feat(kv): watch_prefixes — single multi-filter consumer (v0.3.2)#4
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KvWatcher::watch_prefixes(&[prefix]) watches the union of several prefixes through ONE NATS consumer, using server 2.10 multi-filter consumers (async-nats watch_many -> OrderedConfig.filter_subjects), instead of one consumer per prefix. This matters because JetStream consumers are a per-stream resource. Measured on a single stream: ~tens of KB of server RSS per consumer, growing super-linearly past a few thousand (1.5 -> 22 -> 33 -> 56 KB/consumer at 200/600/1200/2400). A node that watches K prefixes must therefore cost 1 consumer, not K — at fleet scale K-per-node is multiple TB of consumer state on one stream leader; 1-per-node is the only shape that fits. Proven by an integration test against a real nats-server (watch_prefixes_unions_on_a_single_consumer): the KV stream carries exactly one consumer, both watched prefixes are delivered, and a third prefix is filtered server-side (never delivered). Empty prefixes watch nothing (NOT the whole bucket). Trait default is required; all impls updated. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What
Adds
KvWatcher::watch_prefixes(&[prefix]): watch the union of several prefixes through ONE NATS consumer (server 2.10 multi-filter consumers, via async-natswatch_many→OrderedConfig.filter_subjects), instead of one consumer per prefix. Bumps to 0.3.2.Why (measured)
JetStream consumers are a per-stream resource. Measured server RSS per durable consumer on a single stream — and it's super-linear:
A node scoped to K prefixes must cost 1 consumer, not K. At fleet scale (e.g. 500k nodes × ~50 vpcs), K-per-node ≈ 25M consumers ≈ >1 TB of consumer state on one stream leader (impossible); 1-per-node is the only shape that fits.
Proof (integration test, real nats-server)
watch_prefixes_unions_on_a_single_consumer:consumer_count == 1),Empty prefixes watch nothing (explicitly guarded — an empty filter set would otherwise watch the whole bucket).
Notes
watch_prefixesis a required trait method;NatsKvWatcher(real), theappliedmocks, and the bench mock all implement it.server_2_10(a default feature).cargo testgreen;clippy --all-targets --features fjall -D warningsclean.🤖 Generated with Claude Code