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EventLoopBlocked events on docker container in Azure App service - without visible reason #17914
SimonSimCity
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General
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In the past, I got a bunch of EventLoopBlocked events but wasn't really able to figure out what had happened in this time.
It's about the API that I supervise, which runs on node inside a Docker container, hosted using Azure App service on an App Service plan having a pricing of B2:
Screenshot 2025年10月13日 at 09 59 08
In our Sentry configuration, we have a sampling rate of 1, means, we are currently reporting every request going to the server.
As of this EventLoopBlocked event, I couldn't find any request anywhere close to it happening:
{
"event_id": "---",
"project": "---",
"release": "---",
"dist": null,
"platform": "node",
"message": "",
"datetime": "2025年10月10日T04:05:21.518000+00:00",
"tags": [
["environment", "development"],
["level", "error"],
["mechanism", "ANR"],
["os", "Debian 12.10"],
["os.name", "Debian"],
["release", "---"]
],
"_metrics": { "bytes.ingested.event": 2406, "bytes.stored.event": 3584 },
"contexts": {
"app": {
"app_start_time": "2025年10月09日T08:16:37.290Z",
"free_memory": 1775820800,
"type": "app"
},
"cloud_resource": {
"cloud.provider": "azure",
"cloud.region": "westeurope",
"type": "cloud_resource"
},
"culture": { "locale": "en-US", "timezone": "UTC", "type": "culture" },
"device": {
"arch": "x64",
"memory_size": 4060696576,
"boot_time": "2025年09月06日T22:16:18.781Z",
"processor_count": 2,
"cpu_description": "Intel(R) Xeon(R) Platinum 8272CL CPU @ 2.60GHz",
"processor_frequency": 2978,
"type": "device"
},
"os": {
"os": "Debian 12.10\n",
"name": "Debian",
"version": "12.10\n",
"kernel_version": "6.6.96.1-1.azl3",
"type": "os"
}
},
"culprit": "emitInitNative(node:internal/async_hooks)",
"environment": "development",
"exception": {
"values": [
{
"type": "EventLoopBlocked",
"value": "Event Loop Blocked for at least 1000 ms",
"stacktrace": {
"frames": [
{
"function": "emitInitNative",
"filename": "node:internal/async_hooks",
"abs_path": "node:internal/async_hooks",
"lineno": 192,
"colno": 24,
"in_app": false,
"data": { "client_in_app": false }
}
]
},
"thread_id": "0",
"mechanism": { "type": "ANR" }
}
]
},
"fingerprint": ["{{ default }}"],
"grouping_config": {
"enhancements": "---",
"id": "newstyle:2023年01月11日"
},
"hashes": ["---"],
"ingest_path": [
{
"version": "25.9.0",
"public_key": "---"
}
],
"key_id": "---",
"level": "error",
"location": "node:internal/async_hooks",
"logger": "",
"metadata": {
"filename": "node:internal/async_hooks",
"function": "emitInitNative",
"in_app_frame_mix": "system-only",
"type": "EventLoopBlocked",
"value": "Event Loop Blocked for at least 1000 ms"
},
"nodestore_insert": 1760069127.915031,
"received": 1760069121.518078,
"sdk": {
"name": "sentry.javascript.node",
"version": "10.14.0",
"integrations": [
"InboundFilters",
"FunctionToString",
"LinkedErrors",
"RequestData",
"NodeSystemError",
"Console",
"OnUncaughtException",
"OnUnhandledRejection",
"ContextLines",
"LocalVariablesAsync",
"Context",
"ChildProcess",
"ProcessSession",
"Modules",
"Http",
"NodeFetch",
"Express",
"Fastify",
"Graphql",
"Mongo",
"Mongoose",
"Mysql",
"Mysql2",
"Redis",
"Postgres",
"Prisma",
"Hapi",
"Koa",
"Connect",
"Tedious",
"GenericPool",
"Kafka",
"Amqplib",
"LruMemoizer",
"VercelAI",
"OpenAI",
"PostgresJs",
"Firebase",
"Anthropic_AI",
"Google_GenAI",
"ProfilingIntegration",
"ThreadBlocked"
],
"packages": [{ "name": "npm:@sentry/node", "version": "10.14.0" }]
},
"threads": {
"values": [
{
"id": "0",
"name": "main",
"crashed": true,
"current": true,
"main": true
}
]
},
"timestamp": 1760069121.518078,
"title": "EventLoopBlocked: Event Loop Blocked for at least 1000 ms",
"type": "error",
"user": {
"geo": {
"country_code": "NL",
"city": "Amsterdam",
"subdivision": "North Holland",
"region": "The Netherlands"
}
},
"version": "7"
}Note: Some of the values have been replaced by ---. If those contain valuable insights, please let me know.
Does someone know what a reason for this could be? I'm on the verge of replacing Sentry with Azure Metrics in the hope of getting more information there. It doesn't seem though, that they have something like EventLoopBlocked events built in.
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