(Ab)using Disposables for Integration Testing

(Ab)using Disposables for Integration Testing

(Looking for just the code? Disposable examples)


Have you written unit tests against your code but still had some type of issue in production? For example, your code reads and writes to a file. In your unit tests parsing and printing are great. But, when you go to production, something happens out of your control. File permissions, encoding, and faulty assumptions about third-party tools are all common issues. Or in another case, wouldn’t it be great to test those queries you’ve written for your database? I know you’ve written an anti-corruption layer to create your domain objects. But still, shouldn’t we be doing integration tests in some of these scenarios?

Integration tests seem to get a bad reputation, and to be fair it’s understandable why. They tend to be hard to setup, brittle, and high maintenance when underlying assumptions change about your code base. Another big issue with them tends to be isolation and cleanup. There is a solution to some of these problems. We can create disposable objects that generate unique, isolated environments and clean themselves up.

What are disposables?

Dotnet has this concept of the “disposable” pattern. Digging deep into this is outside the scope of this blog. Here’s the sales pitch: wouldn’t it be nice for something to clean itself up after it has left a scope and you don’t have to remember to call it yourself? Now, lets see a super simple example in F#:

let myFunction () =
    use myresource = new Resource()

This function news up an object called Resource then calls DoOperation on that object. After that function is called, the Dispose method of the Resource object will be called. Many dotnet developers are familiar doing this pattern with database connections, commands, and readers. For more in-depth examples please refer to F# for Fun and Profit.

The setup

My F# test framework of choice is Expecto. By default it runs all tests in parallel so it’s difficult to use any global or shared state. Our goal is to have isolated environment for our tests, so this constraint helps us. To aid testing using disposables with async, I had to create a few helpers.

type ParameterizedTest<'a> =
    | Sync of string * ('a -> unit)
    | Async of string * ('a -> Async<unit>)

let testCase' name test =

let testCaseAsync' name test  =

let inline testFixture'<'a> setup =
    Seq.map (fun ( parameterizedTest : ParameterizedTest<'a>) ->
        match parameterizedTest with
        | Sync (name, test) ->
            testCase name <| fun () -> test >> async.Return |> setup |> Async.RunSynchronously
        | Async (name, test) ->
            testCaseAsync name <| setup test


Disposable Directory

For the first example given, let’s create a disposable directory. It’s requirements are that it should be isolated, cleanup after itself, and tell us which directory it created.

type private DisposableDirectory (directory : string) =
    static member Create() =
        let tempPath = IO.Path.Combine(IO.Path.GetTempPath(), Guid.NewGuid().ToString("n"))
        printfn "Creating directory %s" tempPath
        IO.Directory.CreateDirectory tempPath |> ignore
        new DisposableDirectory(tempPath)
    member x.DirectoryInfo = IO.DirectoryInfo(directory)
    interface IDisposable with
        member x.Dispose() =
            printfn "Deleting directory %s" x.DirectoryInfo.FullName

I defined a function to create the object Create, it uses dotnet’s GetTempPath to find the designated temporary directory for your operating system, generates a guid and then creates that directory.

The implementation of the IDisposable will delete this directory. The true part of the parameter is for recursive deletion.

For exposing the directory path, I like using DirectoryInfo. We as an industry still have an obsession with primitives and its better to use the type system to describe your intent.

The print statements are for verbosity of this demo.

Lets show off the tests.

let withDisposableDirectory (test) = async {
  use disposableDirectory = DisposableDirectory.Create()
  do! test disposableDirectory.DirectoryInfo

let disposableDirectoryTests = [
  testCase' "Write file to directory" <| fun (directory : IO.DirectoryInfo) ->
        let fileName = IO.Path.Combine(directory.FullName, "Foo.txt")
        let contents = [
  testCaseAsync' "Write file to directory async" <| fun (directory : IO.DirectoryInfo) -> async {
        let fileName = IO.Path.Combine(directory.FullName, "Foo.txt")
        let contents = [
        do! IO.File.WriteAllLinesAsync(fileName,contents) |> Async.AwaitTask

let tests = 
  testList "Disposable Directory tests" [
    yield! testFixture' withDisposableDirectory disposableDirectoryTests 

The function withDisposableDirectory is a helper to create disposable directories for each test.

disposableDirectoryTests is a list of testCase' or testCaseAsync' that takes in a IO.DirectoryInfo so the test has access to the file path it created.

