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OSSM-4815: Document HA for a mesh
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‎install/ossm-installing-openshift-service-mesh.adoc

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* link:https://docs.redhat.com/en/documentation/openshift_container_platform/latest/html/networking/configuring-ingress-cluster-traffic[Configuring ingress cluster traffic]
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include::modules/ossm-customizing-istio-configuration.adoc[leveloffset=+1]
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include::modules/ossm-about-istio-high-availability.adoc[leveloffset=+1]
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include::modules/ossm-configuring-istio-ha-autoscaling.adoc[leveloffset=+2]
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include::modules/ossm-configuring-istio-ha-replicacount.adoc[leveloffset=+2]

‎modules/ossm-about-istio-high-availability.adoc

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// This module is used in the following assemblies:
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// * service-mesh-docs-main/install/ossm-installing-service-mesh.adoc
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:_mod-docs-content-type: CONCEPT
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[id="ossm-about-istio-high-availability_{context}"]
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= About Istio High Availability
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Running the {istio} control plane in High Availability (HA) mode prevents single points of failure, and ensures continuous mesh operation even if an `istiod` pod fails. By using HA, if one `istiod` pod becomes unavailable, another one continues to manage and configure the {istio} control plane, preventing service outages or disruptions. HA provides scalability by distributing the control plane workload, enables graceful upgrades, supports disaster recovery operations, and protects against zone-wide mesh outages.
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There are two ways for a system administrator to configure HA: by defining replica count or by using autoscaling.

‎modules/ossm-configuring-istio-ha-autoscaling.adoc

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// This procedure is used in the following assembly:
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// * service-mesh-docs-main/install/ossm-installing-service-mesh.adoc
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:_mod-docs-content-type: PROCEDURE
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[id="ossm-configuring-istio-ha-autoscaling_{context}"]
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= Configuring Istio HA by using autoscaling
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Configure the {istio} control plane in High Availability (HA) mode to prevent a single point of failure, and ensure continuous mesh operation even if one of the `istiod` pods fails. Autoscaling defines the minimum and maximum number of {istio} control plane pods that can operate. {ocp-product-title} uses these values to scale the number of control planes in operation in response to the varying number of workloads in the mesh.
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.Prerequisites
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* You are logged in to the {ocp-product-title} web console as a user with the `cluster-admin` role.
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* You installed the {SMProductName} Operator.
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* You deployed the {istio} resource.
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.Procedure
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. Obtain the name of the {istio} resource by running the following command:
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[source,terminal]
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----
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$ oc get istio -n istio-system
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----
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.Example output
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[source,terminal]
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----
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NAME REVISIONS READY IN USE ACTIVE REVISION STATUS VERSION AGE
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default 1 1 0 default Healthy v1.24.6 24m
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----
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The name of the {istio} resource is `default`.
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. Update the {istio} custom resource by adding the `autoscaleMin` and `autoscaleMax` parameters by running the following command:
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[source,terminal]
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----
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$ oc patch istio default -n istio-system --type merge -p '
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spec:
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values:
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pilot:
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autoscaleMin: 2 <1>
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autoscaleMax: 5 <2>
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'
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----
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<1> Defines the minimum number of {istio} control plane replicas that always run.
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<2> Defines the maximum number of {istio} control plane replicas, allowing for scaling based on load. To support HA, there must be at least two replicas.
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.Verification
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. Verify the status of the {Istio} control pods by running the following command:
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[source,terminal]
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----
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$ oc get pods -n istio-system -l app=istiod
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----
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.Example output
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[source,terminal]
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----
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NAME READY STATUS RESTARTS AGE
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istiod-7c7b6564c9-nwhsg 1/1 Running 0 70s
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istiod-7c7b6564c9-xkmsl 1/1 Running 0 85s
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----
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Two `istiod` pods are running, which indicates HA was successfully configured.

‎modules/ossm-configuring-istio-ha-replicacount.adoc

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// This procedure is used in the following assembly:
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// * service-mesh-docs-main/install/ossm-installing-service-mesh.adoc
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:_mod-docs-content-type: PROCEDURE
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[id="ossm-configuring-istio-ha-replicacount_{context}"]
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= Configuring Istio HA by using replica count
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Configure the {istio} control plane in High Availability (HA) mode to prevent a single point of failure, and ensure continuous mesh operation even if one of the `istiod` pods fails. The replica count defines a fixed number of {istio} control plane pods that can operate. Use replica count for mesh environments in which the number of workloads does not scale.
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.Prerequisites
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* You are logged in to the {ocp-product-title} web console as a user with the `cluster-admin` role.
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* You installed the {SMProductName} Operator.
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* You deployed the {istio} resource.
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.Procedure
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. Obtain the name of the {istio} resource by running the following command:
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[source,terminal]
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----
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$ oc get istio -n istio-sytem
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----
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.Example output
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[source,terminal]
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----
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NAME REVISIONS READY IN USE ACTIVE REVISION STATUS VERSION AGE
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default 1 1 0 default Healthy v1.24.6 24m
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----
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The name of the {istio} resource is `default`.
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. Update the {istio} custom resource by adding the `autoscaleEnabled` and `replicaCount` parameters by running the following command:
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[source,terminal]
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----
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$ oc patch istio default -n istio-system --type merge -p '
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spec:
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values:
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pilot:
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autoscaleEnabled: false <1>
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replicaCount: 2 <2>
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'
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----
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<1> Disables autoscaling and ensures that the number of replicas remains fixed.
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<2> Defines the number of {istio} control plane replicas. To support HA, there must be at least two replicas.
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.Verification
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. Verify the status of the {Istio} control pods by running the following command:
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[source,terminal]
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----
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$ oc get pods -n istio-system -l app=istiod
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----
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.Example output
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[source,terminal]
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----
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NAME READY STATUS RESTARTS AGE
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istiod-7c7b6564c9-nwhsg 1/1 Running 0 70s
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istiod-7c7b6564c9-xkmsl 1/1 Running 0 85s
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----
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Two `istiod` pods are running, which indicates HA was successfully configured.

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