Stacks Issue: Missing Content-Length Header Affecting Web Archives

Introduction

This article addresses a critical issue affecting the Stanford University Press web archives, specifically the absence of the Content-Length header in HTTP responses served by Stacks. This problem, discovered by Jasmine Mulliken, has rendered the web archives inaccessible, impacting users attempting to access archived content. The absence of this header prevents the ReplayWebPage component from determining the file size, leading to errors and a degraded user experience. This article will delve into the details of the problem, its potential causes, and the implications for accessing web archives.

The primary focus of this article is to provide a comprehensive understanding of the missing Content-Length header issue within the Stacks environment. We will explore the technical aspects, analyze the potential impact on users, and discuss possible solutions to restore the functionality of the Stanford University Press web archives. This investigation is crucial for maintaining the integrity and accessibility of archived digital content. It highlights the importance of proper HTTP header configuration in ensuring reliable delivery of web resources. We aim to provide valuable insights into the challenges of managing and serving large digital archives, offering practical guidance for addressing similar issues in other contexts.

The Problem: Missing Content-Length Header

The core issue lies in the absence of the Content-Length header in HTTP responses from Stacks when serving Web Archive Collection Zipped (WACZ) files. The Content-Length header is a crucial part of the HTTP protocol. It specifies the size, in bytes, of the body of the HTTP response. When this header is missing, clients, such as the ReplayWebPage component, cannot accurately determine the size of the file being transferred. This leads to errors and prevents the content from being properly loaded.

Jasmine Mulliken's observation highlights a critical disruption in the functionality of the Stanford University Press web archives. The absence of the Content-Length header is causing the ReplayWebPage component to throw an error, rendering the archived content inaccessible to users. This issue directly impacts the usability of the web archives. It prevents users from accessing and exploring the historical digital collections preserved within the WACZ files. The error message, "Sorry, this URL could not be loaded because the size of the file is not accessible," clearly indicates the dependency of the ReplayWebPage component on the Content-Length header.

Furthermore, the problem extends beyond just the ReplayWebPage component. Other clients and applications that rely on the Content-Length header for file size determination may also be affected. This could include download managers, indexing services, and other tools that process web content. Therefore, resolving this issue is crucial for ensuring the broader interoperability and accessibility of the archived content.

Technical Details and Observations

When examining the HTTP HEAD request for a WACZ file, such as https://archive.supdigital.org/filming-revolution.html, it becomes evident that the Content-Length header is indeed missing. This can be verified using command-line tools like curl. The following command demonstrates the absence of the Content-Length header in the response:

$ curl -I https://stacks.stanford.edu/file/druid:kv106fw2233/fr.wacz

HTTP/1.1 200 OK
Date: Fri, 05 Dec 2025 14:10:39 GMT
Server: Apache/2.4.52 (Ubuntu)
access-control-allow-origin: *
cache-control: max-age=600, private
strict-transport-security: max-age=31536000; includeSubDomains
x-request-id: 6c33f3d8-6866-420e-a309-4e78992683dd
content-disposition: inline; filename="fr.wacz"; filename*=UTF-8''fr.wacz
accept-ranges: bytes
x-runtime: 0.075060
Last-Modified: Wed, 26 May 2021 22:51:50 GMT
Transfer-Encoding: chunked
etag: W/"17f23166b0e8b11198316c73a51f6d47"
Status: 200 OK
Content-Type: application/zip

As shown in the curl output, while other headers are present, the Content-Length header is conspicuously absent. Instead, the Transfer-Encoding header is set to chunked, which indicates that the response body is being sent in chunks. While chunked transfer encoding is a valid mechanism for sending data, it does not provide the client with the total content length beforehand. This is the root cause of the issue, as the ReplayWebPage component requires the Content-Length to function correctly.

It's also worth noting that the missing Content-Length header appears to affect GET requests as well, further exacerbating the problem. This means that even when attempting to download the WACZ file directly, the client may not be able to determine the file size accurately. The absence of the Content-Length header in GET requests can lead to unexpected behavior in various applications and tools that rely on this information.

Potential Causes: S3 Support and Configuration Changes

The introduction of S3 support within the Stacks infrastructure is a potential factor contributing to the missing Content-Length header issue. The changes associated with S3 integration may have inadvertently altered the way HTTP responses are constructed, leading to the omission of the Content-Length header.

