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Working with Spool Servers

Objectives

After completing this lesson, you will be able to:

Explain the use of logical spool servers
Create a logical spool server
Summarize the classification of spool servers and output devices

The Concept of Logical Spool Servers

In this lesson, you will learn about the concept of logical spool servers (or – for short – logical servers) and how to set them up.

In the SAP printing concept, an output device is assigned to just one spool server. A spool server, on the other hand, can have multiple output devices assigned to it, which raises the risk of this server becoming overloaded. You should, therefore, have a mechanism for load balancing across multiple servers. Having groups of spool servers is also recommended for reliability reasons. Both of these aspects are taken into account in the implementation of logical spool servers, also called logical servers.

A logical server stands for one or more other logical and/or real SAP spool servers. Logical servers can be used in place of real SAP spool servers anywhere in the spool system. The logical server is a placeholder for the spool server assigned to it, while a real server is an actual application server with a spool work process and can output print requests.

Hint

Include logical servers from the outset when planning your printer landscape. When scaling your SAP system later (additional instances with spool work processes), logical servers make it easier to adapt the printer landscape.

The figure Logical Servers: Concept illustrates a basic scenario involving logical servers. A (real) spool server is an SAP application server with at least one spool work process. Every output request is processed on a real spool server of this type. An output device created in the SAP system can be assigned to a real spool server directly. However, there are many advantages associated with an additional logical layer between the output device and the spool server. You can use logical (spool) servers for this purpose. These stand for a hierarchy of other logical servers and/or real spool servers.

Hint

Another idea might be to group the SAP output devices by defining different logical spool servers for devices connected over a network and for local devices. Both logical spool servers could point to the same real SAP spool server. However, you can manipulate the devices separately for such purposes as switching devices to another spool server.

Features of Logical Spool Servers

Failure Safety

When you create a spool server (both logical and real spool servers), you can specify an alternative server. If the normal server is not available, the SAP system attempts to use this alternative.

You cannot define more than two spool servers for a logical server. However, because a logical server can reference other logical servers, you can also create extensive spool server hierarchies.

In the figure "Failure Safety" above, output device Prod 1 is assigned to logical spool server Logi_Prod_A which in turn is mapped to spool server s4dhost_S4D_11 and has logical spool server Logi_Prod_B as alternative server. In the same way Logi_Prod_B is mapped to s4dhost_S4D_12 and has alternative server Logi_Prod_C. Logi_Prod_C in turn is mapped to s4dhost_S4D_12 and has alternative server Logi_Prod_C (closing the loop). If now, for a document coming from Prod 1, s4dhost_S4D_11 is not available, the spool system will redirect the output request to the (available) spool server s4dhost_S4D_12.

Note

If this spool server was not available as well, it would be redirected to s4dhost_S4D_13.

Caution

You must ensure that all output devices that may be used by a different spool server can be controlled in the same way by every spool server. For example, in the figure "Failure Safety", if the output device Prod 1 points at operating system level to a printer Printer1 that is controlled locally, an operating system printer Printer1 must be available on server s4dhost and be accessible as a local printer from all possible application servers (that is both from s4dhost_S4D_11 and s4dhost_S4D_12 and s4dhost_S4D_13).

Load Balancing

You can allow load balancing for every spool server with an alternative server by selecting the field Allow Load Balancing. The load of a spool server is calculated from the number of spool work processes, output requests, and printed pages.

You can create an output request for a spool server with load balancing on either a logical or a real spool server, as the figure "Load Balancing" shows. The system determines which server has the smallest load. The algorithm is recursive: The same selection criteria are used on the mapping and the alternative server, which both could be logical servers themselves.

If load balancing is activated, then output requests for a particular device may be processed by different spool servers. Potentially, a short output request that was generated later could overtake an earlier but longer request.

Note

The spool system does allow you to enforce print in order of generation for particular output devices. To do this, on the Output Attributes tab for the device definition, select Sequentially Request Processing. This option forces "print in order of generation" for output requests for the device, even if the definition of the assigned Spool Server definition allows dynamic server selection: Sequential request processing (property of an output device) has priority over the load balancing shown here (property of a spool server). This option makes it possible to use load balancing for most of your printers, but keep the "print in order of generation" for critical printers.

