resonate-external-system-of-record-pattern-typescript

Resonate External System of Record Pattern (TypeScript)

Overview

The External System of Record pattern enables consistent state across multiple systems without distributed transactions. By designating one system as the "system of record" (source of truth) and treating others as "systems of reference" (cached/staged data), you can maintain consistency through careful ordering of writes and reads.

Core principle: Write to the system of record last. Read from the system of record first. Use Resonate's durable execution to guarantee eventual completion and automatic recovery.

Mental Model

WITHOUT TRANSACTIONS:

  System of Reference         System of Record
  (Staging/Cache)            (Source of Truth)
         │                          │
         │   Write Last →           │
         │   Read First ←           │
         │                          │
   [May be stale]           [Determines existence]

Resonate guarantees eventual completion:
- Crash after reference write? Resumes, completes record write
- Crash after record write? Resumes, skips completed steps

System Roles

System of Record

• The champion: If it exists here, it exists in reality
• Determines existence: The authoritative source of truth
• Examples: TigerBeetle ledger, payment processor, blockchain

System of Reference

• The supporter: Staging area or cache
• Does NOT determine existence: A record here without the system of record means nothing
• Examples: PostgreSQL, Redis, application database

Core Pattern: Write Last, Read First

Writing (Creating Records)

function* createEntity(ctx: Context, id: string, data: any) {
  // 1. Write to system of REFERENCE first (staging)
  yield* ctx.run(writeToReference, id, data);

  // 2. Write to system of RECORD last (commits existence)
  yield* ctx.run(writeToRecord, id, data);

  // Only NOW does the entity truly exist
  return { id, status: "created" };
}

Why this order?

• Reference write is "safe" - doesn't commit anything
• Only when record write succeeds does the entity spring into existence
• If crash happens between writes, Resonate will resume and complete the record write