How should Event Sourcing event handlers be hosted to construct a read model?

In our project we use subscription-based projections. The reasons for this are:

• Committing to the write-side must be transactional and if you use two pieces of infrastructure (event store and message bus), you have to start using DTC or otherwise you risk your events to be saved to the store but not published to the bus, or the other way around, depending on your implementation. DTC and two-phase commits are nasty things and you do not want to go this way
• Events are usually published in the message bus anyway (we do it via subscriptions too) for event-driven communication between different bounded contexts. If you use message subscribers to update your read model, when you decide to rebuilt the read model, your other subscribers will get these messages too and this will bring the system to invalid state. I think you have thought about this already when saying you must only have one subscriber for each published message type.
• Message bus consumers have no guarantee on message order and this can bring your read model to mess.
• Message consumers usually handle retries by sending the message back to the queue, and usually by the end of the queue, for retrying. This means that your events can become heavily out of order. In addition, usually after some number of retries, message consumer gives up on the poison message and puts it to some DLQ. If this would be your projection, this will mean that one update will be ignored whilst others will be processed. This means that your read model will be in inconsistent (invalid) state.

Considering these reasons, we have single-threaded subscription-based projections that can do whatever. You can do different type of projections with own checkpoints, subscribing to the event store using catch-up subscriptions. We host them in the same process as many other things for the sake of simplicity but this process only runs on one machine. Should we want to scale-out this process, we will have to take the subscriptions/projections out. It can easily be done since this part has virtually no dependencies to other modules, except the read model DTOs itself, which can be shared as an assembly anyway.

By using subscriptions you always project events that have been already committed. If something goes wrong with the projections, the write side is definitely the source of truth and remains so, you just need to fix the projection and run it again.

We have two separate ones - one for projecting to the read model and another one for publishing events to the message bus. This construct has proven to work very well.