> # SequeLink® Architecture

> This section shows examples of SequeLink environments implemented with two-tier and n-tier architectures. These configurations explain the data access flow through the SequeLink components.

> Additionally, the configurations show the SequeLink Manager and SequeLink Agent, which, together, allow the SequeLink administrator to control data access activities. For example, using the SequeLink Manager, an administrator can end an active data access user session. All actions the administrator performs on a data access service are handled by the SequeLink Agent.

> ### SequeLink® Two-Tier Architecture

> SequeLink Server is often installed on the same server on which the database engine resides, and the SequeLink Client is often installed on the same workstation as the user application, as shown in Figure 1-4. This is a two-tier architecture because only two machines are needed for the configuration.

> Figure 1-4. SequeLink Two-Tier Architecture

> Data Access Architecture:

> 1. SequeLink Clients can be configured to connect directly to a specific SequeLink Server or, for ODBC, ADO, or JDBC Clients, retrieve connection information from an LDAP directory.
> 2. User applications use the SequeLink Client to connect to the SequeLink Server. Applications make SQL calls from the SequeLink Client to the SequeLink Server using standard APIs, such as ODBC, ADO, or JDBC.
> 3. The SequeLink data access service passes the SQL request to the database engine.
> 4. The database engine processes the SQL request and passes results back to the SequeLink data access service.
> 5. The SequeLink data access service returns the results directly to the SequeLink Client and the user application.

> ### SequeLink® n-Tier Architecture

> For maximum flexibility and centralized access, SequeLink Client can be installed on a middle-tier server between the client and a database server as shown in Figure 1-5. An example of this configuration is when a client (tier 1) runs a Web browser that downloads and displays a Web page stored on the Web/Application Server (tier 2). On the Web page, the user clicks a button that launches an application (component) on the Web/Application Server. This application uses ODBC, ADO, JDBC, or .NET to access a SequeLink Client that is also on the Web/Application Server. The SequeLink Client accesses data from the data store that is serviced by the SequeLink data access service residing on the Database Server (tier 3).

> Figure 1-5. SequeLink n-Tier Architecture

> Data Access Architecture:

> 1. In a three-tier architecture, a user application (for example, a Web browser) in tier 1 may invoke components residing on the Web/Application Server that need to load the SequeLink Client to gain access to the data store on the Database Server.
> 2. A SequeLink Client is installed on the Web/Application Server. Applications make SQL calls from the Web/Application Server running the SequeLink Client to the SequeLink Server using standard APIs, such as ODBC, ADO, JDBC, or .NET.
> 3. The SequeLink data access service passes the SQL request to the database engine.
> 4. The database engine processes the SQL request and passes results back to the SequeLink data access service.
> 5. The SequeLink data access service returns the results to the Web/Application Server.
> 6. The client receives the results from the Web/Application Server.