Fluentd YAML Config Error: Single Int Value In Array Option

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Fluentd YAML Config Error: Single Int Value in Array Option

Hey guys! Have you ever run into a weird error while configuring Fluentd with YAML, specifically when you're trying to use a single integer value for an option that expects an array? It's a quirky issue, and we're going to dive deep into why it happens and how to fix it. So, buckle up, and let's get started!

Understanding the Issue

The problem arises when you specify a single integer value for an array option in your Fluentd YAML configuration. For example, consider the retryable_response_codes option in a match block. If you try to set it to a single integer like 503, Fluentd throws a Fluent::ConfigError. This behavior is inconsistent because it works fine if you provide a string, a comma-separated list, or use the traditional XML-like syntax. Let's break this down with some code examples.

The Unnatural Behavior

It's kind of strange, right? Here's what makes this behavior so unnatural:

  • Error with Single Int: An error pops up only when you use a single integer value for an array option.
  • String Workaround: You can avoid the error by setting the value as a string (e.g., "503").
  • No String Array Issue: Options that are initially string arrays don't have this problem.
  • XML Syntax Works: The traditional XML-like config syntax doesn't have this issue.

Reproducing the Bug

Let's look at some examples to see this in action. The following YAML configuration will cause a config error:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: 503

When Fluentd tries to parse this, it'll throw an error that looks something like this:

2025-11-11 14:27:25 +0900 [error]: config error file="/test/fluentd/config/yaml/tmp.yaml" error_class=Fluent::ConfigError error="array required but got 503"

The Workarounds

Okay, so how do we get around this? There are a few ways. First, you can simply wrap the integer in quotes, making it a string:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: "503"

This works because Fluentd interprets the string value correctly. Another workaround is to use a comma. Even a trailing comma makes Fluentd see it as an array:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: 503,

If you're using the older, normal syntax (like XML), you won't encounter this issue:

<match test>
  @type http
  endpoint "http://localhost:9880/tag"
  retryable_response_codes 503
</match>

And, if you're on Fluentd version 1.19.1 or later, you can use the more natural YAML array syntax:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: [503]

Or even this:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes:
        - 503

Expected Behavior

Ideally, Fluentd should be able to parse a single integer value for an array option without throwing an error. It would make the configuration cleaner and more intuitive, like this:

config:
  - match:
      $type: http
      $tag: test
      endpoint: http://localhost:9880/tag
      retryable_response_codes: 503

This is what most users would expect, and it aligns with how other configuration systems handle similar situations. However, until this is fully addressed, we need to use the workarounds mentioned above.

Diving Deeper into the Technical Details

To truly grasp why this happens, let's geek out a bit and peek under the hood. The issue stems from how Fluentd's configuration parser interprets YAML. When it encounters a key-value pair where the value is a simple integer, it doesn't automatically assume it should be treated as an array with a single element. Instead, it sees it as a scalar integer value.

The configuration system expects an array, and when it gets an integer, it throws a tantrum—or, more accurately, a Fluent::ConfigError. This is a type mismatch, plain and simple. The parser is saying, "Hey, I needed an array, but you gave me an integer!" 😲

The reason why wrapping the integer in quotes works is that it transforms the value into a string. The parser then sees a string, which it can handle, and the configuration proceeds without a hitch. Similarly, adding a comma, even if it's a trailing one, signals to the parser that you're dealing with a list, and it correctly interprets the value as an array.

Why XML Syntax is Immune

You might be wondering, "Why doesn't this happen with the XML syntax?" Great question! The XML syntax in Fluentd is more forgiving in this regard. It often implicitly converts scalar values into arrays when needed. So, when you write retryable_response_codes 503 in XML, Fluentd's parser is smart enough to treat 503 as a single-element array.

The v1.19.1 Savior

The good news is that Fluentd version 1.19.1 and later have improved YAML parsing capabilities, thanks to the changes introduced in PR #5126. This version allows you to use the standard YAML array syntax (like [503] or using hyphens to list elements) without any issues. So, if you're running an older version, upgrading to v1.19.1 or later is a great way to sidestep this problem.

Real-World Scenarios and Use Cases

Let's put this into a practical context. Imagine you're setting up Fluentd to forward logs to an HTTP endpoint and you want to retry requests that fail with specific HTTP status codes. The retryable_response_codes option is perfect for this. You might want to retry on 503 (Service Unavailable), 504 (Gateway Timeout), or other codes.

If you're dealing with a single retryable code, the temptation is to just write retryable_response_codes: 503. But, as we've seen, this will lead to a configuration error. Instead, you need to use one of the workarounds, such as retryable_response_codes: "503" or retryable_response_codes: [503]. If you have multiple codes, you'd naturally use the array syntax: retryable_response_codes: [503, 504, 500]. 🎉

This issue can also crop up in other scenarios where array options are used, such as defining multiple filters or specifying multiple endpoints. The key takeaway is to always be mindful of how Fluentd interprets your YAML, and when in doubt, use the explicit array syntax to avoid surprises.

Steps to Resolve the Issue

So, what should you do if you encounter this error in your Fluentd configuration? Here’s a step-by-step guide to help you resolve it:

  1. Identify the Offending Line: Look at the error message carefully. It will usually point you to the line in your configuration file that's causing the problem. Pay attention to options that expect arrays.
  2. Apply a Workaround:
    • If you're using a single integer, wrap it in quotes: retryable_response_codes: "503"
    • Alternatively, use the array syntax: retryable_response_codes: [503] or retryable_response_codes: 503,
  3. Test Your Configuration: After making the changes, restart Fluentd and check the logs to ensure the configuration is loaded without errors.
  4. Upgrade Fluentd (If Possible): If you're running an older version, consider upgrading to v1.19.1 or later. This will give you better YAML parsing and eliminate the need for these workarounds.
  5. Review Your YAML: Double-check your YAML syntax. Ensure that you're using the correct indentation and that all your key-value pairs are properly formatted.

Example: Fixing a Broken Configuration

Let's say you have this configuration that's throwing an error:

config:
  - match:
      $type: http
      $tag: api.requests
      endpoint: http://api.example.com/logs
      retryable_response_codes: 503

To fix it, you can modify the retryable_response_codes line like this:

config:
  - match:
      $type: http
      $tag: api.requests
      endpoint: http://api.example.com/logs
      retryable_response_codes: "503"

Or, you can use the array syntax:

config:
  - match:
      $type: http
      $tag: api.requests
      endpoint: http://api.example.com/logs
      retryable_response_codes: [503]

After making these changes and restarting Fluentd, your configuration should load without any errors. 🎉

Fluentd Environment

To give you a complete picture, here’s the environment where this bug was reproduced:

  • Fluentd version: v1.19.1
  • Package version: None (implying it's a direct install or built from source)
  • Operating system: Ubuntu 22.04.5 LTS
  • Kernel version: 6.8.0-85-generic

This information can be helpful if you're trying to replicate the issue or if you want to compare your environment with one where the bug is known to occur.

Wrapping Up

So, there you have it! We've explored the quirky issue of using single integer values for array options in Fluentd YAML configurations. We've seen why it happens, how to work around it, and what the ideal behavior should be. Remember, the key is to be mindful of how Fluentd interprets your YAML and to use the appropriate syntax to avoid errors. And, if you're on an older version, consider upgrading to Fluentd v1.19.1 or later to take advantage of the improved YAML parsing.

By understanding these nuances, you can ensure your Fluentd configurations are robust and error-free. Happy logging, folks! 🚀