Skip to content

Commit

Permalink
Updated eventually for 5.7
Browse files Browse the repository at this point in the history
  • Loading branch information
@sksamuel
sksamuel committed Sep 11, 2023
1 parent 516ae9f commit a801069
Show file tree
Hide file tree
Showing 3 changed files with 171 additions and 299 deletions.
238 changes: 163 additions & 75 deletions documentation/versioned_docs/version-5.7/assertions/eventually.md
View file
Expand Up @@ -4,126 +4,214 @@ title: Eventually
slug: eventually.html
---

:::note New improved module
Starting with Kotest 5.7, the non-deterministic testing functions have moved to the `kotest-assertions-core` module, and
are available under the new package `com.sksamuel.kotest.assertions.nondeterministic`. The previous iterations of these
functions are still available, but deprecated.
:::

When testing non-deterministic code, a common use case is "I expect this code to pass after a short period of time".
Testing non-deterministic code can be hard. You might need to juggle threads, timeouts, race conditions, and the
unpredictability of when events are happening.

For example, if you were testing a IO operation, you might need to wait until the IO operation has flushed.
For example, if you were testing that an asynchronous file write was completed successfully, you need to wait until the
write operation has completed and flushed to disk.

Sometimes you can do a Thread.sleep but this is isn't ideal as you need to set a sleep threshold high enough so that it
won't expire prematurely on a slow machine. Plus it means that your test will sit around waiting on the timeout even if
the code completes quickly on a fast machine.
Some common approaches to these problems are:

Or you can roll a loop and sleep and retry and sleep and retry, but this is just boilerplate slowing you down.
* Using callbacks which are invoked once the operation has completed. The callback can be then used to assert that the
state of the system is as we expect. But not all operations provide callback functionality.

Another common approach is to use countdown latches and this works fine if you are able to inject the latches in the
appropriate places but it isn't always possible to have the code under test trigger a latch.
* Block the thread using `Thread.sleep` or suspend a function using `delay`, waiting for the operation to complete.
The sleep threshold needs to be set high enough to be sure the operations will have completed on a fast or slow
machine. Plus it means that your test will sit around waiting on the timeout even if
the code completes quickly on a fast machine.

As an alternative, kotest provides the `eventually` function and the `Eventually` configuration which periodically test
the code ignoring your specified exceptions and ensuring the result satisfies an optional predicate, until the timeout
is eventually reached or too many iterations have passed. This is flexible and is perfect for testing nondeterministic
code.
* Use a loop with a sleep and retry and a sleep and retry, but then you need to write boilerplate to track number of
iterations, handle certain exceptions and fail on others, ensure the total time taken has not exceeded the max and so
on.

* Use countdown latches and block threads until the latches are released by the non-determistic operation. This can
work well if you are able to inject the latches in the appropriate places, but just like callbacks, it isn't always
possible to have the code to be tested integrate with a latch.

### Examples
As an alternative to the above solutions, kotest provides the `eventually` function which solves the common use case of
_"**I expect this code to pass after a short period of time**"_.

Eventually works by periodically invoking a given lambda, ignoring specified exceptions, until the lambda passes, or a
timeout is reached, or too many
iterations have passed. This is flexible and is perfect for testing nondeterministic code. Eventually can be customized
with regards to the types of exceptions to handle, how the lambda is considered a success or failure, with a listener,
and so on.

#### Simple examples
## API

Let's assume that we send a message to an asynchronous service.
After the message is processed, a new row is inserted into user table.
There are two ways to use eventually. The first is simply providing a duration, using the Kotlin `Duration` type,
followed by the code that should eventually pass without an exception being raised.

We can check this behaviour with our `eventually` function.
For example:

```kotlin
class MyTests : ShouldSpec() {
init {
should("check if user repository has one row after message is sent") {
sendMessage()
eventually(5.seconds) {
userRepository.size() shouldBe 1
}
}
}
eventually(5.seconds) {
userRepository.getById(1).name shouldBe "bob"
}
```

#### Exceptions
The second is by providing a config block. This method should be used when you need to
set more options than just the duration. It also allows the config to be shared between multiple invocations of
eventually.

For example:

```kotlin
val config = eventuallyConfig {
duration = 1.seconds
interval = 100.milliseconds
}

By default, `eventually` will ignore any `AssertionError` that is thrown inside the function (note, that means it won't catch `Error`).
If you want to be more specific, you can tell `eventually` to ignore specific exceptions and any others will immediately fail the test.
eventually(config) {
userRepository.getById(1).name shouldBe "bob"
}
```

Let's assume that our example from before throws a `UserNotFoundException` while the user is not found in the database.
It will eventually return the user when the message is processed by the system.
## Configuration Options

In this scenario, we can explicitly skip the exception that we expect to happen until the test passed, but any other exceptions would
not be ignored. Note, this example is similar to the former, but if there was some other error, say a ConnectionException for example, this would cause
the eventually block to immediately exit with a failure message.
### Durations and Intervals

The duration is the total amount of time to keep trying to pass the test. The `interval` allows us to
specify how often the test should be attempted. So if we set duration to 5 seconds, and interval to 250 millis,
then the test would be attempted at most `5000 / 250 = 20` times.

```kotlin
class MyTests : ShouldSpec() {
init {
should("check if user repository has one row") {
eventually(5.seconds, UserNotFoundException::class.java) {
userRepository.findBy(1) shouldNotBe null
}
}
}
val config = eventuallyConfig {
duration = 5.seconds
interval = 250.milliseconds
}
```

Alternatively, rather than specifying the interval as a fixed number, we can pass in a function. This allows us to
perform some kind of backoff, or anything else we need.

