Free Practice Questions for the MongoDB Certified DBA Associate C100DBA Exam (2026 Updated)

connections.totalCreated is a cumulative count of incoming connections created during the lifetime of the running MongoDB server process. It includes connections that are still open as well as connections that have already closed. That is why the example can show totalCreated: 5 while current: 4: five connections have been created since the process started, but only four remain current. Option A is incorrect because the counter is not limited to closed connections. Option C corresponds more closely to connections.current, which reports presently existing incoming client connections. Option B is imprecise because “since the server was set up” could imply the lifetime of the installation or deployment across restarts, whereas server-status counters generally reflect the current process lifetime. Tracking totalCreated alongside current, active, and connection-pool behavior helps administrators identify excessive connection churn, poorly configured client pools, or bursts of new connections. The most accurate interpretation among the choices is therefore the number of connections created since the server process was started, making D correct.

Study Guide reference/topic:Performance Monitoring and Tuning - serverStatus, connection metrics, and cumulative connection counters.

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db.collection.getIndexes() is the mongosh method used to return the definitions of all indexes that currently exist on a collection. The result is an array of index specification documents that can show fields such as the index name, key pattern, index version, uniqueness, partial-filter settings, sparse configuration, collation, and whether the index is hidden. This makes the method useful for index audits, troubleshooting, and confirming the physical indexes available to the query planner. Option A is not a documented method for enumerating collection indexes. Option B resembles the $indexStats aggregation stage, which reports usage statistics for indexes, but db.collection.indexStats() is not the method used to list full index definitions. Option C, findIndexes(), is also not the documented mongosh collection helper. Administrators should distinguish between listing index definitions, examining index usage, and inspecting an individual query ' s winning plan: getIndexes() addresses the first requirement, $indexStats addresses usage metrics, and explain() addresses query-planner behavior. Therefore option D is the correct command for listing all indexes on the collection.

Study Guide reference/topic:Indexing and Query Optimization - index administration, index metadata, and db.collection.getIndexes().

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MongoDB exposes server parameters that can be configured through the parameter framework. Among the choices, --setParameter is the option specifically designed to set a server parameter on the mongod command line, using syntax such as mongod --setParameter < name > = < value > . A configuration file can also contain a setParameter section, but --config by itself merely tells mongod which configuration file to load; it is not the parameter-setting mechanism identified by the question. Connection-string options control how clients connect and cannot be used to configure arbitrary server runtime parameters. Option D is also incorrect because the server must already be running for mongosh to issue administrative commands. A technical nuance is important: parameters that support modification while the process is already running are changed with the administrative setParameter command, for example db.adminCommand({setParameter:1, ...}). Such runtime changes may not persist across a restart unless the value is also configured for startup. Given the options provided and the exam wording, --setParameter is the intended method, so A is correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - server parameters, --setParameter, configuration files, and runtime administration.

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The operation must update every matching document and use the $inc update operator with correct syntax. updateMany() applies the update to all documents matching { color: " green " }. The $inc operator must be the top-level update operator and receive a field-to-amount mapping, so the valid update document is { $inc: { price: 5 } }. Option C satisfies both requirements. The sweater price increases from 20 to 25, and the hat price increases from 12 to 17. Option B uses a syntactically correct $inc expression, but the legacy update() call shown does not explicitly request a multi-document update, so it does not meet the stated requirement to modify all green products. Options A and D place $inc beneath the price field instead of placing $inc at the update-operator level, which is not valid update syntax. $inc operates atomically on each affected document and can be used with positive or negative numeric increments. Therefore, the correct multi-document update is db.products.updateMany({color: " green " }, {$inc:{price:5}}), making C correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - updateMany(), update operators, filters, and $inc.

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A hidden index remains physically present and continues to be maintained as documents are inserted, updated, or deleted, but the query planner does not consider that index when choosing normal execution plans. This makes hidden indexes especially useful for safely testing whether an index can be removed. An administrator can hide the index and observe production query behavior as though the index were absent. If performance or plan quality degrades, the index can be unhidden immediately without rebuilding it because MongoDB has continued to maintain it in the background. Option A is incorrect because a hidden index is still an index and is not simply removed from index statistics. Option B is also incorrect; hiding an index can actually force collection scans or alternative plans if that index was previously important. Option D misunderstands the feature entirely: hiding an index has no effect on document visibility. Hidden indexes are therefore a risk-reduction mechanism for index lifecycle management, especially before dropping an index in a production environment. Option C accurately states that purpose.

Study Guide reference/topic:Indexing and Query Optimization - hidden indexes, query planner behavior, and safe index-removal testing.

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MongoDB provides db.grantRolesToUser() specifically for assigning one or more additional roles to an existing user. The method takes the username as its first argument and an array of roles as its second argument, so db.grantRolesToUser( " user1 " , [ " customRole1 " ]) is the correct choice. A role can be supplied as a simple string when it belongs to the current database or as an explicit document such as { role: " customRole1 " , db: " admin " } when the database context must be specified. Option B uses db.createUser(), which is used to create a new user rather than modify the role membership of an account that already exists. Options A and C use method names that are not the documented mongosh interfaces for this operation. Administrators should also distinguish grantRolesToUser() from grantRolesToRole(): the latter makes a user-defined role inherit other roles, whereas the former directly changes a user ' s assigned roles. Least-privilege practice requires granting only roles necessary for the user ' s responsibilities. For the requested update to the existing user, option D is correct.

Study Guide reference/topic:Security, Networking, and Encryption - user administration, role assignment, RBAC, and grantRolesToUser().

