Toxiproxy: Applied Toxics Incorrectly Listed Downstream

This article addresses an issue encountered in toxiproxy-netcore, specifically version 2.12.0, where all applied toxics are being categorized under Downstream toxics regardless of their intended stream direction. This behavior deviates from the expected functionality, where toxics should be correctly classified as either Upstream or Downstream. This article aims to provide a comprehensive understanding of the issue, its implications, and potential solutions.

Understanding the Issue

The core of the problem lies in how the ToxiDirection enum is serialized within the toxiproxy-netcore application. The ToxiDirection enum, which defines whether a toxic affects the upstream or downstream connection, is defined as follows:

[JsonConverter(typeof(StringEnumConverter))]
public enum ToxicDirection
{
    UpStream,
    DownStream
}

The [JsonConverter(typeof(StringEnumConverter))] attribute instructs the JSON serializer to convert enum values to their string representations. However, the default behavior of the StringEnumConverter in many JSON serialization libraries is to preserve the casing of the enum names, which in this case is PascalCase (UpStream, DownStream).

When applying toxics using the toxiproxy-cli, the expectation is that the stream direction will be correctly interpreted. However, the issue arises when the toxiproxy-netcore application receives the serialized toxic data. If the application is expecting the stream direction in a different casing (e.g., lowercase or snake_case), it may fail to correctly parse the stream direction, leading to the misclassification of toxics.

The primary symptom of this issue is that when inspecting toxics using the CLI, all toxics appear under the Downstream toxics category, even if they were intended to be Upstream toxics. This can be observed in the following example:

PS C:\git\hxp-idp-e2es> toxiproxy-cli.exe inspect mongo_linux
Name: mongo_linux       Listen: [::]:27018      Upstream: mongo_linux:27017
======================================================================
Upstream toxics:

Downstream toxics:
proxy:mongo_linux|type:latency|attributes:latency:1000,jitter:100|toxicity:1|stream:UpStream:   type=latency    stream=UpStream toxicity=1.00   attributes=[    jitter=100      latency=1000    ]
proxy:mongo_linux|type:latency|attributes:latency:1000,jitter:100|toxicity:1|stream:DownStream: type=latency    stream=DownStream       toxicity=1.00   attributes=[    jitter=100      latency=1000    ]

In the above output, both toxics, one intended for the UpStream and one for the DownStream, are listed under Downstream toxics. This indicates that the application is not correctly interpreting the stream direction when it is serialized in PascalCase.

Impact and Implications

The misclassification of toxics can have significant implications for testing and debugging applications that rely on Toxiproxy. Toxiproxy is a powerful tool for simulating various network conditions, such as latency, bandwidth limitations, and connection failures. By injecting these toxics into the network traffic, developers can test the resilience and fault tolerance of their applications.

If toxics are not being applied in the intended direction, it can lead to inaccurate test results and a false sense of security. For example, if a developer intends to simulate latency on the upstream connection but the toxic is applied downstream, the application may not be adequately tested for its ability to handle latency in the upstream direction. This can result in unexpected behavior or failures in production environments.

Furthermore, the misclassification of toxics can make it difficult to diagnose and troubleshoot network issues. If the observed behavior does not match the intended toxic configuration, it can lead to confusion and wasted time in the debugging process.

Root Cause Analysis

The root cause of this issue is the mismatch between the serialization format of the ToxiDirection enum and the format expected by the toxiproxy-netcore application. As mentioned earlier, the StringEnumConverter by default preserves the PascalCase casing of the enum names. If the application is expecting lowercase or snake_case, it will fail to correctly parse the stream direction.

The fact that the CLI is able to add Upstream toxics using the -u flag suggests that the CLI itself is correctly handling the stream direction. However, the issue arises when the toxiproxy-netcore application processes the toxic data received from the CLI.

The Wireshark capture provided in the original issue further supports this analysis. The capture shows that when the CLI is used to apply an upstream toxic, the request sent to the toxiproxy-netcore application includes the stream direction in a format that the application may not be correctly interpreting.

Potential Solutions

To address this issue, there are several potential solutions:

1. Customize the JSON Serialization: The most straightforward solution is to customize the JSON serialization settings to ensure that enum values are serialized to lowercase. This can be achieved by using a custom JsonConverter or by configuring the serialization settings of the JSON library being used (e.g., Newtonsoft.Json or System.Text.Json).

   For example, using Newtonsoft.Json, you can create a custom JsonConverter that converts enum values to lowercase:

   public class LowercaseStringEnumConverter : StringEnumConverter
   {
       public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
       {
           if (value == null)
           {
               writer.WriteNull();
               return;
           }
   
           var enumType = value.GetType();
   
           if (!enumType.IsEnum)
           {
               throw new JsonSerializationException($