Amazon Web Services (AWS) remains the No. 1 infrastructure-as-a-service (IaaS) public cloud vendor, according to Gartner’s latest numbers, maintaining a massive lead over the competition in both revenue and market share. But its cloud share dominance is shrinking as Microsoft, Alibaba, and Google apply pressure.

The worldwide IaaS market grew 29.5 percent in 2017 to $23.5 billion, up from $18.2 billion in 2016. AWS remained the top vendor in 2017, followed by Microsoft, Alibaba, Google, and IBM. Those five vendors accounted for 75 percent of total IaaS revenues last year.

AWS’ revenue jumped 25 percent year over year, reaching an estimated $12.2 billion in 2017. In addition to being the largest IaaS provider, it’s the most mature, enterprise-ready vendor with the strongest track record of customer success and the most useful partner ecosystem, said Sid Nag, research director at Gartner. Its growth in 2017 was driven by customers that are migrating from traditional data centers to the public cloud as well as customers implementing digital business projects like analytics and mobility, he said.

While its revenue grew, AWS’ market share did decline slightly in 2017, from 53.7 percent in 2016, to 51.8 percent last year.

“AWS growth trajectory slowing down is not unexpected — it’s typical of a maturing market,” Nag said. “AWS is going to continue to see pressure in holding their lead given that Microsoft, especially, is coming on strong and the rest are all gaining share.”

Microsoft landed the No. 2 spot with growth of more than 98 percent on its Azure public cloud product and revenue surpassing $3.1 billion in 2017.

Alibaba once again was the third-largest provider in terms of market share, though was the second-fastest growing at 62.7 percent in 2017. Gartner says this reflects the company’s successful investment in research and development, and added that Alibaba has the financial capability to continue this trend and invest in global expansion. This gives the company potential to become an alternative to the global hyperscale cloud providers in Asia and Europe.

Google was No. 4 on the list in terms of market share and posted 56-percent growth last year. IBM placed fifth, growing almost 54 percent in 2017.