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HTTP Load Distribution Using SAP Message ServerLocate this document in the navigation structure

Prerequisites

Recommendation

SAP recommends that you use the SAP Web Dispatcher as the entrance point for your Web queries. This is then used as the access point for your network and also executes load balancing for HTTP requests.

See SAP Note 1040325 Information published on SAP site.

Caution

You can use HTTP load balancing on the SAP Message Server for stateless applications only. The message server cannot be used in the portal environment, nor as a load balancer for stateful applications.

Instead, use SAP Web Dispatcher.

To use the SAP Message Server for load balancing in an SAP system (AS ABAP), the following prerequisites must be met.

  • HTTP ports, through which the message server receives information from the servers, must be set up for all application servers.

    Parameter ms/ server_port_<xx> must be set in the profile of the message server.

    Note

    You can also transfer the information via HTTPS. Then you have to ensure that HTTPS ports are set up and that the following parameters are set in the profile of the message server:

    ms/urlmap_secure = 1

    ms/urlprefix_secure = 1

  • The following ICF services must be activated (transaction SICF).

    /default_host/sap/public/icf_info

    /default_host/sap/public/icf_info/logon_groups

    / default_host/sap/public/icf_info/urlprefix

  • You cannot work with additional virtual hosts if you want to use HTTP load distribution managed by the message server.

Context

Procedure

  • Functions

    Usually, an SAP system consists of several application servers which share the network load.

    Load distribution is handled by the message server (there is one message server in an SAP System). When a user logs on, the message server assigns him or her to an application server that currently has the smallest load. This procedure is also used for incoming HTTP requests.

    Every application server that is participating in load sharing tells the message server how many work processes it can provide and whether it accepts HTTPS connections. At the start of a connection, an Internet client first connects to a message server (which must also be HTTP-compatible). The message server tells the client via a redirect which application server it should connect to. Obviously, HTTPS requests are only sent to HTTPS-compatible application servers.

  • Configuration

    For this procedure to work, the message server has to open one or several additional ports. Use the following profile parameters to do this:

    ms/server_port_<xx>

    This parameter specifies the HTTP(s) ports that the message server additionally opens to redirect HTTP requests.

    Additional SSL parameters must be set for HTTPS, so that SSL can be used. Also, the message server must be run as a multi-threaded program, as every HTTPS request is processed in its own thread.

    You can add or delete ports (services) dynamically in the Message Server Monitor (transaction SMMS).

    The outdated parameters ms/http_port and ms/https_port do still function for compatibility reasons, but should not be used any longer.

    Caution

    If you are using the SAP Web Dispatcher, bear in mind that you need parameters ms/http_port and ms/https_port in the profile of the Web Dispatcher, so that the Web Dispatcher can open the connection to the message server.

    There are also the following parameters with default values that do not need to be changed.

    • ms/http_max_ports: Maximum number of additional HTTP ports that the message server can open. The default is 20.

    • ms/http_max_clients: Maximum number of HTTP clients that can log on simultaneously to the message server. The default is 1000.

    • ms/http_timeout: Timeout for network operations. If there is no activity on the connection within this time period, the connection to the HTTP client is terminated. The timeout value is expressed in seconds. The default is 20.

    • ms/http_bufferln: Maximum readable length of the HTTP header. This value is expressed in bytes. The default is 65636.

    Caution

    If SSL is being used, the SSL parameters must be set correctly (see SSL documentation).

  • Working with Logon Groups

    If a log on group is defined for the URL in question (defined in transaction SMLG), the message sever searches for a definition in the list of logon groups. In then performs the load synchronization in this group.

    Since with HTTP there are many requests, this logon group must have the attribute "external RFC" so that no one server is flooded with requests.

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