Clarify the meaning of db.instance tag

[lujiajing1126]

In the Java agent, db.instance tag is extensively used in many plugins,

• For JDBC: database name,
• For ElasticSearch: index name,
• For Cassandra: keyspace name,
• For HBase: Table name
• ...

But I found in the redisson which is a high level Redis SDK, ip:port is used for this tag. As I understand, database number should be used in all redis client cases, e.g. Jedis, Lettuce, Redisson, etc.

Also, the semantics of this tag is not clearly documented, https://github.com/apache/skywalking-java/blob/6fce184cfe95b3a4b7cf97421ee4efdb03d2c7cc/apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/tag/Tags.java#L53

[wu-sheng]

The cache has been changed, please take a check again.

https://github.com/apache/skywalking/blob/master/oap-server/analyzer/agent-analyzer/src/main/java/org/apache/skywalking/oap/server/analyzer/provider/trace/parser/listener/vservice/VirtualCacheProcessor.java

[lujiajing1126]

I didn't find db.instance usage in this case?

[wu-sheng]

No, there is no use in the analysis. It is just a tag.

[wu-sheng]

Sometimes, tags are added randomly by plugin, but not used in the analysis.

[wu-sheng]

Do you have any concerns about this?

[lujiajing1126]

Since different plugins are setting values with different meanings, like Redisson, I'm curious if we have agreement how we use this tag.

[wu-sheng]

db.instance actually should not be used in the cache relative plugins actually.

ip:port should already be set through peer.
In this particular case, you could remove it.