What is the difference between a pod and a deployment?

I have been creating pods with type:deployment but I see that some documentation uses type:pod, more specifically the documentation for multi-container pods:

apiVersion: v1
kind: Pod
metadata:
  name: ""
  labels:
    name: ""
  namespace: ""
  annotations: []
  generateName: ""
spec:
  ? "// See 'The spec schema' for details."
  : ~

But to create pods I can just use a deployment type:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: ""
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: ""
    spec:
      containers:
        etc

I noticed the pod documentation says:

> The create command can be used to create a pod directly, or it can > create a pod or pods through a Deployment. It is highly recommended > that you use a Deployment to create your pods. It watches for failed > pods and will start up new pods as required to maintain the specified > number. If you don’t want a Deployment to monitor your pod (e.g. your > pod is writing non-persistent data which won’t survive a restart, or > your pod is intended to be very short-lived), you can create a pod > directly with the create command. > > Note: We recommend using a Deployment to create pods. You should use > the instructions below only if you don’t want to create a Deployment.

But this raises the question of what kind:pod is good for? Can you somehow reference pods in a deployment? I didn't see a way. It looks like what you get with pods is some extra metadata but none of the deployment options such as replica or a restart policy. What good is a pod that doesn't persist data, survives a restart? I think I'd be able to create a multi-container pod with a deployment as well.

Radek's answer is very good, but I would like to pitch in from my experience, you will almost never use an object with the kind pod, because that doesn't make any sense in practice.

Because you need a deployment object - or other Kubernetes API objects like a replication controller or replicaset - that needs to keep the replicas (pods) alive (that's kind of the point of using kubernetes).

What you will use in practice for a typical application are:

1. Deployment object (where you will specify your apps container/containers) that will host your app's container with some other specifications.

2. Service object (that is like a grouping object and gives it a so-called virtual IP (cluster IP) for the pods that have a certain label - and those pods are basically the app containers that you deployed with the former deployment object).

You need to have the service object because the pods from the deployment object can be killed, scaled up and down, and you can't rely on their IP addresses because they will not be persistent.

So you need an object like a service, that gives those pods a stable IP.

Just wanted to give you some context around pods, so you know how things work together.

Hope that clears a few things for you, not long ago I was in your shoes :)

Both Pod and Deployment are full-fledged objects in the Kubernetes API. Deployment manages creating Pods by means of ReplicaSets. What it boils down to is that Deployment will create Pods with spec taken from the template. It is rather unlikely that you will ever need to create Pods directly for a production use-case.

Kubernetes has three Object Types you should know about:

• Pods - runs one or more closely related containers
• Services - sets up networking in a Kubernetes cluster
• Deployment - Maintains a set of identical pods, ensuring that they have the correct config and that the right number of them exist.

Pods:

• Runs a single set of containers
• Good for one-off dev purposes
• Rarely used directly in production

Deployment:

• Runs a set of identical pods
• Monitors the state of each pod, updating as necessary
• Good for dev
• Good for production

And I would agree with other answers, forget about Pods and just use Deployment. Why? Look at the second bullet point, it monitors the state of each pod, updating as necessary.

So, instead of struggling with error messages such as this one:

> Forbidden: pod updates may not change fields other than spec.containers[*].image

So just refactor or completely recreate your Pod into a Deployment that creates a pod to do what you need done. With Deployment you can change any piece of configuration you want to and you need not worry about seeing that error message.

Pod is container instance.

That is the output of replicas: 3

Think of one deployment can have many running instances(replica).

//deployment.yaml
apiVersion: apps/v1beta2
kind: Deployment
metadata:
  name: tomcat-deployment222
spec:
  selector:
    matchLabels:
      app: tomcat
  replicas: 3
  template:
    metadata:
      labels:
        app: tomcat
    spec:
      containers:
      - name: tomcat
        image: tomcat:9.0
        ports:
        - containerPort: 8080

I want to add some informations from Kubernetes In Action book, so you can see all picture and connect relation between Kubernetes resources like Pod, Deployment and ReplicationController(ReplicaSet)

> Pods

are the basic deployable unit in Kubernetes. But in real-world use cases, you want your deployments to stay up and running automatically and remain healthy without any manual intervention. For this the recommended approach is to use a Deployment, which under the hood create a ReplicaSet.

> A ReplicaSet, as the name implies, is a set of replicas (Pods) maintained with their Revision history.

(ReplicaSet extends an older object called ReplicationController -- which is exactly the same but without the Revision history.)

A ReplicaSet constantly monitors the list of running pods and makes sure the running number of pods matching a certain specification always matches the desired number.

Removing a pod from the scope of the ReplicationController comes in handy
when you want to perform actions on a specific pod. For example, you might 
have a bug that causes your pod to start behaving badly after a specific amount 
of time or a specific event.

