Add metadata ADR

[Jarema]

Metadata for Stream and Consumer

Metadata	Value
Date	2023-01-23
Author	@Jarema
Tags	jetstream, client, server

Context and Problem Statement

Until now, there was no way to easily add additional information about Stream or Consumer.
The only solution was using Description field, which is a ugly workaround.

Server PR

nats-io/nats-server#3797

Design

The solution is to add new metadata field to both Consumer and Stream config.
The metadata field would by a map of string keys and string values.

The map would be represented in json as object with nested key/value pairs, which is a default
way to marshal maps/hashmaps in most languages.

Example

{
  "durable_name": "consumer",
  ... // other consumer/stream fields
  "metadata": {
    "owner": "nack",
    "domain": "product"
  }
}

Open questions

Do we want to limit the size of metadata?

Signed-off-by: Tomasz Pietrek tomasz@nats.io

[wallyqs]

considering we could add some private internal metadata namespace when it starts with underscore like "_created_nats_version":"2.10.0" that is immutable and we can pinpoint the origin of the objects for debugging, also via the metadata fields would be useful to stamp whether it was created for a KV/ObjectStore usage for example as a hint for debugging and troubleshooting.

[Jarema]

The internal namespace is a good idea!

Maybe _nats ?

Those could be use by both client internals and the server.
Server could ensure immutability of its entries.

[wallyqs]

_nats sounds good 👍

[Jarema]

Improved ADR after discussions:

1. Set limit to 128kB
2. Added reserved prefix for internals _nats.

[Jarema]

@ripienaar @wallyqs ADR updated, PR updated.

[ripienaar]

LGTM