Enhance Envoy Proxy HTTP Set-Metadata Filter With Format Expansion

Hey guys! Today, we're diving deep into an exciting enhancement for Envoy Proxy, specifically focusing on the HTTP Set-Metadata filter. This is a feature request that could significantly improve how we manage and propagate metadata within our Envoy deployments. Let's break down the problem, the proposed solution, and why it matters.

Understanding the HTTP Set-Metadata Filter

The HTTP Set-Metadata filter in Envoy Proxy is a powerful tool that allows you to add or modify metadata associated with HTTP requests and responses. Think of metadata as extra information tags that you can attach to your traffic. These tags can be incredibly useful for a variety of purposes, such as routing decisions, access control, and logging.

Currently, the Set-Metadata filter allows you to set metadata keys and values. You can specify static values or use simple string manipulations. However, the filter has a limitation: it doesn't support dynamic value expansion using formatters. This means you can't directly embed Envoy's built-in variables, like %VIRTUAL_CLUSTER_NAME%, into your metadata values. Instead, if you try to set a metadata value to %VIRTUAL_CLUSTER_NAME%, it will be treated as a literal string, not the actual virtual cluster name.

This limitation can be a real pain point when you need to propagate dynamic information through your system. For example, if you want to include the virtual cluster name in your access logs, you'd ideally want to use the %VIRTUAL_CLUSTER_NAME% variable. But with the current filter, that's not directly possible. Let's explore why this is important and how we can address it.

The Need for Format Expansion

So, why is format expansion such a crucial feature for the Set-Metadata filter? Imagine you're operating a complex microservices architecture with numerous virtual clusters. Each cluster might handle different types of traffic or have unique configurations. When debugging issues or analyzing traffic patterns, it's incredibly helpful to have context about which virtual cluster a request belongs to.

This is where metadata comes in handy. By attaching the virtual cluster name to requests, you can easily filter and analyze logs, trace requests across services, and make informed decisions about routing and traffic management. However, if you can't dynamically populate the virtual cluster name using a formatter, you're forced to resort to workarounds.

One common workaround is to use Lua scripting within Envoy. Lua allows you to perform complex logic and string manipulations, including expanding variables. While Lua is powerful, it adds complexity to your configuration and can impact performance if not used carefully. It's also an extra dependency to manage.

Another approach might involve modifying your application code to add the metadata. This, however, isn't ideal as it tightly couples your application logic with your infrastructure concerns. You want your applications to focus on business logic, not on Envoy-specific details.

Therefore, enhancing the Set-Metadata filter with format expansion capabilities would streamline this process. It would allow you to embed dynamic values directly into your metadata, eliminating the need for complex workarounds and making your Envoy configurations cleaner and more maintainable. This brings us to the proposed solution: adding a format_string type.

The Proposed Solution: A format_string Type

The proposed solution is to introduce a new format_string type within the Set-Metadata filter configuration. This new type would allow you to specify a string that can contain Envoy formatters, such as %VIRTUAL_CLUSTER_NAME%, %REQUEST_ID%, and others. When the filter processes a request or response, it would expand these formatters into their actual values, and the resulting string would be used as the metadata value.

For example, you could configure the filter to set a metadata key called virtual_cluster with a format_string value of %VIRTUAL_CLUSTER_NAME%. When a request enters Envoy, the %VIRTUAL_CLUSTER_NAME% formatter would be replaced with the actual name of the virtual cluster, and that name would be added as the value for the virtual_cluster metadata key.

This approach offers several advantages:

• Simplicity: It's a straightforward and intuitive way to add dynamic metadata. You can use familiar Envoy formatters without needing to learn new syntax or scripting languages.
• Efficiency: Format expansion is a built-in feature of Envoy, so it's likely to be more performant than executing Lua scripts for the same purpose.
• Maintainability: Your Envoy configurations become cleaner and easier to understand, as you don't have to embed complex logic within them.
• Flexibility: You can use a wide range of Envoy formatters to capture various aspects of the request, response, and Envoy's internal state.

