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The only difference between - `pageserver_api::models::TenantConfig` and - `pageserver::tenant::config::TenantConfOpt` at this point is that `TenantConfOpt` serializes with `skip_serializing_if = Option::is_none`. That is an efficiency improvement for all the places that currently serde `models::TenantConfig` because new serializations will no longer write `$fieldname: null` for each field that is `None` at runtime. This should be particularly beneficial for Storcon, which stores JSON-serialized `models::TenantConfig` in its DB. # Behavior Changes This PR changes the serialization behavior: we omit `None` fields instead of serializing `$fieldname: null`). So it's a data format change (see section on compatibility below). And it changes API responses from Storcon and Pageserver. ## API Response Compatibility Storcon returns the location description. Afaik it is passed through into - storcon_cli output - storcon UI in console admin UI These outputs will no longer contain `$fieldname: null` values, which de-bloats the output (good). But in storcon UI, it also serves as an editor "default", which will be eliminated after a storcon with this PR is released. ## Data Format Compatibility Backwards compat: new software reading old serialized data will deserialize to the same runtime value because all the field types are exactly the same and `skip_serializing_if` does not affect deserialization. Forward compat: old software reading data serialized by new software will map absence fields in the serialized form to runtime value `Option::None`. This is serde default behavior, see this playground to convince yourself: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=f7f4e1a169959a3085b6158c022a05eb The `serde(with="humantime_serde")` however behaves strangely: if used on an `Option<Duration>`, it still requires the field to be present, unlike the serde default behavior shown in the previous paragraph. The workaround is to set `serde(default)`. Previously it was set on each individual field, but, we do have the container attribute, so, set it there. This requires deriving a `Default` impl, which, because all fields are `Option`, is non-magic. See my notes here: https://gist.github.com/problame/eddbc225a5d12617e9f2c6413e0cf799 # Future Work We should have separate types (& crates) for - runtime types configuration (e.g. PageServerConf::tenant_config, AttachedLocationConf) - `config-v1` file pageserver local disk file format - `mgmt API` - `pageserver.toml` Right now they all use the same, which is convenient but makes it hard to reason about compatibility breakage. # Refs - corresponding docs.neon.build PR https://github.com/neondatabase/docs/pull/470