ROOT vs Dagster

ROOT maintains an unusual number of parallel branches — patches for 6.26, 6.28, 6.30, 6.32, 6.36 and 6.38 all landed within weeks of each other in June 2026 — because CERN experiments pin to versions for years at a time. The 6.40 LTS released in May is where the actual work is: an opt-out from automatic object registration, experimental multithreaded histograms, a new RFile interface, an libcurl-based HTTP I/O layer with S3 support, and a rebuilt ML data loader. June's patch wave is the routine consequence, closing out the 6.26 and 6.38 series while fixing the first regressions in 6.40.

The ROOT 7 redesign is arriving piecemeal under ROOT::Experimental rather than as a version bump: RNTuple replacing TTree, RDataFrame replacing TTree::Draw, RFile replacing TFile, and now the ability to switch off the global-directory ownership model that has defined ROOT's ergonomics for decades. The team is easing migration with documentation rather than deprecation — 6.40 ships a TTree::Draw-to-RDataFrame translation guide instead of a removal notice. Build weight is being cut in the same pass, with bundled builtin packages reduced roughly fourfold in favour of system libraries.

Expect further 6.40.x patches through the year as experiments migrate onto the LTS, concentrated in RNTuple and the new histogram paths where the code is youngest. The experimental namespace is the thing to watch: features that stabilize there are the clearest available signal of what a ROOT 7 would actually contain.