Cal Henderson
Born 1981 · Age 44
British computer programmer and author; co‑founder & CTO of Slack; early Flickr engineer; author of Building Scalable Web Sites. Based in San Francisco.
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Life & Career Timeline
Born in the United Kingdom
Callum James Henderson-Begg (known as Cal Henderson) was born.
Early programming interest (first programs)
As a child/teen he began programming (Basic/text-adventure games) after his cousin introduced him to a home computer; the mid‑1990s web inspired him to publish software online.
Completed secondary schooling at Sharnbrook
Attended Sharnbrook Upper School and Community College (dates not explicitly stated; estimated completion around age 18).
Began playing Ludicorp's Game Neverending
Played the MMORPG Game Neverending and snuck onto Ludicorp's internal mailing list, starting his connection to Stewart Butterfield and Ludicorp.
Graduated Birmingham City University (Software Engineering)
Graduated with a degree in software engineering from Birmingham City University.
Hired by Ludicorp / first Flickr employee
Pestered Stewart Butterfield and was given a job at Ludicorp; became the first person ever employed to work on Flickr.
Co-owner / developer of B3ta (online creative community)
Co‑owning and developing the online creative community B3ta alongside Denise Wilton and Rob Manuel (B3ta referenced in sources; active in early 2000s).
City Creator featured by Yahoo! picks
City Creator (one of Henderson's projects) was picked/featured by Yahoo! (Yahoo! picks for September 16, 2003).
Pioneered web APIs (oEmbed / OAuth basis)
Contributed to early web APIs and practices used by platforms such as YouTube and Twitter (oEmbed / OAuth roots attributed to his Flickr-era work).
Flickr acquired by Yahoo!
Yahoo! purchased Flickr (reported around $25 million). Henderson had led/build the engineering team through the acquisition.
Published 'Building Scalable Web Sites' (O'Reilly)
Authored the best‑selling technical book Building Scalable Web Sites for O'Reilly Media (published 2006).
Ran small utility sites and open‑source contributions (ongoing)
Launched and ran utility websites (e.g., Unicodey) and regularly contributed to open‑source projects to make programming tasks easier.
Worked for EMAP as Technical Director (special web projects)
Served as technical director of special web projects for EMAP (exact years not specified; added here as an approximate placement in career timeline).
Co‑founded Tiny Speck
Left Yahoo! with Stewart Butterfield to co‑found Tiny Speck and build an MMORPG (Glitch).
Tiny Speck: engineering and internal tooling work
Tiny Speck developed its game Glitch while also building internal tools; Henderson was VP of engineering; internal Slack-like tool began as a Tiny Speck communication tool.
Glitch project closed / Tiny Speck pivoted
After about three years, the Glitch game failed; the team pivoted from the game to focus on the internal communication tool which became Slack.
Co‑founded Slack (company pivot/name change)
Tiny Speck's internal communication tool was turned into a product and the company rebranded/was co‑founded as Slack (BCU and other sources cite 2013 as co‑founding year).
Early engineering rituals and culture-building at Slack
Introduced engineering practices and rituals (e.g., focused working times, social rituals) that shaped Slack's culture as it grew (timing evolved during early growth years).
Recipient: Birmingham City University Alumni of the Year
Recognised by his alma mater as Alumni of the Year (BCU award cited in 2015 coverage).
Slack valuation reported around $3 billion
Coverage cited Slack as a US company valued at approximately $3 billion (BCU news item referencing Slack's value at that time).
Caloween tradition observed at Slack (employees dress as Cal)
Slack engineers celebrated 'Caloween' where employees dressed like Cal Henderson — tradition visible in photos from around 2015–2017.
Recognised by Birmingham City University (profiled)
BCU later highlighted Henderson's achievements and continued relationship with the university; he was profiled and lauded as an alumnus (ongoing engagement).
Scaled engineering practices: 'Maker Time' and career paths
As CTO, instituted practices such as Maker Time (no‑meeting coding blocks), Career Paths docs, and peer recognition programs to scale engineering effectiveness.
Parenthood (younger child born, estimated)
Index Ventures article (2021) states Henderson has two children ages ~3 and ~7; approximate birth years inferred (one child born ~2018).
Slack direct listing / NYSE debut & valuation > $17B
Slack made its debut on the New York Stock Exchange with an estimated valuation of over $17 billion; BCU referenced >10 million users at this time.
Awarded Honorary Doctorate from Birmingham City University
BCU awarded Henderson an Honorary Doctorate in recognition of his contribution to technology; he collected the accolade during a ceremony at Symphony Hall.
Conducted pandemic-era company AMAs and remote culture practices
During the COVID-19 pandemic Henderson scheduled regular online AMAs and shared personal posts to stay visible and accessible as an engineering leader.
Scaled DEI programs: 'Elevate' sponsorship model
Created/allocated resources for the Elevate Sponsorship program to support women and nonbinary employees by creating circles of support rather than one‑on‑one sponsorship.
Slack acquired by Salesforce (company-level event referenced)
Index Ventures profile (Sept 2021) references that Slack had recently been acquired by Salesforce and that the companies were moving forward together (acquisition occurred in 2021).
Index Ventures profile published (engineering leadership profile)
Index Ventures published a detailed profile on Cal Henderson as an engineering leader (contains many quotes and cultural/practice descriptions).
Children referenced (older child ~7, younger ~3)
Index article notes Henderson has two children ages 7 and 3 as of the article date (used to infer approximate birth years of ~2014 and ~2018).
Ongoing contributions to open source & side projects
Continues to run utility sites (Unicodey etc.) and contribute to open-source projects; many small sites exist that he spins up to test ideas.
Political donation to The Next 50 PAC
Contributed $50,000 to The Next 50, a liberal political action committee (reported August 2022).
Public recognition and legacy as engineering leader
Continuing recognition in tech press for his role in building Flickr infrastructure, authoring a popular scaling book, and co‑founding Slack; ongoing advisory/mentor relationships referenced by investors.
Key Achievement Ages
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