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Abstract

The SAP NetWeaver Application Servers ABAP and SAP NetWeaver Application Server for Java often consist of multiple instances located on different hosts in order to scale the system and assure its availability. Each instance acts as an HTTP server of its own. HTTP requests have to be balanced between the instances.

There are two essential solutions:

  • SAP Web Dispatcher is a self-configuring, out-of-the-box solution for HTTP/HTTPS load balancing in SAP NetWeaver Application Server landscapes.

  • Other HTTP load balancing solutions, be it hardware or software.

The SAP NetWeaver Application Server offers a web service that provides a list of currently active application servers to the load balancer. The information provided by this web service can be used to automate the load balancer configuration and react dynamically to changes in the system, like instances being created or shut down.

This documentation provides a detailed specification of this web service, which is also used by the SAP Web Dispatcher.

This documentation will focus on the basic load balancing of HTTP requests. This load balancing can be performed for stateless requests as well as for the first request of a stateful session. All subsequent request belonging to this session have to be directed to the same application server.

Note

  • This document does not cover concepts how the stickiness of a request is enabled.

  • Certain aspects of the complex SAP request routing logic such as the mapping between URLs and SAP Logon Groups cannot be simply mapped onto a standard web switch configuration.

Basic terms

Knowledge of the SAP NetWeaver application server architecture as well as concepts and basic settings of the SAP Web Dispatcher helps to understand this documentation. You find a reference to the documentation at the end of this document. For the purpose of this document, mainly the following terms need to be understood:

  • An SAP system consists of one or more dialog instance(s) and one special instance, which offers central services to all other instances. One of these is the so called Message Server (executable name: msg_server) which provides information about all other instances of the SAP system and is referred to in the following.

  • Each SAP instance has a sub-structure, referred to as “stacks”. The possible stacks of SAP instances are an ABAP and a Java stack. An SAP instance can include either an ABAP or a Java stack alone or both stacks together ( “dual-stack instance”).

Concept

The load balancer periodically fetches a list of all active application servers of an SAP system from its message server via HTTP. This list includes the host names as well as a static value indicating the capacity of each server (see below). This capacity is separated by stacks, i.e. a dual-stack instance has an ABAP capacity and a Java capacity.

When an instance is shutting down or scheduled for shutdown, the capacity of the instance will be set to 0. This means that stateful requests for existing sessions can still be sent to this instance but the instance should be taken out of load balancing, because it will not accept any new logons.

Servers will be dynamically added and removed from the list whenever the SAP message server detects a new server starting up or a running server shutting down (or crashing), respectively.

It is required that a load-balancing device removes crashed servers from its internal server list immediately for the following reasons:

  • The message server may take up to ten minutes to detect a server crash. Therefore, the server list may not reflect the current state of the servers

  • The load balancer should only update the list periodically (e.g. every several minutes)

Availability

The specification is valid for all SAP products based on application server release 6.20 upwards with SAP kernel 6.40 upwards. This corresponds to SAP NetWeaver '04 upwards.

Capacity

The capacity value is an integer, which gives an estimate of the servers' strength. The value 0 (zero) means that the application server is shutting down and has to be taken out of the load balancing.

Currently, the heuristic used by the SAP system determines the static capacity values as follows:

  • AS ABAP: number of dialog processes of the instance.

  • AS Java: number of Java server processes.

Caution

This heuristic may be changed by SAP without further notice.

The load balancer should never use the absolute value of these capacity values at all or associate any meaning with them. Only the ratio of capacities between the different servers is significant.

  • If there is an HTTP(S) port in the list, the system is either of the ABAP-only or dual-stack type. In this case, send all requests to the HTTP(S) port and use the LB value from the DIAG line for calculating server capacity.

  • If there is no HTTP(S) port in the list, the system is of the Java-only type. In this case, send all requests to the J2EE(S) port and use the LB value from the J2EE line for calculating server capacity.

It is not mandatory for the load balancer to use the capacity values for making the load balancing decision. Other mechanisms, like response time measurements or host load, can be used as well. However, the meaning of the value 0 (zero) has to be observed (see above).

Host and Port Configuration

The network host name and HTTP port of the message server is configured via SAP configuration parameters rdisp/mshost in the default profile and ms/http_port in the profile of the message server.

For simplicity and performance, a plain text format is used in the HTTP message body to describe the server list as a set of server description records (separated by empty lines) as follows ( CRLF denotes the Control/LineFeed as end-of-line indicator).

