hologit

Hologit lets you declaratively define virtual sub-branches (called holobranches) within any Git branch that mix together content from their host branch, content from other repositories/branches, and executable-driven transformations.

Features

• Track and merge remote code from multiple sources
• Advanced merge, filter, and sourcing strategies
• Apply arbitrary executable steps efficiently and consistently via Chef Habitat packages
• Content-based git-distributed caching of build steps
• GitHub Action for materializing holobranches to real branches
• --watch command to produce live updates (currently lazy/slow, theoretically can be made near-instant)

Introduction

Hologit is a free and open framework for code and content automation inside your local git repository. It makes it simple, fast, and reliable for projects to automate complex editing and publishing workflows that can involve multiple source repositories, languages, and build tools. Aiming to make working on software easier for everyone—pro and beginner alike—hologit gets rid of the need to think about or even know what needs to happen after you change files. There should just be content, and it goes places when you change it.

This works by enabling a project's git repository to define virtual "holobranches" that can be continuously and efficiently "projected" from any source branch. The projection process handles combining code from remote sources ("compositing") and executing build tools on the result ("lensing") to produce an output file tree and optionally commit it to a branch/ref.

Compositing offers deeper control over which files are pulled from a remote repository and where they are integrated than git submodules alone, while being more dependable and tracable than language-specific package managers like npm and composer. Instead of copying and moving files around on disk, hologit takes a git-native approach to minimize disk activity by computing new git trees in memory. Computed trees may be written to disk later or used as input to another process without the overhead.

Lensing can execute any existing code or build tool consistently by leveraging habitat and using containers where necessary. However, it also opens the door to a new generation of git-native build tools that do as much of their work as possible in memory, reading and writing to git's object database instead of a working tree on disk.

Quickstart

The guide will walk you through an illustrative minimal use of hologit to publish a GitHub Pages branch.

Each heading links to branches in the hologit/examples repository showing the final state of the example project at the end of the section.

Create a repository with some minimal code [example branch]

To start this example, we'll use the starter template from Bootstrap's Getting Started guide to create a website:

$ git init holo-example
Initialized empty Git repository in /Users/chris/holo-example/.git/
$ cd holo-example/
$ curl -s https://raw.githubusercontent.com/hologit/examples/basic/01-init-repo/index.html > index.html
$ git add index.html
$ git commit -m "Add Bootstrap's starter template as index.html"
[master (root-commit) 9fe77ec] Add Bootstrap's starter template as index.html
 1 file changed, 22 insertions(+)
 create mode 100644 index.html

Install hologit

See docs/grand-tour/installation.md

Initialize .holo/ configuration [example branch]

Hologit configuration is stored under the .holo/ tree at the root of a repository. Initialize it in each branch that will generate projections:

$ git holo init
name=holo-example
initialized .holo/config.toml
$ cat .holo/config.toml
[holo]
name = "holo-example"
$ git commit -m "Initialize .holo/ configuration"
[master 881b0b6] Initialize .holo/ configuration
 1 file changed, 2 insertions(+)
 create mode 100644 .holo/config.toml

To start, this configuration file only assigns a name for the code in the current source branch, which can be used later as an alternative to remote sources. The name holo-example was detected from the name of the repository's working tree, but could have been chosen by passing --name ${my_project_name} for the init command or just by editing the ./holo/config.toml file later.

