Azure DevOps Hidden Gems #4 – Understand Build Agents by Installing One Locally on Your Development Machine
I've been working with Azure DevOps quite a lot recently (having used its predecessors for many years) and I'm constantly amazed by features I never knew existed or which I vaguely knew existed but hadn't fully appreciated. In this blog post series I'm attempting to shine a light on some of these hidden gems for the benefit of others. The full list of posts is here and if you have any suggestions for other posts please leave a comment!
If you've ever examined the logs generated by the agents in Azure Pipelines that do all the actual work you might have puzzled over what exactly is going on behind the scenes as your code is built and deployed. I know I have! We can see from build and release tasks that there are variables such as $(Build.ArtifactStagingDirectory) and (System.DefaultWorkingDirectory) that point to folders where things are happening and that it's all taking place in a folder hierarchy with seemingly cryptic folder names such as D:\a\1\s orD:\a\r1\a. But what exactly is happening in all these different folders?
If you are using Microsoft-hosted agents then they are pretty much black boxes and there is no way to peel back the covers and see what's going on. You can though output a list of all the variables and their values—see below. If you are using Self-hosted agents and you have appropriate permissions to remote to the server then you might have better luck in being able to see what's going on but if your server is a critical part of your build and release process or maybe it's a headless server or perhaps the agent is running in a docker container, then maybe it's not a good idea to go poking around or perhaps there are extra hurdles that you don't want to contend with.
A simple answer to this is to install an agent on your local machine. You can then play around to your heart's content safe in the knowledge that you have full visibility of what's happening and that you won't break a critical system. The process is pretty straightforward as follows:
- Create a dedicated Agent Pool in Azure DevOps at Organization Settings > Pipelines > Agent Pools > New agent pool.
- From the same location download the agent for your OS.
- Create a folder (such as c:\build-agent if you are on Windows) and unzip the contents of the agent download to this folder.
- Follow the instructions for configuring the agent which are available for Linux, macOS and Windows. Don't forget to choose the Agent Pool you created earlier and run the agent as a service as recommended.
Those steps are all that's required to get an agent up-and-running. Next up is to start using the agent to build and perhaps release an application. Chances are that you have a test project handy but if not it's quick to create one and get it configured for build—I usually create an ASP.NET Core application. I won't go through that process here except to say that whether you use something that already exists or you create something from scratch obviously you need to configure your build (and release) to use the Agent Pool you created earlier.
You will also need the tools that are used to build (and release) your application installed locally on your workstation. In the .NET world, if you have Visual Studio installed then you've probably got everything you need for a simple demo application. However if you are using any specialist tools such as Selenium for automated tests then there will be more to do. Exactly what is obviously tool specific, but you can get an idea from the Microsoft-hosted agents. For example, if you need chromedriver.exe then by looking at the Details tab of one of the hosted agents you can see that the path of chromedriver.exe is set by an environment variable called ChromeWebDriver:
In this case all you need to do is create a folder, copy chromedriver.exe to the folder and create a system environment variable to point to the folder. (You might have to reboot for the new variable to be recognised.)
With build (and perhaps release) configured you can now poke around in the folder structure of your agent to see exactly what is happening and where. A great diagnostic tip for any build or release is to output all the environment variables and their values to the logs. On Windows simply create a command line task and have it execute cmd /k set. On Linux use printenv | sort with a Bash script. I use this technique as a standard component of builds and releases and if you are using Microsoft-hosted agents printenv | sort works universally as presumably on Windows agents there is some sort of PowerShell alias at work.
Hope this helps!
Cheers -- Graham