Two writers, one CAS

Two processes must append without silent clobbering. Dacite’s move: compare-and-set is the whole distributed update. ref-swap! retries on conflict; SSE (GET /events) tells a third process the root moved. There is no lock and no CRDT in the core.

Two writers append to the event log on one HTTP root. This is the fourth claim-proving app in the roadmap.

You will:

1. Start clojure -M:service.
2. Append from two terminals without lost events.
3. Watch the root change without polling GET /root yourself.

Prerequisites

• JDK 17+ and the Clojure CLI
• The event-log app from the previous tutorial

Terminal 1 — the service

cd impl/clojure
clojure -M:service --port 8080 --store mem

Terminal 2 and 3 — two writers

cd impl/clojure
clojure -M:log -- --url http://127.0.0.1:8080 --n 0 append credit 1 from-a

cd impl/clojure
clojure -M:log -- --url http://127.0.0.1:8080 append debit 1 from-b

Each append uses v/ref-swap-info!: read the current ledger, conj, CAS the new root. If the other writer landed first, CAS fails, the fn runs again on the new ledger, and both events stay in the log. When a retry happened the CLI prints cas retried N time(s).

To force collisions:

clojure -M:log -- --url http://127.0.0.1:8080 contend 10

Two remote clients each append 10 events. The final size is start+20. cas-retries is how many times a writer rebuilt on a newer root.

Terminal 4 — watch

clojure -M:log -- --url http://127.0.0.1:8080 watch

This is GET /events (SSE), not a sleep-and-poll loop. The first frame is the current root; later frames fire after a successful CAS. Append in another terminal and this one reprints.

What this pulled

Async browser networking stayed on the shelf. Two JVM remotes plus an SSE watch prove the claim; sync XHR is still the browser demo.

Next

Sync a tree of blobs — list metadata; fetch one file.