After being announced in 2015, Microsoft's Azure Stack—which offers a wide range of Azure services for on private, on-premises hardware—is now available.

Azure Stack is positioned as a major part of Microsoft's hybrid cloud offering. It offers the same management tools, straightforward provisioning, and usage-based licensing as the public Azure cloud, but it runs on premises. This makes Stack suitable for organizations with security, privacy, regulatory compliance, or legacy integration constraints that preclude the use of the public cloud.

When announced, Microsoft's intent was to enable organizations to build private Azure clouds on any suitable hardware. It had an initial release date in 2016, and this would have made Azure Stack a direct competitor and alternative to OpenStack. Last July, those plans were changed as Microsoft switched to an appliance model and a 2017 release date. Rather than constructing their own infrastructure, Azure Stack customers must now buy specific hardware from select Microsoft hardware partners, with Dell EMC, HPE, and Lenovo all having systems available to order today. Later in the year, those companies will be joined by Cisco and Huawei. Shipments will start in September.

These appliances won't come cheap: HPE says that, depending on configuration, its systems will start at $300,000 to 400,000.

In switching to this appliance model, Microsoft is actually returning to a much older plan. Back in 2010, the company announced the Windows Azure Appliance, a kind of cloud-in-a-box. Like Azure Stack, this would use prebuilt hardware from specific partners, with Dell, HP, and Fujitsu promising to ship the appliances. Initially these were to be housed in data centers owned by the respective hardware companies, with self-hosting in private data centers coming later.

The full Azure Appliance promise was never delivered. A similar but scaled back offering, the Cloud Platform System, was made available in 2014. This used Dell hardware exclusively.

Today's Azure Stack is closer in spirit to the original Azure Appliance plan. So though the name has changed, Azure Stack is, more or less, the Azure Appliance.