nips/0A.md

NIP-0A
======

Follow List v2
--------------

An event with kind `103` means "follow list" which communicates the set of profiles
(represented by public keys) that the author is currently following.

The public keys followed are specified in "p" tags.

Additional "np" tags are present only to help with synchronization as described below.

The `.content` is not used.

## Tags

Each profile followed is reprsented in a "p" tag as follows:

```
    ["p", <pubkey>, <relay>, <petname>, <timestamp>]
```

Where

* `pubkey` is the hex encoded public key of the profile
* `relay` is an optional relay where the `pubkey` is known to post, or an empty string
* `petname` is an optional petname that the author uses for the contact, or an empty string
* `timestamp` is the time when the entry was added

## Removal of entries

Entries are not removed.  Instead they are converted into "np" entries and their
timestamp is updated to time they are considered removed.

```
    ["np", <pubkey>, <relay>, <petname>, <timestamp2>]
```

Pubkeys MUST only be listed once in either a "p" tag or an "np" tag.

`p` means the person is counted as a contact. `np` means they are not counted as a contact
but the entry serves to help remember when they were removed so that conflicts can be
resolved.

Note that `np` is not searchable on relays.

All contacts should be listed. These events are complete lists, they are not change sets.

## Owner Handling

When you add a person, you find their entry and make sure the tag is "p" and not "np" and then you
update the timestamp to the current time. If missing, you create the entry.

When you remove a person, you find their entry and make sure the tag is "np" and not "p" and then
you update the timestamp to the current time. If missing, you create the entry.

When you receive your own contact list (presumably created by a different client), you merge it
with your local one and if the data has changed you publish the result. See
[Merge Operation](#merge-operation) below.

## Observers

A client that does not store any data can simply accept the latest contact list as probably
up to date.

Clients that do store data can instead choose to merge newer contact lists with the data they
already hold for the person's contacts that was created from previous events. The benefit of
doing this is only slight.

## Merge Operation

The merge operation is as follows: On a tag-by-tag basis, if only one event has a line for a pubkey,
you accept that line. If both events have a line for that pubkey, you take the line with the largest
timestamp.

## Rationale

This is functionally implements a Last-Write-Wins Element Set, which is a conflict-free replicated
data set with eventual consistency.