关于 [TNS-12516 TNS:listener could not find instance with matching protocol stack ]

Title: Intermittent TNS-12516 or TNS-12519 Errors Connecting Via
Net
Symptom(s)
~~~~~~~~~~
Client connections may intermittently fail with either of the following errors:

TNS-12516 TNS:listener could not find instance with matching protocol stack
TNS-12519 TNS:no appropriate service handler found

Additionally, a TNS-12520 error may appear in the listener log.

TNS:listener could not find available handler for requested type of server

The output of the lsnrctl services command may show that the service handler
is in a "blocked" state.

e.g. '"DEDICATED" established:1 refused:0 state:blocked'

Change(s)
~~~~~~~~~~

None necessarily. Perhaps increase in load.

Cause
~~~~~~~

By way of instance registration, PMON is responsible for updating the listener
with information about a particular instance such as load and dispatcher
information. Maximum load for dedicated connections is determined by the
PROCESSES parameter. The frequency at which PMON provides SERVICE_UPDATE
information varies according to the workload of the instance. The maximum
interval between these service updates is 10 minutes.

The listener counts the number of connections it has established to the instance
but does not immediately get information about connections that have terminated.
Only when PMON updates the listener via SERVICE_UPDATE is the listener
informed of current load. Since this can take as long as 10 minutes, there can be
a difference between the current instance load according to the listener
and the actual instance load.

When the listener believes the current number of connections has reached maximum
load, it may set the state of the service handler for an instance to "blocked"
and begin refusing incoming client connections with either of the following
errors:

TNS-12516 TNS:listener could not find instance with matching protocol stack
TNS-12519 TNS:no appropriate service handler found

Additionally, a TNS-12520 error may appear in the listener log.

The output of lsnrctl service may show that the service handler is "blocked".

e.g. '"DEDICATED" established:1 refused:0 state:blocked'

Fix
~~~~

Increase the value for PROCESSES.