nips/45.md

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 for the verb REQ. Multiple filters are OR'd together and aggregated into a single count result.

["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>}.

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

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

HyperLogLog

Relays may return an HyperLogLog value together with the count, hex-encoded.

["COUNT", <subscription_id>, {"count": <integer>, "hll": "<hex>"}]

This is so it enables merging results from multiple relays and yielding a reasonable estimate of reaction counts, comment counts and follower counts, while saving many millions of bytes of bandwidth for everybody.

Algorithm

This section describes the steps a relay should take in order to return HLL values to clients.

1. Upon receiving a filter, if it is eligible (see below) for HyperLogLog, compute the deterministic offset for that filter (see below);
2. Initialize 256 registers to 0 for the HLL value;
3. For all the events that are to be counted according to the filter, do this:
   1. Read the byte at position offset of the event pubkey, its value will be the register index ri;
   2. Count the number of leading zero bits starting at position offset+1 of the event pubkey and add 1;
   3. Compare that with the value stored at register ri, if the new number is bigger, store it.

That is all that has to be done on the relay side, and therefore the only part needed for interoperability.

On the client side, these HLL values received from different relays can be merged (by simply going through all the registers in HLL values from each relay and picking the highest value for each register, regardless of the relay).

And finally the absolute count can be estimated by running some methods I don't dare to describe here in English, it's better to check some implementation source code (also, there can be different ways of performing the estimation, with different quirks applied on top of the raw registers).

Filter eligibility and offset computation

This NIP defines (for now) two filters eligible for HyperLogLog:

• {"#p": ["<pubkey>"], "kinds": [3]}, i.e. a filter for kind:3 events with a single "p" tag, which means the client is interested in knowing how many people "follow" the target <pubkey>. In this case the offset will be given by reading the character at the position 32 of the hex <pubkey> value as a base-16 number then adding 8 to it.
• {"#e": ["<id>"], "kinds": [7]}, i.e. a filter for kind:7 events with a single "e" tag, which means the client is interested in knowing how many people have reacted to the target event <id>. In this case the offset will be given by reading the character at the position 32 of the hex <id> value as a base-16 number then adding 8 to it.

Attack vectors

One could mine a pubkey with a certain number of zero bits in the exact place where the HLL algorithm described above would look for them in order to artificially make its reaction or follow "count more" than others. For this to work a different pubkey would have to be created for each different target (event id, followed profile etc). This approach is not very different than creating tons of new pubkeys and using them all to send likes or follow someone in order to inflate their number of followers. The solution is the same in both cases: clients should not fetch these reaction counts from open relays that accept everything, they should base their counts on relays that perform some form of filtering that makes it more likely that only real humans are able to publish there and not bots or artificially-generated pubkeys.

hll encoding

The value hll value must be the concatenation of the 256 registers, each being a uint8 value (i.e. a byte). Therefore hll will be a 512-character hex string.

Examples

Count posts and reactions

["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}]

Followers count with HyperLogLog

["COUNT", <subscription_id>, {"kinds": [3], "#p": [<pubkey>]}]
["COUNT", <subscription_id>, {"count": 16578, "hll": "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"}]

Relay refuses to count

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