Define a holobranch [example branch]

A holobranch can be defined by creating a holobranch config file at .holo/branches/${my_holobranch_name}.toml or any number of holomapping config files within .holo/branches/${my_holobranch_name}/**.toml. Generate a minimal "passthrough" holobranch that will copy all files from the current source branch:

$ git holo branch create --template=passthrough gh-pages
initialized .holo/branches/gh-pages/_holo-example.toml
$ cat .holo/branches/gh-pages/_holo-example.toml
[holomapping]
files = "**"
$ git commit -m "Initialize .holo/branches/gh-pages configuration"
[master 4b9aa68] Initialize .holo/branches/gh-pages configuration
 1 file changed, 2 insertions(+)
 create mode 100644 .holo/branches/gh-pages/_holo-example.toml

This defines a holobranch named gh-pages with all files from holosource holo-example matching the glob pattern ** populating its root directory. There are several elements of convention on display here:

• The underscore prefixing the filename of/_holo-example.toml indicates that any files produced by the holomapping should be merged into the root directory of the projected holobranch.
  • If the filename were just /holo-example.toml, a subdirectory name /holo-example/ would be created to contain all the files produced by the holomapping.
  • A holomapping config prefixed with an underscore could be named anything, all such holomappings at the same path will have their files merged to populate the directory.
• There are only two required configuration options for each holomapping:
  • holosource: The name of a configured holosource referencing a repository to pull files from
    • Ommitted in the generated holomapping config
    • Defaults to the name of the file with the .toml extension and any _ prefix stripped
  • files: A string or array for strings containing glob patterns for matching or excluding files
    • A value of just '**', as in the generated config, matches all files in the source

Project holobranch for first time

With a holobranch defined with at least one holomapping, we have enough for our first tree projection:

$ git holo project gh-pages
info: reading mappings from holobranch: gitDir=/Users/chris/holo-example/.git, ref=HEAD, workTree=false, name=gh-pages
info: compositing tree...
info: merging holo-example:{**} -> /
info: stripping .holo/ tree from output tree...
info: writing final output tree...
info: projection ready:
ff954bb0a1e4878db424cb1033a0c356dac8d350
$ git cat-file -t ff954bb0a1e4878db424cb1033a0c356dac8d350
tree
$ git ls-tree -r ff954bb0a1e4878db424cb1033a0c356dac8d350
100644 blob 8092fa2adb4a9a395ac291fbdc9717b68be669aa    index.html

The output of the project command seen above is the git hash of a tree object that has been generated, if needed, within your git repository's object database. This hash does not reference a commit object like most git hashes most commonly seen. A tree object is the main ingrediant of a commit obect: the tree represents a complete unique state of all the files and a commit attaches the tree to a point in your chain of commits with timestamp and authorship information.

A tree can be used directly:

$ git archive --format=zip $(git holo project gh-pages) > website.zip
info: reading mappings from holobranch: gitDir=/Users/chris/Repositories/holo-example/.git, ref=HEAD, workTree=false, name=gh-pages
info: compositing tree...
info: merging holo-example:{**} -> /
info: stripping .holo/ tree from output tree...
info: writing final output tree...
info: projection ready:
$ unzip -l website.zip
Archive:  website.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
     1230  12-23-2018 20:32   index.html
---------                     -------
     1230                     1 file

or wrapped in a commit:

$ git commit-tree -m "Update gh-pages"  $(git holo project gh-pages)
info: reading mappings from holobranch: gitDir=/Users/chris/Repositories/holo-example/.git, ref=HEAD, workTree=false, name=gh-pages
info: compositing tree...
info: merging holo-example:{**} -> /
info: stripping .holo/ tree from output tree...
info: writing final output tree...
info: projection ready:
846a551ce356d5fa4088e58b3ad0f0d05aa6d389
$ git cat-file -t 846a551ce356d5fa4088e58b3ad0f0d05aa6d389
commit
$ git cat-file -p 846a551ce356d5fa4088e58b3ad0f0d05aa6d389
tree ff954bb0a1e4878db424cb1033a0c356dac8d350
author Chris Alfano <[email protected]> 1545615571 -0500
committer Chris Alfano <[email protected]> 1545615571 -0500

Update gh-pages

With the --commit-branch option, you can commit the generated tree to a give branch and output the new commit's hash instead:

