Free MongoDB Certified Database Administrator C100DBA Exam Questions

The replication factor is the number of data-bearing nodes that contain copies of the data. In this cluster: 1 primary + 3 secondary + 1 arbiter = 5 nodes total. However, the arbiter does not store data, and one secondary is hidden (but it still stores data). So the replication factor is 4: 1 primary + 3 secondaries (including the hidden one). The arbiter is not counted as it does not hold data.

In MongoDB, a collection is equivalent to a SQL table, and a document is equivalent to a SQL row. Collections are groupings of documents (similar to how tables are groupings of rows), and documents are individual records with field-value pairs (similar to how rows have column values).

When a secondary is prompted by another secondary for an election in a MongoDB replica set, the first task the secondary performs is to check its eligibility for election. It verifies whether it has the most recent oplog entries and meets the criteria to be a candidate (priority > 0, not hidden, not arbiter). If eligible, it increments its term and requests votes from other members. If not eligible, it rejects the request.

A MongoDB replica set should have an odd number of members to ensure proper election of a primary. Common replica set configurations use 3, 5, or 7 members. An odd number ensures that there is always a clear majority that can be achieved during elections. For example, with 3 members, 2 members form a majority; with 5 members, 3 form a majority. If a replica set has an even number of members, there can be a tie situation where neither partition can achieve a majority, resulting in no primary being elected. Therefore, odd numbers of members ensure the replica set can always select a primary even if some members fail.

Adding the index {a: 1} can potentially decrease the speed of the following operations:

• Insert operations: MongoDB must update the index for every inserted document, adding overhead
• Update operations: If the field 'a' is modified, the index must be updated accordingly
• Delete operations: The index entry must be removed for each deleted document
• Write-heavy workloads: If the collection experiences heavy write traffic but light read traffic on field 'a', the index maintenance cost may outweigh the benefits

Conversely, indexes improve query performance. The tradeoff is slower writes but faster reads on indexed fields. If reads on field 'a' are infrequent, the index may not justify its write-time cost.

The query db.sample.find({'$or': [{'a': {'$in': [3, 10]}}, {'b': {'$lt': 2}}]}) uses the $or operator to return documents matching EITHER condition:

• First condition: Field 'a' has a value of 3 or 10
• Second condition: Field 'b' has a value less than 2

A document is returned if it satisfies at least one of these conditions. The $in operator checks if the field value is in the specified array, and $lt checks if the value is less than the specified value.

The db.collection.getIndexes() command (or db.collection.getIndices()) returns all indexes on a collection. This displays the index names, keys, and options for each index, including the default _id index.

In a replica set with 5 members where the primary fails and operations exist in the oplog that only replicated to one secondary, the most likely scenario is:

• The remaining 4 secondaries (and the arbiter if present) will hold an election
• One of the secondaries with more recent oplog entries will likely be elected primary
• The secondary that received the operations that were not replicated to others may have to rollback those operations to stay consistent with the new primary
• This demonstrates why majority writes and journaling are important for data durability in replica sets

The totalKeysExamined field returned by the explain() method indicates how many index entries were examined while executing the query. A high number of keys examined relative to documents returned suggests the index is not selective enough. Ideally, totalKeysExamined should be close to the number of documents returned for an efficient query.

To find posts with likes greater than or equal to 100 AND less than or equal to 200, you use the $gte (greater than or equal) and $lte (less than or equal) operators. Both inclusive means you include the boundary values (100 and 200), so $gte and $lte are the correct operators to use. The query db.posts.find({likes: {$gte: 100, $lte: 200}}) returns all documents matching these criteria.

To save a GIF file (or any binary file) in MongoDB, you would use GridFS.

GridFS is a MongoDB specification for storing and retrieving large files that exceed the BSON document size limit of 16 MB. It works by:

• Dividing the file into chunks, each stored as a separate document
• Storing metadata about the file in a separate document
• Using two collections: one for file chunks and one for file metadata

GridFS is particularly useful for:

• Storing large binary files like images, videos, and PDFs
• Storing files larger than 16 MB
• Accessing portions of files without loading the entire file into memory

Tools like mongofiles provide command-line utilities for uploading and downloading files using GridFS.

The mongoimport command is used for importing data from external file formats (JSON, CSV, or TSV) into a MongoDB collection. It's the counterpart to mongoexport and is useful for loading data from external sources into your database. The basic syntax is: mongoimport --db database_name --collection collection_name --file filename.json.