Langfuse Bug: Trace Deletion Fails In Self-Hosted Instance

Hey guys! We've got a bit of a situation on our hands with Langfuse, specifically a bug where trace deletion just doesn't seem to want to complete. Let's dive into the details and see what's going on.

The Issue: Trace Deletion Stalling

So, the main problem reported is that when trying to delete a large number of traces (in this case, a whopping 1.5 million!) from the Langfuse UI in a self-hosted environment, the deletion process gets stuck. Like, seriously stuck. Even after several hours, nothing happens. This can be a major headache, especially when you're trying to clean up your data or manage storage.

Trace deletion is a crucial feature for maintaining a clean and efficient Langfuse instance. When you're dealing with large datasets, the ability to remove old or irrelevant traces becomes essential. Imagine running numerous experiments or handling a high volume of user interactions; the accumulated data can quickly become overwhelming. Without a reliable trace deletion mechanism, your storage can fill up, and the performance of your Langfuse instance might suffer. Therefore, a bug that prevents trace deletion from completing is a significant issue that needs immediate attention.

This issue also impacts the overall usability of Langfuse. If users can't delete traces, they might hesitate to use the platform for fear of accumulating too much data. This can lead to a reluctance to experiment with new features or to fully utilize Langfuse's capabilities. Moreover, the inability to delete traces can complicate debugging and analysis. When you're trying to identify the root cause of an issue, sifting through millions of traces can be incredibly time-consuming. The ability to delete irrelevant traces allows you to focus on the data that matters, making the debugging process much more efficient. In a nutshell, a functioning trace deletion feature is not just a nice-to-have; it's a critical component of a robust and user-friendly Langfuse instance.

Steps to Reproduce the Bug

The user provided a screenshot that clearly shows the attempt to delete the traces. To break it down:

1. Navigate to the Langfuse UI.
2. Select a large number of traces for deletion (in this case, 1.5 million).
3. Initiate the deletion process.
4. Wait... and wait... and wait...
5. Observe that the deletion never completes.

Error Logs: Diving into the Technical Details

Now, let's get a little technical. The error logs from the worker provide some clues about what's going wrong. The key error message is:

Error: Poco::Exception. Code: 1000, e.code() = 0, HTML Form Exception: Field value too long (version 25.9.2.1 (official build))

This error suggests that there's an issue with the length of a field value being passed in an HTML form. In the context of deleting a large number of traces, it's possible that the list of trace IDs being sent to the server is exceeding some kind of limit.

The error message "Field value too long" indicates a common problem when dealing with large datasets in web applications. When you're trying to delete 1.5 million traces, the list of trace IDs can become extremely long. This list is typically passed as a parameter in an HTTP request, often within the URL or the request body. However, web servers and browsers have limitations on the maximum length of URLs and request bodies. When the list of trace IDs exceeds these limits, the server throws an error, preventing the deletion process from completing. This issue is not unique to Langfuse; it's a general challenge in web development when handling large amounts of data.

To understand why this error occurs, it's helpful to think about how web servers process requests. When a request is sent to a server, it includes headers and a body. The headers contain metadata about the request, such as the content type and the length of the body. The body contains the actual data being sent, like the list of trace IDs in this case. Web servers allocate memory and resources based on the expected size of the request. If the request exceeds the configured limits, the server may refuse to process it to prevent potential security vulnerabilities or performance issues. The "Field value too long" error is a safeguard to ensure that the server doesn't get overwhelmed by excessively large requests.

Langfuse Environment: Self-Hosted, Version 3.112

This bug was reported on a self-hosted instance of Langfuse running version 3.112. Knowing this helps narrow down the potential causes, as self-hosted environments can have unique configurations and limitations compared to cloud-hosted solutions.

The fact that this issue is occurring in a self-hosted environment is significant. In self-hosted deployments, the infrastructure and configuration are managed by the user, which means there are more variables that can influence the behavior of the application. For example, the web server configuration, database settings, and available resources can all play a role in how Langfuse handles large deletion requests. In contrast, cloud-hosted environments typically have a more standardized setup, which can make it easier to identify and resolve issues. When troubleshooting problems in self-hosted instances, it's essential to consider the specific environment and any customizations that might be in place.

Furthermore, the version of Langfuse being used (3.112) is an important piece of information. Each version of Langfuse includes bug fixes, performance improvements, and new features. Knowing the version helps developers determine whether the issue has already been addressed in a later release or whether it's a new problem that needs to be investigated. In this case, if the bug is specific to version 3.112, upgrading to a newer version might resolve the issue. However, it's also possible that the bug exists in multiple versions, so a thorough investigation is necessary to ensure a proper fix.

Potential Causes and Solutions

Based on the error message and the context, here are a few potential causes and solutions:

1. ClickHouse Limitation: The error message mentions ClickHouse, which is likely being used as the database for Langfuse. ClickHouse might have a limit on the size of values that can be inserted or used in queries.
   • Solution: Investigate ClickHouse configurations and limits. Consider batching the deletion into smaller chunks to avoid exceeding the limit. Batching the deletion process involves dividing the 1.5 million traces into smaller groups and deleting them in sequence. For example, instead of trying to delete all traces at once, you could delete them in batches of 10,000 or 50,000. This approach reduces the size of the data being processed at any given time, which can help prevent the "Field value too long" error. Batching also allows the server to process the deletion requests more efficiently, as it doesn't have to handle a massive amount of data in a single transaction.
2. HTTP Request Limit: As mentioned earlier, web servers have limits on the size of HTTP request headers and bodies.
   • Solution: Reduce the number of trace IDs being sent in a single request. Implement pagination or batching in the UI to delete traces in smaller groups. Pagination involves dividing the list of traces into pages and allowing users to delete traces page by page. This approach is user-friendly and helps prevent the server from being overwhelmed. Batching, as described earlier, is a more programmatic way of dividing the deletion process into smaller chunks. Both pagination and batching are effective strategies for handling large datasets and preventing HTTP request limits from being exceeded.
3. Langfuse Code: There might be a bug in the Langfuse code itself that's causing the issue.
   • Solution: Review the Langfuse code responsible for trace deletion. Look for any potential issues with how trace IDs are being handled or how queries are being constructed. Code review is a critical part of software development and maintenance. It involves carefully examining the code to identify potential bugs, performance issues, and security vulnerabilities. In the context of the trace deletion bug, a code review might reveal that the way trace IDs are being processed or the queries being constructed is inefficient or incorrect. For example, the code might be creating unnecessarily large SQL queries or not handling errors properly. A thorough code review can help pinpoint the root cause of the issue and guide the development of a proper fix.

Next Steps

For now, it seems like batching the deletion process might be a good workaround. But, the Langfuse team needs to dig deeper into the code and ClickHouse configurations to find a permanent solution.

Conclusion

Dealing with bugs like this can be frustrating, but hopefully, this breakdown gives you a better understanding of the issue and potential solutions. We'll keep you updated as we learn more!

If you're interested in learning more about database management and ClickHouse, check out the official ClickHouse documentation: ClickHouse Documentation