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# Lock Files

Stack attempts to provide reproducible build plans. This involves reproducibly
getting the exact same contents of source packages and configuration options
(like Cabal flags and GHC options) for a given set of input files. There are a
few problems with making this work:

* Entering all of the information to fully provide reproducibility is tedious.
  This would include things like Hackage revisions, hashes of remote tarballs,
  etc. Users don't want to enter this information.
* Many operations in Stack rely upon a "snapshot hash," which transitively
  includes the completed information for all of these dependencies. If any of
  that information is missing when parsing the `stack.yaml` file or snapshot
  files, it could be expensive for Stack to calculate it.

To address this, we follow the (fairly standard) approach of having a
_lock file_. The goal of the lock file is to cache completed locations of
project, snapshot packages and snapshots themselves so that:

* These files can be stored in source control
* Users on other machines can reuse these lock files and get identical build
  plans given that the used local packages and local snapshots are the same on
  those machines
* Rerunning `stack build` in the future is deterministic in the build plan, not
  depending on mutable state in the world like Hackage revisions

    !!! note

        If, for example, a tarball available remotely is deleted or the hash
        changes, it will not be possible for Stack to perform the build.
        However, by deterministic, we mean it either performs the same build or
        fails, never accidentally doing something different.

This document explains the contents of a lock file, how they are used, and how
they are created and updated.

## stack.yaml and snapshot files

Relevant to this discussion, Stack's project-level configuration file
(`stack.yaml`) specifies:

* the parent snapshot (`resolver` or `snapshot`)
* extra-deps

Some of this information can be incomplete. Consider this `stack.yaml` file:

~~~yaml
resolver: lts-19.22
packages:
- .
extra-deps:
- acme-missiles-0.3
~~~

This information is _incomplete_. For example, the extra-deps may change in the
future. Instead, you could specify enough information in the `stack.yaml` file
to fully resolve that package. That looks like:

~~~yaml
extra-deps:
- hackage: acme-missiles-0.3@sha256:2ba66a092a32593880a87fb00f3213762d7bca65a687d45965778deb8694c5d1,613
  pantry-tree:
    size: 226
    sha256: 614bc0cca76937507ea0a5ccc17a504c997ce458d7f2f9e43b15a10c8eaeb033
~~~

The `lts-19.22` information is also incomplete. While we assume in general that
Haskell LTS snapshots never change, there's nothing that prohibits that from
happening. Instead, the complete version of that key is:

~~~yaml
resolver:
- url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/19/22.yaml
  size: 619399
  sha256: 5098594e71bdefe0c13e9e6236f12e3414ef91a2b89b029fd30e8fc8087f3a07
~~~

Users don't particularly feel like writing all of that. Therefore, it's common
to see _incomplete_ information in a `stack.yaml` file.

## Recursive snapshot layers

Snapshot files can be _recursive_, where `stack.yaml` refers to `foo.yaml`,
which refers to `bar.yaml`, which refers to `baz.yaml`. A local snapshot file
can refer to a remote snapshot file (available via an HTTP(S) URL).

We need to encode information from _all_ of these snapshot layers and the
`stack.yaml` file in the lock file, to ensure that we can detect if anything
changes.

## Performance

In addition to acting as a pure correctness mechanism, the design of a lock file
given here also works as a performance improvement. Instead of requiring that
all snapshot files be fully parsed on each Stack invocation, we can store
information in the lock file and bypass parsing of the additional files in the
common case of no changes.

## Lock file contents

The lock file contains the following information:

* Completed package locations for extra deps and packages in snapshot files

    !!! note

        This only applies to _immutable_ packages. Mutable packages are not
        included in the lock file.

* Completed information for the snapshot locations

It looks like the following:

~~~yaml
# Lock file, some message about the file being auto-generated
snapshots:
  # Starts with the snapshot specified in stack.yaml,
  # then continues with the snapshot specified in each
  # subsequent snapshot file
  - original:
      foo.yaml # raw content specified in a snapshot file
    completed:
      file: foo.yaml
      sha256: XXXX
      size: XXXX
  - original:
      lts-13.9
    completed:
      size: 496662
      url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/13/9.yaml
      sha256: 83de9017d911cf7795f19353dba4d04bd24cd40622b7567ff61fc3f7223aa3ea

packages:
- original: https://hackage.haskell.org/package/acme-missiles-0.3.tar.gz
  completed:
    size: 1442
    url: https://hackage.haskell.org/package/acme-missiles-0.3.tar.gz
    name: acme-missiles
    version: '0.3'
    sha256: e563d8b524017a06b32768c4db8eff1f822f3fb22a90320b7e414402647b735b
    pantry-tree:
      size: 226
      sha256: 614bc0cca76937507ea0a5ccc17a504c997ce458d7f2f9e43b15a10c8eaeb033
~~~

## Creation

Whenever a project-level configuration file (`stack.yaml`) is loaded, Stack
checks for a lock file in the same file path, with a `.lock` extension added.
For example, if you command:

~~~text
stack build --stack-yaml my-stack.yaml
~~~

Stack will use a lock file in the location `my-stack.yaml.lock`. For the rest of
this document, we'll assume that the files are simply `stack.yaml` and
`stack.yaml.lock`.

If the lock file does not exist, it will be created by:

* Loading the `stack.yaml`
* Loading all snapshot files
* Completing all missing information
* Writing out the new `stack.yaml.lock` file

## Update procedure

When loading a Stack project all completed package or snapshot locations (even
when they were completed using information from a lock file) get collected to
form a new lock file in memory and compare against the one on disk, writing if
there are any differences.
