Mixing serialized and concurrent suites in Swift Testing

Development Swift Testing
1

Hi,

I’m trying to express the following structure in Swift Testing:

@Suite(.serialized)
struct Root {
    @Suite(.serialized)
    struct SuiteA { /* ... */ }

    @Suite(.concurrent)
    struct SuiteB { /* ... */ }
}

Goal

  • SuiteA and SuiteB should be serialized (and not overlap with each other)
  • SuiteB should be fully concurrent internally, but also must not run at the same time as A

Problem

  • .concurrent doesn’t exist
  • Without it, SuiteB inherits .serialized from the parent
  • If I remove .serialized from the parent, then SuiteA and SuiteB can run in parallel with each other

So I can’t seem to express:

serialized between some suites, but concurrent within another

Question

Is there a way to model this today?

Thanks!

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2

A suite nested in a serialized suite cannot be run concurrently. This is by design, although it is not a technical constraint. It would help to know more about the real-world scenario you are testing that requires this configuration. :slightly_smiling_face:

Of interest to you (although I don't think it solves your specific problem) may be the pitched data-dependent serialization feature.

3

@grynspan Thank you for your reply.

My use case is SwiftData migration tests that need a file-backed SQLite store, living alongside a large E2E test suite that uses in-memory stores.

Migration tests must use file-backed storage because SwiftData migrations require closing and reopening the ModelContainer — you can't migrate an in-memory database since it's destroyed on close:

  @Suite(.serialized) class MigrationTests {
    var url: URL!
    var container: ModelContainer!
    var context: ModelContext!

    init() {
      url = FileManager.default.temporaryDirectory.appending(component: "default.store")
      // clean up any leftover DB files
      try? FileManager.default.removeItem(at: url)
    }

    @Test func migrateTo1_2_0() throws {
      // 1. Create a container with the OLD schema, insert test data
      container = try initModelContainer(for: OldSchema.self, with: nil, storeUrl: url)
      context = ModelContext(container)
      OldSchema.insertMocks(context: context)

      // 2. Close and reopen with the NEW schema + migration plan
      //    (this is why in-memory doesn't work — the DB must survive the close)
      container = try initModelContainer(for: NewSchema.self, with: MyMigrationPlan.self, storeUrl: url)
      context = ModelContext(container)

      let records = try context.fetch(FetchDescriptor<NewSchema.SaveData>())
      #expect(records.count == 1)
    }
  }

These tests are .serialized because they share a file path and each test writes → closes → reopens the SQLite store.

E2E tests use in-memory stores and are fully independent — each nested suite creates its own ModelContainer, so they can safely run concurrently:

  @Suite struct E2ETests {
    @Suite struct Moves {
      let session = GameSession(store: inMemoryModelContainer())
      @Test func magnetMove() async throws { /* ... */ }
      @Test func positionMove() async throws { /* ... */ }
    }

    @Suite struct Physics {
      let session = GameSession(store: inMemoryModelContainer())
      @Test func gravity() async throws { /* ... */ }
    }

    // ~12 more nested suites, all independent
  }

The conflict: if MigrationTests and E2ETests run at the same time, the file-backed migration tests can interfere with the global SwiftData state (schema registration, container caching). Today I work around this with .disabled on MigrationTests and run them separately, but what I actually want is:

  @Suite(.serialized) struct AllTests {
    @Suite(.serialized) struct MigrationTests { /* file-backed, must be serial */ }

    @Suite(.concurrent)  // ← this doesn't exist
    struct E2ETests { /* in-memory, safe to parallelize */ }
  }

The root .serialized would ensure migrations finish before E2E starts (or vice versa), while .concurrent on E2E would let its ~200 tests run in parallel as they do today. Without this, my options are either disable migration tests (current workaround) or serialize everything (which makes the E2E suite painfully slow).

4

Can you instead teach each test to use a different URL? i.e.:

- url = FileManager.default.temporaryDirectory.appending(component: "default.store")
+ let uuid = UUID().uuidString
+ url = FileManager.default.temporaryDirectory.appending(component: "\(uuid).store")
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