VMware vSphere is a suite of software components for virtualization. These include ESXi, vCenter Server, and other software components that fulfill a number of different functions in the vSphere environment.

vSphere includes the following software components:


A virtualization platform that you use to create the virtual machines as a set of configuration and disk files that together perform all the functions of a physical machine.

Through ESXi, you run the virtual machines, install operating systems, run applications, and configure the virtual machines. Configuration includes identifying the virtual machine’s resources, such as storage devices.

The server provides bootstrapping, management, and other services that manage your virtual machines.

Each ESXi host has a vSphere Client available for your management use. If your ESXi host is registered with vCenter Server, a vSphere Client that accommodates vCenter Server features is available.

vCenter Server

A service that acts as a central administrator for VMware ESXi hosts that are connected on a network. vCenter Server directs actions on the virtual machines and the virtual machine hosts (the ESXi hosts).

vCenter Server is a single Windows Service and is installed to run automatically. vCenter Server runs continuously in the background. It performs its monitoring and managing activities even when no vSphere Clients are connected and when no one is logged on to the computer where it resides. It must have network access to all the hosts it manages and be available for network access from any machine where the vSphere Client is run.

You can install vCenter Server in a Windows virtual machine on an ESXi host, allowing it to take advantage of the high-availability that is provided by VMware HA. See the vSphere Installation and Setup documentation for details about setting up this configuration.

You can join multiple vCenter Server systems using Linked Mode to allow them to be managed using a single vSphere Client connection.

vCenter Single Sign On

A service that is part of the vCenter Server management infrastructure. The vCenter Single Sign On authentication service makes the VMware cloud infrastructure platform more secure by allowing the various vSphere software components to communicate with each other through a secure token exchange mechanism, instead of requiring each component to authenticate a user separately with a directory service like Active Directory.

When you install vCenter Single Sign-On, the following components are deployed.

STS (Security Token Service)

The STS service issues Security Assertion Markup Language (SAML) tokens. These security tokens pass information about a system user between an identity provider and a web service. This service enables a user who has logged on through vCenter Single Sign-On to use multiple web-service delivered applications without authenticating to each one.

Administration server

The Administration Server configures the vCenter Single Sign-On server and manages users and groups.

vCenter Lookup Service

The Lookup Service contains topology information about the vSphere infrastructure, enabling vSphere components to connect to each other securely.

RSA SSPI service

The Security Support Provider Interface is a Microsoft Windows-based API used to perform authentication against Security Support Providers such as NTLM and Kerberos.

vCenter Server plug-ins

Applications that provide additional features and functionality to vCenter Server. Typically, plug-ins consist of a server component and a client component. After the plug-in server is installed, it is registered with vCenter Server and the plug-in client is available to vSphere clients for download. After a plug-in is installed on a vSphere client, it might alter the interface by adding views, tabs, toolbar buttons, or menu options related to the added functionality.

Plug-ins leverage core vCenter Server capabilities, such as authentication and permission management, but can have their own types of events, tasks, metadata, and privileges.

Some vCenter Server features are implemented as plug-ins, and can be managed using the vSphere Client Plug-in Manager. These features include vCenter Storage Monitoring, vCenter Hardware Status, and vCenter Service Status.

vCenter Server database

A persistent storage area for maintaining the status of each virtual machine, host, and user managed in the vCenter Server environment. The vCenter Server database can be remote or local to the vCenter Server system.

The database is installed and configured during vCenter Server installation.

If you are accessing your ESXi host directly through a vSphere Client, and not through a vCenter Server system and associated vSphere Client, you do not use a vCenter Server database.

Tomcat Web server

Many vCenter Server functions are implemented as Web services that require the Tomcat Web server. The Tomcat Web server is installed on the vCenter Server machine as part of the vCenter Server installation.

Features that require the Tomcat Web server to be running include: Linked Mode, CIM/Hardware Status tab, Performance charts, WebAccess, vCenter Storage Monitoring/Storage Views tab, Storage profile services,and vCenter Service status.

vCenter Server agent

On each managed host, the software that collects, communicates, and executes the actions received from vCenter Server. The vCenter Server agent is installed the first time any host is added to the vCenter Server inventory.

Host agent

On each managed host, the software that collects, communicates, and executes the actions received through the vSphere Client. It is installed as part of the ESXi installation.


vCenter Server uses LDAP (Lightweight Directory Access Protocol) to synchronize data such as license and role information across vCenter Server systems joined in Linked Mode.