MongoDB Associate Developer flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

ObjectId structure

A 12-byte BSON type made of a 4-byte Unix timestamp, a 5-byte per-process random value, and a 3-byte incrementing counter, producing roughly time-ordered unique identifiers.

WiredTiger storage engine

MongoDB's default storage engine since version 3.0; it provides document-level concurrency control, checkpoints, journaling, and optional snappy or zstd compression for collections and indexes.

Replica set primary

The single replica-set member that accepts writes. Secondaries replicate its oplog and may serve reads depending on read preference, and an election chooses a new primary if it fails.

write concern majority

A write concern where the write is acknowledged only after a majority of voting replica-set members have applied it, protecting acknowledged writes from rollback during failover.

insertMany ordered:false

Option that makes a bulk insert continue processing remaining documents after an error, returning an array of write errors instead of stopping at the first failure.

replaceOne semantics

Replaces the entire matched document with the supplied replacement while preserving the existing _id; any field absent from the replacement is removed from the document.

upsert option

When an update or replace finds no matching document, upsert:true inserts a new document built from equality conditions in the filter combined with the update operators.

$addToSet versus $push

Both add values to an array, but $push always appends while $addToSet adds a value only if it is not already present, giving set-like deduplication.

Compound index prefix rule

A compound index supports queries on any leading subset of its fields in order; a query that skips the first indexed field cannot use the index efficiently.

Covered query

A query whose filter and projection use only indexed fields, so the server answers entirely from the index and explain shows no document fetch stage.

Multikey index

An index on a field that holds an array; MongoDB writes one index entry per array element, and a compound index may include at most one array field.

TTL index

A single-field index on a date field with expireAfterSeconds; a background thread deletes documents once the indexed date is older than the time to live.

$lookup stage

Aggregation stage that performs a left outer join with another collection, matching localField to foreignField or using a sub-pipeline with let variables.

preserveNullAndEmptyArrays

An option of $unwind that keeps documents whose array field is missing, null, or empty instead of dropping those documents from the pipeline output.

$group accumulators

Operators used inside $group, such as $sum, $avg, $min, $max, $push, $addToSet, $first, and $last, which compute values across documents sharing the same _id key.

allowDiskUse

An aggregation option that lets stages exceeding the 100 MB per-stage memory limit spill intermediate results to temporary files on disk instead of returning an error.

Embedding versus referencing

Embed related data when it is read together and bounded in size; reference by _id when the relationship is large, unbounded, or shared across many parent documents.

Bucket pattern

A modeling approach that groups many small, time-ordered events into a bounded number of documents, avoiding unbounded arrays and keeping index size small.

Unbounded array risk

Arrays that grow without limit can approach the 16 MB document limit, inflate index size, and force expensive full-document rewrites on every update.

Extended reference pattern

Copies a few frequently needed fields from a referenced document into the parent to avoid joins, accepting duplicated data that must be resynchronized periodically.

withTransaction helper

A driver helper that runs a callback inside a transaction and automatically retries on TransientTransactionError and UnknownTransactionCommitResult, simplifying correct transaction code.

Snapshot read concern

The read concern used in multi-document transactions; it returns data as of a single point in time so every read in the transaction sees a consistent snapshot.

Single-document atomicity

Every write to an individual document is atomic in MongoDB even without transactions, which is why embedded data models often avoid multi-document transactions entirely.

transactionLifetimeLimitSeconds

Server parameter defaulting to 60 seconds that aborts transactions running too long; long transactions also hold snapshots and prevent oplog truncation.

MongoClient singleton

A thread-safe driver object that owns a connection pool; applications create one per process and reuse it across requests rather than connecting per request.

mongodb+srv connection string

A URI scheme that resolves a DNS SRV record to discover replica-set hosts and default options, commonly used for Atlas cluster connections.

maxPoolSize

Connection pool setting that caps connections per server; the driver default is 100, and pools should be sized against the server's connection limits.

Duplicate key error 11000

Error code returned when a write violates a unique index; drivers surface it as a write error that applications must handle rather than blindly retry.

mongosh

The modern MongoDB shell, a Node.js-based REPL that supports JavaScript with async APIs, EJSON output, startup scripts, and loading files through load().

Compass schema analysis

A feature of the MongoDB Compass GUI that samples documents to display field types, value distributions, and index usage for schema and query design.

Atlas Search $search

An aggregation stage available in Atlas that runs full-text Lucene-based queries against an Atlas Search index using operators such as text, autocomplete, and compound.

mongodump and mongorestore

Database Tools utilities that produce and restore a binary BSON export of a database; used for backups and migrations, distinct from JSON-based mongoexport and mongoimport.