WEBVTT

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-Hello, and welcome to this lecture
on labels and selectors.

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My name is Mumshad Mannambeth.

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What do we know about labels
and selectors already?

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Labels and selectors are a standard method
to group things together.

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Say you have a set of different species,

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a user wants to be able to filter them
based on different criteria,

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such as based on their class or kind,

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if they are domestic or wild,
or say by their color.

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Not just group,
you want to be able to filter them

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based on a criteria
such as all green animals,

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or with multiple criteria,
such as everything green,

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that is also a bird.

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Whatever that classification may be,
you need the ability

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to group things together
and filter them based on your needs.

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The best way to do that is with labels.

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Labels are properties attached
to each item.

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You add properties
to each item for their class,

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kind, and color.

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Selectors help you filter these items.

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For example, when you say Class = Mammal,
we get a list of mammals.

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When you say Color = Green,
we get the green mammals.

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We see labels
and selectors used everywhere,

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such as the keywords you tag
to YouTube videos

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or blogs that help users filter
and find the right content.

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We see labels added to items
in an online store that help you add

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different kinds of filters
to view your products.

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How are labels and selectors
used in Kubernetes?

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We have created a lot
of different types of objects

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in Kubernetes,

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Pods, services, ReplicaSets,
deployments, et cetera.

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For Kubernetes,
all of these are different objects.

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Over time, you may end up
having hundreds or thousands

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of these objects in your cluster.

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Then you will need a way
to filter and view different objects

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by different categories,

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such as to group objects by their type,

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or view objects by application,
or by their functionality.

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Whatever it may be, you can group
and select objects using labels

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and selectors.

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For each object,
attach labels as per your needs,

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like app, function, et cetera.

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Then while selecting, specify
a condition to filter specific objects.

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For example, app = App1.

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How exactly do you specify labels
in Kubernetes?

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In a Pod definition file, under metadata,
create a section called labels.

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Under that, add the labels
in a key-value format like this.

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You can add as many labels as you like.

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Once the Pod is created,
to select the Pod with the labels,

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use the kube control get pod command
along with the selector option

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and specify the condition like app = App1.

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Now, this is one use case
of labels and selectors.

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Kubernetes objects use labels
and selectors internally

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to connect different objects together.

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For example, to create a ReplicaSet
consisting of three different Pods,

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we first label the Pod definition
and use selector in a ReplicaSet

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to group the Pods.

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In a ReplicaSet definition file,
you will see labels defined in two places.

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Note that this is an area
where beginners tend to make a mistake.

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The labels defined
under the template section

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are the labels configured on the Pod.

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The labels you see at the top are
the labels of the ReplicaSet itself.

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We're not really concerned
about the labels of the ReplicaSet for now

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because we're trying to get
the ReplicaSet to discover the Pods.

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The labels on the ReplicaSet
will be used if you were to configure

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some other object
to discover the ReplicaSet.

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In order to connect the ReplicaSet
to the Pod,

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we configure the selector field
under the ReplicaSet specification

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to match the labels defined on the Pod.

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A single label will do
if it matches correctly.

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However, if you feel there could be
other parts with the same label,

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but with a different function,
then you could specify both the labels

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to ensure that the right Pods
are discovered by the ReplicaSet.

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On creation, if the labels match,
the ReplicaSet is created successfully.

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It works the same
for other objects like a service.

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When a service is created.

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It uses the selector defined
in the service definition file

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to match the labels set on the Pods
in the ReplicaSet definition file.

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Finally, let's look at annotations.

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While labels and selectors are used
to group and select objects,

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annotations are used to record
other details for informatory purpose.

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For example, two details like name,
version, build information,

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et cetera, or contact details,
phone numbers,

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email IDs, et cetera,
that may be used for some kind

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of integration purpose.

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That's it for this lecture
on labels and selectors and annotations.

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Head over to the coding exercises section
and practice working with labels

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and selectors