Configure Dynamic Logon Load Distribution for RFC Calls

Dynamic logon load distribution should ensure that the load is distributed as evenly as possible over all available application servers in a logon group. To achieve this, a program runs automatically every five minutes, determines the logon quality for each application server, and writes the quality to a table on the message server. This quality is a numerical value that is calculated from various performance values, such as the number of users logged on and the response time. The server with the best quality within a logon group is used for the next logon.

This mechanism is sufficient for dialog logons; for external RFC logons, which often have thousands of logons per second, this is not the case. The information about quality in the message server is too static, meaning that the RFC server with the best quality would have to process too many requests before the table is updated. The RFC client would also have to query with the message server which RFC server had the best quality for every logon, which would create additional load.

Instead of this, an additional mechanism with the following properties was introduced as an extension with SAP Web Application Server 6.10:

• For the first logon of an RFC client, the client queries the message server, as previously, for information about the quality of the RFC servers in the selected logon group. This information now also contains a value for the change in quality for an additional logon (Delta Quality). This negative value is added to the quality after a logon, which gives a lower quality for the server.
• For further RFC client logons, the client uses the quality and the delta quality to calculate locally which RFC server has the best quality for this logon. An equal distribution of the logons is ensured in this way.
• Only after the interval specified in a logon group, or after a specified number of RFC client logons does the RFC client read the values for quality (and delta quality) from the message server again.

The mechanism for dynamic logon load distribution for RFC logons does not replace the distribution mechanism for dialog logons, but rather extends it in the respect that an RFC client can find the RFC server with the best quality locally.

Prerequisites

You have already defined logon groups for dialog applications in transaction SMLG (CCMS: Logon Group Maintenance) (see Configuring Logon Groups). You can also use these logon groups for RFC logons.

Activating Dynamic RFC Load Distribution for a Logon Group

1. Choose CCMS ® Configuration ® Logon Groups, or call transaction SMLG.
2. Choose the required logon group by double clicking it. The system displays the Change Assignment dialog box.
3. Choose the Properties tab page and activate the Extern. RFC enabled indicator. Choose Copy.

Display Parameters for Dynamic RFC Load Distribution

1. Choose CCMS ® Configuration ® Logon Groups, or call transaction SMLG.
2. Choose Msg-Srv Status Area (). The system displays the Display Control/Status Area screen. In the Integer Storage list, the system displays, among other information, the current quality for every system (6^th numerical column) and the delta quality (8^th numerical column).
3. To display the parameters for the dynamic logon load distribution for RFC calls, choose the required logon group in the Logon Favorite Storage list by double clicking it. The parameters displayed in this dialog window have the following meanings:

Parameter	Meaning
Version	Version = 1: External RFC is not enabled Version = 2. External RFC is enabled
Selection criteria	Best: The server for which the best quality was calculated is used for the next logon Round Robin: Each server in the group is used in turn for the next logon.
Time to read	Time after which the RFC client reads new values for quality and delta quality from the message server
Number logons to read	Number of logons after which the RFC client reads new values for quality and delta quality from the message server

You can only display the parameters in this window. To make changes to the parameters, follow this procedure:

Change Parameters for Dynamic RFC Load Distribution

1. Call Transaction SE16 (Data Browser).
2. Enter the table name RZLLICLASS and choose Enter ().
3. The system displays the screen Data Browser: Table RZLLICLASS: Selection Screen. Choose Execute (), to display all table entries.
4. To change an entry, select the entry and choose Change (). The column titles correspond to the following parameters:

Column	Parameter	Default Value
TIMERERD	Time to read	120 seconds
LOGRERD	Number logons to read	200
FAVTYPE	Selection criteria (possible values are B for Best or R for Round Robin)	B

5. Save your entries.