Count the number of active servers (`status: "running"`) in the `servers` collection.

Problem Statement

<p>Count the number of active servers (<code>status: "running"</code>) in the <code>servers</code> collection.</p>

Examples

Input: servers collection: +-----+----------+---------+ | _id | name | status | +-----+----------+---------+ | 1 | web-01 | running | | 2 | web-02 | stopped | | 3 | db-01 | running | | 4 | cache-01 | running | +-----+----------+---------+

Output: +-------+ | count | +-------+ | 3 | +-------+

Explanation: The query calculates the total count of documents that satisfy the given filter criteria.

Complexity

Time Complexity: -

Space Complexity: -

Hints

💡 Hint 1: Use db.<collection>.find(<filter>) to query the documents in the collection. 💡 Hint 2: Pass an empty object {} to retrieve all documents, or specify key-value pairs for exact matching. 💡 Hint 3: Chain .toArray() to resolve the MongoDB cursor into a JavaScript array: return db.<collection>.find(...).toArray();

Editorial & Approach

Problem Overview & Intuition

To solve "Count Documents (countDocuments)", we query the MongoDB document store. The goal is to count the number of active servers (`status: "running"`) in the `servers` collection. Using a targeted find query, the database engine filters and structures the BSON documents efficiently.

Step-by-Step Approach

Identify Target Collection: Access the collection through the db instance.
Construct Query / Pipeline: Build the query filter with appropriate comparison operators.
Resolve Cursor: Invoke .toArray() to transform the query cursor into the required array of documents.

Optimal Implementation (MongoDB)

function solve(db) {
  return db.servers.countDocuments({ status: "running" });
}

Complexity Analysis

Time Complexity O(N) collection scan (O(log N) if index is present on filtered fields).

Space Complexity O(K) where K is the number of returned documents in memory.

Key Considerations & Edge Cases

Empty Collections: If no documents match, the query cleanly returns an empty array [].
Missing / NULL Fields: Missing fields in documents are handled safely without throwing runtime exceptions.
Type Coercion: BSON types (ObjectId, Numbers, Strings) are compared strictly according to MongoDB specifications.

Count Documents (countDocuments)

Easy

Count the number of active servers (status: "running") in the servers collection.

Example Scenarios

1Example 1

Input:

servers collection

_id	name	status
1	web-01	running
2	web-02	stopped
3	db-01	running
4	cache-01	running

Output:

count
3

Explanation:

The query calculates the total count of documents that satisfy the given filter criteria.

MongoDB Editor

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Query Results (JSON)

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