Persistent Staging Area
The Persistent Staging Area (PSA) is the inbound storage area in BW for data from the source systems. The requested data is saved, unchanged from the source system.
Request data is stored in the transfer structure format in transparent, relational database tables in BI. The data format remains unchanged, meaning that no summarization or transformations take place, as is the case with InfoCubes.
When loading flat files, the data does not remain completely unchanged, since it is adjusted by conversion routines, where necessary (for example, the date format 31.21.1999 is converted to 19991231 in order to ensure that the data is uniform).
You define the PSA transfer method in transfer rule maintenance.
If you set the PSA when extracting data, performance will be improvided if you use TRFCs to load the data. The temporary storage facility in the PSA also allows you to check and change the data before the update to data targets. Isolating the load process from further processing in BW also helps to improve load performance. Unlike a data request with IDocs, a data request in the PSA also provides you with variousoptions for further updating data to the data targets. Isolating the load process from further processing in BW also helps to improve loading performance. The operative system is not affected if errors occur during further processing of data.
The PSA delivers the backup status for the ODS (until the total staging process is confirmed). The duration of the data storage in the PSA is medium-term, since the data can still be used for reorganization. For updates to ODS objects however, data is only stored for the short term.
In the PSA tree in the Administrator Workbench, a PSA is displayed for every InfoSource. You can access the PSA tree in the Administrator Workbench using either Modeling or Monitoring. The requested data records appear, divided according to request, under the source system they belong to for an InfoSource in the PSA tree.
The data records in BW are transferred to the transfer structure when you load data with the PSA transfer method. A TRFC is performed for each data package. Data is written to the PSA table from the transfer structure, and stored there. A transparent PSA table is created for each transfer structure that is activated. The PSA tables each have the same structure as their transfer structures. They are also flagged with key fields for the request ID, the data package number, and the data record number.
Since the requested data is stored unchanged in the PSA, it might contain errors if there were errors in the source system. If the requested data records have been written to the PSA table, you cancheck the data for the request and change incorrect data records.
Depending on the type of update, data is transferred from the PSA table into the communication structure using the transfer rules. From the communication structure, the data is updated to the corresponding data target.
Using partitioning, you can separate the dataset of a PSA table into several smaller, physically independent, and redundancy-free units. This separation can mean improved performance when you update data from the PSA. In the BW Customizing Implementation Guide, under Business Information Warehouse → Connections to Other Systems → Maintain Control Parameters for Data Transfer, you set the number of data records to create a partition from. Only data records from a complete request are stored in a partition. The specified value is a threshold value.
As of SAP BW 3.0, you can use the PSA to load hierarchies from the DataSources released for this purpose. The corresponding DataSources will be delivered with Plug-In (-A) 2001.2, at the earliest. You can also use a PSA to load hierarchies from files.
The maximum number of fields is 255 when using TRFCs to transfer data. The maximum length of the data record is 1962 bytes when using TRFCs.
Data transfer with IDocs cannot be used in connection with the PSA.