For example, to use a fibonacci increasing interval, starting with 100ms:

#### Predicates
```kotlin
val config = eventuallyConfig {
duration = 5.seconds
intervalFn = 100.milliseconds.fibonacci()
}
```

### Initial Delay

In addition to verifying a test case eventually runs without throwing, we can also verify the result and treat a non-throwing result as failing.
Usually `eventually` starts executing the test block immediately, but we can add an initial delay before the first
iteration using `initialDelay`, such as:

```kotlin
class MyTests : StringSpec({
"check that predicate eventually succeeds in time" {
var i = 0
eventually<Int>(25.seconds, predicate = { it == 5 }) {
delay(1.seconds)
i++
}
}
})
val config = eventuallyConfig {
initialDelay = 1.seconds
}
```

#### Sharing configuration
### Retries

Sharing the configuration for eventually is a breeze with the `Eventually` data class. Suppose you have classified the operations in your
system to "slow" and "fast" operations. Instead of remembering which timing values were for slow and fast we can set up some objects to share between tests
and customize them per suite. This is also a perfect time to show off the listener capabilities of `eventually` which give you insight
into the current value of the result of your producer and the state of iterations!
In addition to bounding the number of invocations by time, we can do so by iteration count. In the following example
we retry the operation 10 times, or until 8 seconds has expired.

```kotlin
val slow = EventuallyConfig<ServerResponse, ServerException>(5.minutes, interval = 25.milliseconds.fibonacci(), exceptionClass = ServerException::class)
val fast = slow.copy(duration = 5.seconds)
val config = eventuallyConfig {
initialDelay = 8.seconds
retries = 10
}

eventually(config) {
userRepository.getById(1).name shouldBe "bob"
}
```

### Specifying the exceptions to trap

By default, `eventually` will ignore any `AssertionError` that is thrown inside the function (note, that means it won't
catch `Error`). If you want to be more specific, you can tell `eventually` to ignore specific exceptions and any others
will immediately fail the test. We call these exceptions, the _expected exceptions_.

For example, when testing that a user should exist in the database, a `UserNotFoundException` might be thrown
if the user does not exist. We know that eventually that user will exist. But if an `IOException` is thrown, we don't
want to keep retrying as this indicates a larger issue than simply timing.

class FooTests : StringSpec({
val logger = logger("FooTests")
val fSlow = slow.copy(listener = { i, t -> logger.info("Current $i after {${t.times} attempts")})
We can do this by specifying that `UserNotFoundException` is an exception to suppress.

```kotlin
val config = eventuallyConfig {
duration = 5.seconds
expectedExceptions = setOf(UserNotFoundException::class)
}

"server eventually provides a result for /foo" {
eventually(fSlow) {
eventually(config) {
userRepository.getById(1).name shouldBe "bob"
}
```

As an alternative to passing in a set of exceptions, we can provide a function which is invoked, passing in the throw
exception. This function should return true if the exception should be ignored, or false if the exception should bubble
out.

```kotlin
val config = eventuallyConfig {
duration = 5.seconds
expectedExceptions = { it is UserNotFoundException }
}

eventually(config) {
userRepository.getById(1).name shouldBe "bob"
}
```

### Listeners

We can attach a listener, which will be invoked on each iteration, with the current iteration count and the
exception that caused the iteration to fail. Note: The listener will not be fired on a successful invocation.

```kotlin
val config = eventuallyConfig {
duration = 5.seconds
listener = { k, throwable -> println("Iteration $k, with cause $throwable") }
}

eventually(config) {
userRepository.getById(1).name shouldBe "bob"
}
```

### Sharing configuration

Sharing the configuration for eventually is a breeze with the `eventuallyConfig` builder.
Suppose you have classified the operations in your system to "slow" and "fast" operations. Instead of remembering
which timing values were for slow and
fast we can set up some objects to share between tests and customize them per suite. This is also a perfect time to show
off the listener capabilities of `eventually` which give you insight into the current value of the result of your
producer and the state of iterations!

```kotlin
val slow = eventuallyConfig {
duration = 5.minutes
interval = 25.milliseconds.fibonacci()
listener = { i, t -> logger.info("Current $i after {${t.times} attempts") }
}

val fast = slow.copy(duration = 5.seconds)

class FooTests : FunSpec({
test("server eventually provides a result for /foo") {
eventually(slow) {
fooApi()
}
}
})

class BarTests : StringSpec({
val logger = logger("BarTests")
val bFast = fast.copy(listener = { i, t -> logger.info("Current $i after {${t.times} attempts")})

"server eventually provides a result for /bar" {
eventually(bFast) {
class BarTests : FunSpec({
test("server eventually provides a result for /bar") {
eventually(fast) {
barApi()
}
}
})

```

Here we can see sharing of configuration can be useful to reduce duplicate code while allowing flexibility for things like
custom logging per test suite for clear test logs.

0 comments on commit a801069

Please sign in to comment.