Constraints

Load balancing for a single-stack system (ABAP only or Java only) is much easier than for a dual-stack system, as the request type is clear. If you want to do load balancing for a dual-stack system, the following prerequisites must be met:

  • The system is homogeneous, i.e. all instances contain AS ABAP and AS Java.

    Note

    In case that two message servers are configured (one for ABAP, one for Java), the information has to be retrieved from the ABAP message server.

  • Before a request is dispatched to a server, the load balancer checks that both stacks are available in the application server list.

Structure

The body of the HTTP response message is structured according to the following syntax.

        
<server-list> = "version" <major>"."<minor> CRLF
        
<sap-server-name> CRLF
        
<protocol-type> <host> <port> <description> CRLF
        
...
        
<protocol-type> <host> <port> <description> CRLF
        
CRLF
        
<sap-server-name> CRLF
        
<protocol-type> <host> <port> <description> CRLF
        
...
        
<protocol-type> <host> <port> <description> CRLF

CRLF stands for “carriage return line feed” (new line).

The following table explains the possible values for the variables. Examples are given below.

Note

  • The following explanations refer to version 1.2 of the protocol (see first line of the HTTP response).

  • You can also use HTTPS to retrieve the data from the message server. To do this, the message server has to be configured for HTTPS. See the documentation links at the end of the document for further information.

You get the application server list from the message server (release 6.40 onwards) with the following HTTP request:

http://<msg_server_host>: <msg_server_port>/msgserver/text/logon?version=1.2

The list is in plain text format. The following table describes the response in detail.

Variable

Value

Description

<major>.<minor>

two numbers separated by a period

Typical version description: the first number denotes the major version, the second number the minor version.

We use version 1.2 in the following.

<sap-server-name>

character string

Instance name.

  • AS ABAP or dual-stack instances: <host>_<SID>_<inst.no>

  • AS Java instances: J2EE<node_id>

Note

Don't use these strings to get information about the server, the information is contained in the lines below.

The instance name can be an opaque sting, if hash-encryption is configured with the profile parameter is/instname_encoding in the system profile DEFAULT.PFL. In the default setting, the parameter has the value none, which means instance names are transferred in clear text.

<protocol- type>

AS ABAP

HTTP

Server has an HTTP port configured in the ICM.

HTTPS

Server has an HTTPS port configured in the ICM.

DIAG

Server has dialog work processes; this line contains the LB capacity of the AS ABAP.

...

Other values may be listed here: RFC, RFCS, SMTP among others. These protocols are not relevant for load balancing.

AS Java

J2EE

AS Java is active and HTTP port is configured.

J2EES

AS Java is active and SSL-anabled. HTTPS port is configured.

...

Other values may be listed here: P4, P4S, IIOP, IIOPS among others. These protocols are not relevant for load balancing.

<host>

character string

host name

<port>

number

port number

<description>

character string

The following strings are possible:

  • LB=<n> denotes the capacity of the application server for load balancing; applicable for DIAG and all AS Java protocols.

    Note

    For DIAG, the capacity is denoted with DIA=<n> in AS ABAP Release 6.40 (SAP NetWeaver 04)

  • SNC string (applicable for DIAGS and RFCS, see example below)

  • Virtual host index VHOST=<n> (applicable for version 1.2 only)

    Note

    The virtual host string is only used for AS ABAP. For every virtual host configured in the application server (parameter is/<prot>/virt_host_<n> in the instance profile), the corresponding ports are listed.
Example

Java System

The following example shows the application server list of a Java-only system with two instances.

The HTTP request http://ld8060: 8081/msgserver/text/logon?version=1.2 to the message server with HTTP port 8081 could get a response like this:

          
version 1.2
          
J2EE187834720
          
J2EE ld8060.wdf.sap.corp 51800 LB=2
          
J2EES ld8060.wdf.sap.corp 51801 LB=2
          
P4 ld8060.wdf.sap.corp 51804 LB=2
          
P4S ld8060.wdf.sap.corp 51805 LB=2
          
IIOP ld8060.wdf.sap.corp 51806 LB=2
          
IIOPS ld8060.wdf.sap.corp 51807 LB=2
          
TELNET ld8060.wdf.sap.corp 51808 LB=2
          


          
J2EE507834720
          
J2EE ld8060.wdf.sap.corp 55000 LB=1
          
P4 ld8060.wdf.sap.corp 55004 LB=1
          
IIOP ld8060.wdf.sap.corp 55007 LB=1
          
TELNET ld8060.wdf.sap.corp 55008 LB=1

Here, two AS Java instances are configured on one physical host. One of them is SSL-enabled and offers ports for all protocols. The instance J2EE187834720 has double capacity (two Java server processes active) and thus gets twice as many requests (as far as the requests are stateless) from the Web Dispatcher as the other instance J2EE507834720.

