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The Multi-Tier Drivers comprise client and server components, being uniquely equipped with an in-built high-performance database-independent networking layer and a session rule-book. These drivers are network-ready out of the box, and capable of dynamically discovering matching server components anywhere in a LAN/subnet using the Rendezvous service-discovery protocol.

These drivers are described as "Multi-Tier" due to the fact that they include interface implementations at both the client and the server levels - a connection can "bounce" to a final destination using the native comms layer of the database, if appropriate.

In today's world of client/server and web/application server-based programming paradigms, with remote clients attempting to access your company's corporate data from a number of locations, security becomes a major concern of any organization in terms of controlling the access to this information.

The OpenLink? Multi-Tier architecture uses a server-side sessions Rule Book to enforce access-controls to the Database from incoming client requests based on multiple-access criteria across the Domain (typically the RDBMS type they''re requesting), Database, User, Application, OS or hostname being presented by the client. This gives the company Network/Database Administrator ultimate control of who or what groups of users are allowed access to the database(s).

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The Multi-Tier Drivers include the following components:

• Generic Client(the entry for service consumption) this is component that provides the high level implementation of the relevant data-access mechanism (ODBC, JDBC, OLE DB, or .NET Provider) within the multi-tier component stack.
  
• OpenLink? RPC Clientthis is the client side of the database-independent networking layer, and it is inextricably associated with the generic client at runtime (so you do not physically see this component as a separate library or class file etc).
  
• OpenLink? Request Brokerone of the server-side components that implements server-side OpenLink? RPC functionality. This is the part of the Multi-Tier architecture responsible for session instantiation, configuration management, and overall system security. It is the heart and soul of the Multi-Tier component stack.
  
• Database AgentAnother? server-side component that implements both the server-side OpenLink? RPC functionality and the actual OpenLink? Data Access functionality. This is the only database-specific component in the Multi-Tier component stack; it is also the set of interfaces implemented via the database vendor-provided CLI.
  The architectures of the Multi-Tier and Single-Tier drivers are different, but not as different as instinctively assumed. This is because the Single-Tier database specific driver and the Multi-Tier Database Agent share a common core. What does not change is they both implement the call-level interfaces, albeit at different places. The call-level interfaces take the following forms:

• Type A - C-based dynamic SQL interface that inextricably includes client and server networking components
  
• Type B - C-based remote procedure calls (RPC) interface to the wire-protocol of the underlying database. This is a client-only interface that communicates directly with the remote database server. These interfaces are not typically available to third-party developers. To date the Open Source projects such as FreeTDS? , MySQL? , PostgreSQL? , and Interbase are the only publicly accessible and freely available versions of such interfaces.
  
• Type C - Generic bridges, these are ODBC, JDBC, OLE DB, and .NET providers that act as implementation proxies, such that bridging can be achieved in the manner depicted in the matrix below:
  OpenLink? provides Multi-Tier Drivers built using the Type A, B, and C call-level interfaces formats, depending on what is publicly available to third-party developers by the vendors of the respective database engines.

Please view your respective Data Access Mechanism for more information.