Open Source Infrastructure: Version Control Systems Jim Blandy [email protected] Version Control Systems ● SCCS ● RCS ● CVS ● network-transparent CVS ● Subversion ● BitKeeper ● Git ● Mercurial ● Darcs SCCS ● Mark Rochkind, 1972 ● Included in early Unix systems from AT&T ● Managed single files ● “Interleaved” delta format SCCS: Interleaved deltas A int main() { puts (“a”); } SCCS: Interleaved deltas B A int main() int main() { { puts (“b”); puts (“a”); puts (“a”); } } SCCS: Interleaved deltas B A B' int main() int main() int main() { { { puts (“b”); puts (“a”); puts (“a”); puts (“a”); } puts (“b' ”); } } SCCS: Interleaved deltas +A int main() +A { +B puts (“b”); +A puts (“a”); +B' puts (“b' ”); +A } RCS ● Walter Tichy, 1980's ● designed to address SCCS performance issues RCS Creator:inkscape 0.45.1 RCS Creator:inkscape 0.45.1 CVS ● Dick Grune et. al., ~1988 ● full directory trees, not single files ● copy/modify/merge/commit model CVS Creator:inkscape 0.45.1 CVS Creator:inkscape 0.45.1 CVS Creator:inkscape 0.45.1 Network-transparent CVS ● Jim Kingdon, 1990 (Cygnus Solutions) Subversion ● Blandy, Collins-Sussman, Fitzpatrick, Fogel 2000 ● Retains CVS model ● Better performance ● Better features ● Better architecture Subversion Subversion Creator:inkscape 0.45.1 Subversion Creator:inkscape 0.45.1 Subversion Creator:inkscape 0.45.1 BitKeeper ● BitMover (McVoy), 1998 ● Distributed ● Uses SCCS weave file format ● Used for Linux kernel collaboration until 2005 Hash naming Hash naming 76 ● Is 2 a big number? Hash naming 76 22 ● Is 2 a big number? Roughly = 7.5 * 10 Hash naming 32 ● Roughly 2 people (for now) 32 ● generating 2 strings per year 20 ● for 2 years 84 ● makes 2 strings generated in the likely history of the species Hash naming 160 ● SHA1 has 2 different hash values 84 160 76 ● 2 /2 means we'll hit 1/2 of the hash values Git ● Linus Torvalds, 2005 ● Influenced by BitKeeper and Monotone (Graydon Hoare) Git ● a blob is a string of bytes identified by its hash ● a tree is a series of names, flag bits, and blob hashes of files or subdirectories Git Creator:inkscape 0.45.1 Git ● each working copy includes complete history ● commits are local ● commits can have parents – zero or more! ● history is a DAG; multiple heads ● hash naming makes for fast net synchronization ● trading histories is the essential net operation Mercurial ● Matt Mackall, 2005 ● Similar to Git ● Uses a manifest instead of a tree Leading Question ● What is the correct number of context lines to include in a patch? Darcs ● David Roundy ● metadata is a set of patches (not historical!) Darcs ● Write the effect of applying patch P1, and then P2, as: P1 P2 ● Suppose I have P1 P2 P3 P4 ● It's easy to compute P1 P2 P3 ● But P1 P3 P4 may or may not apply cleanly Darcs ● Some pairs of patches P1 and P2 commute: there is some pair of patches P1' and P2' such that P1 P2 = P2' P1' The Toronto Idea ● Karl Fogel, in dire circumstances .