MongoDB Developer Associate case studies
MongoDB Developer Associate · Put your knowledge to work in context.
The MongoDB Associate Developer Exam is MongoDB's certification for developers who build applications against MongoDB. MongoDB describes it as validating "your MongoDB Developer skills by certifying your mastery of building modern applications with MongoDB and Node.js, Python, Java, PHP or C#." It is aimed at working developers rather than administrators or data modelers - MongoDB lists it alongside separate Associate Data Modeler, Associate Atlas Administrator, and Associate Database Administrator exams, so do not expect deep coverage of cluster operations or schema design methodology here. The MongoDB Developer Associate case studies are 5 cases and about 4,759 words.
1Ledger double-spend: replication lag and a retry that mints a new _id
A payments ledger runs on a three-member replica set (MongoDB 6.0, no arbiter). During month-end batch reporting, secondaries lag the primary by up to six seconds.
921 words, about 5 min2Hot ticket query: compound index order and a blocking sort at 32 MB
An internal ticketing SaaS stores 42 million documents (average 1.1 KB) in one tickets collection on a three-node replica set. The hottest query, run about 400 times per minute, is db.tickets.find({tenantId: "t 88", status: "open", assigneeId: "u 412"}).sort({createdAt: -1}).limit(50).
975 words, about 5 min3Product catalog with an unbounded reviews array approaching 16 MB
An e-commerce catalog has 3.5 million product documents, each embedding a reviews array. The median product holds 35 reviews, but the top 0.4% (about 14,000 products) hold more than 20,000 reviews, and 300 documents have already reached the 16 MB BSON limit, causing writes to fail with "object to insert too large".
940 words, about 5 min4Node.js on Lambda: connection storm and a doubly applied $merge
A Node.js 18 service on AWS Lambda using the MongoDB Node.js driver 6.x talks to a three-node Atlas replica set. The handler constructs new MongoClient(uri) at the top of every invocation, calls connect() , runs one aggregation that ends in $merge into a daily stats collection, then calls client.close() .
968 words, about 5 min5Java transactions: two error labels, a duplicated outbox event, and a 60-second batch
A Java 21 service using the synchronous MongoDB driver moves money between accounts on a three-node replica set. Each transfer calls startTransaction() with readConcern SNAPSHOT and writeConcern MAJORITY, debits one account, credits another, and inserts a document into an outbox collection that a downstream service polls.
955 words, about 5 min