Parquet.NET Bug: Deserializing Required Field To Nullable Fails

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Hey everyone! Today, let's dive into a fascinating bug we've uncovered in the Parquet.NET library. Specifically, it involves the deserialization of a required Parquet field into a nullable value type. Trust me, this is more interesting than it sounds! We're going to break down the issue, show you how to reproduce it, and discuss why it's happening. So, buckle up and let's get started!

The Issue: InvalidDataException During Deserialization

So, what's the big deal? Well, the core problem is that when you try to deserialize a required field (meaning it always has a value) from a Parquet file into a nullable value type (like bool?) in your C# code, you might encounter an InvalidDataException. This exception throws a message that sounds like something went wrong in the Matrix: class definition level (X) does not match file's definition level (Y) in field 'FieldName'. This usually means nullability in class definiton is incompatible.

Diving Deeper into the Exception

Let's break that down a bit. The "definition level" refers to how Parquet handles null values. In Parquet, each field has a definition level that indicates whether it can be null or not. A required field, naturally, has a definition level indicating it cannot be null. Now, when you try to map this required field to a nullable type in your C# class (like bool?), the deserializer gets confused. It sees the mismatch in nullability and throws a fit – hence the InvalidDataException.

The problematic line of code, as pointed out, is within the ParquetSerializer.cs file in the Parquet.NET library. This is where the deserialization logic lives, and it's where the mismatch between the field's required status and the nullable type in your class definition causes the exception to be thrown.

Why This Matters

Now, you might be thinking, "Okay, so it throws an exception. Why should I care?" Well, this can lead to some serious headaches, especially when dealing with data pipelines and large datasets. Imagine you have a Spark job or some other process generating Parquet files, and sometimes it decides to serialize non-null values as bool? instead of bool (even though the column is technically required). If your downstream application expects a bool? and you hit this bug, your deserialization will fail, and your data processing pipeline will grind to a halt. Nobody wants that, right?

Reproducing the Bug: A Failing Test Case

Alright, enough talk – let's see this bug in action! The best way to understand a bug is to reproduce it, so let's walk through the failing test case provided. This test case is written in C# using xUnit and FluentAssertions, which are popular libraries for unit testing and making assertions in a more readable way.

The Test Setup

The test defines two classes: SparkType and DeserializeType. SparkType represents the structure of the data being written to the Parquet file, while DeserializeType represents the structure we're trying to deserialize the data into. The key difference here is that SparkType has a bool IsServer property (a required boolean field), while DeserializeType has a bool? IsServer property (a nullable boolean field).

private sealed class SparkType
{
    public bool IsServer { get; set; }
}

private sealed class DeserializeType
{
    public bool? IsServer { get; set; }
}

The Test Logic

The test does the following:

  1. Creates an array of SparkType objects, each with a bool value for IsServer.
  2. Serializes these objects into a MemoryStream using ParquetSerializer.SerializeAsync. This simulates writing the data to a Parquet file.
  3. Resets the stream position to the beginning, so we can read from it.
  4. Deserializes the data from the stream into an IAsyncEnumerable<DeserializeType> using ParquetSerializer.DeserializeAllAsync<DeserializeType>. This is where the magic (or rather, the bug) happens.
  5. Collects the deserialized DeserializeType objects into a list.
  6. Asserts that the deserialized results are equivalent to the original SparkType objects using FluentAssertions' Should().BeEquivalentTo() method.

The Moment of Truth

When you run this test, it fails with the infamous InvalidDataException. This proves that the bug is real and reproducible. You can copy and paste this code into a test project and see it fail for yourself. It's always a good idea to get your hands dirty and experience the bug firsthand.

