Single Sign-On (SSO) allows a user to authenticate once and access multiple applications and systems without reauthenticating. SSO uses a centralized authentication repository, the Oracle Internet Directory (OID), to validate users as they move between applications. The SSO component interacts with Oracle HTTP Server and formats SSO information for use by external systems using protocols like XML. The Metadata Repository (Infrastructure) provides common management of Application Server instances and components. Oracle Management Server (OMS) allows centralized management of Application Server farms using Oracle Enterprise Manager. TopLink enables object persistence by mapping Java objects to database tables.