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

SAP Netweaver ICM Fast Channel Architecture

SAP Netweaver has been around for many, many years now. In fact we have had very nearly 20 years of Netweaver.
Back in March 2001, SAP acquired TopTier and went on to use TopTier’s application as the underpinning to the SAP Netweaver application server (WebAS).
Now this would not have been the Netweaver Java stack, that was to come later in the form of WebAS 6.30.
My point is, you would imagine by now that Netweaver is known inside and out by most BASIS professionals, but this is just not the case. It’s a complex and very capable application server and there are things that we know and things that we know in detail.
One of the things that seems to be little known is the FCA and it’s role within the ICM of the Netweaver Java stack.

In this post I want to explain the function of the SAP Netweaver Internet Communication Manager (ICM) Fast Channel Architecture (FCA) and how this is responsible for routing the HTTP communications to your Netweaver Java stack.

As usual, a little context will help set the scene.

A History of Netweaver Java

Before Netweaver 7.1, the Java stack did not have an Internet Communication Manager (ICM). This was reserved only for the Netweaver ABAP stack.
Instead, these old Netweaver Java versions had additional Java nodes (JVMs) called dispatcher nodes (in addition to the server0 node).

The dispatcher node was responsible for receiving and dispatching the inbound HTTP requests to the server nodes of the instance.

The ICM Was Added

Since Netweaver 7.1, the Java stack was given the ICM, which runs from the Kernel binaries, instead of a JVM.


The benefits of this change were:

  • Faster startup and response time (Kernel is C++ compiled binary code).
  • Smaller memory requirements.
  • Same ICM in Netweaver ABAP and Netweaver Java (same Kernel DB independent part).
  • Use of profile files for configuration (for SSL, security, memory params) instead of ConfigTool.

Identifying the FCA

We know the ICM is visible as a separate binary executable process at the operating system level.
In Windows we see “icman.exe” and in Unix/Linux we see “icman”.
At execution, the icman program reads the instance profile to determine it’s configuration.

The Fast Channel Architecture (FCA) is a specific, dedicated set of memory pipes (MPIs) in the shared memory region, accessible by both the ICM and the Java server nodes and used as a method of super fast inter-process communication between the ICM and the Java server nodes.
In Linux, shared memory segments are visible using the “ipcs -m” command, in Windows these are memory mapped files and you cannot see them so easily, you would need a 3rd party tool.

By using shared memory and the concept of memory pipes, it avoids the need for the data in a HTTP request/response to be sent from the ICM to the Java Server node. Instead of sending the actual data, a simple memory pointer can be sent (smaller and consistent in size), telling the Java Server node where to look in memory, for the data.
Effectively what this means is that the shared memory area for the MPIs, sits logically between the ICM and the Java Server nodes.

According to the Netweaver AS Java documentation, the FCA is itself just another MPI, that acts as a FIFO queue.
The HTTP requests coming into the ICM via a TCP port, travel through a regular (anonymous) MPI, before the ICM dispatches the request into a specific FCA queue.
If you have two server nodes on your Java stack (server0 and server1), then the ICM will query the server node to determine the back-end load, then push the request to the specific FCA queue of the target server node that has capacity to handle the request.
Therefore, if you have two server nodes, you will have a dedicated FCA queue for each.
It is the responsibility of the Java server node, to create the FCA queue in the ICM shared memory during start-up.

Once the HTTP request (or rather, the memory pointer to the request) hits the FCA, it becomes the responsibility of the Java server node to pull the request off the queue into a thread for processing.
Inside the Java Server node, these threads are known as the FCA threads or HTTP Worker Threads.
If you run a SAP PI/PO system, then you may already be familiar with these threads and their configuration.
You may have seen these threads when running thread dumps for SAP support incidents.

There are two methods to actually see the FCA Queues:

  • Within the SAP ICM Web Administration page.
  • Using the “icmon” command line tool.

We can call the icmon tool as follows:

icmon pf=<path-to-instance-profile>

then from the menu select "m"
then from the menu select "y"

Once the MPI list is dumped (option “y”), the the FCA queues are visible at the end of the output:

...
MPI<174>: 4d50494d 'ANON' 11 50 0 0 0 0(4996) 1(30001) 1(30001)
MPI<173>: 4d50494d 'ANON' 10 50 0 0 0 0(4996) 1(30001) 1(30001)
MPI<60>: 4d50494d 'TS1_00_1234650_HTTP_WAIT' 5 -1 20 0 0 0(4996) 1(10002) 0(-1)
MPI<5f>: 4d50494d 'TS1_00_1234650_HTTP' 4 -1 20 0 0 0(4996) 1(10002) 1(30001)
MPI<58>: 4d50494d 'TS1_00_1234651_HTTP_WAIT' 2 -1 20 0 4406 0(4996) 1(10003) 0(-1)
MPI<57>: 4d50494d 'TS1_00_1234651_HTTP' 7 -1 20 0 0 0(4996) 1(10003) 1(30001)
MPI<52>: 4d50494d 'TS1_00_1234650_P4' 6 -1 20 0 0 0(4996) 1(10002) 1(30001)
MPI<4d>: 4d50494d 'TS1_00_1234651_P4' 3 -1 20 0 0 0(4996) 1(10003) 1(30001)
MPI<4>: 4d50494d 'ANON' 1 1 0 0 0 0(4996) 1(30001) 1(30001)
MPI<2>: 4d50494d 'ANON' 0 1 0 0 0 0(4996) 1(30001) 1(30001)
 
    q - quit
    m - menue 

NOTE: For those interested, the 4d 50 49 4d at the beginning of each line, translates from HEX to ASCII as “MPIM”.

In my example, you can see I have 2 Java server nodes registered at this ICM: 1234650 and 1234651.
You will notice that there are 3 queues for each Java server node.
The P4 queue is self explanatory, it is used to talk to the Java server node on it’s P4 port (SAP proprietary protocol) and is probably used to acquire capacity/load information from the server node.
Of the other 2 queues, one queue is the “WAIT” queue and is where (I think) the inbound requests (destined to the Java server node) are held, before they enter the other request queue which is where (I think) the Java server node is waiting to process the requests.
(There is not a great deal of documentation on the above, but I have seen instances where the WAIT queue fills, which makes me believe it’s a holding area).

In the dev_icm trace we can also see the joining of the server nodes to the ICM for the HTTP protocol (other protocols are supported, such as Telnet, P4):

[Thr 140608759801600] Wed Mar 17 22:59:32:934 2021
[Thr 140608759801600] JNCMIHttpCallLBListener: node 1234650, service Http joins load balancing
[Thr 140608759801600] HttpJ2EELbPut: server 1234650 started protocol HTTP, attached to request queue TS1_00_1234650_HTTP
[Thr 140608759801600] JNCMIHttpMsPutLogon: set http logon port (port:50000) (lbcount: 2)
[Thr 140608759801600] JNCMIHttpMsPutLogon: set https logon port (port:50001) (lbcount: 2)

In the Java server node developer trace files (e.g. dev_server0 and dev_server1), we can see the name of the node (JNODE_10002 for server0) which is also visible in the dev_icm trace output in column 10:

F [Thr 139637668607872] Wed Mar 17 22:53:49 2021
F [Thr 139637668607872] JSFSetLocalAddr: using NI defaults for bind()
I [Thr 139637668607872] MtxInit: JNODE_10002 0 2

The relevant dev_icm output:

MPI<60>: 4d50494d ‘TS1_00_1234650_HTTP_WAIT’ 5 -1 20 0 0 0(4996) 1(10002) 0(-1)
MPI<5f>: 4d50494d ‘TS1_00_1234650_HTTP’ 4 -1 20 0 0 0(4996) 1(10002) 1(30001)

Sizing the FCA

The size of the FCA is not directly configurable.
Instead, we can configure the size of the shared memory area (total area) for all the MPIs using parameter “mpi/total_size_MB“, then from this total size, the maximum possible size of any individual MPI is fixed to 25% of the total area size.

In later Netweaver versions (7.40+), it is not recommended to adjust “mpi/total_size_MB“, instead, adjust the “icm/max_conn” parameter, which is then used to calculate “mpi/total_size_MB“.
The internal formula is described as:
mpi/total_size_MB = min(0.06 * $(icm/max_conn) + 50, 2000)

There is another undocumented (apart from SAP notes) parameter, which can allow you to increase the max size of an MPI. However it means any one MPI can consume more of the total area than the default 25%.
It is therefore not advised to be adjusted.

We can see the value of the parameter “mpi/total_size_MB” in the ICM developer trace file (dev_icm) during it’s start up. This is useful as it shows us the calculation based on the formula mentioned above.
We are looing at “total size MB” right at the end of the line:

[Thr 140610607359872] MPI init, created: pipes=40010 buffers=19985 reserved=5995 quota=10%, buffer size=65536, total size MB=1250

Common FCA Errors

There are a dedicated set of SAP notes for FCA errors, such as 1867119.
Based on the architecture we can see that they describe issues with throughput (through the FCA Queue), and with issues in the Java server node threads causing the FCA Queues to fill.
They also show issues with sizing of the MPIs, and the number of the worker threads (for high throughput scenarios).

