Web Server Logging, IIS logs, deployment slots and swaps


I wrote a number of articles related to this topic, have a look to get a better context in regards to this one.

The following is a scenario I worked on, which was a little confusing, but understandable, nonetheless.

The misunderstanding was that when a production slot is swapped with a staging slot, the worker is not restarted and therefore, the production slot, after a slot swap is running in a different W3WP process.  When the swap happens, the process does not change only the routing of requests changes.  Because of that, you might see different information in your IIS / Web Server logs after a swap.  Let me try to explain why now.

Production – Pre swap

This is how the configuration looked liked before I swapped my slots.  As you can see in Figure 1, the Azure Blob Storage account is iislogs and the container is blackforest-iislogs.  I also made this slot setting sticky.  This means, when I swap, this setting will not be moved.

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Figure 1, swapping slots and IIS Web Server logs Azure App Service

I look in KUDU/SCM and see the process user_name is BLACKFOREST, see Figure 2.

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Figure 2, swapping slots and IIS Web Server logs Azure App Service, KUDU/SCM

I make some requests to the PRODUCTION slot and see that a folder named BLACKFOREST is created, Figure 3, which contains my IIS / Web Server logs.

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Figure 3, swapping slots and IIS Web Server logs Azure App Service

Then, looking into the logs I see that the s-sitename is also BLACKFOREST.  *NOTE – the s-sitename with the tilde “~” is a request to the KUDU/SCM site.

#Fields: date time s-sitename
2018-04-18 10:45:29 BLACKFOREST GET
2018-04-18 10:42:36 ~1BLACKFOREST

No let’s look at the staging slot.

Staging – pre swap…

This is how the configuration looked liked before I swapped my slots.  As you can see in Figure 4, the Azure Blob Storage account is iislogs and the container is blackforest-iislogs-staging.  I also made this slot setting sticky.  This means, when I swap, this setting will not be moved.

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Figure 4, swapping slots and IIS Web Server logs Azure App Service

I look in KUDU/SCM for the staging slot and see the process user_name is BLACKFOREST_2DC8, see Figure 5.

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Figure 5, swapping slots and IIS Web Server logs Azure App Service, KUDU/SCM

I make some requests to the STAGING slot and see that a folder named BLACKFOREST_2DC8 is created, Figure 6, which contains my IIS / Web Server logs.

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Figure 6, swapping slots and IIS Web Server logs Azure App Service

Then, looking into the logs I see that the s-sitename is also BLACKFOREST_2DC8.

#Fields: date time s-sitename
2018-04-18 10:49:53 BLACKFOREST__2DC8
2018-04-18 10:50:15 ~1BLACKFOREST__2DC8

No let’s do a swap.

After the slot swap

The Application Setting is sticky to the slot and therefore when I swap I would expect the Application Setting “WEBSITE_HTTPLOGGING_CONTAINER_URL” not to change.  Let’s confirm.

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It did not change, it remained the same as is seen in Figure 1 and Figure 4.

However, now my production slot is running with a user_name value of the previous staging slot, I saw this by logging into the PRODUCTION KUDU/SCM site and I see the following, Figure 7, in Process Explorer.

Production after swap with staging

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Figure 7, swapping slots and IIS Web Server logs Azure App Service, KUDU/SCM

Additionally, when I look at the storage container, I see a new folder which matched the user-name, Figure 8.

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Figure 8, swapping slots and IIS Web Server logs Azure App Service

This means that Web Server / IIS logs are being written into that folder and the s-sitename in the log file contains the same name too, I.e. BLACKFOREST_2DC8.

Staging slot after the swap with production

You see the same when accessing the staging slot, it is now staging, but has a user_name of BLACKFOREST, Figure 9.

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Figure 9, swapping slots and IIS Web Server logs Azure App Service, KUDU/SCM

And, as expected, a directory named BLACKFOREST is existing in the storage container, Figure 10.

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Figure 10, swapping slots and IIS Web Server logs Azure App Service

If you are doing analysis of your IIS / Web Application logs, then you need to keep this in mind if you use s-sitename as a filter.  You might instead consider using cs-host.

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