poco/MongoDB/samples/ReplicaSet

MongoDB Replica Set Examples

This directory contains comprehensive examples demonstrating Poco::MongoDB replica set support with automatic failover, read preferences, and connection pooling.

Minimum MongoDB Version: MongoDB 5.1 or later (for replica set features)

Examples Overview

Sample	Description
ReplicaSetMonitor	Production-ready monitoring tool for deployment verification and continuous health monitoring
ReplicaSet	Feature demonstrations with multiple commands (basic, readpref, failover, pool, topology)
URIExample	MongoDB URI parsing and connection demonstration

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ReplicaSetMonitor - Deployment Health Check Tool

A continuous monitoring tool that performs regular read/write operations and displays replica set topology status. Ideal for deployment verification and health monitoring.

Features

• Continuous Health Checks: Performs writes to primary and reads from replica set at configurable intervals
• Real-time Topology Display: Shows current replica set status, server roles, and round-trip times
• Statistics Tracking: Monitors success rates for read and write operations
• Deployment Verification: Quickly verify replica set is functioning correctly
• Automatic Failover Testing: Continues working even during primary elections

Usage

# Basic usage with defaults
./ReplicaSetMonitor

# Using MongoDB URI (recommended)
./ReplicaSetMonitor -u 'mongodb://mongo1:27017,mongo2:27017/?replicaSet=rs0'

# Specify replica set and hosts (traditional method)
./ReplicaSetMonitor -s rs0 -H mongo1:27017,mongo2:27017,mongo3:27017

# Run with 10-second intervals for 100 iterations
./ReplicaSetMonitor -i 10 -n 100

# Verbose mode for detailed operation output
./ReplicaSetMonitor -v

# Full example with URI
./ReplicaSetMonitor \
    --uri 'mongodb://mongo1:27017,mongo2:27017,mongo3:27017/?replicaSet=rs0&readPreference=primaryPreferred' \
    --interval 5 \
    --database test \
    --collection health_check \
    --verbose

# Full example with traditional options
./ReplicaSetMonitor \
    --set rs0 \
    --hosts mongo1:27017,mongo2:27017,mongo3:27017 \
    --interval 5 \
    --database test \
    --collection health_check \
    --verbose

Command-Line Options

Option	Description	Default
-h, --help	Show help message	-
-u, --uri URI	MongoDB connection URI (takes precedence)	-
-s, --set NAME	Replica set name	rs0
-H, --hosts HOSTS	Comma-separated host:port list	localhost:27017,localhost:27018,localhost:27019
-i, --interval SECONDS	Check interval in seconds	5
-d, --database NAME	Database name	test
-c, --collection NAME	Collection name	poco_monitor
-v, --verbose	Verbose output	false
-n, --iterations N	Number of iterations	unlimited

Note: The --uri option takes precedence over --set and --hosts options.

Environment Variables

• MONGODB_URI: MongoDB connection URI (takes precedence)
• MONGODB_REPLICA_SET: Replica set name
• MONGODB_HOSTS: Comma-separated host:port list

Example Output

Connecting to replica set: rs0
Seed servers: mongo1:27017, mongo2:27017, mongo3:27017
Check interval: 5 seconds

Replica set connected successfully!
Background monitoring active.

================================================================================
TOPOLOGY STATUS
================================================================================
Replica Set: rs0
Type:        Replica Set (with Primary)
Has Primary: Yes

Servers: 3
--------------------------------------------------------------------------------
Address                        Type        RTT (ms)  Status
--------------------------------------------------------------------------------
mongo1:27017                   PRIMARY     2.34      OK
mongo2:27017                   SECONDARY   3.12      OK
mongo3:27017                   SECONDARY   2.89      OK
================================================================================

[2025-11-26T21:15:00Z] Check #1
Write (Primary): ✓ OK (12 ms)
Read (PrimaryPreferred): ✓ OK (8 ms)
Statistics: Writes: 1/1 (100.0%), Reads: 1/1 (100.0%)
--------------------------------------------------------------------------------
[2025-11-26T21:15:05Z] Check #2
Write (Primary): ✓ OK (9 ms)
Read (PrimaryPreferred): ✓ OK (7 ms)
Statistics: Writes: 2/2 (100.0%), Reads: 2/2 (100.0%)
--------------------------------------------------------------------------------

Use Cases

1. Deployment Verification

Run the monitor immediately after deploying a replica set:

# Run for 60 iterations (5 minutes with 5-second intervals)
./ReplicaSetMonitor -s production-rs -H prod1:27017,prod2:27017,prod3:27017 -n 60

2. Load Testing

Combine with multiple instances to generate load:

# Run multiple monitors in parallel
for i in {1..10}; do
    ./ReplicaSetMonitor -i 1 &
done

3. Failover Testing

Run the monitor while performing failover operations:

# Start monitor
./ReplicaSetMonitor -v

# In another terminal, step down the primary:
# mongo --eval "rs.stepDown()"

# Monitor will automatically failover and continue operations

4. Continuous Monitoring

Run as a long-term health check:

# Run indefinitely with 30-second intervals
./ReplicaSetMonitor -i 30 > replica_set_health.log 2>&1

Docker Compose Testing

Create a local replica set for testing:

# docker-compose.yml
version: '3'
services:
  mongo1:
    image: mongo:7.0
    command: ["--replSet", "rs0", "--bind_ip_all", "--port", "27017"]
    ports: ["27017:27017"]

  mongo2:
    image: mongo:7.0
    command: ["--replSet", "rs0", "--bind_ip_all", "--port", "27017"]
    ports: ["27018:27017"]

  mongo3:
    image: mongo:7.0
    command: ["--replSet", "rs0", "--bind_ip_all", "--port", "27017"]
    ports: ["27019:27017"]

Initialize the replica set:

docker-compose up -d
docker exec -it $(docker ps -q -f name=mongo1) mongosh --eval "
rs.initiate({
  _id: 'rs0',
  members: [
    { _id: 0, host: 'localhost:27017' },
    { _id: 1, host: 'localhost:27018' },
    { _id: 2, host: 'localhost:27019' }
  ]
})"

# Wait a few seconds for election
sleep 5

# Run the monitor
./ReplicaSetMonitor -s rs0 -H localhost:27017,localhost:27018,localhost:27019

---

ReplicaSet - Feature Examples

Demonstrates various replica set features with multiple commands.

