# FAQs
These are known and common FAQs. We welcome you to contribute yours.

## Design
* [Why doesn't SkyWalking involve MQ in the architecture?](why_mq_not_involved.md)

## Compiling
* [Protoc plugin fails in maven build](Protoc-Plugin-Fails-When-Build.md)
* [Required items could not be found, when import project into Eclipse](Import-Project-Eclipse-RequireItems-Exception.md)
* [Maven compilation failure with `python2 not found` error](maven-compile-npm-failure.md)

## Runtime
* [8.x+ upgrade](v8-version-upgrade.md)
* [Why metrics indexes(ElasticSearch) in Hour and Day precisions stop update after upgrade to 7.x?](Hour-Day-Metrics-Stopping.md)
* [6.x version upgrade](v6-version-upgrade.md)
* [Why only traces in UI?](Why-have-traces-no-others.md)
* [The trace doesn't continue in kafka consumer side](kafka-plugin.md)
* [Agent or collector version upgrade,  3.x -> 5.0.0-alpha](v3-version-upgrade.md)
* [EnhanceRequireObjectCache class cast exception](EnhanceRequireObjectCache-Cast-Exception.md)
* [ElasticSearch server performance FAQ, including ERROR CODE:429](ES-Server-FAQ.md)
* [IllegalStateException when install Java agent on WebSphere 7](install_agent_on_websphere.md)
* ["FORBIDDEN/12/index read-only / allow delete (api)" appears in the log](https://discuss.elastic.co/t/forbidden-12-index-read-only-allow-delete-api/110282)
* [No data shown and backend replies with "Variable 'serviceId' has coerced Null value for NonNull type 'ID!'"](time-and-timezone.md)
* [**Unexpected endpoint register** warning after 6.6.0](Unexpected-endpoint-register.md)
* [Use the profile exporter tool if the profile analysis is not right](../guides/backend-profile-export.md)
* [Compatible with other javaagent bytecode processing](Compatible-with-other-javaagent-bytecode-processing.md)
* [**Java agent memory leak** when enhance `Worker thread` at use Thread Pool](Memory-leak-enhance-Worker-thread.md)
* [Thrift plugin](thrift-plugin.md)

## UI
* [What is **VNode**? And why does SkyWalking have that?](vnode.md)