natsio-counters

Distributed counters over JetStream counter streams (ADR-49), mirroring orbit.go/counters. Distribution natsio-counters, imported as natsio.counters. Zero runtime dependencies beyond natsio. Pre-1.0, no API-stability promises.

Requires nats-server 2.12+ (the allow_msg_counter stream feature).

What it is

A counter stream is a normal JetStream stream created with allow_msg_counter. Every subject the stream captures becomes an independent, arbitrary-precision counter. Incrementing is an ordinary publish carrying a Nats-Incr header; the server folds the delta into a running total, stores it as the subject's latest message, and returns the new value in the PubAck — so a bare add needs no follow-up read.

Python int is itself unbounded, so it is used directly for both deltas and values — no big-integer wrapper.

Usage

import natsio
from natsio.counters import CounterConfig, create_counter, get_counter

nc = await natsio.connect("nats://localhost")
js = nc.jetstream()

# Create a counter stream (allow_msg_counter + allow_direct are forced on).
counter = await create_counter(js, CounterConfig(name="COUNTS", subjects=["events.>"]))
# ...or bind an existing one:  counter = await get_counter(js, "COUNTS")

# Increment / decrement — returns the new running total, straight from the PubAck.
await counter.add("events.orders", 1)      # -> 1
await counter.add("events.orders", 10)     # -> 11
await counter.add("events.orders", -4)     # negative deltas are fine  -> 7
await counter.add("events.orders", 0)      # zero is a valid no-op increment

# Read a single counter's current value.
value = await counter.load("events.orders")          # -> 7

# Full entry: value + most-recent increment + source history (aggregation only).
entry = await counter.get("events.orders")
print(entry.value, entry.incr, entry.sources)

# Enumerate many counters in one batch Direct Get (wildcards allowed).
async for entry in counter.get_multiple(["events.>"]):
    print(entry.subject, entry.value)

API

Errors (all subclass natsio.jetstream.JetStreamError): CounterNotEnabledError, DirectAccessRequiredError, CounterNotFoundError, CounterSubjectNotInitializedError, InvalidCounterValueError.

ADR-49 wire contract

Pinned by the wire-contract tests and exported as constants:

• Increment header Nats-Incr (COUNTER_INCREMENT_HEADER) — a base-10 integer of any size, optional leading sign. A counter stream rejects any publish without it (message counter increment is missing).
• Sources header Nats-Counter-Sources (COUNTER_SOURCES_HEADER) — server-set JSON {source_stream: {subject: contribution}}; present only on aggregating counters.
• Stored payload {"val": "<int-as-string>"} — the value is a string so it survives languages whose JSON numbers cap at 2^53.
• PubAck carries the new total in its val field (fast feedback, no Get).

Notes: allow_msg_counter can be set only at stream creation (it is read-only afterwards) and is incompatible with per-message TTLs and message schedules, so CounterConfig intentionally does not expose those knobs. Reads use Direct Get, so allow_direct is required (and forced on by create_counter).

Example

A runnable script is at examples/basic.py — start a server with just server, then:

python extensions/natsio-counters/examples/basic.py