In my experience the following types of FCA errors can be seen in the Java server developer traces “dev_server<n>” files:

  • “-3” error: The Java server node is unable to put a response back onto the FCA Queue, probably because the MPI area is full from a full FCA Queue. This can happen if one of the Java server node HTTP Worker threads has become stuck (waiting) for resources or for the database.
    As you will see from my previous diagram, a full MPI area will then start to affect HTTP access to both Java server nodes as they share the ICM (it’s a single point of failure).
  • “-7” error: This affects one individual Java server node and prevents it from pulling requests off the FCA queue in a timely manner. This specific issue is usually a timeout mismatch between the HTTP provider and the ICM.

Both of the above errors look similar, but one is a lack of resources in the Java stack and the other is a full FCA Queue (in shared memory) due to inaction (stuck threads) in the Java stack.
The “-7” error can therefore present itself as an issue in the ICM or in the Java stack, but it is usually a problem in the Java stack that causes it to close the connection early.

Summary

There you have it, the simple FCA queue that serves HTTP requests to your Java Server nodes.
We learned:

  • Netweaver Java was given the ICM in 7.1 onwards.
  • The ICM in the Netweaver Java and ABAP stacks is the same binary.
  • The ICM uses shared memory for the MPIs.
  • The shared memory area is controlled via a parameter of which it’s value is controlled via 1 parameter (in NW 7.40+).
  • The FCA queues are MPIs.
  • Only memory pointers are passed through the FCA Queues.
  • The Java server nodes are responsible for creating the FCA queues in the ICM shared memory.
  • There are 2 FCA queues for each server node.
  • The developer traces store information about the size of the ICM shared memory and the registration of the Java Server nodes to a queue.
  • There are a known set of errors that can occur and are documented in SAP notes.
Useful SAP References
  • SAP Note 1867119 – No more memory for FCA
  • SAP Note 2417488 – Resource leak for MPI buffers in FCA communication
  • SAP Note 1945745 – How to increase HTTP Worker (FCA) threads in PI
  • SAP Note 2579836 – AS Java system has performance problem – FCAException – Best practices and tuning recommendations.
  • SAP Note 2997765 – AS Java system has performance problem – FCAException – Best practices for analysis
  • SAP Note 2276273 – AS Java – How to identify the largest MPI buffer consumer by MPI dump

HowTo: Check Netweaver 7.02 Secure Store Keyphrase

For Netweaver 7.1 and above, SAP provide a Java class that you can use to check the Secure Store keyphrase.
See SAP note 1895736 “Check if secure store keyphrase is correct”.
However, in the older Netweaver 7.02, the Java check function does not exist.

In this post I provide a simple way to check the keyphrase without making any destructive changes in Netweaver AS Java 7.02.

Why Check the Keyphrase?

Being able to check the Netweaver AS Java Secure Store keyphrase is useful when setting up SAP ASE HADR. The Software Provisioning Manager requests the keyphrase when installing the companion database on the standby/DR server.

The Check Process

In NW 7.02, you can use the following method, to check that you have the correct keyphrase for the Secure Store.
The method does not cause any outage or overwrite anything.
It is completely non-destructive, so you can run it as many times as you need.
I guess in a way it could also be used as a brute force method of guessing the keyphrase.

As the adm Linux user on the Java Central Instance, we first set up some useful variables:

setenv SLTOOLS /sapmnt/${SAPSYSTEMNAME}/global/sltools
setenv LIB ${SLTOOLS}/sharedlib
setenv IAIK ${SLTOOLS}/../security/lib/tools

Now we can call the java code that allows us to create a temporary Secure Store using the same keyphrase that we think is the real Secure Store keyphrase:
NOTE: We change “thepw” for the keyphrase that we think is correct.

/usr/sap/${SAPSYSTEMNAME}/J*/exe/sapjvm_*/bin/java -classpath "${LIB}/tc_sec_secstorefs.jar:${LIB}/exception.jar:${IAIK}/iaik_jce.jar:${LIB}/logging.jar" com.sap.security.core.server.secstorefs.SecStoreFS create -s ${SAPSYSTEMNAME} -f /tmp/${SAPSYSTEMNAME}sec.properties -k /tmp/${SAPSYSTEMNAME}sec.key -enc -p "thepw"

The output of the command above is 2 files in the /tmp folder, called sec.key and sec.properties.
If we now compare the checksum of the new temporary key file, to the current system Secure Store key file (in our case this is called SecStore.key):

cksum /sapmnt/${SAPSYSTEMNAME}/global/security/data/SecStore.key 
cksum /tmp/${SAPSYSTEMNAME}Sec.key


If both the check sum values are the same, then you have the correct keyphrase.

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.


Useful Links:

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.

Useful Links

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.

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