> A Deployment

is a higher-level resource meant for deploying applications and updating them declaratively.

When you create a Deployment, a ReplicaSet resource is created underneath (eventually more of them). ReplicaSets replicate and manage pods, as well. When using a Deployment, the actual pods are created and managed by the Deployment’s ReplicaSets, not by the Deployment directly

Let’s think about what has happened. By changing the pod template in your Deployment resource, you’ve updated your app to a newer version—by changing a single field!

Finally, Roll back a Deployment either to the previous revision or to any earlier revision so easy with Deployment resource.

These images are from Kubernetes In Action book, too.

Pod is a collection of containers and basic object of Kuberntes. All containers of pod lie in same node.

• Not suitable for production
• No rolling updates

Deployment is a kind of controller in Kubernetes.

Controllers use a Pod Template that you provide to create the Pods for which it is responsible.

Deployment creates a ReplicaSet which in turn make sure that, CurrentReplicas is always same as desiredReplicas .

Advantages :

• You can rollout and rollback your changes using deployment

• Monitors the state of each pod

• Best suitable for production

• Supports rolling updates

Try to avoid Pods and implement Deployments instead for managing containers as objects of kind Pod will not be rescheduled (or self healed) in the event of a node failure or pod termination.

A Deployment is generally preferable because it defines a ReplicaSet to ensure that the desired number of Pods is always available and specifies a strategy to replace Pods, such as RollingUpdate.

May be this example will be helpful for beginners !!

1) Listing PODs

controlplane $ kubectl -n my-namespace get pods
NAME                            READY   STATUS    RESTARTS   AGE
mysql                           1/1     Running   0          92s
webapp-mysql-75dfdf859f-9c54j   1/1     Running   0          92s

2) Deleting web-app pode - which is created using deployment

controlplane $ kubectl -n my-namespace delete pod webapp-mysql-75dfdf859f-9c54j
pod "webapp-mysql-75dfdf859f-9c54j" deleted

3) Listing PODs ( You can see, it is recreated automatically)

controlplane $ kubectl -n my-namespace get pods
NAME                            READY   STATUS    RESTARTS   AGE
mysql                           1/1     Running   0          2m42s
webapp-mysql-75dfdf859f-mqrcx   1/1     Running   0          45s

4) Deleting mysql POD whcih is created directly ( with out deployment)

controlplane $ kubectl -n my-namespace delete pod mysql
pod "mysql" deleted

5) Listing PODs ( You can see mysql POD is lost for ever )

controlplane $ kubectl -n my-namespace get pods
NAME                            READY   STATUS    RESTARTS   AGE
webapp-mysql-75dfdf859f-mqrcx   1/1     Running   0          76s

In kubernetes Pods are the smallest deployable units. Every time when we create a kubernetes object like Deployments, replica-sets, statefulsets, daemonsets it creates pod.

As mentioned above deployments create pods based on desired state mentioned in your deployment object. So for example you want 5 replicas of a application, you mentioned replicas: 5 in your deployment manifest. Now deployment controller is responsible to create 5 identical replicas (no less, no more) of given application with all metadata like RBAC policy, networks policy, labels, annotations, health check, resource quotas, taint/tolerations and others and associate with each pods it creates.

There are some cases when you wants to create pod, for example if you are running a test sidecar where you don't need to run application forever, you don't need multiple replicas, and you run application when you wants to execute in that case pod is suitable. For example helm test, which is a pod definition that specifies a container with a given command to run.

I am also a beginner in k8s so correct me if I am wrong.

We know that a pod is created when we create a deployment. What I observed is that if you see the YAML file of the deployment, you can see its kind:deployment. But if you see the YAML file of the pod, you see its kind:pod.

In Kubernetes we can deploy our workloads using different type of API objects like Pods, Deployment, ReplicaSet, ReplicationController and StatefulSets.

Out of those Pods are the smallest deployable unit in Kubernetes. Any workload/application that runs in Kubernetes, has to run inside a container part of a Pod. A Pod could run multiple containers (meaning multiple applications) within it. A Pod is a wrapper on top of one/many running containers. Using a Pod, kubernetes could control, monitor, operate the containers.

Now using stand alone Pods we can't do lot of things. We can't change configurations, volumes inside Pods. We can't restart the Pod if one is down. So there is another API Object called Deployment comes into picture which maintains the desired state (how many instances, how much compute resource application uses) of the application. The Deployment maintaines multiple instances of same application by running multiple Pods. Deployments unlike Pods are mutable. Deployments uses another API Object called ReplicaSet to maintain the desired state. Deployments through ReplicaSet spawns another Pod if one is down.

So Pod runs applications in containers. Deployments run Pods and maintains desired state of the application.