Distributed Transaction In MTS Using Shared Dblink Fails With Ora-00022

SYMPTOMS

When using shared dblinks to do a distributed transaction from
MTS to MTS the client might see errors of the form

ORA-02050: transaction 840.21.16 rolled back, some remote DBs may be in-doubt
ORA-02068: following severe error from …
ORA-00022: invalid session ID; access denied

CAUSE

Shared servers closed down there by closing the dblink

SOLUTION

Set shared_servers = max_shared_servers in the remote instance