> # Planning SequeLink® Data Access Services

> A SequeLink data access service, which resides on the server, allows a SequeLink Client to connect to a database and is required for your SequeLink configuration. On Windows and UNIX, a SequeLink data access service is automatically created and configured when you install SequeLink Server, so you do not have to define one. On z/OS, defining a SequeLink data access service is part of the installation process as described in the SequeLink Installation Guide.

> After the installation, only the default service will be defined and running. If you need support for internationalization, you must add a service with enhanced code page support and modify that service for the needs of your environment.

> ### Defining Server Data Sources

> Part of the definition of each data access service is a server data source. A data access service has one default server data source defined. You can define additional server data sources, but a SequeLink Client data source or connection string can reference only one server data source. Therefore, if you create a second server data source for a data access service and you want the SequeLink Client to reference the second server data source, you must create a new client data source or connection string and configure it to reference the new server data source.

> See "Default Behavior of Data Access Services" for the definition of the default behavior of SequeLink data access services.

> ### Changing Default Behavior

> The examples in this section demonstrate possible scenarios and reasons that you may have to modify the default behavior of a data access service. In addition, this section describes reasons why you may need more than one server data source for a data access service.

> Example A:

> You may want most of your client applications that use a specific data access service (such as SequeLink Server for DB2) to connect to the database with read-only functionality. Therefore, you would change the value of the DataSourceReadOnly service attribute to read-only.

> The DataSourceReadOnly service attribute is an attribute of a server data source. Because changing the value of the default server data source would cause all connections to the database to be read-only, you would choose to create a second server data source for the data access service and change the value of the DataSourceReadOnly service attribute of the new server data source as shown:

> Example B:

> You may want to configure your data access service to use data scrambling. By default, messages between the client and server are transmitted over the network as cleartext.

> In this case, you would change the value of the ServiceEncryptionAlgorithm service attribute to one of the three available data scrambling options: byte swapping, DES, or 3DES. The ServiceEncryptionAlgorithm attribute is set at the data access service level and is not part of the definition of a server data source. Therefore, any client application that uses the data access service will have data scrambling set.

> Example C:

> ODBC Clients support multiple workarounds to circumvent limitations in some applications (such as Microsoft Access). Using SequeLink, these workarounds can be configured at the server for a specific server data source. This example shows why you may want to configure multiple server data sources for a data access service.

> The service attribute DataSourceFetchTimeStampAsString specifies whether a workaround for a Microsoft Access problem with timestamps is turned on. The workaround is turned off by default (FALSE). You can create a new server data source and set the DataSourceFetchTimeStampAsString attribute to TRUE to turn on this workaround. With the workaround turned on, the Microsoft Access application can use the client data source that references this server data source.