Finally, we’re telling expecto to make these as tests in the last section.

Lets run them and see the output:

[15:45:30 INF] EXPECTO? Running tests... <Expecto>
Creating directory /var/folders/14/mp4bnvkn3fq5mqcgqscm5c040000gn/T/eeb2aac569b8417a8035c2d10c08c3bb
Creating directory /var/folders/14/mp4bnvkn3fq5mqcgqscm5c040000gn/T/bfa68a425def41e88e8c227f87cfd4b5
Deleting directory /var/folders/14/mp4bnvkn3fq5mqcgqscm5c040000gn/T/eeb2aac569b8417a8035c2d10c08c3bb
Deleting directory /var/folders/14/mp4bnvkn3fq5mqcgqscm5c040000gn/T/bfa68a425def41e88e8c227f87cfd4b5
[15:45:30 INF] EXPECTO! 2 tests run in 00:00:00.1009410 for Disposable Directory tests – 2 passed, 0 ignored, 0 failed, 0 errored. Success! <Expecto>

We can see our print statements telling us where it created the directories and when it deleted them. Each test has their own isolated folder. Neat!

Disposable Database

For databases, we’re going to take a similar approach. I’ll be using Postgres with Npgsql in this example. Setting up Postgres is outside the scope of this blog post. We’ll be doing something similar to the Disposable Directory example.

type private DisposableDatabase (superConn : NpgsqlConnectionStringBuilder, databaseName : string) =
    static member Create(connStr) =
        let databaseName = System.Guid.NewGuid().ToString("n")
        DatabaseTestHelpers.createDatabase (connStr |> string) databaseName
        new DisposableDatabase(connStr,databaseName)
    member x.SuperConn = superConn
    member x.Conn =
        let builder = x.SuperConn |> DatabaseTestHelpers.cloneConnectionString
        builder.Database <- x.DatabaseName
    member x.DatabaseName = databaseName
    interface IDisposable with
        member x.Dispose() =
            DatabaseTestHelpers.dropDatabase (superConn |> string) databaseName

The Create function generates us a new database name, then creates that database with another helper called createDatabase.

The Dispose method will delete that database.

We’re exposing both the SuperConn (The connection with the correct permissions to create that database) and the Conn (the connection to the database).

Since we’ll want to run migrations we’ll have to add a hook to running migrations

let withDisposableDatabase connectionString runMigration (test) = async {
  use disposableDirectory = DisposableDatabase.Create(connectionString)
  runMigration disposableDirectory.Conn
  do! test disposableDirectory.Conn

In our case, we’ll be using Simple.Migrations.

let runMigration (migrationsAssembly : Assembly) (connStr : NpgsqlConnectionStringBuilder) =
    use connection = new NpgsqlConnection(connStr.ToString())
    let databaseProvider = new PostgresqlDatabaseProvider(connection)
    let migrator = new SimpleMigrator(migrationsAssembly, databaseProvider)

Now that the boiler plate is out of the way, we can write some tests!

let disposableDirectoryTests = [
  testCase' "Write to database" <| fun (connStr : NpgsqlConnectionStringBuilder) ->
        use conn = new NpgsqlConnection(connStr.ToString())
        use cmd = new NpgsqlCommand("INSERT INTO animals (name, birthday) VALUES ('Spunky', now())", conn)
        let added = cmd.ExecuteNonQuery()
        Expect.equal added 1 "Should have added 1 row"
  testCaseAsync' "Read count from database" <| fun (connStr : NpgsqlConnectionStringBuilder) -> async {
        use conn = new NpgsqlConnection(connStr.ToString())
        use cmd = new NpgsqlCommand("SELECT COUNT(*) FROM animals)", conn)
        use! reader = cmd.ExecuteReaderAsync() |> Async.AwaitTask
        let! canRead = reader.ReadAsync() |> Async.AwaitTask
        let count = reader.GetInt64(0)
        Expect.equal count 0L "Should have 0 rows"

Every test will now get its own isolated environment to run queries in. Neat!


By now you can see it does take some work to get integration tests setup. But with some effort, you can make them less brittle and behave as isolated environments. You could expand on this to even include spinning up and down Docker containers, or virtual machines with Vagrant. Now I admit, this feels like abusing disposables. Yet there are benefits in doing this for test purposes. Here’s the repo with Disposable examples.

Written on December 12, 2018