This blog contains experience gained over the years of implementing (and de-implementing) large scale IT applications/software.

Category: SAP

Cookies, SAP Analytics Cloud and CORS in Netweaver & HANA

Back in 2019 (now designated as 2019AC – Anno-Covid19), I wrote a post explaining in simple terms what CORS is and how it can affect a SAP landscape.
In that post I showed a simple “on-premise” setup using Fiori, a back-end system and how a Web Dispatcher can help alleviate CORS issues without needing too much complexity.
This post is about a recent CORS related issue that impacts access to back-end SAP data repositories.

Back To The Future

If we hit the “Fast-Forward” button to 2020MC (Mid-Covid19), CORS is now an extremely important technical setup to enable Web Browser based user interfaces to be served from Internet based SAP SaaS services (like SAP Analytics Cloud) and communicate with back-end on-premise/private data sources such as SAP BW systems or SAP HANA databases.

We see that CORS is going to become ever more important going forward, since Web Browser based user interfaces will become more abundant (due to the increase of SaaS products) for the types of back-end data access. The old world of installing a software application on-premise takes too much time and effort to keep up with changing technology.
Using SaaS applications as user interfaces to on-premise data allows a far more agile delivery of user functionality.

The next generation of Web Interfaces will be capable of processing ever larger data sets, with richer capabilities and more in-built intelligence. We’re talking about the Web Browser being a central hub of cross-connected Web Based services.
Imagine, one “web application” that needs a connection to a SaaS product that provides the analytical interface and version management, a connection to one or more back-end data repositories, a connection to a separate SaaS product for AI data analysis and pattern matching (deep insights), a connection to a separate SaaS product for content management (publishing), a connection to a separate SaaS product for marketing and customer engagement.

All of that, from one central web “origin” will mean CORS will become critical to prevent unwanted connections and data leaks. The Web Browser is already the target of many cyber security exploits, therefore staying secure is extremely important, but security is always at the expense of functionality.

IETF Is On It

The Internet Engineering Task Force already have this in hand. That’s how we have CORS in the first place (tools.ietf.org/html/rfc6454).
The Web Origin Concept is constantly evolving to provide features for useability and also security. Way back in 2016 an update to RFC 6265 was proposed, to enhance the HTTP state management mechanism, which is commonly known to you and I as “cookies”.

This amendment (the RFC details are here: tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00) was the SameSite attribute that can be set for cookies.
Even in this RFC, you can see that it actually attributes the idea of “samedomain-cookies” back to Mozilla, in 2011. So this is not really a “new” security feature, it’s a long time coming!

The Deal With SAC

The “problem” that has brought me back around to CORS, is recent experience with a CORS issue and SAP Analytics Cloud (SAC).
The issue led me to a blog post by Dong Pan of SAP Canada in Feb 2020 and a recent blog post by Ian Henry, also of SAP in Aug 2020.

Dong Pan wrote quite a long technical blog post on how to fix or work-around the full introduction of the SameSite cookie attribute in Google Chrome version 80 when using SAP Analytics Cloud (SAC).

Ian Henry’s post is also based on the same set of solutions that Dong Pan wrote about, but his issue was accessing a backend HANA XS Engine via Web Dispatcher.

The problem in both cases is that SAP Analytics Cloud (SAC) uses the Web Browser as a middleman to create a “Live Connection” back to an “on-premise” data repository (such as SAP BW or SAP S/4HANA), but the back-end SAP Netweaver/SAP ABAP Platform stack/HANA XS engine, that hosts the “on-premise” data repository does not apply the “SameSite” attribute to cookies that it creates.

You can read Dong Pan’s blog post here: www.sapanalytics.cloud/direct-live-connections-in-sap-analytics-cloud-and-samesite-cookies/
You can read Ian Henry’s blog post here: https://blogs.sap.com/2020/08/26/how-to-fix-google-chrome-samesite-cookie-issue-with-sac-and-hana/

By not applying the “SameSite” attribute to the cookie, Google Chrome browsers of version 80+ will not allow SAC to establish a full session to the back-end system.
You will see an HTTP 400 “session expired” error when viewing the HTTP browser traffic, because SAC tries to establish the connection to the back-end, but no back-end system cookies are allowed to be visible to SAC. Therefore SAC thinks you have no session to the back-end.

