The following terms are used in the documentation for package management and the pub tool.

Application package

A package that contains a program or app, with a main entrypoint. Meant to be run directly, either on the command line or in a browser.

Application packages may have dependencies on other packages, but are never depended on themselves. Unlike regular packages, they are not intended to be shared.

Application packages should check their lockfiles into source control, so that everyone working on the application and every location the application is deployed has a consistent set of dependencies. Because their dependencies are constrained by the lockfile, application packages usually specify any for their dependencies’ version constraints.

Content hashes

The pub.dev repository maintains a sha256 hash of each package version it hosts. Pub clients can use this hash to validate the integrity of downloaded packages, and protect against changes on the repository.

When dart pub get downloads a package, it computes the hash of the downloaded archive. The hash of each hosted dependency is stored with the resolution in the lockfile.

The pub client uses this content hash to verify that running dart pub get again using the same lockfile, potentially on a different computer, uses exactly the same packages.

If the locked hash doesn’t match what’s currently in the pub cache, pub redownloads the archive. If it still doesn’t match, the lockfile updates and a warning is printed. For example:

$ dart pub get
Resolving dependencies...
Cached version of foo-1.0.0 has wrong hash - redownloading.
 ~ foo 1.0.0 (was 1.0.0)
The existing content-hash from pubspec.lock doesn't match contents for:
 * foo-1.0.0 from "pub.dev"
This indicates one of:
 * The content has changed on the server since you created the pubspec.lock.
 * The pubspec.lock has been corrupted.
 
The content-hashes in pubspec.lock has been updated.

For more information see:
https://dart.dev/go/content-hashes

Changed 1 dependency!

The updated content hash will show up in your version control diff, and should make you suspicious.

To make a discrepancy become an error instead of a warning, use dart pub get --enforce-lockfile. It will cause the resolution to fail if it cannot find package archives with the same hashes, without updating the lockfile.

$ dart pub get --enforce-lockfile
Resolving dependencies...
Cached version of foo-1.0.0 has wrong hash - redownloading.
~ foo 1.0.0 (was 1.0.0)
The existing content-hash from pubspec.lock doesn't match contents for:
 * foo-1.0.0 from "pub.dev"

This indicates one of:
 * The content has changed on the server since you created the pubspec.lock.
 * The pubspec.lock has been corrupted.

For more information see:
https://dart.dev/go/content-hashes
Would change 1 dependency.
Unable to satisfy `pubspec.yaml` using `pubspec.lock`.

To update `pubspec.lock` run `dart pub get` without
`--enforce-lockfile`.

Dependency

Another package that your package relies on. If your package wants to import code from some other package, that package must be a dependency. Dependencies are specified in your package’s pubspec and described in Package dependencies.

To see the dependencies used by a package, use pub deps.

Entrypoint

In the general context of Dart, an entrypoint is a Dart library that is directly invoked by a Dart implementation. When you reference a Dart library in a <script> tag or pass it as a command-line argument to the standalone Dart VM, that library is the entrypoint. In other words, it’s usually the .dart file that contains main().

In the context of pub, an entrypoint package or root package is the root of a dependency graph. It will usually be an application. When you run your app, it’s the entrypoint package. Every other package it depends on will not be an entrypoint in that context.

A package can be an entrypoint in some contexts and not in others. Say your app uses a package A. When you run your app, A is not the entrypoint package. However, if you go over to A and execute its tests, in that context, it is the entrypoint since your app isn’t involved.

Entrypoint directory

A directory inside your package that is allowed to contain Dart entrypoints.

Pub has a list of these directories: benchmark, bin, example, test, tool, and web (and lib, for Flutter apps). Any subdirectories of those (except bin) may also contain entrypoints.

Immediate dependency

A dependency that your package directly uses itself. The dependencies you list in your pubspec are your package’s immediate dependencies. All other dependencies are transitive dependencies.

Library

A library is a single compilation unit, made up of a single primary file and any optional number of parts. Libraries have their own private scope.

Lockfile

A file named pubspec.lock that specifies the concrete versions and other identifying information for every immediate and transitive dependency a package relies on.

Unlike the pubspec, which only lists immediate dependencies and allows version ranges, the lockfile comprehensively pins down the entire dependency graph to specific versions of packages. A lockfile ensures that you can recreate the exact configuration of packages used by an application.

The lockfile is generated automatically for you by pub when you run pub get, pub upgrade, or pub downgrade. Pub includes a content hash for each package to check against during future resolutions.

If your package is an application package, you will typically check this into source control. For regular packages, you usually won’t.

Package

A collection of libraries under a directory, with a pubspec.yaml in the root of that directory.

Packages can have dependencies on other packages and can be dependencies themselves. A package’s /lib directory contains the public libraries that other packages can import and use. They can also include scripts to be run directly. A package that is not intended to be depended on by other packages is an application package. Shared packages are published to pub.dev, but you can also have non-published packages.

Don’t check the lockfile of a package into source control, since libraries should support a range of dependency versions. The version constraints of a package’s immediate dependencies should be as wide as possible while still ensuring that the dependencies will be compatible with the versions that were tested against.

Since semantic versioning requires that libraries increment their major version numbers for any backwards incompatible changes, packages will usually require their dependencies’ versions to be greater than or equal to the versions that were tested and less than the next major version. So if your library depended on the (fictional) transmogrify package and you tested it at version 1.2.1, your version constraint would be ^1.2.1.

SDK constraint

The declared versions of the Dart SDK itself that a package declares that it supports. An SDK constraint is specified using normal version constraint syntax, but in a special environment section in the pubspec.

Source

A kind of place that pub can get packages from. A source isn’t a specific place like the pub.dev site or some specific Git URL. Each source describes a general procedure for accessing a package in some way. For example, git is one source. The git source knows how to download packages given a Git URL. Several different supported sources are available.

System cache

When pub gets a remote package, it downloads it into a single system cache directory maintained by pub. On Mac and Linux, this directory defaults to ~/.pub-cache. On Windows, the directory defaults to %LOCALAPPDATA%\Pub\Cache, though its exact location may vary depending on the Windows version. You can specify a different location using the PUB_CACHE environment variable.

Once packages are in the system cache, pub creates a package_config.json file that maps each package used by your application to the corresponding package in the cache.

You only have to download a given version of a package once and can then reuse it in as many packages as you would like. If you specify the --offline flag to use cached packages, you can delete and regenerate your package_config.json files without having to access the network.

Transitive dependency

A dependency that your package indirectly uses because one of its dependencies requires it. If your package depends on A, which in turn depends on B which depends on C, then A is an immediate dependency and B and C are transitive ones.

Uploader

Someone who has administrative permissions for a package. A package uploader can upload new versions of the package, and they can also add and remove other uploaders for that package.

If a package has a verified publisher, then all members of the publisher can upload the package.

Verified publisher

One or more users who own a set of packages. Each verified publisher is identified by a verified domain name, such as dart.dev. For general information about verified publishers, see the verified publishers page. For details on creating a verified publisher and transferring packages to it, see the documentation for publishing packages.

Version constraint

A constraint placed on each dependency of a package that specifies which versions of that dependency the package is expected to work with. This can be a single version (0.3.0) or a range of versions (^1.2.1). While any is also allowed, for performance reasons we don’t recommend it.

For more information, see Version constraints.

Packages should always specify version constraints for all of their dependencies. Application packages, on the other hand, should usually allow any version of their dependencies, since they use the lockfile to manage their dependency versions.

For more information, see Pub Versioning Philosophy.