Usage

./ReplicaSet <command>

Commands

Command	Description
basic	Basic replica set connection and operations
readpref	Read preference examples (primary, secondary, nearest)
failover	Automatic failover demonstration
pool	Connection pooling example
topology	Topology discovery and monitoring

Examples

# Basic connection
./ReplicaSet basic

# Try different read preferences
./ReplicaSet readpref

# Demonstrate automatic failover
./ReplicaSet failover

# Show connection pooling
./ReplicaSet pool

# Display topology information
./ReplicaSet topology

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URIExample - URI Parsing Demonstration

Demonstrates MongoDB URI parsing and connection to replica sets.

Features

• Parse MongoDB connection URIs
• Display parsed configuration (hosts, replica set name, read preference, timeouts)
• Connect to replica set and show topology
• Query server information
• Validate URI format

Usage

./URIExample <uri>

Examples

# Basic replica set URI
./URIExample 'mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaSet=rs0'

# With read preference
./URIExample 'mongodb://mongo1:27017,mongo2:27017/?replicaSet=rs0&readPreference=primaryPreferred'

# With custom timeouts and heartbeat
./URIExample 'mongodb://host1:27017,host2:27017/?replicaSet=rs0&connectTimeoutMS=5000&socketTimeoutMS=30000&heartbeatFrequencyMS=5000'

Supported URI Options

• replicaSet=name - Replica set name
• readPreference=mode - Read preference (primary|primaryPreferred|secondary|secondaryPreferred|nearest)
• connectTimeoutMS=ms - Connection timeout in milliseconds (for custom SocketFactory implementations)
• socketTimeoutMS=ms - Socket timeout in milliseconds (for custom SocketFactory implementations)
• heartbeatFrequencyMS=milliseconds - Heartbeat frequency in milliseconds (default: 10000)
• reconnectRetries=n - Number of reconnection retries (default: 10)
• reconnectDelay=seconds - Delay between reconnection attempts in seconds (default: 1)

Example Output

Parsing MongoDB Replica Set URI
================================================================================
URI: mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaSet=rs0

✓ URI parsed successfully!

Configuration:
--------------------------------------------------------------------------------
Replica Set Name: rs0
Read Preference:  primary
Seed Servers:     localhost:27017, localhost:27018, localhost:27019
Monitoring:       Active

Connecting to replica set...
✓ Connected to primary: localhost:27017

Server Information:
--------------------------------------------------------------------------------
MongoDB Version: 7.0.5
Git Version:     7809d71e84e314b497f282ea52598668b08b84dd

Replica Set Topology:
--------------------------------------------------------------------------------
Set Name:    rs0
Has Primary: Yes
Servers:     3

  localhost:27017 [PRIMARY] RTT: 2.34 ms
  localhost:27018 [SECONDARY] RTT: 3.12 ms
  localhost:27019 [SECONDARY] RTT: 2.89 ms

✓ Success!

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Building the Examples

With CMake

cd poco
mkdir build && cd build
cmake .. -DENABLE_MONGODB=ON -DENABLE_SAMPLES=ON
cmake --build . --target ReplicaSetMonitor
cmake --build . --target ReplicaSet
cmake --build . --target URIExample

# Executables are in bin/
./bin/ReplicaSetMonitor --help
./bin/ReplicaSet basic
./bin/URIExample 'mongodb://localhost:27017/?replicaSet=rs0'

With Make

cd MongoDB/samples/ReplicaSet
make
./ReplicaSetMonitor --help

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Troubleshooting

"No suitable server found in replica set"

• Verify MongoDB servers are running: nc -zv localhost 27017
• Check replica set is initialized: mongosh --eval "rs.status()"
• Verify network connectivity between hosts
• Check replica set name matches: -s option should match rs.status()._id

"Connection failed" errors

• Ensure MongoDB is binding to the correct interface (--bind_ip_all)
• Check firewall rules allow connections to MongoDB ports
• Verify authentication is disabled or credentials are provided

High latency or timeouts

• Check network conditions between client and MongoDB servers
• Increase timeout values in replica set configuration
• Verify MongoDB servers are not overloaded

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Advanced Configuration

Custom Read Preference with Tags

// Target servers in specific datacenter
Document tags;
tags.add("dc", "east");
tags.add("rack", "1");
ReadPreference pref(ReadPreference::Nearest, tags);

Custom Heartbeat Frequency

ReplicaSet::Config config;
config.heartbeatFrequencySeconds = 30;  // 30 seconds (default: 10)

Disable Background Monitoring

ReplicaSet::Config config;
config.enableMonitoring = false;  // Manual topology refresh only

Custom Reconnection Settings

ReplicaSet::Config config;
config.serverReconnectRetries = 5;         // Number of retries (default: 10)
config.serverReconnectDelaySeconds = 2;    // Delay between retries in seconds (default: 1)

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Additional Resources

• MongoDB Replica Set Documentation
• Server Discovery and Monitoring Spec
• Poco Documentation