How to See the Problem

You will need to be proficient at tracing HTTP requests to be able to capture the problem, but it looks like the following in the HTTP response from the back-end system:

You will see (in Google Chrome) two yellow warning triangles on the “set-cookie” headers in the response from the back-end during the call to “GetServerInfo” to establish the actual connection.
The call is the GET for URL “/sap/bw/ina/GetServerInfo?sap-client=xxx&sap-language=EN&sap-sessionviaurl=X“, with the sap-sessionviaurl in the query-string being the key part.
The text when you hover over the yellow triangle is: “This Set-Cookie didn’t specify a “SameSite” attribute and was defaulted to “SameSite=Lax,” and was blocked because it came from a cross-site response which was not the response to a top-level navigation. The Set-Cookie had to have been set with “SameSite=None” to enable cross-site usage.“.

The Fix(es)

SAP Netweaver (or SAP ABAP Platform) needs some code fixes to add the required cookie attribute “SameSite”.

A workaround (it is a workaround) is possible by using the rewrite module capability of the Internet Communication Management (ICM) or using a rewrite rule in a Web Dispatcher, to re-write the responses and include a generic “SameSite” attribute on each cookie.
This is a workaround for a reason, because using the rewrite method causes unnecessary extra work in the ICM (or Web Dispatcher) for every request (matched or not matched) by the rewrite engine.

It’s always better (more secure, more efficient) to apply the code fix to Netweaver (or ABAP Platform) so the “SameSite” attribute is added at the point of the cookie creation.
For HANA XS, it will need a patch to be applied (if it ever gets fixed in the XS since it is soon deprecated).
With the workaround, we are forcing a setting onto cookies outside of the creation process of those cookies.

Don’t get me wrong, I’m not saying that the workaround should not be used. In some cases it will be the only way to fix this problem in some older SAP systems. I’m just pointing out that there are consequences and it’s not ideal.

Dong Pan and Ian Henry have done a good job of providing options for fixing this in a way that should work for 99% of cases.

Is There a Pretty Picture?

This is something I always find useful when I try and work something through in my mind.
I’ve adjusted my original CORS diagram to include an overview of how I think this “SameSite” attribute issue can be imagined.
Hopefully it will help.

We see the following architecture setup with SAC and it’s domain “sapanalytics.cloud”, issuing CORS requests to back-end system BE2, which sits in domain “corp.net”:

Using the above picture for reference, we can now show where the “SameSite” issue occurs in the processing of the “Resource Response” when it comes back to the browser from the BE2 back-end system:

The blocking, by the Chrome Web browser, of the cookies set by the back-end system in domain “corp.net”, means that from the point of view of SAC, no session was established.
There are a couple more “Request”, “Response” exchanges, before the usual HTTP Authorization header is sent from SAC, but at that point it’s really too late as the returned SAP SSO cookie will also be blocked.

At this point you could see a number of different error messages in SAC, but in the Chrome debugging you will see no HTTP errors because the actual HTTP request/response mechanism is working and HTTP content is being returned. It’s just that SAC will know it does not have a session established, because it will not be finding the usual cookies that it would expect from a successfully established session.

Hopefully I’ve helped explain what was already a highly technical topic, in a more visual way and helped convey the problem and the solution.


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Azure Front Door in a SAP Context

In April 2019, Microsoft announced the general availability of the Azure Front Door service.
The highlight of this service is layer 7 (HTTP/S) load balancing.
In this post I want to briefly explore how Azure Front Door could sit in an example SAP landscape.

But We Have Azure Application Gateway…

Yes, while the Azure Front Door service does provide similar capabilities with regards to load balancing an HTTP/s based back-end service, the similarities end when we start to consider multi-regional distribution of services. That is, multiple Azure regions actively servicing global clients.

Azure Application Gateway

The Azure Application Gateway service is the go-to service for HTTP/S load balancing for your Azure hosted HTTP/S IaaS or Container based services that are contained within a region.

Event for some, limited, SAP uses, the Azure Application Gateway may be sufficient, but you really need an experienced SAP Solution Architect to help you plan your SAP landscape architecture at this point. The consequences of doing it wrong, could cause you to completely re-implement a new architecture pattern in your landscape and, of course, additional cost.

… and SAP Web Dispatcher

I have discussed the features of the SAP Web Dispatcher before.
The need for a SAP Web Dispatcher in a SAP landscape is clear and even more appropriate in a cloud deployment of SAP.
Just like Azure Application Gateway, the SAP Web Dispatcher’s context should be limited to a single region. This is especially true because it is IaaS, which means the VMs on which the Web Dispatcher is deployed, are themselves bound to a specific region.

