Enterprise applications are becoming so complex that more than one VM may be required to service the entire application....
In other cases, applications are becoming more reliant on one another. These scenarios present serious management implications for VM startup, placement, management and protection.
VMware has responded to these challenges by developing the idea of a virtual appliance (vApp), which collects multiple VMs into a single logical entity to provide a service, while allowing all of the constituent VMs to be managed together. VMware vSphere users can create and provision vApps in the vSphere Client or Web client.
A common application of vApp might be for enterprise email. A product such as Microsoft Exchange 2013 relies on multiple servers, such as client access servers (CAS) and database availability group (DAG) servers or mailbox servers where each server can be instanced in a VM. Rather than managing each CAS array and DAG server separately, they might be grouped as a virtual email appliance through vApp, which can manage and protect all of the related VMs as a single, logical entity.
So vApps allow better management of complex or inter-related VMs. For example, a vApp not only lists the VMs involved, but stipulates where each VM can be placed, the required startup sequence involved, and other details. In addition, vApps can be included in high-availability clusters and protected through DR tools. VMware vApps also support distributed resource scheduler (DRS), allowing vApps to be migrated and powered on or off as usage demands dictate.
Virtual appliances can be imported and exported, but virtual appliance packages are imported from -- or exported to -- industry-recognized standards, including the Distributed Management Task Force (DMTF), Open Virtualization Format (OVF) or Open Virtual Appliance (OVA) formats. The benefit of these formats is that VMs packaged into virtual appliances can be exchanged across virtualization platforms, allowing vApps to function under other hypervisors.
The choice between OVF and OVA formats depends on flexibility. OVF produces a series of uncompressed files that are flexible and quicker to download individually, while OVA produces a single large file that is easier to use but takes longer to move. The native distribution format for VMware vApps is OVF.
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