NService Bus: Nitty-Gritty Deployment Issues

Question

Please consider the following questions in the context of multiple publications from a scaled out publisher (using DB subscription storage) and multiple subscriptions with scaled out subscribers (using distributors) where installs and uninstalls happen regularly for initial deployments, upgrades, etc. using automated MSI's.

1. Using DB subscription storage, what happens if the DB goes down? If access to the Subscription DB is required in order to Publish a message, how will it be delivered? Will it get lost? Will the call to Bus.Publish throw an exception?
2. Assuming you need to have no down-time deployments: What if you want to move your subscription DB for a particular publication to a different server? How do you manage a transition like this?
3. Same question goes for a distributor on the subscriber side: What if you want to move your distributor endpoint? One scenario I can think of is if you have multiple subscriptions utilizing a single distributor machine, it might be hard if you want to move some of them to another distributor server to reduce load.
4. What would the install/uninstall scenarios look like for a setup like this (both initially, and for continuous upgrades)? It seems like you would want to have some special install/uninstall scripts for deployment of the "logical publication" and subscription DB, as well as for the "logical subscriptions" and the distributors. The publisher instances wouldn't need any special install/uninstall logic (since they just start publishing messages using the configured subscription DB, and then stop when they are uninstalled). The subscriber worker nodes wouldn't need anything special on install other than the correct configuration of the distributor endpoint, but would need uninstall logic to make sure they are removed from the distributors list of worker nodes.

Answer 1

1. Eventually the publisher will fail and the messages will build up in the internal queue. You will have to plan the size of disk you need to handle this based on the message size and how long you want to wait for a DB to come up. From there it is based how much downtime you can handle. You can use DB mirroring or clustering to make the DB have less downtime.
2. Mirroring and clustering technologies can also help with this. Depends on if you want to do manual or automatic failover and where your doing it(remote sites?).
3. Clustering MSMQ could help you here. If you want to drop a distributor and move it within a cluster you'd be ok. Another possibility is to expose your distributors via HTTP and load balance them behind either a software or hardware load balancing solution. Behind the load balancer you'd be more free to move things around.
4. Sounds like you have a good grasp on this one already :)

Answer 2

To your first question, about the high availability of the subscription DB, you can use a cluster for failover. If the DB is down, then the Bus.Publish will throw an exception, yes. It is recommended to keep the subscription DB separate from your applicative DB to avoid having to bring it down when upgrading your app. This doesn't have to be a separate DB server, a separate DB on the same DB server will be fine.

About moving servers, this is usually managed at a DNS level where for a certain period of time you'll have both running, until communication moves over.

On your third question about distributors - don't share a distributor between different publishers or subscribers.

As a rule of thumb, it is recommended to not add/remove subscribers when doing these kinds of maintainenance activities. This usually simplifies things quite a bit.