However, what is not clear is how disparate Web Dispatcher systems (i.e. different SAPGLOBALHOST values) can be used in different regions to correctly load balance. This is not the same as a single system with different instances in different regions!

How It All Hangs Together

If we go back to the purpose of this post, I wanted to show how Azure Front Door could be used within the context of a SAP system deployment in Azure.

To help convey the idea, I’ve put together a simple diagram:

In the above diagram, you can see that the Azure Front Door service is used to balance inbound requests from a customer booking system, across multiple Azure regions, directly from the internet. This means that Azure Front Door is most definitely suited as a global customer facing load balancer.
An example scenario is a 2 (or more) region architecture with primary region and disaster recovery region. If the primary region for our customer booking system is unavailable, a DR could be invoked and customers could be routed to the DR region, allowing customer bookings to be taken.

In the diagram, traffic routed from Azure Front Door, is then (for the sake of example) routed through Azure Application Gateway. This is just for example, but in reality it’s not really needed. It could be that you have a real mixture of SAP and non-SAP in some converged sub-domain, and it may be easier to load balance this mix of URLs at this level.
The main point at this point is, you are committed to returning data from a single region.

In our example diagram, the Azure Application Gateway then routes traffic to the SAP Web Dispatcher, which load balances the traffic over the back-end SAP ECC system available application servers using the ABAP stack message server (a feature that is not easily replicated in any other load balancer).

Where Does Azure Traffic Manager Sit?

The Azure Traffic Manager service is a DNS based routing and distribution service. If your company is a multinational conglomerate with a latency sensitive web based customer service, then the Azure Traffic Manager can be used to route customers to their nearest region, where you have your web service hosted and where they can potentially get the speedist and most appropriate content.
If you have only 2 or 3 regions, do not have latency issues and have no need to provide region specific content, then Azure Front Door is probably what you need.

Summary:

I’ve tried to show how the Azure Front Door service can provide your internet sourced, customer entry point into your multi-region web service.
The diagram I’ve provided hopefully shows how Azure Front Door can be distinguished from other similar technologies in a SAP landscape including how Azure Application Gateway could also be in the mix (although rare).
Finally I discuss how Azure Traffic Manager may not always be appropriate for load distribution.

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SAP Web Dispatcher Reverse Proxy Features

If you read through any SAP documentation, you may be forgiven for thinking that the SAP Web Dispatcher is just a reverse HTTP proxy.
It can be located in front of a SAP WebAS and balance the load.
Therefore, it is a simple reverse proxy, right?

In this post, I am going to highlight some of the core features of the SAP Web Dispatcher, so that you may understand its strengths in comparison with other solutions such as Azure Application Gateway and even Azure Front-Door.

Heavily Engineered

There’s a common misconception that SAP is just another piece of software using an array of different components lumped together with some bits of Open Source. In some small cases this may be true of acquired software.
However, the core SAP software offerings are actually far more coherent and intricately linked than you may first imagine.

Ask any Oracle EBS administrator about their software stack and you will be impressed at how well the SAP software stack has been engineered.
This is especially true for the lower SAP Kernel level software components. The older parts of the software stack, are reused so often because of their robustness.

3 Routing Principals

The main thing to remember is that the SAP Web Dispatcher can route requests according to 3 main principles:

  1. Capability
    Is the desired target URL path served by the configured target back-end system(s).

  2. Availability
    Is the desired target URL path served by a configured back-end system that is available (i.e. not in maintenance mode).

  3. Capacity
    Are there more than one target back-end servers capable of handling the request and which one has more capacity.

Load Balancing Act

The SAP Web Dispatcher takes the HTTP/S request from the end-user and as part of the routing determination it analyses the target back-end system load.
It’s actually continually aware of the back-end systems.

There’s a great picture here, which highlights the load balancing methods used for the different types of SAP back-end: https://help.sap.com/viewer/683d6a1797a34730a6e005d1e8de6f22/7.40.18/en-US/4899c3d999273987e10000000a421937.html

What is not mentioned on the help.sap.com page linked above, is target back-end systems configured as “EXTSRV” (non-SAP routing) and also the “flat-file” routing method (very rarely used – at least, I’ve not used it).

The “EXTSRV” back-end systems will use basic round-robin to distribute the request between a comma separated list of target servers. Sticky-ness is achieved through HTTP headers, allowing the Web Dispatcher to determine which back-end system it routed your previous request to.

