Schedule job that will run as specific user - oracle

Question

I'm trying to create a job that will run a stored procedure as a specific user which I can create and provide with the limited permissions to do only what is required.
From what I have read I need to create the user (pretty standard) and then create a credential entry by performing;
dbms_scheduler.create_credential('[cred name]', '[Oracle User]', '[Oracle Password]');
Once I have the credential created I try to use it in the job creation script;
dbms_scheduler.create_job(
job_name => 'myJobName',
job_type => 'stored_procedure',
job_action => 'myStoredProcedure',
credential_name => 'jobRunningUserCredential',
destination_name => NULL);
The problem that I am having is that when this is run I get the following error message;
ora-27351 conflicting values of job attributes credential_name and job_type
I was wondering whether anyone can see something that I've missed out or whether I am just barking up the wrong tree with this approach in which case any recommendations are welcome.
Thanks for any help/advice in advance.
Sam
UPDATE:
After receiving the suggestion to use alter_chain I have implemented the following code;
dbms_scheduler.create_chain('myjob_chain', NULL, NULL, NULL);
dbms_scheduler.define_chain_step('myjob_chain', 'step_name', 'schemaName');
dbms_scheduler.define_chain_step('myjob_chain', 'parallel_instances', 'schemaName');
dbms_scheduler.define_chain_step('myjob_chain', 'enable_job', 'schemaName');
dbms_scheduler.enable('myjob_chain');
dbms_scheduler.alter_chain('myjob_chain', 'step_name', 'credential_name', 'myCredential');
dbms_scheduler.create_job(
job_name => 'myJobName',
job_type => 'stored_procedure',
job_action => 'myStoredProcedure',
credential_name => 'jobRunningUserCredential',
destination_name => NULL);
dbms_output.put_line('job created');
dbms_scheduler.alter_chain('myjob_chain', 'set_parallel_instances', 'credential_name', 'myCredential');
dbms_scheduler.set_attribute(
name => 'myJobName',
attribute => 'parallel_instances',
value => TRUE);
dbms_output_put_line('parallel_instances');
dbms_scheduler.alter_chain('myjob_chain', 'enable_job', 'credential_name', 'myCredential');
dbms_scheduler.enable(name => 'myJobName');
dbms_output_put_line('enable');
The output that this gives me is;
job created
parallel_instances
BEGIN
*
ERROR at line 1:
ORA-27351: conflicting values of job attributes CREDENTIAL_NAME and JOB_TYPE
ORA-06512: at "SYS.DBMS_ISCHED", line 4395
ORA-06512: at "SYS.DBMS_SCHEDULER", line 2803
ORA-06512: at line 67
Any suggestions are greatly appreciated.

Answer 1

IF you want to run a job as a specific user ,
create credential for the particular OS not for the Database (Because the credential is pointing to the OS not to DB)
Create the credential
BEGIN
dbms_scheduler.create_credential(
username => 'OS Username',
password => 'OS password',
credential_name => 'TEST_CREDENTIAL'
);
END;
Create the Job
dbms_scheduler.create_job(
job_name => 'myJobName',
job_type => 'stored_procedure',
job_action => 'myStoredProcedure',
credential_name => 'TEST_CREDENTIAL',
destination_name => NULL);
Now Run the Job ..

Answer 2

What you probably want to do is set the credential for the step.
You can do this using alter_chain on the particular step with
attribute set to credential_name .
http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28419/d_sched.htm#CHDIAICJ
Credit: Ravi

Answer 3

If you want for the job to be run under userm, say, AJAX - you have to create the job under ths user. For that you have to grant him CREATE JOB privilege.
According to Oracle's documentaion credentials are created as a comntainer for user/password information that is to be passed by Ora scheduler to external programs.
Therefore Your initial script had a misconception problem, trying to create a job with credentials (that is with parameter to call external program) and at the same time with job_type => 'stored_procedure' (and stored_procedure is a job type for calling internal pl/sql procedures).