Should your daily batch job live inside your main application?

Most Spring Boot services end up with a scheduled job in them somewhere. A nightly reconciliation, a report, an export to some partner system. It starts small, and it goes in the main app because that's where the domain code already is. One artifact, one deployment, one pipeline. That's a real advantage and it's why most teams do it. This post is about when that stops being a good trade, how to split the job out, and when you shouldn't. The memory problem Look at how much memory each workload uses over a day. The API is fairly flat. Warm heap, connection pool, some caches. It moves with traffic but it doesn't swing much. The batch job uses close to nothing for 23 hours, jumps while it runs, then drops back to nothing. When both live in the same JVM, the pod has to be sized for the peak. So every replica of your API holds batch-sized memory all day, for a job that runs once. With three replicas you're reserving that headroom three times over so one job can use it once, at 2am. Memory limits are not like CPU limits CPU is compressible. Go over your CPU limit and the kernel throttles you. The app gets slower and keeps running. Memory doesn't work that way. There's no "run with less" mode. If the container goes over its memory limit, the kernel kills the process. What you get is a container that exited with code 137 (that's 128 + 9, where 9 is SIGKILL). What you don't get is anything useful in the logs. No OutOfMemoryError , no stack trace, no heap dump unless you configured one and it had time to write, no shutdown hook. The JVM was running fine, asked for another page of memory, and got killed for it. So a batch job sharing a pod with your API is a way for a nightly job to take down the pods serving traffic. If the job's working set grows (bigger dataset, a table that keeps growing, one unusually heavy day) the thing that dies is the API. There's a quieter version of the same problem. Even when the job stays under the limit, it allocates heavily and triggers longer GC