Observability of the Java Virtual Machine

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The JVM is one of the most observable runtimes. It provides us lots of tools for troubleshooting a JVM application in production. 1. Thread observability Threads are how the JVM actually does work. When something is wrong in production, the symptom is almost always a thread: stopped, blocked, leaking etc. Thread dumps work on any JVM with no  instrumentation, no agents, no restarts. <Example project link with /threaddump endpoint>         // (1) Deadlock — two threads grab the same pair of locks in opposite order.         new Thread(() -> grab(LOCK_A, LOCK_B), "deadlock-A-then-B").start();         new Thread(() -> grab(LOCK_B, LOCK_A), "deadlock-B-then-A").start(); http://localhost:8080/actuator/threaddump To list the JVMS, we can use the command below. PS C:\observe-jvm> jps -lv 25296 jdk.jcmd/sun.tools.jps.Jps -Dapplication.home=C:\Program Files\Microsoft\jdk-21.0.3.9-hotspot -Xms8m -Djdk.module.main=...
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nijhof2axon

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Axon framework kullanıcılarına katkıda bulunmak amacıyla paylaştığım proje ile ilgili yorum ve sorular gelmeye başladı.

http://code.google.com/p/nijhof2axon/

>>You share the Data between the Domain and the Query Model?
When a command is dispatched, a CommandHandler uses the aggregate root to handle the command. Aggregate root fires DomainEvents.
=> Query layer related code listens to domain events and updates the query model. (Example: ClientTableUpdater)

Domain entities are first class objects but Query Model classes are just DTOs with getters and setters. We have Client as entity and ClientDetailsEntry as DTO.

>>The (generated?) table domainevententry looks like you can not replay events? missing the event data?

To observe how events are replayed by the framework, you can follow these steps:

Click on sample client
change name
stop server
start server
put a breakpoint to EventSourcingRepository.doLoad() method
put a breakpoint to Client.handleClientNameChangedEvent() method
start server in debug mode
open the client that you changed
change name
=> you stop at the doLoad() breakpoint
reach to this line:
aggregate.initializeState(...
Here the events will be replayed to initialize aggregate state.
=> hit the breakpoint of Client.handleClientNameChangedEvent()

So, we have to implement the handlers that handle state changes properly.

>>if i can force the query database to have a invalid state...
You can do that by not listening to domain events and not updating query tables. So updating the query tables properly is our duty.

>>how to replay events to integrate new features
I added a test class to nijhof2axon. Please have a look at SampleEventVisitorTest

>>how i can monitor the system (maybe the command and event queue)
We can look at org.axonframework.auditing.AuditingInterceptor

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