pulumi/docs/design/deps.md

Coconut Dependencies

This is a short note describing how CocoPack dependencies are managed and resolved. This design has been inspired by many existing package managers, and is a mashup of the approaches taken in Go, NPM/Yarn, and Docker.

Packages

The unit of dependency in Coconut is a package, encoded in the CocoPack format.

Each has a Coconut.yaml (or .json) manifest, which lists, among other things, that package's own set of dependencies.

Each may also carry arbitrary assets, such as a Dockerfile and associated source code; serverless source code representing lambdas and API implementations; and so on.

The dependency management system is opinionated about how directories are laid out, however most CocoLangs will project CocoPack dependencies into the native package management system using proxy packages that the CocoLang compilers understand how to recognize. The details of how a language does this is outside of the scope of this document.

References

Each package is referenced using a URL-like scheme, facilitating multiple package management distribution schemes. For example, the URL https://cocohub.com/aws#^1.0.6 references the aws package on CocoHub's built-in package manager, and askes specifically for version 1.0.6 or higher using semantic versioning resolution. Note that package names may have multiple parts, delimited by /, as part of the URL; for example https://cocohub.com/a/b/c.

Specifically, the reference has up to four parts: a protocol, base URL, name, and version:

PackName = [ Protocol ] [ BaseURL ] NamePart [ Version ]
Protocol = "http://" | "https://" | "ssh://" | ...
BaseURL  = URL* "." (URL | ".")* "/"
URL      = (* valid URL characters *)
NamePart = (Identifier "/")* Identifier
Version  = "#" (* valid version numbers *)

Although there are four parts, three of them are optional, because because Coconut uses these defaults:

• https:// is the default protocol.
• cocohub.com/ is the default base URL.
• latest is the default version number (a.k.a., "tip").

Although we're concerned with package references right now, we'll see soon that a similar reference scheme is used to address elements exported from a package, like a module, class, function, or variable. The package part of the reference uses the above grammar, however members inside of it are preceded by a :. Furthermore, such references do not have version numbers. These references are not strictly URLs and must be interpreted by the Coconut toolchain:

MemberName = [ Protocol ] [ BaseURL ] NamePart MemberPart
MemberPart = ":" NamePart

For example, to reference the VM class from a CocoIL token -- assuming we have a dependency declared on https://cocohub.com/aws#^1.0.6 as shown above -- we would most likely say aws:ec2/VM. A fully qualified, but versionless, reference is also permitted, as in https://cocohub.com/aws:ec2/VM, although this is less conventional. The self-referential package plus module identifier . can be used to reference members in the current package.

The way these URLs are resolved to physical CocoPacks is discussed later on in this document.

Versions

Each physical CocoPack can be tagged with one or more versions. How this tagging process happens is left to the specific package provider. Each version can either be a semantic version number or arbitrary string tag.

For example, the Git provider allows dependency on a specific Git SHA hash. For example, https://github.com/coconut/aws/ec2#1895753f53a63c055e7cae81ebe4ea5d5805584f depends on a CocoPack published in a GitHub repo at commit 1895753. Alternatively, Git tags can be used to give CocoPacks friendly names. So, for example, https://github.com/coconut/aws/ec2#beta1 uses on the arbitrary tag beta1; the same scheme can be used to denote semantic versions simply by using Git tags, for instance https://github.com/coconut/aws/ec2#^1.0.6.

Flexible vs. Pinned

If the reference uses a semantic version range, the toolchain is given some wiggle room in how it resolves the reference (in the usual ways). Such a reference is said to be "flexible." If the reference uses a non-range semantic version, Git commit hash, or Git tag, on the other hand, the reference is said to be "pinned" to a specific version and can never bind to anything else.

At development-time, flexible versions are nice, because you're often getting the latest-and-greatest that a library has to offer, without having to spend a great deal of time manually managing version numbers. Flexible versions are also nice for libraries, as the package manager can resolve multiple close, but different, semantic versions of a given library to a single physical incarnation of it. But when it comes to managing a production system, flexible versions can cause problems, since upgrading to a new version may change a topology unexpectedly and/or at an inopportune time.

It is up to you, the package author, to decide whether to use flexible or pinned versions. The recommended practice is, however, for package manifests to specify flexible semantic version ranges. This ensures development-time is flexible. These package manifests should be published as-is, permitting more versioning flexibility on the consumer side. Blueprints, however, should be pinned to specific versions, both in version control, and in the default developer workflow. This pinning is important to ensure that deployments are repeatable, and is encouraged by the command line tools; in particular, generating a plan automatically first generates a so-called "pinfile."

