Private, open source Slack competitor Mattermost today closed a $20 million funding round to help members of its community create new plugins and integrations.

The news comes a day after Slack shared plans to go public and workplace collaboration app Coda launched out of beta.

Mattermost has shaped itself in Slack’s image. Like that popular real-time workplace messaging app, Mattermost began as a provider of communications for gamers, CEO Ian Tien told VentureBeat in a phone interview, but was later sold as a team communications service to enterprise customers.

Mattermost looks a lot like Slack, uses similar keyboard shortcuts, and has a range of open source automated bots made by other users that can complete tasks — just like Slack App Store offerings from SaaS and productivity providers.

Like Slack, Mattermost also sees a road to success leading through developers within companies.

“We think that the way that this experience gets to enterprises in the proper way is going to be through open-source, and it’s going to be through the DevOps communities in those enterprises,” Tien said.

Unlike Slack, Mattermost is for internal company use only and is not made for collaboration across teams. (Slack added shared channels in fall 2017.)

Also unlike Slack, Mattermost was created by an open source software project, allows access to source code for customization, and can sync with existing company chat systems.

The open source version of Mattermost is free to download, while the more feature-rich options — like corporate directory integration — are part of the paid commercial license version.

While declining to share the number of users from organizations using its chat app, Mattermost said it sees 10,000 server downloads a month. Customers include teams at Uber, Samsung, and the Department of Defense.

More than 1,000 developers have contributed to the open source project, Tien said.

This $20 million funding round was led by Redpoint Ventures, with participation from Y Combinator and S28 Capital. Mattermost has raised $23.5 million to date.

Mattermost was established in November 2015. It is based in Palo Alto, California and currently has about 50 employees.

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