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MongoDbContainer

A single-node MongoDB container started as a one-member replica set (named docker-rs) — required for transactions and change streams, which is why this module doesn’t just boot a bare standalone mongod.

Default image: mongo:8.0 Guest port: 27017

MethodOnEffect
MongoDbContainer::new()builderPinned default image.
MongoDbContainer::with_image(image)builderCaller-chosen image.
.start()builder → Result<MongoDbGuard>Boots the container; does not return until the replica set has an elected primary.
.connection_string()guardmongodb://host:port/test?directConnection=true.
.replica_set_url()guardAlias for connection_string().
.stop()guardStops and removes the container, releases its port.

The post-start hook

start() runs a Container::with_post_start hook (see Containers & Guards) that, once the plain listening-port wait passes:

  1. Runs rs.initiate() via mongosh — checking rs.status() first, so a retry after a partial initiate doesn’t re-initiate a set that’s already forming.
  2. Polls db.hello().isWritablePrimary until it reports true.

Both steps retry through the proxy-accepts-before-mongod-listens race (the same read-probe-relevant timing gap covered in Wait Strategies): the listening-port wait can return before mongod is far enough along to accept a real client command, so the first few mongosh invocations failing is expected and retried, not a fatal error. The practical upshot: connection_string() is always immediately usable once start() returns Ok — no separate “wait for the replica set” step needed in your own test.

Complete example

use rightsize_modules::MongoDbContainer;

#[tokio::test]
async fn mongo_accepts_a_write() -> rightsize::Result<()> {
    let guard = MongoDbContainer::new().start().await?;

    // start() already awaited a writable primary — connection_string() is usable now.
    let insert = guard
        .exec(&["mongosh", "--quiet", "--eval", "db.smoke.insertOne({ok: 1})"])
        .await?;
    assert_eq!(insert.exit_code, 0);

    let count = guard
        .exec(&["mongosh", "--quiet", "--eval", "db.smoke.countDocuments({ok: 1})"])
        .await?;
    assert!(count.stdout.trim().ends_with('1'));

    guard.stop().await?;
    Ok(())
}

Swap the exec-based mongosh calls above for the mongodb driver crate of your choice against guard.connection_string() in your own tests — this crate’s own integration suite uses exec specifically to avoid taking on a MongoDB client-library dev-dependency just for a smoke test.

Backend notes

No memory-limit override. The replica-set initiation retries absorb the same proxy-timing gap that affects wait strategies generally on both backends — nothing MongoDB-specific beyond that.