Pinning

A pinfile exists alongside a package's manifest to lock all versions to specific pinned versions; it is called Cocodeps.yaml (or Cocodeps.json). Using a separate pinfile enables an independent pinning step without modifying the package manifest, which can continue using flexible versions if appropriate. This file contains the entire transitive closure of a package, its dependencies, their dependencies, and so on, pinned to specific versions.

All CLI commands respect the pinfile when it exists. There are CLI commands to manage this pinfile too. For example:

• coco pin will generate a new pinfile.
• coco upgrade will upgrade all packages to new versions where available.
• And so on.

To encourage the recommended workflow above, a few other behaviors are worth noting.

First, all deployment CLI commands will generate a pinfile if it doesn't exist (coco plan, coco apply, etc). This ensures that blueprints are generally pinned to versions during, between, and after deployment activities. Such pinfiles should be checked into blueprint source control repositories and versioned intentionally. The option --no-pin suppresses the automatic generation of pinfiles fo rthe deployment commands.

Second, publishing a library package, by default, omits the pinfile. This encourages publication of libraries that are not pinned to specific versions. If that is desired, the pinned versions belong in the package manifest.

Package Resolution

Now let us see how references are resolved to physical CocoPacks.

CocoPacks may be found in multiple places, and, as in most dependency management schemes, some locations are preferred over others. This is to ease the task of local development while also providing rigorous dependency management.

Roughly speaking, these locations are are searched, in order:

1. The current workspace, for intra-workspace but inter-package dependencies.
2. The current workspace's .coconut/packs/ directory.
3. The global Workspace's .coconut/packs/ directory.
4. The Coconut runtime libraries: $COCOROOT/lib/packs/ (default /usr/local/coconut/lib/packs).

In each location, Coconut prefers a fully qualified hit if it exists -- containing both the base of the reference plus the name -- however, it also accept name-only hits. This allows developers to organize their workspace without worrying about where their CocoPacks will be published. Most of the Coconut tools, however, prefer fully qualified paths.

To be more precise, given a reference r and a workspace root w, we look in these locations, in order:

1. w/base(r)/name(r)
2. w/name(r)
3. w/.coconut/packs/base(r)/name(r)
4. w/.coconut/packs/name(r)
5. ~/.coconut/packs/base(r)/name(r)
6. ~/.coconut/packs/name(r)
7. $COCOROOT/lib/base(r)/name(r)
8. $COCOROOT/lib/name(r)

To illustrate this process, let us imagine we are looking up the package https://cocohub.com/aws/ec2.

In the illustration, let us imagine we're the author of the package, and so it is in our workspace. We have things organized so that it can be easily found, eliminating the need for us to frequently publish changes during development:

<Workspace>
|   .coconut/
|   |   workspace.yaml
|   aws/
|   |   ec2/
|   |   |   Coconut.yaml
|   |   |   ...other assets...

The workspace.yaml file may optionally specify a "namespace" property, as in:

namespace: aws

In this case, the following simpler directory structure would also do the trick:

<Workspace>
|   .coconut/
|   |   workspace.yaml
|   ec2/
|   |   Coconut.yaml
|   |   ...other assets...

It is possible to simplify this even further by specifying the namespace as aws/ec2, leading to:

<Workspace>
|   .coconut/
|   |   workspace.yaml
|   Coconut.yaml
|   ...other assets...

Notice that we didn't have to mention the cocohub.com/ part in our workspace, although we can if we choose to.

In the second illustration, let us imagine we have used coco get to download the dependency from a package manager:

$ coco get https://cocohub.com/aws/ec2

In this case, our local workspace's package directory will have been populated with a copy of aws/ec2:

<Workspace>
|   .coconut/
|   |   packs/
|   |   |   cocohub.com/
|   |   |   |   aws/
|   |   |   |   |   ec2/
|   |   |   |   |   |   Coconut.yaml
|   |   |   |   |   |   ...other assets...

Notice that in this case, the full base part cocohub.com/ is part of the path, since we downloaded it from that URL.

Now in the third and final illustration, let us imagine that we've installed a global copy of the package. This might have been thanks to use using coco get's --global (or -g) flag:

$ coco get --global https://cocohub.com/aws/ec2

The directory structure will look identical to the second example, except that it is rooted in ~/:

~
|   .coconut/
|   |   packs/
|   |   |   cocohub.com/
|   |   |   |   aws/
|   |   |   |   |   ec2/
|   |   |   |   |   |   Coconut.yaml
|   |   |   |   |   |   ...other assets...

Fetching

TODO(joe): describe package fetching protocols.

TODO(joe): on-demand compilation (for easier Git fetching).

TODO(joe): describe how semantic versioning resolution works.

TODO(joe): describe how all of this interacts with Git repos (locally; e.g., git pull).