The global market for public cloud services will be worth $302 billion by 2021, when just 10 companies will command almost three-quarters of the market’s sales, up from 50 percent in 2016.

That’s according to Gartner Inc.’s updated public cloud revenue forecast, which it released today. Gartner breaks down the cloud market into five distinct segments and said all of them will expand in the next four years.

The fastest-growing of these will be the familiar Cloud System Infrastructure Services segment, which is where industry leader Amazon Web Services Inc. has found its large and growing niche. This segment will grow by 36 percent in 2018 to become a $40.8 billion market, rising to $83.5 billion by 2021.

That’s an extremely healthy forecast and one that should have Amazon and its rivals Microsoft Corp. and Google LLC rubbing their hands with glee. However, Gartner warned that the increasing dominance of these companies in the infrastructure-as-a-service market could present challenges for the companies that use their services. It all comes down to the threat of “vendor lock-in,” the potential for companies to get tied to the bulk of one provider’s services because it’s expensive and cumbersome to move large amounts of data.

“While it enables efficiencies and cost benefits, organizations need to be cautious about about IaaS providers potentially gaining unchecked influence over customers and the market,” warned Sid Nag, a research director at Gartner.

Nag said organizations will most likely heed his warning and “demand a simpler way to move workloads, applications and data across cloud providers’ IaaS offerings without penalties.” Whether organizations will get their wish remains to be seen, but a number of startups including Velostrata Inc. have already identified this opportunity and are attempting to build low-cost cloud migration services that can meet this growing need.

The largest and fastest-growing segment in Gartner’s forecast is software as a service, which will rise 22.2 percent to hit $73.6 billion by the end of this year. Further, the SaaS segment will grow to account for 45 percent of all application software spending by 2021, when it’s forecast to become a $117.1 billion market.

Gartner also singled out the database platform as a service category for special attention, since it will be the fastest-growing niche within the wider PaaS segment. The market for dbPaaS will hit $10 billion by 2021, and once again Gartner warned that hyperscale cloud providers will be looking for opportunities here to lock in their customers.

“Although these large vendors have different strengths, and customers generally feel comfortable that they will be able to meet their current and future needs, other dbPaaS offerings may be good choices for organizations looking to avoid lock-in,” said Nag.

The PaaS segment as a whole will hit $27.3 billion by 2012, Gartner said.

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