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Mock API server

README · Docs · Guides · Packages

Deterministic HTTP mock: same URL, same response, every run, every machine. Use the library inside a minimal server, or generate a static fixture and serve it with an existing mock tool.

Contents

Express / FastAPI endpoint

// server.js
const express = require("express");
const { SeedFaker } = require("@opendsr/seedfaker");
const app = express();
const sf = new SeedFaker({ seed: "mock", locale: "en", until: "2025" });

app.get("/api/users", (req, res) => {
  const n = Math.min(parseInt(req.query.limit) || 20, 100);
  res.json(sf.records(["name", "email", "phone"], { n, ctx: "strict" }));
});

app.listen(3000);
# FastAPI equivalent
from fastapi import FastAPI
from seedfaker import SeedFaker
app = FastAPI()
sf = SeedFaker(seed="mock", locale="en", until="2025")

@app.get("/api/users")
def users(limit: int = 20):
    return sf.records(["name", "email", "phone"], n=min(limit, 100), ctx="strict")

Same seed → same response bytes on every restart.

Stable entity by id

Seed a fresh SeedFaker from the entity id. /api/users/42 always returns the same user.

app.get("/api/users/:id", (req, res) => {
  const f = new SeedFaker({ seed: `user-${req.params.id}`, locale: "en", until: "2025" });
  res.json(f.record(["name", "email", "phone", "address"], { ctx: "strict" }));
});

Deterministic pagination

Seed per page — GET /api/users?page=5 returns identical rows on every call.

app.get("/api/users", (req, res) => {
  const page = parseInt(req.query.page) || 1;
  const size = Math.min(parseInt(req.query.size) || 20, 100);
  const f = new SeedFaker({ seed: `users-page-${page}`, locale: "en", until: "2025" });
  res.json({ page, data: f.records(["name", "email", "phone"], { n: size, ctx: "strict" }) });
});

For global sequential access (row N of the full set), pre-generate a JSONL with the CLI + --shard and serve slices from memory.

Relational mock

Multi-table YAML gives linked entities without a database:

# shop.yaml
options: { seed: shop-mock, until: "2025" }

users:
  columns: { id: serial, name: first-name, email: email }
  options: { count: 1000, ctx: strict }

orders:
  columns:
    id: serial
    user_id: users.id:zipf
    user_email: user_id->email
    total: amount:usd:1..5000
    status: enum:pending=30,paid=60,cancelled=10
  options: { count: 5000 }
const fs = require("fs");
const { execSync } = require("child_process");
execSync("seedfaker run shop.yaml --all --output-dir ./mock-data --format jsonl");

const users  = fs.readFileSync("mock-data/users.jsonl",  "utf8").trim().split("\n").map(JSON.parse);
const orders = fs.readFileSync("mock-data/orders.jsonl", "utf8").trim().split("\n").map(JSON.parse);

app.get("/api/users/:id/orders", (req, res) => {
  res.json(orders.filter(o => String(o.user_id) === req.params.id));
});

orders[k].user_email equals the email of the user with orders[k].user_id. FK semantics: docs/multi-table.

Coherent identities

ctx: "strict" locks every field in a record to one identity per row — name, email, phone, address, gov ID match. Without it, each field is drawn independently (a Japanese name next to a German phone number). Details: docs/context.

Custom JSON shape

Inline template:

seedfaker name email -t '{"user": "{{name}}", "contact": "{{email}}"}' \
  --seed api --until 2025 -n 3

Nested via config template: docs/templates.

Static fixture file

For json-server, MSW, Playwright route mocks:

seedfaker name email phone role=enum:admin=1,user=9 \
  --format jsonl --seed api --until 2025 -n 100 > users.jsonl

Commit the file or regenerate in CI with the same seed. Pin --fingerprint to catch algorithm drift.

Error and latency injection

Deterministic chaos from the URL, independent of seedfaker:

app.get("/api/users", async (req, res) => {
  const hash = s => [...s].reduce((h, c) => ((h << 5) - h) + c.charCodeAt(0), 0);
  if ((hash(req.url) & 0xFF) < 13) return res.status(500).json({ error: "simulated" });
  await new Promise(r => setTimeout(r, (hash(req.url) >> 8) & 0xFF));
  res.json(sf.records(["name", "email"], { n: 20 }));
});

For streaming load tests with --rate and --corrupt: api-load-testing.

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