Description
Description
Intermittent request failures have been observed on the Previewnet environment (6%-9% of the requests). These requests fail with a status code of 567 (the fallback code for unknown server errors in MirrorNodeClient), indicating the error likely originates in the Relay service rather than the Mirror Node remote server.
Error Symptoms
The error logs show vague and inconsistent messages:
Error invoking RPC: Unexpected 'S'Error invoking RPC: Unexpected '<'
These errors:- Occur in rapid succession during short timeframes
- Are exclusive to the Previewnet environment
- Cannot be reproduced in local development environments
- Do not contain sufficient context in logs for immediate diagnosis
Root Cause
The error is caused by HTML content being returned instead of expected JSON responses. At the time of the issue, MN was returning 502 Server Error with the below content:
<html><head>
<meta http-equiv="content-type" content="text/html;charset=utf-8">
<title>502 Server Error</title>
</head>
<body text=#000000 bgcolor=#ffffff>
<h1>Error: Server Error</h1>
<h2>The server encountered a temporary error and could not complete your request.<p>Please try again in 30 seconds.</h2>
</body></html>The error messages Unexpected 'S' and Unexpected '<' occur when the MirrorNodeClient attempts to parse HTML with JSONBigInt.parse() in the Axios response interceptor:
axiosRequestConfig['transformResponse'] = [
(data) => {
// if the data is not valid, just return it to stick to the current behaviour
if (data) {
return JSONBigInt.parse(data);
}
return data;
},
];This is a classic case of receiving HTML error pages (starting with <!DOCTYPE html> or <html>) when JSON is expected. In the Previewnet environment, one of the following is occurring:
- The Mirror Node service is returning HTML error pages during service degradation
- A proxy/WAF in the request path is intercepting and returning HTML error pages
- Network configurations specific to Previewnet are causing redirects to HTML status pages
Suggested Fix
- Enhance error logging to capture the initial portion of failing responses to confirm HTML content
- Add logic for the parsing to handle the case when the response is an http page instead of valid JSON object. If response is not a valid JSON object, skip parsing so the error can be thrown properly.