Demystified - Why is there no ALTER CONTRACT statement?

The concept of "contracts" in Service Broker is initially confusing to most data professionals.  I like to think of a contract as a constraint applied to a message.  Then why can't you ALTER a contract?  Because a contract is really more like a "legally-binding contract."  There are good reasons why contracts can't/shouldn't be altered.

People tell me all the time that they don't want to use Service Broker because it is too confusing.  I started a blog series called Service Broker Demystified because SB really isn't that tough if you don't get lost in the weeds.  Today we are going to cover how you alter a Service Broker (SB) contract. 

Remember from the last post that a SB contract merely lists which services are allowed to use which message types.  Message types and contracts are both totally optional and your SB implementation will run fine without them.  When I'm first learning a new technology I try to avoid overloading my brain with optional features to limit potential confusion.  

A contract can specify multiple message types and who may use them (valid values are ANY, INITIATOR, and TARGET).  Let's start with a little demo.  If you want to follow along the full source code is available here.  Let's say that we've been tasked with designing an asynchronous inventory management system with SB.  Here are some message types that we may decide to create.  Hopefully the names are sufficient to understand the intent of the system.  

And here is the associated contract that handles are inventory workflow (very rudimentary):


Initially the workflow's intention was to have either the inventory requestor or the inventory system be able to cancel an inventory request.  But let's say you decide later that only the sender can cancel an inventory request...the inventory system can never cancel a request, rather it can only delay it. In 
this case you must drop and recreate the can't alter it.  This command clearly will fail:

Instead we must DROP and reCREATE the CONTRACT.  Something like this:  

Note that we have changed the contract such that the INITIATOR is the only party that can post messages of type CancelInventoryItemRequest.  

Why is there no ALTER CONTRACT?

In the real world you don't alter a legal contract.  You generally make a new version with a new set of signatures.  Certainly there are exceptions but contracts aren't meant to be broken.  It's no different in the Service Broker world.  CONTRACTs are not meant to be broken.  Contracts are immutable.  However, situations such as my inventory example are commonplace.  When requirements, and hence CONTRACTs, must change BEFORE the system is released into the wild it is certainly reasonable to simply DROP/CREATE the contract like the above screenshot.  

However, this is frowned upon once other systems begin to send messages to your service using that contract.  Altering the contract could break other systems that are relying on its functionality.  Since there may be a lot of moving parts and different systems with different release schedules you can't guarantee that everyone will be able to alter their functionality due to the change in your contract.  Instead you should consider versioning your contracts and supporting multiple versions of a contract for a certain amount of time.  A simple naming convention like CancelInventoryItemRequest_v1 is usually sufficient.  

So if we should version our CONTRACTs, why not our SERVICEs...or...why is there an ALTER SERVICE command?

Versioning your SERVICEs is probably a best practice as well, but the fact is SQL Server does allow us an ALTER SERVICE command.  Why?  

  • It may be necessary to temporarily alter a service to point to a different queue with different activation semantics.  
  • you can always ADD CONTRACT to an existing SERVICE...meaning that your service can adjust to support new features without service interruption.  
  • at a later date you can also ALTER SERVICE...DROP CONTRACT to deprecate the old functionality.  


So based on how Microsoft architected services, contracts, and message types you can see that serious thought was given to versioning the critical objects to help you ensure no service disruptions when there are changes to your design.  If you understand why MS has given you these options you start to understand better how Service Broker really works.  


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