Hi,

Thanks for posting.

The SerialAttribute works internally through a combination of ExclusivelyUsesAttribute and InclusivelyUsesAttribute.

When this attribute exists, NCrunch internally does the following:
1. All tests within the project become implicitly marked with InclusivelyUses("Test Runner")
2. All tests within the project using the SerialAttribute become implicitly marked with ExclusivelyUses("Test Runner")

The end result is that on one machine, no test is allowed to run concurrently with another test that declares SerialAttribute.

These concurrency constraints are always at machine level. This makes them advantageous in a situation where the constraint is local - for example, you may have a local file or database server running on your development machine that also exists on every grid node. In such a situation, distributed processing is extremely helpful as it gives you much more capacity even when constraints exist.

The downside is that this doesn't work well for distributed environments that are centrally constrained. For example, you may have a database server that is shared by all nodes on the network. At present, there is no way to add a concurrency constraint that would apply to all these nodes allowing only one to use the database at any one time. The reason this feature doesn't yet exist is because to do it effectively in a team-based environment would require a distributed mutex in which the grid nodes would coordinate with each other to handle the concurrency constraints. At present the distributed processing doesn't have the concept of a controller node to coordinate such a mutex.

I think this is the question you're asking ... Or are you asking in terms of the meaning of the SerialAttribute itself? At the moment, this attribute has no impact on the sequencing or batching of test execution.

Using SerialAttribute with RequiresCapabilityAttribute is the quickest and simplest way to implement this. Unfortunately, it would mean targeting the tests to a specific node, but you likely would receive little benefit from the parallelisation or distributed processing if these tests have a central dependency anyway.

It may be worth using ExclusivelyUsesAttribute instead of SerialAttribute if you have other tests that are able to run concurrently with these database tests. It would be painful to wait for your database tests to finish before any unrelated and unrestricted unit tests could be run.

I recommend examining ways to tweak your test code to remove the concurrency constraint. One possible option could be to have a separate database or schema per grid node, so that the nodes wouldn't be restricted by access to only one database. You could derive the connection string using some parameter on the grid node, like System.Environment.MachineName for example. This would give you significantly higher throughput and better response times from the engine - assuming the database server can keep up with the demand.