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Checkout the latest article by Darryl Griffiths from Aliter Consulting. SAP on Azure Web Dispatcher High Availability provides an overview of how to utilise an Azure Internal Load Balancer in conjunction with the parallel SAP Web Dispatchers to achieve a highly available, load-balanced and scalable solution for fronting SAP Fiori and other SAP components. This deployment is proving very successful on a current SAP Fiori and SAP S/4HANA implementation project for one of our clients.
• Technology improvements mean computing tasks previously requiring interaction with people, can be fully automated.
• Automation brings repeatability, reduced error rates, easy scalability of service provision.
• Future interoperability and open standards will mean businesses can swap easily between cloud providers.
• It is key that solutions are designed to operate in such a platform agnostic manner outside the bounds of normal technical
architecture design (i.e. no fixed O/S choices or fixed DB platforms).
Established Technological Principals
• Solutions today, should be built using already established technological principals.
• Using bleeding edge rarely produces the perceived benefits in places such as core business systems, without significant
buy-in from business leaders.
• Pre-empting standards not already widely adopted, could produce a “Beta-Max” scenario.
• Technology solutions should deliver for a minimum timeframe within the context of the lifecycle of the related business system.
• Example: Re-writing scripts during any platform migration should not just use the coolest scripting language, they should use a commonly known
language widely used and understood.
• Acts as a reverse proxy for HTTP to/from SAP/non-SAP systems.
• Required when HA setup of a Web Server is implemented (i.e. multi-node
• Required when load balancing HTTP traffic to back-end SAP systems.
• Supports 3 different load balancing methods.
• Supports request rewriting through modification handlers.
• Supports serving static content.
• Supports SSL termination and end-to-end SSL pass-through.
• Can also route non-HTTP traffic in RAW mode.
About the Web Dispatcher
• Primarily you need to protect against Azure underlying physical host failures.
They DO happen!
• Secondarily, provision for server downtime during Microsoft physical host
patching/maintenance. There is no magic that moves your VM to another Azure
physical host (not like vMotion).
• To protect your system:
*You* must provision duplicate services into Azure Availability Sets.
For the SAP Web Dispatcher, this means you need at least two virtual machines
in one Azure Availability Set with the Web Dispatcher running on both servers
in active/active (parallel) mode.
About High Availability on Azure
• A logical grouping of your virtual machines.
• Servers within a group are bound not to be running on the same underlying
physical host at the same time (like anti-affinity rules).
• Availability Sets are assigned at Virtual Machine creation only.
Azure Availability Set #1
About Azure Availability Sets
• A software defined load balancer with port routing in SKU basic or standard.
• The ILB SKU must match the SKU of back-end pool member VMs & IP resources.
• Uses probes (like a ping) to test and route network traffic to available back-end
• You are billed for an ILB once you’ve created one
• An ILB has an initial cost, plus potentially a cost for each of the ports you assign to it.
• You can map to any IP address on a NIC of a back-end member.
• You define an IP address for the ILB.
• You define the front-end (listen) ports.
• You define the back-end pools of virtual machines.
• You define a probe port and protocol for testing availability of back-end members.
• You define flow rules that map front-end ports to back-end pools and destination
• Back-end pool members cannot talk to themselves via the ILB (ILB is invisible to
About Azure Internal Load Balancers
ILB IP: 10.0.0.1 (Web Dispatcher IP)
Web Dispatcher Ports: 80, 443 etc
Front-End IP & “listen” Ports
Back-End Pool Probe Ports
health status for
routing to back-end
A back-end member cannot talk to itself via the ILB !
There are 2x SKUs:
STANDARD or BASIC.
For STANDARD SKU IP addresses
you must have STANDARD ILB.
About Azure Internal Load Balancers
https 443 mycorp.int.net
Sample Web Dispatcher Architecture
• DNS is key.
• ILB fronts the Web Dispatchers in active/active setup with a single PSE
containing the SSL cert(s).
• ILB has front-end ports listening on tcp/80 and tcp/443 (well known and
probably firewall friendly).
• ILB routes to backend Web Dispatcher VMs on port 80## and 443##.
• Administrative access to Web Dispatcher is via host:443## and not via ILB (more
• In NetWeaver ABAP AS the HTTPURLLOC table should be configured to ensure
all URLs generated in the ABAP AS are prefixed with the ILB hostname and IP.
• In Web Dispatcher the parameter “wdisp/handle_webdisp_ap_header” needs
configuring to ensure correct ports are advertised in HTTP headers sent to
backend SAP system.
Sample Web Dispatcher Architecture
• Since Web Dispatcher is not listening on ports <1024 no external bind is required
• Co-hosting of the Web Dispatcher on the SAP ASCS/SCS is possible but will require
some additional service IP addresses adding to the ILB to enable tcp routing to the
message server (can’t go via the ILB from a back-end pool member!).
• Co-hosting of the Web Dispatcher using 1 single ILB but different back-end VMs is
not ideal due to mixing application architectural layers (and security in-depth).
• NetWeaver table HTTPURLLOC is truncated emptied in the target client of a SAP
local client copy!
• Secure the Web Dispatcher admin URL behind specific IP addresses and ensure it is
only reachable via HTTPS. It could be on a different HTTPS port not reached via the
Points to Note
• Protection of the SAP Web Dispatcher from faults is critical.
Provisioning for Azure physical host maintenance is critical.
• Apart from a few specific areas, HA of the SAP Web Dispatcher in Azure is
almost the same as on-premise, but with an Azure twist (the ILB).
• Correct use of the ILB and emphasizing the architectural layout is important for
security and correct operation of the Wdisp within the realms of HTTP (AP
access ports is an example).
• Azure virtual machine high availability for SAP NetWeaver:
• Azure Load Balancer Overview: https://docs.microsoft.com/en-us/azure/load-
• Reference architecture: https://docs.microsoft.com/en-
• SAP Web Dispatcher 7.5
• SAP Web Dispatcher Parameters