Azure App Service日志流持续刷屏输出连接欢迎语无法查看日志

问题表现 #

Azure日志流功能以约每秒10次的频率持续输出Welcome, you are now connected to log-streaming service.提示，正常业务日志被完全覆盖，无法查看有效输出，异常样例如下：

2022-07-07T12:48:40  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:40  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:40  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:40  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:40  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:40  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:39  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:39  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:39  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:39  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:39  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:39  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:38  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:38  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:38  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:38  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:38  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:38  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:38  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:37  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:37  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:37  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:37  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:37  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:37  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:37  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:36  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:36  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:36  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:35  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:35  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:35  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:35  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:35  Welcome, you are now connected to log-streaming service.
2022-07-07T12:48:35  Welcome, you are now connected to log-streaming service.

触发原因 #

这是Azure App Service日志流的长连接重连bug，核心是日志流连接反复断开、重建，每次建连成功就会打印一次欢迎提示，高频触发的常见场景：

• 同一应用的日志流页面被多个浏览器标签页同时打开，或者有后台脚本、浏览器插件持续发起日志流连接请求，新连接不断抢占旧连接，触发循环重连
• 应用所在的App Service Plan负载过高，CPU、内存占用长期超过80%，日志流代理进程无法维持长连接，平台侧自动重连机制反复触发

修复方案 #

按以下顺序排查处理即可：

1. 关闭所有打开了该应用日志流的浏览器标签页，清除对应站点的缓存和Cookie后，单独开一个新标签页打开日志流，排除多连接抢占问题
2. 到Azure门户查看App Service Plan的资源监控指标，如果存在CPU、内存持续跑满的情况，先扩容实例规格、优化应用内存/CPU占用，等负载回落到正常区间后再使用日志流
3. 前两步操作无效的话，直接重启对应应用实例，强制重置所有现存的日志流长连接会话即可恢复
4. 紧急排障时可以绕过门户日志流页面，直接进入应用的Kudu管理站点，从LogFiles目录下载原始日志文件查看，不受这个重连bug影响

临时过滤方案：如果需要立刻查看日志、来不及做上述修复，可以在日志流页面按F12打开浏览器控制台，执行以下JS脚本自动过滤掉重复的欢迎提示行，直接展示业务日志：

setInterval(() => {
  document.querySelectorAll('.log-line').forEach(line => {
    if (line.textContent.includes('Welcome, you are now connected to log-streaming service.')) line.remove()
  })
}, 100)

内容的提问来源于stack exchange，提问作者flutterisbae