ABAP System 

          
version 1.2
          
WDFD00146517A_BIN_53
          
DIAG WDFD00146517A 3253 LB=3
          
RFC WDFD00146517A 3353
          
HTTP WDFD00146517A 1080
          
SMTP WDFD00146517A 8025
          
HTTP WDFD00146517A 1080 VHOST=0
          
SMTP WDFD00146517A 8025 VHOST=0
          


          
ld8060_BIN_12
          
DIAG ld8060.wdf.sap.corp 3212 LB=7
          
RFC ld8060.wdf.sap.corp 3312
          
HTTP ld8060.wdf.sap.corp 8888
          
SMTP ld8060.wdf.sap.corp 2500
          
HTTP ld8060.wdf.sap.corp 8888 VHOST=0
          
SMTP ld8060.wdf.sap.corp 2500 VHOST=0
          


          
pwdf2310_BIN_53
          
DIAG pwdf2310.wdf.sap.corp 3253 LB=10
          
DIAGS pwdf2310.wdf.sap.corp 3253 p/secude:CN=BIN, O=SAP-AG, C=DE
          
RFC pwdf2310.wdf.sap.corp 3353
          
RFCS pwdf2310.wdf.sap.corp 4853 p/secude:CN=BIN, O=SAP-AG, C=DE
          
HTTP pwdf2310.wdf.sap.corp 8080
          
HTTPS pwdf2310.wdf.sap.corp 8443
          
SMTP pwdf2310.wdf.sap.corp 25053
          
HTTP pwdf2310.wdf.sap.corp 8080 VHOST=0
          
HTTPS pwdf2310.wdf.sap.corp 8443 VHOST=0
          
SMTP pwdf2310.wdf.sap.corp 25053 VHOST=0

Here, three AS ABAP instances are configured on different hosts. They have different capacities and protocols.

The information needed for a load balancer is the capacity in the first line and the ports for HTTP and HTTPS..

Dual-Stack System 

          
version 1.2
          
us4251_BCO_27
          
DIAG us4251 3227 LB=8
          
RFC us4251 3327
          
HTTP us4251.wdf.sap.corp 50027
          
HTTPS us4251.wdf.sap.corp 44327
          
SMTP us4251.wdf.sap.corp 25027
          
J2EE us4251.wdf.sap.corp 50027 LB=1
          
J2EES us4251.wdf.sap.corp 44327 LB=1
          
P4 us4251.wdf.sap.corp 52704 LB=1
          
IIOP us4251.wdf.sap.corp 52707 LB=1
          
TELNET us4251.wdf.sap.corp 52708 LB=1
          
HTTP us4251.wdf.sap.corp 52700 VHOST=0
          
HTTPS us4251.wdf.sap.corp 44327 VHOST=0
          
SMTP us4251.wdf.sap.corp 25027 VHOST=0
          


          
bcomain_BCO_27
          
DIAG bcomain 3227 LB=20
          
RFC bcomain 3327
          
HTTP bcomain.wdf.sap.corp 52700
          
HTTPS bcomain.wdf.sap.corp 44327
          
SMTP bcomain.wdf.sap.corp 25027
          
J2EE bcomain.wdf.sap.corp 52700 LB=2
          
J2EES bcomain.wdf.sap.corp 44327 LB=2
          
P4 bcomain.wdf.sap.corp 52704 LB=2
          
P4S bcomain.wdf.sap.corp 52705 LB=2
          
IIOP bcomain.wdf.sap.corp 52707 LB=2
          
TELNET bcomain.wdf.sap.corp 52708 LB=2
          
HTTP bcomain.wdf.sap.corp 52700 VHOST=0
          
HTTPS bcomain.wdf.sap.corp 44327 VHOST=0
          
SMTP bcomain.wdf.sap.corp 25027 VHOST=0

This system has two instances providing ABAP and Java. The first one has ABAP capacity 8, Java capacity 1, whereas the second one has 20 and 2. If you do load balancing for a dual-stack system, you can take one of the values as capacity or do a calculation.

Reference