discuss: Remove Metakey Concept

[Xuanwo]

Metakey is designed for users to query metadata from storage services. However, it is so complex that users struggle to use it correctly, while developers find it challenging to implement properly.

When I designed Metakey, I expect users to use them like the following:

let lister = op.lister_with(path).metakey(Metakey::ContentLength | Metakey::ContentType).await?;

Users can select the meta they want, and opendal will determine whether to fetch more as needed.

However, this mechanism has been used incorrectly in many ways.

Extra stat call

Users aren't aware that setting Metakey::Full results in an additional stat call for each entry, which can make list performance up to 1000 times slower.

Unexpected metakey used

Many users will use Metakey in this way:

let entries = op.lister_with(path).metakey(Metakey::ContentLength).await?.collect().await;

...

entry.metadata().content_type(); // This meta is not queried.

Overlap with version

Lister now includes a new argument called version(bool), which determines whether to list versioned objects. This somewhat overlaps with Metakey::Version, potentially causing confusion.

---

Although I still love Metakey, it is complex and can be easily misused in the wrong API, so it should not exist in OpenDAL.

I suggest to remove this concept from the OpenDAL, and with the following changes:

• Add MetadataCapability which a struct like:

struct MetadataCapability {
  content_length: bool,
  content_type: bool,
  ...
}

Services can return MetadataCapability to indicate how much metadata will be included in the returned data. Users can use this capability to decide whether to perform extra stat or not.

• Don't perform stat during list

The list will no longer perform stat operations. Users who want this functionality can easily use the Stream API to compose them instead.

After those changes, OpenDAL's Lister will be much simpler and more predictable.

What do you think? I will propose an RFC if no objection.