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db.collection.insertOne() inserts one BSON document into a collection. In mongosh, string values must be quoted and Boolean literals use JavaScript syntax: lowercase true or false. Therefore the student ' s name must appear as " Randall " , and the passed flag must be true. Option D is the only choice that gets both value types correct. Option A quotes the string properly but uses True, which is not the JavaScript Boolean literal. Options B and C leave Randall unquoted, so mongosh would interpret it as an identifier or variable rather than as the literal string required by the document. Option C additionally uses the invalid capitalized Boolean form. If an _id field is omitted, MongoDB automatically adds a unique _id, typically an ObjectId, before storing the document. That behavior does not affect the two requested fields. This question tests both CRUD syntax and the BSON/JavaScript literal representation used by mongosh. The correct command is db.class101.insertOne({ student: " Randall " , passed: true }), so option D is correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - insertOne(), BSON values, strings, Booleans, and automatic _id.

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MongoDB supports a clean server shutdown through the shutdown administrative command and through the mongosh helper db.shutdownServer(). Option A uses db.adminCommand({ shutdown: 1 }), which executes the shutdown command against the admin database and requests an orderly termination of the server process. Option D explicitly switches to the admin database and calls db.shutdownServer(), the documented shell helper that wraps the same administrative behavior. Option B uses killOp, which terminates a specific database operation identified by an operation ID; port 27017 is not an operation ID and killing an operation is not equivalent to shutting down mongod. Option C uses killAllSessions, which ends logical sessions but leaves the server process running. A controlled shutdown is preferred over abruptly terminating the process because MongoDB can close resources and complete its shutdown procedures cleanly. In replica-set deployments, administrators should also consider whether the member is primary and whether a planned step-down is appropriate before maintenance. Among the commands shown, A and D are the two valid mongosh-based shutdown mechanisms.

Study Guide reference/topic:MongoDB Architecture and Core Operations - clean server shutdown, admin commands, and db.shutdownServer().

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In MongoDB explain output, a COLLSCAN stage means the winning plan is scanning documents in the collection rather than traversing a suitable index. This is the clearest indicator that the query is not being serviced by a normal index scan for the relevant predicate. By contrast, IXSCAN explicitly indicates an index scan and typically includes details such as the index key pattern, direction, and bounds. A FETCH stage represents retrieval of full documents, often after an index has already identified candidate record locations; its presence does not imply that no index was used. GROUP relates to grouping behavior and is not the stage used to identify missing index support. A collection scan is not automatically wrong: for tiny collections or queries that return most documents, it can be efficient. However, on large collections with selective predicates, repeated COLLSCAN plans are a strong signal that the DBA should evaluate indexing. Administrators should compare totalDocsExamined, totalKeysExamined, execution time, and the winning/rejected plans before deciding to add an index. For the terminology tested by the question, COLLSCAN is the correct stage, so C is correct.

Study Guide reference/topic:Indexing and Query Optimization - explain plans, COLLSCAN, IXSCAN, and missing index diagnostics.

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mongosh accepts a MongoDB host address in hostname:port form, so mongosh " node-1.cluster1.mycompany.local:27017 " is the valid choice among the commands shown. A canonical URI form such as mongosh " mongodb://node-1.cluster1.mycompany.local:27017 " is also valid, and the connection can alternatively be expressed using --host together with --port. Option A uses an unsupported --hostname switch rather than the documented --host option. Option B invents a --connect flag that is not the standard mongosh interface. Option C constructs an invalid string using an ampersand-style parameter rather than a MongoDB URI or a normal host:port target. Port 27017 is MongoDB ' s default server port, so it could often be omitted if the deployment uses the default, but explicitly including it is valid and removes ambiguity. In authenticated or TLS-enabled environments, the DBA would also supply credentials, authentication database, certificate settings, or a connection string carrying those parameters. For the simple host-and-port requirement in the question, option D is correct.

Study Guide reference/topic:Security, Networking, and Encryption - mongosh connectivity, host/port addressing, and connection options.

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logLevel is a MongoDB server parameter, and server parameters are retrieved through the administrative getParameter database command. The documented and unambiguous mongosh form is db.adminCommand({ getParameter: 1, logLevel: 1 }), which executes the command against the admin database and requests the current value of logLevel. The first numeric value activates the command, while the logLevel field identifies the parameter to return. Options B and C refer to a db.getParameter() helper that is not the documented interface. Option A uses db.runCommand() without explicitly switching to the admin database. Because getParameter is an administrative command, db.adminCommand() is the correct and reliable form among the choices. Administrators use server parameters to inspect and, where supported, tune aspects of mongod or mongos behavior. logLevel controls global diagnostic verbosity, while component-specific verbosity can be managed through the logging configuration. For this question, option D precisely matches the expected administrative syntax. Checking the current value before changing logging settings is also a sound operational practice during troubleshooting.

Study Guide reference/topic:Performance Monitoring and Tuning - runtime server parameters, logging configuration, and getParameter.

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MongoDB custom roles are defined in terms of privileges, and each privilege specifies a resource plus one or more allowed actions. For read/query access, find is a valid privilege action. To represent all normal collections in a specified database, MongoDB uses a resource document with the target database and an empty collection name, for example { db: " products " , collection: " " }. Option B therefore combines the correct resource scope with the find action and does not grant write capability. The distractors confuse built-in role names with privilege actions. read, readWrite, and readAnyDatabase are built-in roles or role concepts, not the appropriate action values for the privileges[].actions list shown here. Likewise, a literal " * " collection value is not the documented database-wide resource form for this custom privilege. In practice, an administrator could also assign MongoDB ' s built-in read role rather than creating a custom role if its permissions exactly match the requirement. Given the specific createRole() choices, however, option B is the technically correct construction.

Study Guide reference/topic:Security, Networking, and Encryption - RBAC, custom roles, resources, and privilege actions.

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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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