Specific to Java, there are metrics that can be helpful to understand the health, and performance of the application. Guiding future decisions on how to scale and optimise the application. Garbage collection time, heap size, thread count, JIT time are all important and Java specific.

Looking at the worst case latency is more meaningful, and more reflective of the user perceived performance.

When something breaks, troubleshooting the issue should be possible from only the metrics being collected. You should not be to depending on log files, or looking at code, to deal with an outage.

There are many features of the JVM that have a fixed cost per running JVM, such as JIT and garbage collection. Your application may also have fixed overheads, such as resource polling (backend database connections), etc. If you run fewer, but larger (in terms of CPU and RAM) instances, you can reduce this fixed cost, getting an economy of scale.... See more

Make sure the most important and useful metrics are exported from the Java application, are collected and easily graphed.

In a multi-server deployment, it is best practice to slowly ramp up traffic to a newly started task, giving it time to warm up, and to not harm the overall performance of the service. You may be tempted to warm up new tasks by sending it artificial traffic, before it is placed into the user-serving path. Artificial traffic can be problematic if it... See more

When starting up, the Java Virtual Machine (JVM) reserves a large chunk of OS memory and splits it into heap and non-heap. The non-heap contains areas such as Metaspace (formally called Permgen), and stack space. Metaspace is for class definitions, and stack space is for each thread’s stacks. The heap is used for the objects that are created, which... See more

As mentioned earlier, GC is a source of long tail latency, so should be monitored closely. The time taken for each phase of the GC should be recorded, as well as the fullness of heap space (broken down by young/old/etc) before and after GC runs. This provides all the hints needed to either tune, or improve the application to get GC under control.

Keep it simple, and make the application as quick and easy to version, using a single Fat JAR, or executable where possible.