We have developed a MicroService which is accessed by a secondary system. Both systems use a Swagger/OpenApi API to communicate with one-another. We generate the client as "java" (underlying okhttp-Client).
When we are load-testing the system after some time we get java.lang.OutOfMemoryError: unable to create new native thread on the client, although the system has plenty of memory configured (via -Xmx).
How can we avoid this? What is wrong?
OpenApi generates the "java"-Client such that each instance of ApiClient wraps one instance of OkHttpClient. The documentation of OkHttpClient states, that each instance creates a ThreadPool as well as a cache. It also states, that OkHttpClient-Instances should be shared among multiple requests.
If you do not do this and generate one ApiClient per call, you will sort of leak Threads. These are recovered, once a garbage-collection eliminates the unused ApiClient instances, however, if your system is configured with plenty of memory, this gc-call might not occur frequently enough and you might end up with more requested native threads, than the underlying os can deliver.
The solution is to re-use ApiClient-instances within your application or switch to a client based e.g. on Spring RestTemplate.
For those curious: OkHttpClient seems to reserve the threads to deal with async http/2 requests - which is kind of overkill, if your application does not actually do async calls.