MongoDB

A single-node MongoDB container running as a one-member replica set (required for transactions and change streams). A post-start hook runs rs.initiate() and polls for a writable primary before start() returns — so connectionString is always usable immediately, with no manual replica-set setup.

Default image: mongo:8.0Exposed port: 27017Command: mongod --replSet docker-rs --bind_ip_all

Member	Returns
MongoDBContainer.start(image?)	Promise<MongoDBContainer> — boots the container, initiates the replica set, and waits for a primary
.connectionString	A mongodb://host:port/test?directConnection=true connection string
.replicaSetUrl	Alias for .connectionString — the container is always a (single-node) replica set

Example

import { MongoDBContainer } from "rightsize/modules";
import { MongoClient } from "mongodb";

await using mongo = await MongoDBContainer.start();
const client = new MongoClient(mongo.connectionString);
await client.connect();
const db = client.db("test");
await db.collection("docs").insertOne({ k: "v" });
console.log(await db.collection("docs").findOne({ k: "v" }));
await client.close();

Backend notes

The replica-set initiation races the same early-accept behavior every Wait.forListeningPort() container has to account for (see Wait strategies) — the first rs.initiate() attempt can hit the container before mongod is genuinely ready to serve it. This module's post-start hook retries both rs.initiate() and the writable-primary check in a bounded loop for exactly this reason; no action needed on your part.