afterlog

Structured logging for TypeScript. One JSON object per request.

``typescript
import { afterlog } from "afterlog"

const builder = afterlog.createBuilder({
http_method: "GET",
path: "/users/123"
})

builder.set("user_id", "123")

const user = await builder.timing("database", () => db.getUser("123"))

await afterlog.finalize(builder)
// {"request_id":"550e8400-...","trace_id":"a12b34cd-5678-40ef-abcd-...","http_method":"GET",...}
`

Install

Quick Start

Configure once:

`typescript
import { afterlog, createConsoleAdapter } from "afterlog"

afterlog.configure({
adapter: createConsoleAdapter()
})
`

Use in your routes:

`typescript
app.get("/users/:id", async (req, res) => {
const builder = afterlog.createBuilder({
http_method: req.method,
path: req.path
})

const user = await builder.timing("db", () => db.getUser(req.params.id))
builder.set("user_id", user.id)

await afterlog.finalize(builder)
res.json(user)
})
`

What You Get

One JSON object per request with:

- request_id - UUID unique per request
- trace_id - UUID shared across services (auto-generated if not provided)
- timings - how long each operation took
- error - normalized error info
- Your custom fields

`json
{
"request_id": "550e8400-e29b-41d4-a716-446655440000",
"trace_id": "a12b34cd-5678-40ef-abcd-1234567890ab",
"timestamp": "2024-01-15T10:30:00.000Z",
"http_method": "GET",
"path": "/users/123",
"user_id": "123",
"timings": {
"db": 45,
"cache": 5
}
}
`

Why Wide Events?

Traditional logging:
`
[10:30:00] GET /users/123
[10:30:00] Database query: SELECT * FROM users WHERE id=123
[10:30:01] Cache miss
[10:30:02] Response: 200
`

Wide event logging:
`json
{"http_method":"GET","path":"/users/123","timings":{"db":1000,"cache":50}}
`

- Query by any field
- One write per request
- Works with tracing

Documentation

- Getting Started - Setup and basic usage
- Examples - Common patterns
- API Reference - All methods and types

Adapters

afterlog doesn't send logs anywhere by default. You write the adapter:

`typescript
const datadogAdapter = {
emit: async (event) => {
await fetch("https://http-intake.logs.datadoghq.com/v1/input", {
method: "POST",
headers: { "DD-API-KEY": process.env.DD_API_KEY },
body: JSON.stringify(event)
})
}
}

afterlog.configure({ adapter: datadogAdapter })
`

Or use the built-in console adapter for development.

Sampling

Don't log everything. Sample by error, latency, or random:

`typescript
import { errorRule, createLatencyRule } from "afterlog"

afterlog.configure({
adapter: myAdapter,
sampling: {
rules: [
errorRule, // Always log errors
createLatencyRule({ threshold_ms: 1000, sample_rate: 1.0 }) // Always log slow requests
],
default_rate: 0.05 // 5% of the rest
}
})
``

License

MIT