Even though “EXTSRV” is really designed for non-SAP back-ends, I have used “EXTSRV” for SAP systems, especially when using the SAP Web Dispatcher to avoid issues for system-to-system communications and wanting to avoid CORS issues (see CORS in a SAP Netweaver Landscape).

The “flat-file” method simply uses a static text file as a kind of false load response from a Message Server. The flat-file can be generated by anything and the Web Dispatcher configuration is then defined to route to whatever is in the flat-file.

Back-End

Apart from “EXTSRV” and “flat-file”, all other routing mechanisms use SAP proprietary methods to determine the back-end system load.
As you can see in the SAP Help page link referenced above, the SAP Web Dispatcher knows about the back-end because in the SAP Web Dispatcher configuration, we tell it what it is going to be routing to.

As an example, ABAP back-end systems are added to the Web Dispatcher profile file with the ABAP Message Server described in the configuration.
The Web Dispatcher connects to the target system’s Message Server and says “hello”.
Once connected, the SAP Web Dispatcher retrieves the list of URLs that are provided by the ABAP back-end system, the servers that are served by the Message Server and the relative load of those servers.
All of this information is used during the routing determination.

Protocols

The Web Dispatcher can handle HTTP 1.0, 1.1 and 2.0 (HTTP/2) protocols delivered over TLS (SSL).

Since Kernel 7.49, HTTP/2 has been supported in the Web Dispatcher and also in the ABAP Netweaver stack. This is significant for the latest HTTP based SAP UX known as SAP Fiori. The use of HTTP/2 allows request multiplexing over a single continuous TCP connection, reducing latency and increasing throughput.

NOTE: There are some great SAP blogs out on there on how and why to enable HTTP/2 for Fiori!

For many years now, the SAP Web Dispatcher has supported the Web Socket protocol.
The Web Socket protocol allows developers to utilise push-notifications and provide a more real-time interactive experience for HTML 5.0 content.
Bringing a closer level of integration with the consuming Web Browser.

Security

Some of the more complex uses of the SAP Web Dispatcher involve specific security scenarios.

One such scenario that comes to mind, is Principal Propagation, which can use the Web Dispatcher to front a set of common back-end systems.
The whole premise of Principal Propagation, is that the iDP (identity provider) is “impersonating” the authenticated user, by issuing a generated certificate of authenticity to the target system, on behalf of the user.
With a reverse proxy between the Web Browser and the target HTTP service, things can become complex because the generated X.509 client certificate can become consumed by the proxy server, instead of being forwarded to the target HTTP server.
To prevent the certificate from being interpreted in the wrong way, the SAP Web Dispatcher can be configured to shift the client certificate out to a predefined HTTP header., allowing a kind of X.509 client certificate forwarding.
(More information can be found here: Principal Propagation with SAP Cloud Platform).

Update Aug-2020: As pointed out by a reader, the SAP Web Dispatcher is also capable of reverse invocation. This is an added security feature which allows the target SAP system to open the connection to the SAP Web Dispatcher (instead of the other way around). The SAP Web Dispatcher then uses this open connection channel to send load balanced requests back to the target SAP system. The Reverse Invoke feature is obviously meant for scenarios where the Web Dispatcher exists in a separate network segment (DMZ) to the target SAP system, meaning you only need to open the firewall in the outbound (from the target SAP system) direction.
(Details here: https://help.sap.com/doc/7b196aab728810148a4b1a83b0e91070/1511%20000/en-US/frameset.htm)

Manageability

There’s nothing I like about trying to trace a HTTP call through a proxy server.
The SAP Web Dispatcher comes with it’s own secure administration page from where an administrator can enable advanced tracing capabilities.

The SAP Web Dispatcher makes it much easier to trace requests and responses, with the ability to show the complete unencrypted trace of SSL encrypted sessions (not using pass-through encryption).

The trace is able to show the exact ABAP work process number that processed the request in the target back-end system.

An administrator is able to move individual back-end systems into “maintenance mode” and provide a custom HTTP 503 (service unavailable) message, without affecting the other back-end systems serviced by the same Web Dispatcher.

