Large sequences stay cheap

You have thousands of events. You need a page of them and a derived balance, not the whole log in RAM. Dacite’s move: v/subvec and nth on a vector; append is conj. Do not seq the log to print page 0.

This is the third claim-proving app in the roadmap.

You will:

1. Seed 2000 events (not five).
2. Page the log with v/subvec — no seq of the whole vector.
3. Replay the view from a prefix via nth.
4. See that one more append adds a handful of nodes at n=100 and at n=2000.

Prerequisites

• A clone of this repo (see Install)
• JDK 17+ and the Clojure CLI, or Node.js 18+ / babashka for the file store

The shape

{"log"  [{"type" "credit"|"debit" "amount" n "note" s} …]
 "view" {"size" n "credits" n "debits" n "balance" n}}

Append updates the view incrementally. Replay rebuilds the view from log[0, end) without walking the tail.

Domain code lives in dacite.examples.event-log and uses only dacite.value. Pagination pulled v/subvec.

Local file

cd impl/clojure
clojure -M:log -- --reset show

Or from the repo root (file store; seeding 2000 events takes a few seconds):

npm run log -- --reset show
bb log --reset show

--n 200 seeds a smaller log. Expected shape:

seeded new store at target/dacite-log (2000 events)
size:     2000
credits:  …
debits:   …
balance:  …
log:      2000 events
root:     <64 hex chars>

Page, append, replay:

clojure -M:log -- page 0
clojure -M:log -- page 3 10
clojure -M:log -- append credit 5 coffee
clojure -M:log -- replay 100

page prints a window and its content hash. replay 100 replaces the view with a fold of the first 100 events.

The same --url recipe as Persist and update a document works against clojure -M:service.

Append bench

clojure -M:log -- bench

Typical output:

append bench (nodes added by the last conj at each size)
  n=100   +N nodes
  n=500   +N nodes
  n=1000  +N nodes
  n=2000  +N nodes

The deltas stay small. They must not grow linearly with n — that would mean each append rewrote the log.

A prefix slice has the same hash as a log built from those events alone (hash fusion is shape-independent).

What this pulled

dac->clj is not on this path. Missing-node errors stay on the shelf until a remote page actually needs a catchable “not local” signal.

Next

Two writers, one CAS — ref-swap! retries; SSE watches the root.