Wednesday, 22 May 2013

Specifying Environment Variables at Deploy Time not at Build Time

Over a month ago I posted an article in which I revelled in the ability to create a publish package in one step then deploy it in another. Last week I discovered a limitation to that process, as it was, and had to find a way to reconfigure it. This post is about that challenge and the conclusions I've drawn from it.

The difficulty with the process that I described in that article was that it is largely dependent on Configuration Transformations. These have been around for a while now and do a great job of specifying the environment variables with respect to the build configuration, normally Debug or Release. But that is exactly the problem (and my realisation):  Config Transforms blur the boundary between Building and Deploying.

When I'm compiling my application I want to be thinking about the build, about optimisation, about references and dependencies. So unless I'm developing locally and debugging I usually want to build in Release mode, as its probably going to get deployed for testing or production. I might be compiling following performance enhancements - I don't want to have to concern myself with the target environment's connection strings when I'm trying to make my app go faster. I always want to build in Release mode. But with config transformations that are tightly coupled to my build configuration I'm stuck with a built package with production connections strings - I can't performance test against that!

Build configurations are necessary, but I don't want to have to create a new one just so that I can apply a different Config Transformation at Build Time. Nor do I want to specify the Publish Profile at build time either. I want to separate Build configuration and Environment configuration concerns. What I want to do is specify the Environment settings at Deploy Time, after my deploy package has been built.

Thankfully, there is a way to do this. But its much more convoluted then adding a new config transformation file to your project. It seems Microsoft have unfortunately made it easier to do things the wrong way. MS have made it easier to deploy from the desktop, but in an enterprise this is usually not the case. Perhaps the new Publish Profile is more about making it easier to deploy to Azure?

The 'Package' MSBuild target that is only available for Web projects generates a directory containing a zipped up deployable package containing all the assets and dlls required, a MyProject.deploy.cmd file that is a friendly wrapper around the msdeploy.exe and, crucially for us, a MyProject.SetParamters.xml. The xml file gives us the ability to modify the contents of the package at Deploy Time (when we call the .cmd). This is what we will be editing:


The xml file can be manually edited before deploying, but this is rarely practical. The IIS App Name and any connection strings are pulled into this file by default. The connection strings are replaced at deploy time with the actual values found in whatever the result of your configuration transformation was. It is also possible to configure the IIS App Name in your .csproj file, but then we are again dependent on the build configuration. The way to override these settings when we deploy is to call msdeploy.exe via the MyProject.deploy.cmd and make use of the -setParam flag. The syntax of which is rather cumbersome:


The second problem is this: what if you want to specify Application Settings (appSettings) in your web.config? As stated, IIS App Name and Connections strings are all that you get to override out-of-the-box. To be able to configure an App Setting, you must add a new file to the root of your web project: parameters.xml. This file needs to contain any App Settings you want to specify when you deploy. Follow this article for a more detailed explanation of the syntax of a parameter.xml. Essentially, we must specify the XPath to the setting and provide an alias to it so we can refer to it in the generated MyProject.SetParameters.xml that we will go on to edit at Deploy Time:


Now, when we create the package at Build Time there is another parameter specified in our MyProject.SetParameters.xml generated along with the package. Like so:


We then apply the same overriding "-setParam" flag when calling the MyProject.deploy.cmd at Deploy Time to configure the "My Service Url" however we want for out target environment. The final call that overrides the parameters is shown below. I've parameterised the call so that I can use the Team City UI to run a custom build and override the parameters without the chance of missing a closing quote etc.



The added bonus of this configuration is that if you are manually deploying a built package by importing it into an IIS website and using the GUI, you are then prompted to provide the settings. This is useful when only certain members of IT are permitted to know the production connection strings or other sensitive information.

However, should you not be in an enterprise with such tight restrictions, then it might be preferable to store the various environment settings in source control with the project. The benefit of this is that there is less chance of misconfiguring a deployment with a typo. This could be achieved by using the "-setParamFile" flag when calling the cmd. However, the difficulty is that if your custom SetParameters.xml is in source control then you have to go fetch it out of the deployed package, which is already zipped up by the build step! Perhaps TC's artifacts can assist here...

This feels like a much more appropriate way to configure the deployment workflow: Always compiling the code in the most suitable configuration (Release), then dealing with the deployment configuration when the time comes to Deploy (or re-deploy). The trouble is that the steps shown above are a lot more tedious and error prone in comparison to the Configuration Transforms we are already used to. Even so, I think this is the right solution for the job.

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