The SAP Web Dispatcher comes with a vast array of configuration parameters to hone the characteristics of the service you are trying to deliver.
As an example, parameter “wdisp/handle_webdisp_ap_header” can be set to allow the Web Dispatcher to add additional HTTP headers to the request, thereby informing the target back-end system of the Web Dispatcher forward-facing TCP ports. This feature allows the target back-end systems to correctly rewrite HTML links and referral URLs, with the ports on which the SAP Web Dispatcher is listening for requests.
This is just one example of where the back-end SAP system is actually aware that it is being called via a SAP Web Dispatcher.

The Future

With the seemingly constant evolution of cloud based services, what do I imagine the future is for the SAP Web Dispatcher?
In my opinion it is here for another few years yet. The feature list is too specific to SAP landscapes for any real profit to be made by a competitive product.
However, what we may see in this hyper-competitive race for cloud adoption, is the use of a SaaS based version of SAP Web Dispatcher, provided for by the major cloud providers.
Right now, a SAP Web Dispatcher consumes far too much cost/resources/effort than it needs to. Therefore, a simple button click and subsequent configuration in something like the Azure Portal, would be a great saving and more importantly, a great incentive to potential cloud customers with SAP landscapes.

Summary

In this short article, we have discussed how the robust engineering of the SAP Web Dispatcher makes it the ideal front-end reverse proxy for the back-end systems of a SAP landscape.

In fact, in some situations it is the only possibility due to the way the Web Dispatcher is acutely SAP back-end aware, with many features built for native SAP compatibility.

Conversely we’ve seen how, in some situations, the back-end system is actually aware of the presence of the SAP Web Dispatcher and can rewrite HTML links and referral URLs accordingly.

We know the latest HTTP/2 protocol is supported and that this is in line with SAP’s goal of having Fiori as the future SAP presentation layer.

We discussed the extensive tracing capabilities, helping SAP administrators to diagnose complex HTTP connectivity, and authentication issues.

We can conclude that, SAP Web Dispatcher is not just a simple reverse proxy and its use within your SAP landscape is more than likely going to be beneficial in some way or another.
The SAP Web Dispatcher will be with us for a while longer.

References:

Critical SAP Host Agent Security Changes in PL47 – PermissionPolicy

The SAP Host Agent is a critical part of the SAP landscape infrastructure, used to control and, importantly, help automate some aspects of SAP systems.
Generally, writing custom scripts for the Host Agent has been easy.
With experience, it’s easy to see how the Host Agent could be easily abused in such a way that could allow highly privileged access to the server host, without certain security considerations being implemented.

As of the SAP Host Agent 7.21 PL47, the security of the SAP Host Agent and the way that it executes custom scripts is changing.
In this post I will describe how this could break a few things.

What Can The Host Agent Be Used For?

In my experience I have used the Host Agent for the following:

  • Detecting SAP instances on a server host.
  • Patching SAP instances on a server host.
  • Starting/Stopping SAP instances on a server host.
  • Executing scripts on a server host.

Some of the above actions have been performed direct on the server, from SAP BPA (Business Process Automation), from scripts or from tools like Postman, and a lot of the time from SAP LaMa (Landscape Management).

See a previous post for a more detailed example: How an Azure hosted SAP LaMa Controlled SAP System Starts Up

In the majority of cases I have been calling custom scripts, written to perform specific tasks on the target server host.
The scripts are generally hosted in a central location, accessible from all server hosts. This makes it simple to call whichever script.

To be able to execute a custom script, a Host Agent operation descriptor file is required to be deployed into the operations.d directory of the Host Agent home executable directory (usually /usr/sap/hostctrl/exe or C:\Program Files\SAP\hostctrl\exe).
The descriptor allows the Host Agent to understand how to execute the custom script. It contains, for example, the target platform (Windows\Linux), the name and path for the target script, which operating system user is needed to execute the script and any parameters.

On Linux, the descriptor can be specified to execute the target script as any operating system user on Linux, including the root user.
For this reason, the Host Agent and it’s installation directory location are owned by the root user. All files are only modifiable by the root user.

On Windows it is more secure by default.
The Windows security mechanisms prevent the Host Agent from executing any script as any user other than the Computer SYSTEM user (this is the user that the Host Agent executes as). NOTE: I have a workaround for this which I have developed.

Even though the Host Agent installation location and descriptor location and files are not necessarily easily modified, the weakest link in the security chain is the target script/executable and the location of the target script/executable.

What is Changing With Patch Level 47?

From June 2020, with the introduction of Host Agent 7.21 PL47, a new set of security requirements (PermissionPolicy) are introduced, which make the Host Agent more secure when executing custom scripts.

