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This is needed to avoid this https://github.com/mesonbuild/meson/issues/13774 when we go back to making our subproject directory `src`.
128 lines
4.4 KiB
Markdown
128 lines
4.4 KiB
Markdown
# JSON guideline
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Nix consumes and produces JSON in a variety of contexts.
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These guidelines ensure consistent practices for all our JSON interfaces, for ease of use, and so that experience in one part carries over to another.
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## Extensibility
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The schema of JSON input and output should allow for backwards compatible extension.
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This section explains how to achieve this.
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Two definitions are helpful here, because while JSON only defines one "key-value" object type, we use it to cover two use cases:
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- **dictionary**: a map from names to value that all have the same type.
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In C++ this would be a `std::map` with string keys.
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- **record**: a fixed set of attributes each with their own type.
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In C++, this would be represented by a `struct`.
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It is best not to mix these use cases, as that may lead to incompatibilities when the schema changes.
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For example, adding a record field to a dictionary breaks consumers that assume all JSON object fields to have the same meaning and type, and dictionary items with a colliding name can not be represented anymore.
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This leads to the following guidelines:
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- The top-level (root) value must be a record.
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Otherwise, one can not change the structure of a command's output.
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- The value of a dictionary item must be a record.
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Otherwise, the item type can not be extended.
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- List items should be records.
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Otherwise, one can not change the structure of the list items.
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If the order of the items does not matter, and each item has a unique key that is a string, consider representing the list as a dictionary instead.
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If the order of the items needs to be preserved, return a list of records.
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- Streaming JSON should return records.
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An example of a streaming JSON format is [JSON lines](https://jsonlines.org/), where each line represents a JSON value.
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These JSON values can be considered top-level values or list items, and they must be records.
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### Examples
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This is bad, because all keys must be assumed to be store types:
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```json
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{
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"local": { ... },
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"remote": { ... },
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"http": { ... }
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}
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```
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This is good, because the it is extensible at the root, and is somewhat self-documenting:
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```json
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{
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"storeTypes": { "local": { ... }, ... },
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"pluginSupport": true
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}
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```
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While the dictionary of store types seems like a very complete response at first, a use case may arise that warrants returning additional information.
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For example, the presence of plugin support may be crucial information for a client to proceed when their desired store type is missing.
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The following representation is bad because it is not extensible:
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```json
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{ "outputs": [ "out" "bin" ] }
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```
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However, simply converting everything to records is not enough, because the order of outputs must be preserved:
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```json
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{ "outputs": { "bin": {}, "out": {} } }
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```
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The first item is the default output. Deriving this information from the outputs ordering is not great, but this is how Nix currently happens to work.
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While it is possible for a JSON parser to preserve the order of fields, we can not rely on this capability to be present in all JSON libraries.
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This representation is extensible and preserves the ordering:
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```json
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{ "outputs": [ { "outputName": "out" }, { "outputName": "bin" } ] }
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```
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## Self-describing values
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As described in the previous section, it's crucial that schemas can be extended with new fields without breaking compatibility.
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However, that should *not* mean we use the presence/absence of fields to indicate optional information *within* a version of the schema.
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Instead, always include the field, and use `null` to indicate the "nothing" case.
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### Examples
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Here are two JSON objects:
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```json
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{
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"foo": {}
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}
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```
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```json
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{
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"foo": {},
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"bar": {}
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}
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```
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Since they differ in which fields they contain, they should *not* both be valid values of the same schema.
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At most, they can match two different schemas where the second (with `foo` and `bar`) is considered a newer version of the first (with just `foo`).
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Within each version, all fields are mandatory (always `foo`, and always `foo` and `bar`).
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Only *between* each version, `bar` gets added as a new mandatory field.
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Here are another two JSON objects:
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```json
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{ "foo": null }
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```
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```json
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{ "foo": { "bar": 1 } }
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```
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Since they both contain a `foo` field, they could be valid values of the same schema.
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The schema would have `foo` has an optional field, which is either `null` or an object where `bar` is an integer.
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