About the behavior after the connection pool is exhausted and the applicable scenarios?

I have seen all kinds of connection pooling, and some of them will suspend themselves when they are exhausted and will not accept additional connection acquisition requests. When some connection pooling connections are exhausted, additional connection acquisition requests come and new connections will be created.
what I want to ask is, what are the applicable scenarios for each of these two behaviors?

Golang database backend connection-pool java

Mar.20,2021

make sure the database is stable (the number of connections is limited by pooling, and the database will not fail)
try to ensure that the application is stable (new when the pool is full, and the application will not wait, but if there are too many connections, the database may hang, resulting in the application hanging)

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b320ea-2b55f.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b320ea-2b55f.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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