I am currently using ELK along with dotnet and java APM agents for monitoring applications of respective technologies.
I wanted to confirm on below points
Since Elasticsearch uses Lucene documents to save the trace data, How long can I retrieve the trace details for a transaction that are saved in lucene indices. Does it depend on the index size.
I have referred below link for understanding how long I can have the trace data in my system Index lifecycle management | APM User Guide [8.2] | Elastic
Open source for Enterprise Edition:
ELK is open source at enterprise level, Paid Version is available only in case customized features are required for the same.
Please confirm whether my understanding on above points are correct.
First you can keep / retain the APM Data transactions and traces as long as you want as long as you have enough storage / properly configured cluster. Yes all the data is stored in indices.
How long you keep the data depends on the Index Lifecycle Policy You implement, the default are listed on the page you linked to ... but you can easily create your own or override the policy. The ability to do that is not dependent on the subscription tier.
I am not quite clear what you mean by "Open source for Enterprise" please look at this subscription page that shows the different subscriptions Tiers Include Free, Platinum and Enterprise etc.
Free is Free
Platinum and Enterprise require a commercial / paid license.