Fix: Pulsar MongoSourceTest's Ordering Bug

Hey everyone, let's dive into a nifty bug that was causing some headaches in the MongoSourceTest within Apache Pulsar. The core issue? Tests were failing because they were too strict about the order of data, which isn't always guaranteed in how data structures like HashMaps work.

The Heart of the Problem: Nondeterministic Order

So, the MongoSourceTest was written with the assumption that the order of strings in the data would always be the same. But, when you're dealing with things like HashMaps, the order isn't set in stone. HashMaps don't promise a specific order; it depends on the environment and can vary even when the logical content is identical. This meant that the test could fail even when the data was fundamentally correct, just because the order of elements was different. The test compared the raw strings or trees "as-is," which caused harmless re-ordering to flip the test from pass to fail.

In simpler terms, imagine you have two lists of ingredients for a recipe: ["flour", "sugar", "eggs"] and ["eggs", "flour", "sugar"]. The recipe is the same, right? But the test was treating them as different because the order was different. That’s the problem we're addressing here.

Diving Deeper with NonDex

To sniff out this bug, we used a cool tool called NonDex. NonDex is designed to spot these kinds of issues. It detects incorrect tests that rely on non-deterministic behaviors in Java APIs – such as assuming the order of contents in HashMaps. It does this by instrumenting undetermined APIs and randomizing the returned order or behavior within what the spec allows. NonDex exposes failures that reliably indicate flawed assumptions in order that were never promised and should be fixed. NonDex revealed that the MongoSourceTest was making this flawed assumption about the order. NonDex revealed that the MongoSourceTest was making this flawed assumption about the order.

The Error Messages and What They Mean

The error message from the test looked something like this:

[ERROR] org.apache.pulsar.io.mongodb.MongoSourceTest.testWriteBadMessage -- Time elapsed: 0.017 s <<< FAILURE!
java.lang.AssertionError: expected [{"fullDocument":{"hello":"pulsar"},"ns":{"databaseName":"hello","collectionName":"pulsar","fullName":"hello.pulsar"},"operation":"INSERT"}] but found [{"fullDocument":{"hello":"pulsar"},"operation":"INSERT","ns":{"fullName":"hello.pulsar","collectionName":"pulsar","databaseName":"hello"}}]

This message tells us that the test was expecting the data in a specific order but found it in a different order. The core information was the same (the "hello":"pulsar" part), but the arrangement of the fields was different, leading to a test failure.

Reproducing the Issue

To reproduce this, you could use the NonDex tool with the following command:

mvn -pl pulsar-io/mongo -Dtest=org.apache.pulsar.io.mongodb.MongoSourceTest#testWriteBadMessage -DforkCount=1 -DnondexRuns=1 -DnondexSeed=1016066 -DreuseForks=false edu.illinois:nondex-maven-plugin:2.2.1:nondex

This command essentially runs the test with NonDex, which will expose the nondeterministic behavior and cause the test to fail if the order is incorrect. It highlights the test's sensitivity to the order of elements, even when the data's essence remains unchanged.

The Solution: Addressing the Ordering Issue

The fix involved ensuring that the test wasn't overly reliant on the order of the data. Instead of comparing the raw strings directly, we needed to compare the meaning of the data. This could involve things like:

• Sorting the data: Before comparison, sorting the lists or objects being compared. This way, the order doesn't matter.
• Comparing specific fields: Instead of comparing the whole object, focusing on the crucial fields that define the data's meaning. This approach will make the test more robust and less sensitive to ordering changes.
• Using a different comparison method: Employing a comparison method that ignores the order, such as comparing sets or using a custom comparison function that checks if the expected fields exist, irrespective of their order.

By making these changes, we can make the test more reliable and prevent it from failing due to harmless changes in the data's order.

Why This Matters

Fixing these kinds of bugs is crucial because they ensure the reliability of our tests. If tests are too strict about things that shouldn't matter (like the order of elements in a HashMap), they can lead to false positives – tests failing even when the code is working correctly. This can waste time and make it harder to find real bugs.

It is also important to ensure that tests accurately reflect the functionality of the code. By accounting for non-deterministic behaviors, we can make tests that are more robust and give developers more confidence in their code.

Contributing to the Fix

I'm happy to report that a PR (Pull Request) has been submitted to address this issue. The fix will make the MongoSourceTest more reliable and less prone to false positives caused by order-related issues. This work improves the overall quality of the Apache Pulsar project.

In essence, the fix involved making the test less sensitive to the order of elements in the data structures being compared. Instead of directly comparing the strings, the test was updated to compare the essence of the data, so that the test will pass, irrespective of the internal ordering.

Wrapping Up

So there you have it, guys! We've tackled a tricky bug and made our tests more robust. Remember, the goal is to make sure the tests accurately reflect the code's functionality, and by addressing the non-deterministic ordering, we've done just that.

For more details on the fix and related discussions, check out the Apache Pulsar GitHub repository. Also, if you're interested in the NonDex tool, you can find more information at the NonDex GitHub page.

External Links:

• Apache Pulsar: https://pulsar.apache.org/
• NonDex: https://github.com/TestingResearchIllinois/NonDex