Pass the MongoDB MongoDB Certified DBA Associate C100DBA Questions and answers with CertsForce

MongoDB write concern defines the level of acknowledgment requested for a write. Setting the numeric value w: 1 means that a write is considered acknowledged after the primary has accepted it; no secondary acknowledgment is required. In a replica set with one primary and two secondaries, those secondaries can replicate the operation asynchronously after the client has already received success. That configuration is represented by { writeConcern: { w: 1 } }, making option D correct. The numeric type matters: w can be specified as an integer count or certain strings such as " majority " , but the string " 1 " is not the intended numeric node count shown by the question. Options B and C also omit the w field structure expected inside a write concern document. The durability trade-off is important: a w:1 write can be acknowledged before any secondary has replicated it, so a sudden primary failure before replication may result in that write being rolled back after failover. If the workload requires stronger durability, w: " majority " is commonly used. For exactly one acknowledging node, however, the correct setting is D.

Study Guide reference/topic:Cluster Reliability and Data Resilience - write concern, w acknowledgments, replica-set durability, and rollback risk.

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An upsert combines update behavior with conditional insertion. updateOne() first searches for a document matching { product: " laptop " }. If one exists, $set changes unitsInStock to 25. If none exists and the options document contains { upsert: true }, MongoDB inserts a new document based on the equality portions of the filter together with the update result. Option A uses exactly this pattern. Options B and C incorrectly use insertOne(), which inserts a supplied document and does not accept an update-operator document plus upsert semantics in this form. Option D uses the correct method and update structure but specifies True with a capital T. Mongosh uses JavaScript Boolean syntax, so the literal must be lowercase true. Upserts are useful when an application needs to ensure that a logical entity exists after an operation, but DBAs should consider a unique index on the identifying field when duplicate creation by concurrent requests must be prevented. Given the choices, updateOne(..., {$set:...}, {upsert:true}) is the valid operation, so A is correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - updateOne(), $set, upserts, and Boolean syntax.

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replaceOne() replaces the matched document ' s content with the replacement document, except that the immutable _id value is preserved when it is not explicitly changed. Ordinary application fields that are omitted from the replacement disappear. The required final document must retain userid, retain first, and change roles to " SuperUser " . Option B supplies all three fields and therefore produces the requested document structure. Option A omits userid, so that ordinary field would be removed from the replacement. Option C omits both userid and first, leaving only roles in addition to the preserved _id. Option D incorrectly uses the update operator $set inside a replacement document. $set belongs with update operations such as updateOne() or updateMany(); a replaceOne() replacement must be a replacement document rather than an update-operator document. This distinction is important for DBAs and application developers because accidental use of a partial replacement can delete fields that were intended to remain. If the goal is only to change one field, updateOne({userid:1}, {$set:{roles: " SuperUser " }}) would normally be safer. Given the choices, B is correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - replaceOne(), full-document replacement, and update-versus-replacement semantics.

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