Use Case: Propagating Fields to gRPC Backend via Metadata

To illustrate the benefits of this feature, let's consider a specific use case: propagating additional fields to a gRPC backend via metadata. Many organizations use Envoy as a gateway to their gRPC services. Access logs are crucial for monitoring and troubleshooting these services. Envoy's Access Log Service (ALS) allows you to stream access logs to a backend for analysis.

However, sometimes you need to include additional information in your logs beyond the standard fields. For example, you might want to include the request ID, the virtual cluster name, or custom headers. Propagating these fields to your gRPC backend via metadata allows you to enrich your access logs with valuable context.

Currently, achieving this requires workarounds like Lua scripting. With the format_string type in the Set-Metadata filter, you could simply configure the filter to add metadata keys with values that are format strings. For instance, you could add a request_id metadata key with the value %REQUEST_ID%, a virtual_cluster key with the value %VIRTUAL_CLUSTER_NAME%, and so on.

This metadata would then be included in the gRPC request headers, allowing your backend to access it and include it in the access logs. This significantly simplifies the process of enriching access logs and provides valuable insights into your gRPC traffic.

Impact on Access Log Service (ALS)

As mentioned earlier, one of the primary motivations for this feature is to improve integration with Envoy's Access Log Service (ALS). ALS allows you to stream detailed access logs to a centralized backend for analysis and storage. By propagating metadata using the format_string type, you can significantly enrich the data available in your access logs.

Imagine being able to filter your access logs not just by path or status code, but also by virtual cluster, request ID, or any other piece of information you've captured in metadata. This level of granularity can be invaluable for troubleshooting issues, identifying performance bottlenecks, and understanding traffic patterns.

Furthermore, this enhanced metadata propagation can simplify the process of building custom dashboards and alerts based on your access logs. You can easily extract the relevant metadata fields and use them to create visualizations and trigger alerts when specific conditions are met.

In essence, the format_string type in the Set-Metadata filter acts as a bridge, connecting the rich context available within Envoy to your logging and monitoring infrastructure. This results in a more comprehensive and actionable view of your application traffic.

Alternatives Considered

While the format_string type offers a clean and efficient solution, it's worth considering alternative approaches and why they might not be as suitable. One alternative, as discussed earlier, is to rely on Lua scripting.

Lua provides a powerful and flexible way to manipulate requests and responses within Envoy. You can use Lua scripts to extract information, perform complex logic, and modify headers and metadata. However, Lua scripting also comes with its own set of challenges.

Lua scripts can be more complex to write and maintain than simple filter configurations. They also introduce an extra layer of processing, which can potentially impact performance. Additionally, managing and deploying Lua scripts can add complexity to your Envoy deployment pipeline.

Another alternative might involve modifying the application code to add the desired metadata. However, this approach tightly couples your application logic with your infrastructure concerns. It's generally preferable to keep your applications focused on business logic and let Envoy handle the complexities of routing, filtering, and metadata management.

Therefore, the format_string type in the Set-Metadata filter strikes a good balance between simplicity, performance, and flexibility. It allows you to achieve the desired outcome without resorting to complex scripting or modifying application code.

Conclusion

In conclusion, enhancing the Envoy Proxy's HTTP Set-Metadata filter with format expansion capabilities is a valuable improvement. The proposed format_string type offers a simple, efficient, and maintainable way to add dynamic metadata to requests and responses. This feature has significant implications for access logging, monitoring, and overall traffic management within Envoy deployments.

By allowing you to embed Envoy formatters directly into metadata values, the format_string type eliminates the need for complex workarounds like Lua scripting. It simplifies your configurations, improves performance, and provides a more flexible way to propagate valuable context throughout your system. This is particularly beneficial for use cases like enriching access logs with information like the virtual cluster name or request ID.

This enhancement would be a welcome addition to Envoy Proxy, empowering users to leverage metadata more effectively and gain deeper insights into their application traffic. Guys, what are your thoughts on this? Let's discuss in the comments below!