Before performing your migration to OpenShift Container Platform 4.2, it is important to take the time to properly plan for the transition. OpenShift Container Platform 4 introduces architectural changes and enhancements, so the procedures that you used to manage your OpenShift Container Platform 3 cluster might not apply for OpenShift Container Platform 4.

This planning document assumes that you are transitioning from OpenShift Container Platform 3.11 to OpenShift Container Platform 4.2.

This document provides high-level information on the most important differences between OpenShift Container Platform 3 and OpenShift Container Platform 4 and the most noteworthy migration considerations. For detailed information on configuring your OpenShift Container Platform 4 cluster, review the appropriate sections of the OpenShift Container Platform documentation. For detailed information on new features and other notable technical changes, review the OpenShift Container Platform 4.2 release notes.

It is not possible to upgrade your existing OpenShift Container Platform 3 cluster to OpenShift Container Platform 4. You must start with a new OpenShift Container Platform 4 installation. Tools are available to assist in migrating your control plane settings and application workloads.

Comparing OpenShift Container Platform 3 and OpenShift Container Platform 4

With OpenShift Container Platform 3, administrators individually deployed Red Hat Enterprise Linux (RHEL) hosts, and then installed OpenShift Container Platform on top of these hosts to form a cluster. Administrators were responsible for properly configuring these hosts and performing updates.

OpenShift Container Platform 4 represents a significant change in the way that OpenShift Container Platform clusters are deployed and managed. OpenShift Container Platform 4 includes new technologies and functionality, such as Operators, MachineSets, and Red Hat Enterprise Linux CoreOS (RHCOS), which are core to the operation of the cluster. This technology shift enables clusters to self-manage some functions previously performed by administrators. This also ensures platform stability and consistency, and simplifies installation and scaling.

For more information, see OpenShift Container Platform architecture.

Architecture differences

Immutable infrastructure

OpenShift Container Platform 4 uses Red Hat Enterprise Linux CoreOS (RHCOS), which is designed to run containerized applications, and provides efficient installation, Operator-based management, and simplified upgrades. RHCOS is an immutable container host, rather than a customizable operating system like RHEL. RHCOS enables OpenShift Container Platform 4 to manage and automate the deployment of the underlying container host. RHCOS is a part of OpenShift Container Platform, which means that everything runs inside a container and is deployed using OpenShift Container Platform.

In OpenShift Container Platform 4, control plane nodes must run RHCOS, ensuring that full-stack automation is maintained for the control plane. This makes rolling out updates and upgrades a much easier process than in OpenShift Container Platform 3.

For more information, see Red Hat Enterprise Linux CoreOS.

Operators

Operators are a method of packaging, deploying, and managing a Kubernetes application. Operators ease the operational complexity of running another piece of software. They watch over your environment and use the current state to make decisions in real time. Advanced Operators are designed to upgrade and react to failures automatically.

For more information, see Understanding Operators.

Installation and update differences

Installation process

To install OpenShift Container Platform 3.11, you prepared your Red Hat Enterprise Linux (RHEL) hosts, set all of the configuration values your cluster needed, and then ran an Ansible playbook to install and set up your cluster.

In OpenShift Container Platform 4.2, you use the OpenShift installation program to create a minimum set of resources required for a cluster. Once the cluster is running, you use Operators to further configure your cluster and to install new services. After first boot, Red Hat Enterprise Linux CoreOS (RHCOS) systems are managed by the Machine Config Operator (MCO) that runs in the OpenShift Container Platform cluster.

For more information, see Installation process.

If you want to add RHEL worker machines to your OpenShift Container Platform 4.2 cluster, you use an Ansible playbook to join the RHEL worker machines after the cluster is running. For more information, see Adding RHEL compute machines to an OpenShift Container Platform cluster.

Infrastructure options

In OpenShift Container Platform 3.11, you installed your cluster on infrastructure that you prepared and maintained. In addition to providing your own infrastructure, OpenShift Container Platform 4 offers an option to deploy a cluster on infrastructure that the OpenShift Container Platform installation program provisions and the cluster maintains.

Upgrading your cluster

In OpenShift Container Platform 3.11, you upgraded your cluster by running Ansible playbooks. In OpenShift Container Platform 4.2, The cluster manages its own updates, including updates to Red Hat Enterprise Linux CoreOS (RHCOS) on cluster nodes. You can easily upgrade your cluster by using the web console or by using the oc adm upgrade command from the OpenShift CLI and the Operators will automatically upgrade themselves. If your OpenShift Container Platform 4.2 cluster has Red Hat Enterprise Linux worker machines, then you will still need to run an Ansible playbook to upgrade those worker machines.