In fact, the changes were introduced before PL47, probably in PL44 or 45, as I remember seeing the PermissionPolicy check output in a previous trace file. It was obviously disabled by default in those prior patch levels.

The main changes introduced by the new PermissionPolicy are:

  • The target script and its directory must be owned by the same user as is specified in the descriptor file for the execution of the script, or it should be executable by the root user (Linux).
  • The script’s source directory must be writeable by this same user or root (Linux), or be writeable by the primary group of the user.
  • If the script is located on an NFS share, “root squash” must be disabled.

What Is Impacted By the New PermissionPolicy Change?

  • Any descriptor in the Host Agent operations.d directory, will be impacted.
  • Any target script will be checked by the new Host Agent security policy.
  • Only Linux/Unix servers will be affected due to the way that Windows security works (as mentioned before).

Because the new security policy affects Linux and affects any descriptor, this will also have a direct impact on some SAP HANA HSR operations performed from SAP LaMa, plus impact any custom operations that you have created.

By default the new security policy is enabled in the Host Agent as soon as you apply patch level 47.

How to Minimise Disruption?

A lot of customer implement the Host Agent auto-update feature, which saves significant effort when applying the frequent SAP Host Agent patches to the entire landscape.

The auto-update feature has one downside; it’s too easy to apply a patch to the whole landscape without reading the SAP notes to discover the contents of the patch or any changes in the patch. Make sure you always read the notes and make sure your auto-update architecture is designed to allow selective roll-out of the Host Agent patches to a portion of your landscape at a time (not the whole landscape in one go).

See here for a brief overview of SAP Host Agent auto-update.

The SAP note 2932953 mentions a method of adjusting the descriptor file to disable the new PermissionPolicy setting completely.
However, this needs pro-active adjustment, since some of the operations affected may only be used in a HANA HSR failover scenario (you will not know it doesn’t work until you need to use it).

Disabling the new security policy is obviously not a long term solution, since it could be enforced in the future.

Remember: Make your desired PermissionPolicy changes to your descriptor files before you apply the Host Agent patch.

Ultimate Active-Active SAP Web Dispatcher Architecture in Azure?

I have never been fully satisfied with the reference architecture on the Microsoft site for running active-active SAP Web Dispatchers in an Azure IaaS platform.

Don’t get me wrong, from a high-level Azure perspective they are representative of what you will be desiring. However, they just lack enough detail to make you really think about the solution more than I feel you should need to.

To re-cap, in an Active-Active SAP Web Dispatcher in Azure, you rely on the inherent capabilities of the Azure Internal Load Balancer (ILB) to provide availability routing to the active-working VMs that are running the Web Dispatcher instances.

To help you (and me) understand what needs to be configured, I’ve put together what I feel is a pretty good low-level architecture diagram.

It’s almost a version two to SAP on Azure Web Dispatcher High Availability.

Show Us the Picture or It Never Happened!

Below is the diagram that I have created.
There is quite a lot of detail in it, and also quite a lot of detail that is not included (required O/S params, instance params and config for the network layer etc). It is really not as simple as you first imagine to include the required detail in one picture.

It Happened, Now Explain Please

If we look at the diagram above, we can easily see that WD1 is the SAP System name, with 2 instances of WD1, both with an instance number of 98 but installed against 2 virtual hostnames of sapwd1 and sapwd2.

Could we have installed on the server hostname directly? Yes, we could have. But that is not inline with a SAP Adaptive Computing Design principal, which is separation of the SAP instance from the host.

Notice that we have a Highly Available NFS share that hosts our SAP system instance profile files and a single shared PSE (SAPSSLS.pse).
We don’t show that this could be from a HA fileshare or NetApp or some other technology, but please use your imagination here. For production the Azure Files service is not currently supported.

Our ILB is configured to accept HTTP and HTTPS connections on their native ports (80 and 443) and it routes these through to the 8098 and 44398 ports that the Web Dispatchers are configured to listen on. You can configure whatever ports you want, but ultimately, having separately addressable back-end ports allows you to re-use the SSL port for Web Dispatcher administration and tie-down the access to a DMZ hosted Jump Box (definitely not on the diagram).

