Seed data

Seed steps are numbered. Each runs once per database, in order, as the master admin. The database stores the last step it ran (at cms/modules/{name}.json in its storage), so a step added in an update runs alone at the next start. A new step is also the place to change existing data.

The events module's step 2 came in an update. It throws while the host's page is missing, so it runs again at the next start:

public static void ListUnder(CmsModuleContext c, Guid? parentPageId) {
    if (parentPageId is not Guid parent) return;
    if (!c.Store.TryGet<IPage>(parent, out _))
        throw new InvalidOperationException("The page the events go under is not there yet.");
    foreach (var e in c.Store.Query<IEventPage>().Execute())
        if (!e.Parent.IsSet()) c.Store.SetRelation<IEventPage>(e.Id, x => x.Parent, parent);
}

A step that throws stops the steps after it. The error is logged, and the step runs again at the next start.
Make every step safe to run twice. Give seeded nodes fixed ids and check for them first, as VenuesModule does. Later steps and other modules can then find the nodes too.
Never renumber a released step. A database that has run step 2 never runs a step 2 again.
Write synchronously (Insert, Update, CreateAndInsert). Only the store's synchronous writes run its transaction plugins.

CmsModules.SeedVersionOf(name, store) returns the last step a database ran.