A relevant commit, 8317ee4cd9e023dc103087d3ce527be8b037da28, within the sul-dlss/stacks repository, highlights significant modifications related to S3 support. Specifically, the diff in 173ca1ad5e2841f0ab346d694060e523a1c4f86168e2a71387d5504311df5540L19 may contain clues as to why the Content-Length header is no longer being included in the responses. It's possible that the changes introduced in this commit, while intended to improve S3 integration, have had unintended consequences on the HTTP header configuration.

It's important to investigate the specific code changes within this commit to determine if they directly impact the setting of the Content-Length header. For example, the changes may have altered the way files are streamed from S3, or they may have introduced a new middleware component that is inadvertently stripping the Content-Length header. A thorough code review is necessary to identify the root cause of the problem.

Configuration settings related to the web server (e.g., Apache) and the Stacks application itself should also be examined. It's possible that a misconfiguration is preventing the Content-Length header from being set correctly. For example, the web server may be configured to use chunked transfer encoding by default, or the Stacks application may be overriding the web server's default behavior.

Impact and Implications

The absence of the Content-Length header has significant implications for the accessibility and usability of the Stanford University Press web archives. The primary impact is the inability of the ReplayWebPage component to load and display the archived content. This prevents users from accessing and exploring the historical digital collections preserved within the WACZ files.

Furthermore, the missing Content-Length header can affect other applications and tools that rely on this information. Download managers may not be able to accurately track download progress. Indexing services may not be able to properly index the content. And other tools that process web content may encounter unexpected errors or behavior.

The issue also raises concerns about the long-term preservation and accessibility of the web archives. If the Content-Length header is not consistently provided, it could lead to compatibility issues with future versions of web browsers and other software. This could jeopardize the ability to access and preserve the archived content over time.

Proposed Solutions and Recommendations

To address the missing Content-Length header issue, the following solutions and recommendations are proposed:

1. Code Review: Conduct a thorough code review of the relevant commits within the sul-dlss/stacks repository, particularly the one mentioned earlier (8317ee4cd9e023dc103087d3ce527be8b037da28). Identify any code changes that may be inadvertently affecting the setting of the Content-Length header.
2. Configuration Audit: Perform a comprehensive audit of the web server (e.g., Apache) and Stacks application configuration settings. Ensure that the Content-Length header is being set correctly and that there are no conflicting settings that might be causing the issue.
3. S3 Integration Review: Carefully examine the S3 integration code to ensure that it is not interfering with the setting of the Content-Length header. Verify that the files are being streamed correctly from S3 and that the header is being preserved during the transfer.
4. Testing and Validation: Implement thorough testing and validation procedures to ensure that the Content-Length header is consistently present in HTTP responses for WACZ files. Use automated tests to detect any regressions that may occur in the future.
5. Fallback Mechanism: If it is not possible to reliably set the Content-Length header, consider implementing a fallback mechanism that allows the ReplayWebPage component to determine the file size using other methods. For example, the component could make a separate request to retrieve the file size, or it could use a different approach to stream the content.

By implementing these solutions and recommendations, it should be possible to restore the Content-Length header and ensure the accessibility and usability of the Stanford University Press web archives.

Conclusion

The missing Content-Length header in Stacks responses represents a significant challenge to the accessibility and usability of the Stanford University Press web archives. This issue, triggered by recent changes in S3 support, has rendered archived content inaccessible, hindering user engagement and raising concerns about long-term preservation. Addressing this problem requires a multifaceted approach, including a thorough code review, configuration audit, and implementation of robust testing procedures.

The absence of the Content-Length header not only impacts the ReplayWebPage component but also has broader implications for various applications and tools that rely on accurate file size determination. The proposed solutions aim to restore the proper functioning of the Stacks infrastructure. This will ensure the reliable delivery of archived content and preserve the integrity of valuable digital collections.

Ultimately, resolving this issue underscores the importance of careful configuration management, thorough testing, and a deep understanding of the interactions between different components within a complex web infrastructure. By taking a proactive approach to identifying and addressing these challenges, we can ensure the long-term accessibility and preservation of our digital heritage.

For more information on HTTP headers and their importance, visit the Mozilla Developer Network.