For more information, see Updating clusters.

Migration considerations

Review the changes and other considerations that might affect your transition from OpenShift Container Platform 3.11 to OpenShift Container Platform 4.

Storage considerations

Review the following storage changes to consider when transitioning from OpenShift Container Platform 3.11 to OpenShift Container Platform 4.2.

Local volume persistent storage

Local storage is only supported by using the Local Storage Operator in OpenShift Container Platform 4.2. It is not supported to use the local provisioner method from OpenShift Container Platform 3.11.

For more information, see Persistent storage using local volumes.

Container Storage Interface (CSI) persistent storage

Persistent storage using the Container Storage Interface (CSI) was Technology Preview in OpenShift Container Platform 3.11. CSI version 1.1.0 is fully supported in OpenShift Container Platform 4.2, but does not ship with any CSI drivers. You must install your own driver.

Unsupported persistent storage options

Support for the following persistent storage options from OpenShift Container Platform 3.11 has changed in OpenShift Container Platform 4.2:

  • GlusterFS is no longer supported.

  • CephFS is no longer supported.

  • Ceph RBD is no longer supported.

If you used of one these in OpenShift Container Platform 3.11, you must choose a different persistent storage option for full support in OpenShift Container Platform 4.2.

For more information, see Understanding persistent storage.

Networking considerations

Review the following networking changes to consider when transitioning from OpenShift Container Platform 3.11 to OpenShift Container Platform 4.2.

Network isolation mode

The default network isolation mode for OpenShift Container Platform 3.11 was ovs-subnet, though users frequently switched to use ovn-multitenant. The default network isolation mode for OpenShift Container Platform 4.2 is now NetworkPolicy.

If your OpenShift Container Platform 3.11 cluster used the ovs-subnet or ovs-multitenant mode, it is recommended to switch to the NetworkPolicy mode for your OpenShift Container Platform 4.2 cluster. NetworkPolicy is supported upstream, is more flexible, and also provides the functionality that ovs-multitenant does. If you want to maintain the ovs-multitenant behavior while using NetworkPolicy in OpenShift Container Platform 4.2, follow the steps to configure multitenant isolation using NetworkPolicy.

For more information, see About network policy.

Encrypting traffic between hosts

In OpenShift Container Platform 3.11, you could use IPsec to encrypt traffic between hosts. OpenShift Container Platform 4.2 does not support IPsec. It is recommended to use Red Hat OpenShift Service Mesh to enable mutual TLS between services.

Logging considerations

Review the following logging changes to consider when transitioning from OpenShift Container Platform 3.11 to OpenShift Container Platform 4.2.

Deploying cluster logging

OpenShift Container Platform 4 provides a simple deployment mechanism for cluster logging, by using a Cluster Logging custom resource. Once deployed, the cluster logging experience is the same as it was in OpenShift Container Platform 3.11.

Aggregated logging data

You cannot transition your aggregate logging data from OpenShift Container Platform 3.11 into your new OpenShift Container Platform 4 cluster.

For more information, see About cluster logging.

Security considerations

Review the following security changes to consider when transitioning from OpenShift Container Platform 3.11 to OpenShift Container Platform 4.2.

Unauthenticated access to discovery endpoints

In OpenShift Container Platform 3.11, an unauthenticated user could access the discovery endpoints (for example, /api/* and /apis/*). For security reasons, unauthenticated access to the discovery endpoints is no longer allowed in OpenShift Container Platform 4.2. If you do need to allow unauthenticated access, you can configure the RBAC settings as necessary; however, be sure to consider the security implications as this can expose internal cluster components to the external network.

Identity providers

Configuration for identity providers has changed for OpenShift Container Platform 4, including the following notable changes:

  • The request header identity provider in OpenShift Container Platform 4.2 requires mutual TLS, where in OpenShift Container Platform 3.11 it did not.

  • The configuration of the OpenID Connect identity provider was simplified in OpenShift Container Platform 4.2. It now obtains data, which previously had to specified in OpenShift Container Platform 3.11, from the provider’s /.well-known/openid-configuration endpoint.

Monitoring considerations

Review the following monitoring changes to consider when transitioning from OpenShift Container Platform 3.11 to OpenShift Container Platform 4.2.

Alert for monitoring infrastructure availability

The default alert that triggers to ensure the availability of the monitoring structure was called DeadMansSwitch in OpenShift Container Platform 3.11. This was renamed to Watchdog in OpenShift Container Platform 4. If you had PagerDuty integration set up with this alert in OpenShift Container Platform 3.11, you must set up the PagerDuty integration for the Watchdog alert in OpenShift Container Platform 4.

For more information, see Applying custom Alertmanager configuration.