[Podcast] Shared subscriptions between autonomous components

Ask Udi Podcast – Udi Dahan – The Software Simplist

This week we’re discussing scenarios involving the use of multiple autonomous components handling the same event. We also get into the topics of component hosting as well as solution development structure.

Our long-time listener Bill asks the following:

Hi Udi,

Thank you for your podcast clarifying the concept of autonomous components. It certainly cleared up a lot in terms of how you define an autonomous component and helped position the concept better in my mind.

I am considering building our Policy Administration service with 3 autonomous components as you previously suggested – one for each insurance product family. As you previously mentioned, these autonomous components could either share a database instance or have separate database instances, even though they may share schema (being a part of the same service).

What concerns me however is that there is some data that must be shared between these autonomous components. A perfect example is Region. If a new Region is defined somewhere in the enterprise, a RegionCreatedNotification is published onto the bus. If I have 3 autonomous components sharing the same database, is it best I just choose one to be the subscriber for this event? Or should I create a new separate queue to receive these notifications and put a message handler for it in a separate MessageHandlers assembly? Or should I subscribe all 3 autonomous components and have them each check if the Region has already been created in the database before inserting a new one?

Option (1) seems presumptuous because it assumes all 3 autonomous components are sharing the same database. Option (2) seems a bit strange because it is like creating a 4th autonomous component only to receive the RegionCreatedNotification messages. And Option (3) seems wasteful since there is redundant code and the system is doing extra unnecessary lookups. At the moment it looks like Option (2) is my best bet, but I’m very interested to get your take on it.

Another question is whether you would say that autonomous components should be hosted in different processes. Since I don’t need to run these autonomous components on separate servers at this stage, it would seem wasteful to create a service host for each component. I could always create separate service hosts at a later time if I needed to host one or more autonomous components on other servers.

And the final question I have is in terms of solution structure. I was contemplating the following structure:

ProductFamily1
———————-
Company.PolicyAdministration.ProductFamily1.Domain
Company.PolicyAdministration.ProductFamily1.MessageHandlers
Company.PolicyAdministration.ProductFamily1.Messages
Company.PolicyAdministration.ProductFamily1.Persistence
Company.PolicyAdministration.ProductFamily1.Persistence.Implementation
Company.PolicyAdministration.ProductFamily1.Etc…

ProductFamily2
———————-
Company.PolicyAdministration.ProductFamily2.Domain
Company.PolicyAdministration.ProductFamily2.MessageHandlers
Company.PolicyAdministration.ProductFamily2.Messages
Company.PolicyAdministration.ProductFamily2.Persistence
Company.PolicyAdministration.ProductFamily2.Persistence.Implementation
Company.PolicyAdministration.ProductFamily2.Etc…

ProductFamily3
———————-
Company.PolicyAdministration.ProductFamily3.Domain
Company.PolicyAdministration.ProductFamily3.MessageHandlers
Company.PolicyAdministration.ProductFamily3.Messages
Company.PolicyAdministration.ProductFamily3.Persistence
Company.PolicyAdministration.ProductFamily3.Persistence.Implementation
Company.PolicyAdministration.ProductFamily3.Etc…

Common
———————-
Company.PolicyAdministration.Common.Domain
Company.PolicyAdministration.Common.Persistence
Company.PolicyAdministration.Common.Messages
Company.PolicyAdministration.Common.ServiceHost
Company.PolicyAdministration.Common.Etc…

Do you have any thoughts on the above structure?

Once again thank you very much for your extremely valuable advice!

Best Regards,
Bill

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Additional References:

• Ask Udi Podcast #017 – Using Autonomous Components for SLAs in SOA
• Ask Udi Podcast #020 – How to structure .NET solutions and components

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