Correlation

See Correlation, identity, and causation for what a correlation ID is and why Chronicle tracks it. This page covers the TypeScript-specific API in depth. A CorrelationId is sent with every event append as the CorrelationId field.

CorrelationId

import { CorrelationId } from '@cratis/chronicle';

// Generate a new unique correlation ID
const id = CorrelationId.create();

// Wrap an existing string (e.g. from an incoming X-Correlation-Id header)
const fromHeader = new CorrelationId('d4e5f6a7-...');

// Well-known sentinel for an unset ID
const empty = CorrelationId.notSet;

console.log(id.toString()); // 'xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx'

Managing the correlation ID for a call context

The module exports a singleton correlationIdManager backed by Node.js AsyncLocalStorage. The ID is scoped automatically to the active async context — no need to pass it through every function call.

Reading the current ID

import { correlationIdManager } from '@cratis/chronicle';

const id = correlationIdManager.current;
// If nothing has been set in this context, a new GUID is generated on each access.

Setting a fixed ID

import { correlationIdManager, CorrelationId } from '@cratis/chronicle';

correlationIdManager.setCurrent(new CorrelationId('d4e5f6a7-...'));

Resetting to a new ID

correlationIdManager.clear(); // replaces the stored ID with a fresh GUID

Express middleware example

import express from 'express';
import { correlationIdManager, CorrelationId } from '@cratis/chronicle';

app.use((req, res, next) => {
    const header = req.headers['x-correlation-id'];
    const id = header
        ? new CorrelationId(String(header))
        : CorrelationId.create();

    correlationIdManager.setCurrent(id);
    res.setHeader('x-correlation-id', id.toString());
    res.on('finish', () => correlationIdManager.clear());
    next();
});

Interface segregation

CorrelationIdManager implements two separate interfaces, following the same pattern as the .NET client:

Interface	Purpose
ICorrelationIdAccessor	Read-only access — exposes current. Inject this where you only need to read the ID.
ICorrelationIdSetter	Write access — exposes setCurrent() and clear(). Inject this where you need to set or reset the ID.

import { ICorrelationIdAccessor, ICorrelationIdSetter, CorrelationIdManager } from '@cratis/chronicle';

// Narrow the type for read-only consumers
function logCorrelation(accessor: ICorrelationIdAccessor) {
    console.log('Correlation:', accessor.current.toString());
}

// Narrow the type for writers (e.g. middleware, test setup)
function resetForTest(setter: ICorrelationIdSetter) {
    setter.clear();
}

const manager = new CorrelationIdManager();
logCorrelation(manager);
resetForTest(manager);

How correlation flows into events

When you call eventLog.append() or eventLog.appendMany(), the Chronicle client automatically uses correlationIdManager.current as the CorrelationId for the gRPC request unless you provide an explicit correlationId in AppendOptions:

import { CorrelationId } from '@cratis/chronicle';

// Uses correlationIdManager.current automatically
await store.eventLog.append(eventSourceId, new EmployeeHired(...));

// Override for this single append
await store.eventLog.append(eventSourceId, new EmployeeHired(...), {
    correlationId: CorrelationId.create()
});