Why do you specify a pool in a deployment job in Azure Pipelines? - azure-devops

Question

I find it hard to grasp the concept of deployment jobs and environments in Azure Pipelines. From what I understand, a deployment job is a sequence of steps to deploy your application to a specific environment, hence the environment field.
If so, why is there also a pool definition for agent pool for that job definition?
EDIT
What bothers me is that, from what I understand, an Environment is a collection of resources that you can run your application on. So you'll define some for dev, some for stage, prod, etc. So you want to run the job on these targets. So why do we need to specify an agent pool to run the deployment job on? Shouldn't it run on the resources that belong to the specified environment?
EDIT
Take this pipeline definition for example:
jobs:
# Track deployments on the environment.
- deployment: DeployWeb
displayName: deploy Web App
pool:
vmImage: 'Ubuntu-16.04'
# Creates an environment if it doesn't exist.
environment:
name: 'Stage'
resourceType: VirtualMachine
strategy:
# Default deployment strategy, more coming...
runOnce:
preDeploy:
steps:
- script: echo "Hello"
deploy:
steps:
- checkout: self
- script: echo my first deployment
I have an environment called "Stage" with one virtual machine on it.
When I run it, I can see both jobs run on my VM
The agent pool specified is NOT USED at all.
However, if I target another environment with no machines on it, it will run on Azure Pipelines vm

Answer 1

Why do you specify a pool in a deployment job in Azure Pipelines?
That because the environment is a collection of resources that you can target with deployments from a pipeline.
In other words, it is like the machine that hosts our private agent, but it can now be a virtual environment, like K8s, VM and so on.
When we specify an environment, it only provide us one virtual environment(You can think of it as a machine). However, there is no agent installed on these virtual environments for us to run the pipeline, so we need to specify an agent to run the pipeline.
For example, we execute our pipeline in our local machine, we still need to create our private agent, otherwise, we only have the target environment, but there is no agent that hosts the pipeline running.

Answer 2

The environment field denotes the target environment to where your artifact is deployed. There are commonly multiple environments like through which the artifacts flow, for example development -> test -> production. Azure DevOps uses this field to keep track of what versions are deployed to what environment etc, from the docs:
An environment is a collection of resources that you can target with
deployments from a pipeline. Typical examples of environment names are
Dev, Test, QA, Staging, and Production.
The pool is a reference to the agent pool. The agent is the machine executing the logic inside the job. For example, a deployment job might have several logical steps, such as scripts, file copying etc. All this logic is executed on the agent that comes from the agent pool. From the docs:
To build your code or deploy your software using Azure Pipelines, you
need at least one agent. As you add more code and people, you'll
eventually need more.
When your pipeline runs, the system begins one or more jobs. An agent
is computing infrastructure with installed agent software that runs
one job at a time.