The ILB is probing both back-end VM members on tcp/8098 to see if the Web Dispatcher is responding. It’s a basic TCP SYN check (can I open a connection – yes, OK). For a better check, you can use a HTTP health probe on tcp/8098, which would allow you to set the Web Dispatcher to “maintenance” mode, causing a HTTP “service unavailable” response to be returned to the ILB probe, which would remove that particular Web Dispatcher from the ILB routing. If you also followed the other suggestion of accessing the admin page from the 44398 port via the virtual hostname, then you will see that an administrator would still have admin control for maintenance purposes. Nice.

We have a SAN enabled SSL certificate inside our shared PSE, with 3 Common Names associated with that certificate, one for the ILB “host” name (sapwd), and 1 for each of the virtual hostnames against which we have installed the Web Dispatcher instances (sapwd1 and sapwd2).

Our “icm/host_name_full” parameter sets both Web Dispatchers to think that they are accessed through sapwd.corp.net. However, we have to be careful that we do not use EXTBIND in this particular case, because we do not have the IP address of the ILB bound onto the servers (although if you read my post on how to add a secondary IP address on the Loopback device I can show you how it’s possible to do this and why you may want to).

How Do We Cater for DR?

Because we do not have a high disk I/O throughput on a Web Dispatcher VM, it is perfect to be protected by Azure Site Recovery (ASR).

This means the VM is replicated across to the Azure DR region (the region of your choice).

Like this:

But wait, we’re only replicating 1 VM! Yes, we don’t need to pay for both, since a cost-optimised approach would be to just re-install the second Web Dispatcher after a DR failover.

We have a dependency on some sort of NFS share replication to exist in the DR region, but it doesn’t necessarily need to be fancy in the case of the SAP Web Dispatcher, because very little will be changing on the /sapmnt/<SID> area.

NOTE: The replicated VM is not accessible until a failover is instigated.

What Happens In a Failover to DR

In a DR scenario, the decision to failover to the DR region is manual.
The decision maker can choose to failover as soon as the primary region is unavailable, or they can choose to wait to see if the primary region is quickly recovered by Microsoft.

I’ve put together a diagram of how this could affect our simple HA Web Dispatcher setup:

The decision to failover should not be taken lightly, because it will take a lot of effort to failback (for databases).

Generally the recommendation from Microsoft is to use an Azure Automation Runbook to execute a pre-defined script of tasks.

In our case, the runbook will create the ILB above the Web Dispatcher VM and add the replicated VM to the ILB.
Our runbook will also then add secondary IP addresses to the VM and finally adjust DNS for all our hostnames and virtual host names, assigning the new IP addresses to the DNS records.

Once our Web Dispatcher is online and working, we could choose to then build out a further VM and add it into the ILB back-end pool, depending on how long we think we will be living in the DR region.

Summary

Did we successfully include more detail in the architecture diagram? Yes we sure did!
Was it all the detail? No. There’s a lot that I have not included still.
Will I be enhancing this diagram? Probably; I hate leaving holes.

I’ve shown above how an active-active SAP Web Dispatcher architecture can work in Azure and how that could be setup for a DR protection.

We also briefly touched on some good points about separation of administration traffic, using a HTTP health probe for an ILB aware Web Dispatcher maintenance capability, and how the SSL setup uses a SAN certificate.

Would this diagram be more complicated by having an active-active HA Web Dispatcher alongside an ASCS or SCS? Yes, it gets more complicated, but there are some great features in the ILB that allow simplification of the rules which allow you to use the ILB for more than one purpose, saving cost.

Update Jun-2020: This duplicate Web Dispatcher architecture is known in SAP as “Parallel Web Dispatcher” and a basic description is visible here: https://help.sap.com/viewer/683d6a1797a34730a6e005d1e8de6f22/1709%20002/en-US/489a9a6b48c673e8e10000000a42189b.html

Update Mar-2021: Some of you have asked about how the “Maintenance Mode” activation works with the ILB. This is siply that the WDisp returns a HTTP 503 when Wdisp maintenance mode is enabled.
By default the ILB health probe will be “http://<the-vm>:<your-port>/”, but if you don’t have a back-end service allocated to “/” then you will get a HTTP 404 constantly. You need to adjust the URL to an actual working URL location based on the config of your back-end systems.
If you don’t want the health probe to make a call to an actual back-end system (during the health probe ping) then use parameter “icm/HTTP/file_access_<xx>” to define a custom local directory and place a blank file called “health.htm”. Then just adjust the health problem URL with the path to the “health.htm” and the health probe pings will never call a back-end system URL. It also means that you can touch or remove the health.htm to permit the ILB to use or not use that specific WDisp.