When a mapping is created, the developer defines the data flow
from a source to a target with or without transformations in between.
These source and target definitions might have been imported previously using
the source analyzer and the target designer. It is important to note that while
doing so, PowerCenter only imports the table definition and does not retain the
connectivity information such as the database name, user name, password. This
is ideal because when a developer imports table definitions, he/she might have
done so with their individual logins which usually have read only access to
data. However, when the job runs, we would like it to use an application
having write access to it. This keeps the application independent of any
individual credentials. So the connectivity to the source and the target needs to
be specified to be able to execute the mapping. This is exactly where the
session fits in. A session contains connectivity and runtime information for a
mapping. The session itself does not have any code but it is linked to a
mapping and associates the runtime configuration to it. While a mapping
defines "how" the data flows, session defines "where" the data comes from
and where it goes. A session also defines other runtime metadata such as
commit interval, schema/owner name overrides, etc
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