Setup of Logical Spool Servers

You manage spool servers in transaction SPAD, on the Devices / servers tab in the Spool Servers area. The following terminology is relevant when setting up a spool server (see figure "Creating a Logical Server"):

Server name (field AS Instance): The name of the spool server can have a maximum of 20 characters and is case-sensitive. You can use an additional field a short description.
Server class: You can classify the spool server here (for high-volume printing, for example).
Logical server: Select this field when you create a logical server.
Allow load balancing: You use this field to control which method the system uses to determine a spool server for processing of output requests. When the field is set, the system searches for the server with the lowest load among the hierarchy of logical and/or alternative servers you have defined (valid both for logical and physical spool servers).
Mapping: The name of a real or logical server to which this logical server refers.
Alternative server: The server that should be used if the current server is down or not available.

You can use dynamic server selection without restrictions only for output devices that are defined with remote access methods S and U or with the local OMS access method E. The reason for this is that the SAP system accesses remote printers using a network link. This network link is likely to be usable without adjustment by any spool server. Output requests for a remote printer can therefore be moved from one spool server to another without causing any problems. Output Management System (OMS) printers are accessed by a print command that can be executed locally from any spool server. Requests for OMS printers can therefore also be moved from spool server to spool server without problems.

For local printers (access methods C and L), on the other hand, dynamic server selection requires some preparation at the operating system level. In a local connection, the SAP spool server passes an output request directly to the spooler or print manager in the host on which it is running. Such an output request can be processed at another server only if the local spooler or print manager at the second spool server can identify the target output device. This means that the target output device must be defined under the same name in both local spoolers (in the host of the first spool server, and also in the second host). Otherwise, switching an output request for such a device to another spool server could generate the error Device unknown from the local spooler or print manager.

Classification of Spool Servers and Output Devices

You can classify spool servers according to their intended use, such as a spool server for mass printing or for production printing. This classification helps to make your print architecture clearer. It also helps you to realize your planned print architecture, such as assigning classified output devices to the correct spool server.

You can classify output devices and spool servers, for example, for test printing or production printing. To classify an output device, select it (in transaction SPAD under Output Devices) and choose the menu path Edit → Classification.

The SAP system checks the classifications when saving, and displays a warning message if there are deviations: For example, if you attempt to assign a mass printing output device to a production printing spool server, the spool system compares the classification of the output device to the classification of the server. In the case that the classifications do not match, the SAP system informs you.

Create and Classify a Logical Spool Server

As a system administrator, you want to set up logical spool servers in your SAP system to ensure reliability and load balancing. To do so, you need to create a logical server with load balancing and assign an existing output device to it. In addition, you need to classify both the logical spool server and the assigned output device for the use with production printing.

Note

In this exercise, when an object name or value contains ##, replace ## by the number your trainer assigned to you.

Steps

In your SAP system, create a logical server called Production_Print with the mapping to AAS and with PAS as alternative server. Classify this logical server as a server for production printing.

Log on to your assigned SAP System with the credentials provided by your instructor. Start transaction SPAD.
On the Devices / servers tab, in the line Spool Servers, choose Display (leave the input field to the left empty).
If you cannot find the Create button (in the application toolbar), choose Change (in the application toolbar) to switch to change mode.
Choose Create (in the application toolbar).

Create logical spool server Production_Print with the following specifications.

Field	Value
AS Instance	`Production_Print`
Description	<enter an explanatory text>
Server Class	Production printing (for example, documents, cover letters)
Logical Server	<select this checkbox>.
Allow load balancing	<select this checkbox>
Mapping	<use the F4 value help to select the AAS instance of your SAP system>
Alternative server	<use the value help to select the PAS instance of your SAP system>

Save your settings.

Change output device Local## so that the logical server Production_Print is used as the spool server in future. Classify the Local## output device as a production printer.
1. Following the previous step, in your assigned SAP system go back until you come back to the initial screen of transaction SPAD.
2. On the Devices / servers tab, in the line Output Devices, choose Display (leave the input field to the left empty).
3. Open output device Local## in change mode, by double-clicking it and (if you are in display mode) choosing Change from the application toolbar.
4. On the Device Attributes tab page, use the value help for field Spool Server to select the Production_Print logical spool server as assigned spool server for this output device.
5. Classify the output device as a test printer by choosing Edit → Classification → Production printing from the menu.
  Note
  If you chose Edit → Classification → Test printing instead, you would get the message Incorrect classification of the format type server (P) in the status bar.
6. Save your changes.

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