Externalize ASP.NET App Configuration using Cloud Foundry buildpac-Alfus Inigo Jaganathan

Background

This article explains how easily you can externalize your configurations, from web.config to environment variables or any other external sources like Spring Cloud Config Server (which is available in Pivotal Platform(PAS-Cloud Foundry) as marketplace service), using and extension buildpack called Web Config Transform Extension Buildpack. The most interesting thing to note here is, that you need not change any code to make this happen.

Usually it will be a bit challenging process when we try to pull the configuration out, from web.config because of the tight coupling with the framework. That problem will be solved completely using this extension buildpack approach. This especially helps when doing a lift and shift, with minimal or no code change.

If you are familiar with Cloud Foundry, you should be already familiar with buildpacks and its staging process. If not, please refer to cloud foundry, buildpacks and buildpack staging process, before continuing further.

How does this buildpack work?

This is an extension buildpack that will pull the configurations from environment variables and Spring Cloud Config Server (if bound), then modifies the web.config file during the buildpack staging process

Uses Steeltoe Configuration to pull in the configuration from environment variables and Spring Cloud Config Server
Apply web.config transformations (which is more similar to the traditional approach)
Apply token/placeholder replacement to any of the web.config content

Highlights

No code change required, when pushing any ASP.NET application to Pivotal Platform (PAS)
So it reduces the effort in lifting and shifting a legacy ASP.NET application to Pivotal Platform (PAS), more interestingly if you are already using transformation techniques for handling configurations, you can just use it as is.

Pre-Requisites

Pivotal Platform (PAS) environment with cf push access
WEB-CONFIG-TRANSFORM-BUILDPACK should be already installed, use cf buildpacks to verify
Optional – A Spring Cloud Config Server tile in marketplace
Basic understanding of Cloud Foundry and buildpacks

High Level Steps

Info: You can find more detailed information and steps here

Add the buildpack in WEB-CONFIG-TRANSFORM-BUILDPACK to the manifest as below.

---
applications:
- name: redis-buildpack-sample
  memory: 1024M
  stack: windows2016
  buildpacks:
   - WEB-CONFIG-TRANSFORM-BUILDPACK
   - hwc_buildpack
  env:
   ASPNETCORE_ENVIRONMENT: development

Warning: The order of buildpacks matters, always have the system buildpack at the last

If you have a Spring Cloud Config Server instance bounded, that will be added as another configuration source, which you have to mention as a service in the application manifest.yml. If you want to know more about setting up Spring Cloud Config Server, please refer to this recipe
Now we have two choices, using config transformations or token replacement or both. Let’s look at the transformations first.

1. Web Config Transformation

You should have a basic understanding of, how web config transform works, if not please go through Microsoft docs
Usually the transformations will be applied based on the build configuration, but in our case it’s a bit different.
Create the transformation configs as many as environments you have. The name of the file should be web.{app-environment}.config where app-environment is the value of environment variable ASPNETCORE_ENVIRONMENT, in our sample manifest above, I have set it to development. So my config file for development environment names like web.development.config
Modify the transformation files based on the transformations you need
Simply, push the application using cf push command, you should see the transformations applied during the build staging process and the applications will make use of the transformed configuration values
If you have cf ssh access, you can get into into your application’s container and see the transformed web.config file

2. Web Config Token Replacement

This is the most useful functionality from this buildpack as it helps in externalizing the configuration to various sources. This helps the application adhering to one of the cloud native factors Configuration, credentials, and code
Let’s say that you have an appSetting configuration with key foo, to be externalized. So we need to modify the appSettings section as below

<appSettings><add key="foo" value="#{appSettings:foo}"/></appSettings>

appSettings:foo is any name that you can provide, it’s just an environment variable name, but it should match the token name
Now you can set your environment variable, based on the target environment, lets say I have set appSettings:foo to bar
Push the application now, you will be seeing logs as below, where the token #{appSettings:foo} will be replaced with the value bar.

================================================================================
=============== WebConfig Transform Buildpack execution started ================
================================================================================
-----> Using Environment: development
-----> Replacing token `#{appSettings:foo}` in web.config
================================================================================ 
============== WebConfig Transform Buildpack execution completed =============== 
========================================================================

If you have cf ssh access, you can get into into your application’s container and see the appSettings section in web.config file, as below

<appSettings><add key="foo" value="bar"/></appSettings>

Hope you had fun pushing the application to cloud with no or minimal code changes!

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