Welcome to VirtuallyThatGuy – today we are going to discuss how vCenter communicates with ESXi host … when the hosts are been managed by the vCenter , either in cluster or standalone Management. First of all what is the difference between Vsphere , vCenter and ESXi host? Many at times people new to virtualisation get this confused. Well let’s discuss together and I hope you find this useful whether you’re preparing for an interview or your first vCp or just knowledge around the vmware stack.
vSphere is a software suite that has many software components like vCenter, ESXi, vSphere client and so on. So, the combination of all these software components is vSphere.
ESXi, server is the most important part of vSphere. ESXi is the virtualization server. It is type 1 hypervisor. All the virtual machines or Guest OS are installed on ESXi server. To install, manage and access those virtual servers which sit above of ESXi server.
vCenter – VMware vCenter server is a centralized management application that lets you manage virtual machines and ESXi hosts centrally. vCenter server is compulsory for enterprises to have enterprise features like vMotion, VMware High Availability, VMware Update Manager and VMware Distributed Resource Scheduler (DRS). For example, you can easily clone existing virtual machine in vCenter server.
That was pretty cool wasn’t it? Now that we know the difference let’s not digress and get back to our main subject.
How does vCenter communicate with ESXi Server?
vSphere Web Client can be used to communicate with a vCenter Server instance. Access to the ESXi host is provided by a vCenter Server agent named vpxa. The vpxa process is started on the ESXi host when it is added to the vCenter Server inventory. Vpxa communicates with the ESXi host agent named hostd. The hostd process runs on the ESXi host and it is used for managing most of the operations on the host. Most commands come from vCenter Server through hostd(e.g. virtual machine creation, migration, removal, etc…).
vpxa acts as an intermediary between the vpxd process on vCenter Server and the hostd process to relay the tasks to be performed on the host.
When you are logged in vCenter Server through vSphere Web Client, vCenter Server passes commands to the ESXi host through the vpxa process. If you are using the vSphere Client to communicate directly with the ESXi host, the communication goes directly to the hostd process.
ESXi and vCenter Communication Diagram
Very simple and clear explanation.