$ git cat-file -p $(git holo project gh-pages --commit-branch=gh-pages)
info: reading mappings from holobranch: gitDir=/Users/chris/Repositories/holo-example/.git, ref=HEAD, workTree=false, name=gh-pages
info: compositing tree...
info: merging holo-example:{**} -> /
info: stripping .holo/ tree from output tree...
info: writing final output tree...
info: committed new tree to "gh-pages": 734f7dc034868af4e2bd23daf23e119faca1e0b8
info: projection ready:
tree ff954bb0a1e4878db424cb1033a0c356dac8d350
author Chris Alfano <[email protected]> 1545616786 -0500
committer Chris Alfano <[email protected]> 1545616786 -0500

Projected gh-pages from 4b9aa68

Merge external code via a holosource [example branch]

The first step to using external code in your projections is defining a holosource:

$ git holo source create https://github.com/twbs/bootstrap --ref=v4.2.1
info: listing https://github.com/twbs/bootstrap#v4.2.1
info: fetching https://github.com/twbs/bootstrap#refs/tags/v4.2.1@9e4e94747bd698f4f61d48ed54c9c6d4d199bd32
fetched https://github.com/twbs/bootstrap#refs/tags/v4.2.1@9e4e94747bd698f4f61d48ed54c9c6d4d199bd32
initialized .holo/sources/bootstrap.toml
$ cat .holo/sources/bootstrap.toml
[holosource]
url = "https://github.com/twbs/bootstrap"
ref = "refs/tags/v4.2.1"
$ git commit -m "Initialize .holo/sources/bootstrap configuration"
[master 64ef9fc] Initialize .holo/sources/bootstrap configuration
 1 file changed, 3 insertions(+)
 create mode 100644 .holo/sources/bootstrap.toml

Now this source can be referenced in holobranch mappings, this example takes advantage of the holosource being automatically set from the mapping filename:

$ mkdir .holo/branches/gh-pages/{js,css}
$ cat > .holo/branches/gh-pages/css/_bootstrap.toml <<- END_OF_TOML
[holomapping]
root = "dist/css"
files = "*.min.css"
END_OF_TOML
$ cat > .holo/branches/gh-pages/js/_bootstrap.toml <<- END_OF_TOML
[holomapping]
root = "dist/js"
files = "*.min.js"
END_OF_TOML
$ git add --all
$ git commit -am "Add css and js mappings for bootstrap to gh-pages holobranch"
[master 4180e45] Add css and js mappings for bootstrap to gh-pages holobranch
 2 files changed, 6 insertions(+)
 create mode 100644 .holo/branches/gh-pages/css/_bootstrap.toml
 create mode 100644 .holo/branches/gh-pages/js/_bootstrap.toml

Projecting the gh-pages tree now shows the files merged from bootstrap:

$ git ls-tree -r $(git holo project gh-pages)
info: reading mappings from holobranch: gitDir=/Users/chris/Repositories/holo-example/.git, ref=HEAD, workTree=false, name=gh-pages
info: compositing tree...
info: merging holo-example:{**} -> /
info: merging bootstrap:dist/css/{*.min.css} -> /css/
info: merging bootstrap:dist/js/{*.min.js} -> /js/
info: stripping .holo/ tree from output tree...
info: writing final output tree...
info: projection ready:
100644 blob b3e6881a586c99b55e2d1878839eede6fb3fa9d7    css/bootstrap-grid.min.css
100644 blob 0668a8cd93bba140c00bc0c410ad54c61af71d9e    css/bootstrap-reboot.min.css
100644 blob e6b4977799e3a3a377e475ee765eb4a9961c6c71    css/bootstrap.min.css
100644 blob 8092fa2adb4a9a395ac291fbdc9717b68be669aa    index.html
100644 blob 97f14c05c3d5960129caf3e4666f661dfdb8228a    js/bootstrap.bundle.min.js
100644 blob 9df6b6c2ced14a60259171e1fdacc2534ddee183    js/bootstrap.min.js

