NIP-45
======

Event Counts
--------------

`draft` `optional`

Relays may support the verb `COUNT`, which provides a mechanism for obtaining event counts.

## Motivation

Some queries a client may want to execute against connected relays are prohibitively expensive, for example, in order to retrieve follower counts for a given pubkey, a client must query all kind-3 events referring to a given pubkey only to count them. The result may be cached, either by a client or by a separate indexing server as an alternative, but both options erode the decentralization of the network by creating a second-layer protocol on top of Nostr.

## Filters and return values

This NIP defines the verb `COUNT`, which accepts a subscription id and filters as specified in [NIP 01](01.md) for the verb `REQ`. Multiple filters are OR'd together and aggregated into a single count result.

```json
["COUNT", <subscription_id>, <filters JSON>...]
```

Counts are returned using a `COUNT` response in the form `{"count": <integer>}`. Relays may use probabilistic counts to reduce compute requirements.
In case a relay uses probabilistic counts, it MAY indicate it in the response with `approximate` key i.e. `{"count": <integer>, "approximate": <true|false>}`.

```json
["COUNT", <subscription_id>, {"count": <integer>}]
```

Whenever the relay decides to refuse to fulfill the `COUNT` request, it MUST return a `CLOSED` message.

## Examples

### Followers count

```json
["COUNT", <subscription_id>, {"kinds": [3], "#p": [<pubkey>]}]
["COUNT", <subscription_id>, {"count": 238}]
```

### Count posts and reactions

```json
["COUNT", <subscription_id>, {"kinds": [1, 7], "authors": [<pubkey>]}]
["COUNT", <subscription_id>, {"count": 5}]
```

### Count posts approximately

```
["COUNT", <subscription_id>, {"kinds": [1]}]
["COUNT", <subscription_id>, {"count": 93412452, "approximate": true}]
```

### Relay refuses to count

```
["COUNT", <subscription_id>, {"kinds": [4], "authors": [<pubkey>], "#p": [<pubkey>]}]
["CLOSED", <subscription_id>, "auth-required: cannot count other people's DMs"]
```