The Code

Here’s the complete failing test code for your convenience:

using FluentAssertions;
using Parquet.Serialization;
using Xunit;

public sealed class ParquetReaderTests
{
    [Fact]
    public async Task ReproTest()
    {
        SparkType[] inputRecords =
        [
            new SparkType { IsServer = true },
            new SparkType { IsServer = false },
        ];

        using MemoryStream stream = new();
        await ParquetSerializer.SerializeAsync(
            inputRecords,
            stream,
            cancellationToken: TestContext.Current.CancellationToken);
        stream.Position = 0;

        IAsyncEnumerable<DeserializeType> loadedResults = ParquetSerializer.DeserializeAllAsync<DeserializeType>(
            stream,
            cancellationToken: TestContext.Current.CancellationToken);

        List<DeserializeType> results = [];
        await foreach (DeserializeType item in loadedResults)
        {
            results.Add(item);
        }

        results.Should().BeEquivalentTo(inputRecords);
    }

    private sealed class SparkType
    {
        public bool IsServer { get; set; }
    }

    private sealed class DeserializeType
    {
        public bool? IsServer { get; set; }
    }
}

Why Does This Happen? Understanding the Root Cause

Okay, so we've seen the bug, we've reproduced it, but why is it happening? To understand this, we need to delve a bit deeper into how Parquet.NET handles nullability during deserialization.

Parquet's Definition Levels

As mentioned earlier, Parquet uses definition levels to indicate whether a field can be null. A required field has a definition level that essentially says, "This field will never be null." When the deserializer encounters a required field in the Parquet file, it expects a non-null value. If you're trying to deserialize this into a nullable type, the deserializer needs to reconcile these two different concepts of nullability.

The Mismatch

The issue arises because the deserializer isn't correctly handling the case where a required field (definition level indicating non-null) is being mapped to a nullable type in the C# class. It sees the potential for a null value in the bool? property, even though the Parquet file guarantees it will never be null. This mismatch in expectations leads to the InvalidDataException.

Synapse and the bool? Mystery

Now, the original bug report mentions that Synapse (a data warehousing service) sometimes serializes values as bool? even when the columns are defined as non-null. This adds another layer of complexity to the issue. It suggests that there might be inconsistencies in how Synapse handles nullability during Parquet serialization. This inconsistency exacerbates the problem, as it increases the likelihood of encountering this bug in real-world scenarios.

Possible Solutions and Workarounds

So, what can we do about this bug? While we wait for a fix in the Parquet.NET library, there are a few potential solutions and workarounds we can consider.

1. Aligning Types: The Simplest Approach

The most straightforward solution is to ensure that the types in your C# classes match the nullability of the fields in your Parquet schema. If a field is required in Parquet, use a non-nullable type (like bool) in your C# class. This eliminates the mismatch that triggers the bug.

However, this isn't always practical. You might have reasons for using nullable types in your classes, even if the underlying data is guaranteed to be non-null. For example, you might be working with a legacy codebase or using a library that expects nullable types.

2. Custom Deserialization Logic: A More Flexible Option

If you can't change your class definitions, you might consider implementing custom deserialization logic. This involves reading the Parquet data manually and mapping it to your classes. This gives you more control over the deserialization process and allows you to handle nullability mismatches explicitly.

However, custom deserialization can be complex and time-consuming. You'll need to understand the Parquet format and write code to handle different data types and schemas. It's a powerful solution, but it comes with a cost.

3. Workarounds in Synapse (If Applicable)

If the issue is related to how Synapse serializes data, you might be able to work around it within Synapse itself. This could involve configuring Synapse to always serialize non-null boolean values as bool instead of bool?. However, this depends on the specific capabilities of Synapse and might not be possible in all cases.

4. Contributing to Parquet.NET: The Best Long-Term Solution

The best long-term solution is to contribute a fix to the Parquet.NET library itself. This ensures that the bug is resolved for everyone and prevents future occurrences. If you're comfortable with C# and have some experience with Parquet, consider submitting a pull request with a fix. The Parquet.NET maintainers will likely appreciate your contribution.

Conclusion: Bugs Are a Part of Life, Let's Fix Them!

So, there you have it – a deep dive into a fascinating bug in the Parquet.NET library. We've explored the issue, reproduced it with a failing test case, understood the root cause, and discussed potential solutions and workarounds.

Bugs are a part of life in software development. They're inevitable, but they're also opportunities to learn and improve. By understanding the bugs we encounter, we can become better developers and build more robust systems. So, next time you stumble upon a bug, don't despair – embrace it, investigate it, and maybe even fix it!

If you're working with Parquet.NET and encounter this bug, I hope this article has been helpful. And if you're feeling adventurous, consider contributing a fix to the library. Together, we can make Parquet.NET even better! Happy coding, guys!