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What is the purpose of the SCS' Message server in dual stack system?

Former Member
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Hello everyone,

If I am not wrong, in a dual stack system, the web dispatcher communicates with the ASCS' MS when there is a request from a user.

Therefore, I wonder, why there are 2 MS in dual stack system? does the ASCS' MS talk directly to the SCS' MS?

so, in conclusion:

1. why there are 2 MS in dual stack system?

2. what is the purpose of each MS?

3. how can the web dispatcher communicate only with the ASCS' MS? does ASCS' MS has information about the java instances as well as the ABAP ones?

thanks in advance,

Adi Elisha.

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Answers (2)

Answers (2)

Former Member
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I'll rephrase the question:

Why is the SCS' MS required for normal operation in a dual stack system?

which components communicate with the SCS' MS, and when?

any Information from sap about this will be great.

isaias_freitas
Advisor
Advisor
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Hello Adi,

Did you read the link shared by Americo, in this post?

Regards,

Isaías

Former Member
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Hey Isaias,

I have read the link, and I'll quote it:

"The Message Service keeps a list of all server processes in the AS Java cluster and provides information about their availability to the ICM." - which is not true for dual stack, since it's the ASCS' MS job.

"Notification of events that arise in the cluster, for example, when a service is started or stopped" - apparently, the ASCS' MS can gather that information alone, and therefore, he can notify about it aswell.


"Communication between different services" - I assume the "services" are ICM to java dispatcher? well, in dual stack, the ASCS' MS takes that role.


"Forwarding of messages and requests to all participants" - again, irrelevant in ASCS' MS because the messages from the clients (browser) is forwarded to the web dispatcher > ASCS' MS > Java's instance ICM > java dispatcher > Java SP > and back all the way to the browser.


"Guaranteed message transmission" - which messages? when?

"Exchange of cache information in the cluster" - that is the only valid point I see if I am not mistaken about all I wrote.

please let me know if I got it right.

isaias_freitas
Advisor
Advisor
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"Communication between different services"

Java-only, specific services, like FCA, maybe?

"Forwarding of messages and requests to all participants"


This is about the intercommunication of the Java, not related to external clients.


Again, I do not work with the Java components, but if something at a Java instance needs to send a message to another Java instance, this will go through the SCS' MS.


In any case, the SCS' MS is mandatory. The Java-stack will not even start if the SCS' MS is not available.


Kind regards,

Isaías

isaias_freitas
Advisor
Advisor
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Hello,

About:

1) Each Message Server (MS) will be responsible for the intercommunication of the instances of each stack (the MS of the ASCS will be responsible for the intercommunication between ABAP instances, while the MS of the SCS for the intercommunication between Java instances);

2) Already answered at #1, I believe ;

3) Yes. The MS of the ASCS also has the information about the Java instances.

Regards,

Isaías

Former Member
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I am probaly just forgetting something, but when will there be a need for an intercommunication between the java instances? Could you give an example of that?

there is no need for the SCS' MS to tell the web dispatcher to which java instance he should talk, so I don't see when will I need an intercommunication between java instances.

Regards,

Adi Elisha.

Former Member
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Hi,

As I have understood....

In dual stack system with web dispatcher the flow of request is as below

  • Web dispatcher (Decides where the request should go ABAP/JAVA and load balancing with the information from MS' ASCS (ABAP request) and MS' SCS (Java request))
  • ICM of the application server
  • Dispatcher
  • Process (work process/server process)

The main function of Web dispatcher is of reverse proxy.

MS of SCS and MS of ASCS performs load balancing below multiple application servers (say you have dual stack system with more than one application server)

Intercommunication between application servers will go through Gateway and not message server.

https://help.sap.com/saphelp_nw73ehp1/helpdata/en/48/99ac3a7f020e27e10000000a421937/content.htm

Server Selection and Load Balancing Using the SAP Web Dispatcher - SAP Web Dispatcher - SAP Library

Architecture of SAP NetWeaver Application Server Dual Stack - SAP NetWeaver Library: Function-Orient...

Regards,

Prithviraj.

Former Member
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the thing you are wrong about is that in a dual stack system with web dispatcher, the web dispatcher communicates only with the ASCS' MS, and therefore, there is no use in  the SCS' MS.

does anyone know why the SCS' MS really exists? and can you please give me an example?

isaias_freitas
Advisor
Advisor
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Hello,

I do not work with Java systems... So, I cannot send an example of when a Java instance would need to communicate with each other (if this happens).

Anyway, when the Java-stack starts it will connect to the SCS' MS.

The SCS' MS is required for normal system operation (disregarding whether a Web Dispatcher is used or not).

Regards,

Isaías

Former Member
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There is still no real answer for why we need the SCS' MS if the needed information about the java instances can be gathered from the ASCS' MS.

isaias_freitas
Advisor
Advisor
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Hello,

Just because the Web Dispatcher does not need it, it does not mean that the SCS' MS is not required.

As I informed in my previous reply, the SCS' MS is required for normal operation of the Java-stack, no matter whether if it is a dual-stack system or Java-only.

Regards,

Isaías

Former Member
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Then here is my question,

why is the SCS' MS required for normal operation of the java-stack? which components use it, and when?

isaias_freitas
Advisor
Advisor
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Hello Prithviraj,


Intercommunication between application servers will go through Gateway and not message server.

It depends on what communication you are addressing...

Using an ABAP system with no ASCS as example, the so called "classic Central Instance" will have the Message Server and a work process of type ENQ (enqueue).

If a dialog instance wants to read information from the enqueue lock table, it will do so through an RFC call to the CI. This will use the gateway.

However, if the dialog instance wants to write to the lock table (set, change or delete a lock), the communication will go through the Message Server.

Regards,

Isaías

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https://help.sap.com/saphelp_nwesrce/helpdata/en/1d/e9c43dac1bcf51e10000000a114084/frameset.htm

SCS is a mandatory additional infrastructure component handling messaging and locking

Former Member
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I'll rephrase the question:

Why is the SCS' MS required for normal operation in a dual stack system?

which components communicate with the SCS' MS, and when?

any Information from sap about this will be great.