# DAML-LF archive

This component contains the `.proto` definitions specifying the format
in which DAML-LF packages are stored -- the DAML-LF archive.

The entry point definition is `Archive` in `da/daml_lf.proto`. `Archive`
contains some metadata about the actual archive (currently the hashing
function and the hash), and then a binary blob containing the archive.
The binary blob must be an `ArchivePayload` -- we keep it in binary form
to facilitate hashing, signing, etc. The encoding and decoding of the
payload is handled by Haskell and Java libraries in `daml-core-package`,
so that consumers and producers do not really need to worry about it.

`ArchivePayload` is a sum type containing the various DAML-LF versions
supported by the DAML-LF archive. Currently we have two versions:

* `DAML-LF-0`, which is the legacy DAML core;
* `DAML-LF-1`, which is the first version of DAML-LF as specified by
    <https://github.com/digital-asset/daml/blob/master/daml-lf/spec/daml-lf-1.rst>.

## Building

It produces two libraries containing code to encode / decode such
definition, a Haskell one and a Java one:

```
$ bazel build //daml-lf/archive:daml_lf_haskell_proto
$ bazel build //daml-lf/archive:daml_lf_java_proto
```

## Editing the `.proto` definitions

When editing the proto definitions, **you must make sure to not change
them in a backwards-incompatible way**. To make sure this doesn't happen:

* **DO NOT** delete message fields;
* **DO NOT** change the number of a message field or an enum value;
* **DO NOT** change the type of a message field;

Note that "fields" include `oneof` fields. Also note that the "don't
delete fields" rule is there not because they introduce a backwards
incompatible change, but rather because after a field has been deleted
another commiter might redefine it with a different type without
realizing.

What is OK is renaming message fields while keeping the number and semantics unchanged.
For example, if you have

```
message Foo {
  bytes blah = 1;
}
```

it's OK to change it to

```
message Foo {
  // this field is deprecated -- use baz instead!
  bytes blah_deprecated = 1;
  string baz = 2;
}
```

## Conversion from the `.proto` to AST

The `.proto` definitions contain the serialized format for DAML-LF
packages, however the code to convert from the `.proto` definitions to
the actual AST lives elsewhere.