change default task executor setup

Author: derlin

src/main/resources/application.properties

@@ -64,13 +64,15 @@ cache.evict.secret-key=1234
 async.enabled=true
 # according to the springboot source,
 # https://github.com/spring-projects/spring-boot/blob/master/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/task/TaskExecutorBuilder.java#L279
-# the default implementation uses ThreadPoolTaskExecutor under the hood. Thus, create one that mimics a bit the newCacheTheadPool:
-# have a long keep alive (so threads are reused), set limits to the number of threads/tasks (to ensure we don't run out of memory),
-# and ensure we wait for the threads to finish before shutdown.
-spring.task.execution.pool.core-size=1
+# the default implementation uses ThreadPoolTaskExecutor under the hood. See the ThreadPoolTaskExecutor documentation.
+# Main idea: when a task is submitted, the executor will first try to use a free thread if the number of active threads is
+# currently less than the core size. If the core size has been reached, then the task will be added to the queue as long as
+# its capacity has not yet been reached. Only then, if the queue's capacity has been reached, will the executor create a
+# new thread beyond the core size. If the max size has also been reached, then the calling thread will handle the task (CallerRuns policy).
+spring.task.execution.pool.core-size=2
 spring.task.execution.pool.max-size=20
 spring.task.execution.pool.keep-alive=120s
-spring.task.execution.pool.queue-capacity=1000
+spring.task.execution.pool.queue-capacity=10
 spring.task.execution.shutdown.await-termination=true
 spring.task.execution.shutdown.await-termination-period=5m
 spring.task.execution.thread-name-prefix=async-executor-