For reference, here is what the holobranch definition that projected this tree looks like at this point:

$ tree .holo/branches/gh-pages
.holo/branches/gh-pages
├── _holo-example.toml
├── css
│   └── _bootstrap.toml
└── js
    └── _bootstrap.toml

Before projecting again, you might want to update all remote sources to their latest commits:

$ git holo source fetch --all
fetched bootstrap https://github.com/twbs/bootstrap#refs/heads/v4-dev@dc17c924e86948ae514d72f8ccc67f9d77657f6b

Assemble the complete source code via a holo lens

• Apply sass compilation and compression via generic lenses

Work upstream by checking out a holosource

To work on changes to code being pulled in from remote repositories, any or all sources can be checkout out as a git submodule:

$ git holo source checkout --all
checked out .holo/sources/bootstrap from https://github.com/twbs/bootstrap#refs/tags/v4.2.1@9e4e9474
$ git commit -m "Initialize .holo/sources/bootstrap submodule"
[basic/05-checkout-holosource ee39b88] Initialize .holo/sources/bootstrap submodule
 2 files changed, 5 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 .holo/sources/bootstrap

• Make commits inside submodule, project with --working
• Commit gitlink outside submodule, change all projections

Make use of a projected tree

• Archive tree-ish
• Write to a real branch
• Push to github gh-pages

Advanced Usage

Overlay a project

Build new holo lenses

Roadmap

• [~] (in progress) Complete getting started tour, break into sections in another doc format
• [ ] Develop shorter quickstart
• [~] * --ref (in progress) option to use a specific ref instead of HEAD
• [~] * ---no-working (in progress) option to ignore working directory and only use ref
• [ ] project --watch option to keep running and automatically update projection with changes to input
• [ ] project --audit option to produce audit commits chain
• [ ] Implement holoreactor objects: defined like lenses within the projected branches, the handle piping result subtrees into single-run or persistent processes running in the studio via hab exec or hab svc load
  • [ ] Implement a holoreactor for serving static websites
  • [ ] Implement a holoreactor for running/restarting a node app
  • [ ] Implement a holoreactor for running an emergence app locally or on a remote cluster
• [ ] Expose from the studio's HTTP interface a virtual holospace that can be mutated via git push or WebDAV
• [ ] Add option to [holosource] config to override submodule checkout path
• [ ] Leverage lower-overhead chroot environments instead of Docker containers on Linux systems
• [ ] Enable running and connecting to a persistent background studio for quick on-demand projection
  • Build a studio docker image that extends Habitat's studio image with hologit pre-installed
  • Wrap around opening interactive shells
  • Wrap around entrypoint to start holostudio process
• [ ] Implement in-memory tree hashing to avoid calls to git mktree for tree hashes that are known in the tree cache to already exist in the repo
• [ ] Visual Studio Code extension
  • Top-level hologit section with views of sources and branches
  • Commands via context menu and command palette
  • Ability to graphically toggle watch mode for each source
  • Open holobranches in workspace via filesystem provider for read-only browsing of either composited or lensed content
  • Enable writing to mounted holobranches by routing writes to estimated source via reverse-compositing, checking out submodules on-the-fly
• [ ] Isolate lens environments further from source
  • Currently, .git directory is mounted into lensing environment, so working tree is safe but repo is exposed to damage by lens code
  • Lensing only needs to exchange object hashes, so objects tree could be mounted read-write and the rest could be empty
  • More robust options might include using git's fetch/push mechanism and/or mounting objects tree as read-only alternates
    • A git SmartHTTP server could be exposed via the studio socket, but could objects be exchanged as efficiently as just bind-mounting the objects database directly?
• [ ] Enable running web-based code editor as a holoreactor to your code base

Reference

TODO

• [ ] Have project fetch and read source HEAD if no submodule commit is found
• [ ] Refactor source add and source fetch to use common code, leave things in same state