Primary and secondary nodes in MongoDB - Step-by-Step Execution

Concept Flow - Primary and secondary nodes

Start: Replica Set Initiated

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Elect Primary Node

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Primary Accepts Writes

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Primary Replicates Data

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Secondary Nodes Receive Data

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Secondaries Serve Reads (optional)

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If Primary Fails

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Election: New Primary Chosen

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Continue Operations

The replica set starts, elects a primary node that handles writes and replicates data to secondary nodes, which can serve reads. If the primary fails, a new primary is elected to keep the system running.

Execution Sample

MongoDB

rs.initiate()
rs.status()
rs.stepDown()
rs.freeze(0)

Commands to start a replica set, check status, force primary to step down, and unfreeze nodes for election.

Execution Table

Step	Action	Primary Node	Secondary Nodes	Result
1	Initiate replica set	None	None	Replica set starts, no primary yet
2	Elect primary node	Node A	Nodes B, C	Node A becomes primary, others secondary
3	Primary accepts write	Node A	Nodes B, C	Data written to primary
4	Primary replicates data	Node A	Nodes B, C	Secondaries replicate data from primary
5	Secondaries serve reads	Node A	Nodes B, C	Secondaries can serve read requests
6	Primary fails	Node A (down)	Nodes B, C	Primary node goes down
7	Election for new primary	Node B	Node C	Node B elected as new primary
8	Operations continue	Node B	Node C	Writes and replication continue with new primary

💡 Replica set maintains availability by electing new primary if current primary fails

Variable Tracker

Variable	Start	After Step 2	After Step 6	After Step 7	Final
Primary Node	None	Node A	Node A (down)	Node B	Node B
Secondary Nodes	None	Nodes B, C	Nodes B, C	Nodes C	Nodes C
Data Consistency	N/A	Consistent	Consistent	Consistent	Consistent

Key Moments - 3 Insights

Why does the replica set need to elect a primary node?

Because only the primary node accepts write operations and replicates data to secondaries, ensuring data consistency. See execution_table step 2 where Node A becomes primary.

What happens to writes when the primary node fails?

Writes cannot be accepted until a new primary is elected. Execution_table step 6 shows primary down, and step 7 shows election of new primary Node B.

Can secondary nodes serve read requests?

Yes, secondary nodes can serve reads to reduce load on primary, as shown in execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution_table, which node is the primary after step 7?

A Node A

B Node C

C Node B

D No primary

💡 Hint

Check the 'Primary Node' column at step 7 in execution_table

At which step does the primary node fail?

A Step 4

B Step 6

C Step 5

D Step 7

💡 Hint

Look for 'Primary fails' action in execution_table

If the primary node is down, what must happen before writes can continue?

A A new primary must be elected

B Secondaries serve writes immediately

C Replica set shuts down

D Primary node restarts automatically

💡 Hint

Refer to execution_table steps 6 and 7 about primary failure and election

Concept Snapshot

Primary and secondary nodes in MongoDB replica sets:
- Primary node handles all writes and replicates data.
- Secondary nodes replicate data and can serve reads.
- If primary fails, secondaries hold election to choose new primary.
- This ensures high availability and data consistency.
- Use rs.initiate(), rs.status(), rs.stepDown() to manage nodes.

Full Transcript

In MongoDB replica sets, one node is elected as primary to accept writes and replicate data to secondary nodes. Secondary nodes replicate data from the primary and can serve read requests to balance load. If the primary node fails, the replica set automatically holds an election to choose a new primary, ensuring continuous availability. Commands like rs.initiate() start the replica set, rs.status() shows current node roles, and rs.stepDown() forces the primary to step down to test failover. This process keeps data consistent and the database available even if some nodes fail.