Cloud 4.02

Slave Connections

Leveraging slave connections for MySQL and Redis to improve performance with read-only replicas.

Exam Critical: Understanding slave connections for MySQL and Redis is essential for the AD0-E717 exam! Know the environment variables, configuration location, and limitations for Pro vs Integration environments.

Slave Connections

mindmap root((Slave Connections)) Environment Variables MYSQL_USE_SLAVE_CONNECTION REDIS_USE_SLAVE_CONNECTION Benefits Speeds up responses Read-only connection Performance improvement Configuration magento.env.yaml stage deploy section Default false Pro Recommendation Enable both MySQL Enable both Redis Redis Limitations Not in Integration Not with custom CACHE_CONFIGURATION

Slave Connection Overview

What Are Slave Connections?

Slave connections enable read-only database connections that improve performance by offloading read operations from the master database.

Benefits

Faster responses: Read operations are distributed
Improved performance: Master database handles writes only
Load balancing: Reduces load on primary database

Environment Variables

Two Key Variables

There are two environment variables that enable the use of read-only connections:

Variable	Purpose	Default
`MYSQL_USE_SLAVE_CONNECTION`	Enable MySQL read-only connection	false
`REDIS_USE_SLAVE_CONNECTION`	Enable Redis read-only connection	false

Default Behavior

Both variables default to false. You must explicitly enable them to use slave connections.

Configuration in .magento.env.yaml

Basic Configuration

Slave connections are configured in .magento.env.yaml:

stage:
    deploy:
        MYSQL_USE_SLAVE_CONNECTION: true

Enabling Both MySQL and Redis

stage:
    deploy:
        MYSQL_USE_SLAVE_CONNECTION: true
        REDIS_USE_SLAVE_CONNECTION: true

Configuration Location

File: .magento.env.yaml
Section: stage → deploy

Pro Account Recommendations

Magento's Recommendation

For Pro accounts, Magento recommends enabling both Redis and MySQL slave connections:

# .magento.env.yaml
stage:
    deploy:
        MYSQL_USE_SLAVE_CONNECTION: true
        REDIS_USE_SLAVE_CONNECTION: true

Why Enable Both?

Performance: Maximizes read performance across all services
Scalability: Better handles high traffic loads
Resource optimization: Distributes database load efficiently

Redis Slave Connection Limitations

Important Restrictions

Critical Limitations for Redis

1. Not Available in Integration Environment

Redis read-only connection is NOT available in Integration environment.

2. Not Available with Custom CACHE_CONFIGURATION

Redis slave connection is NOT available if you specify custom details in the CACHE_CONFIGURATION environment variable.

When Redis Slave Connection Works

Redis slave connection is available when:

Pro Staging: ✓ Available
Pro Production: ✓ Available
Integration: ✗ NOT Available
With default Redis config: ✓ Available
With custom CACHE_CONFIGURATION: ✗ NOT Available

MySQL Slave Connection

MySQL Configuration

stage:
    deploy:
        MYSQL_USE_SLAVE_CONNECTION: true

Availability

Environment	MySQL Slave Connection
Integration	✓ Available
Pro Staging	✓ Available
Pro Production	✓ Available

No Restrictions

Unlike Redis, MySQL slave connections work in all environments without restrictions.

Complete Configuration Example

Recommended Pro Account Setup

# .magento.env.yaml
stage:
    global:
        # Global environment variables
        SCD_ON_DEMAND: true
    deploy:
        # Enable slave connections for performance
        MYSQL_USE_SLAVE_CONNECTION: true
        REDIS_USE_SLAVE_CONNECTION: true
        
        # Other deploy configurations
        CACHE_CONFIGURATION:
            _merge: true
        SESSION_CONFIGURATION:
            _merge: true

Important Note

If you're using custom CACHE_CONFIGURATION, remember that REDIS_USE_SLAVE_CONNECTION will NOT work!

Comparison: MySQL vs Redis Slave Connections

Aspect	MySQL	Redis
Variable	MYSQL_USE_SLAVE_CONNECTION	REDIS_USE_SLAVE_CONNECTION
Default Value	false	false
Integration Environment	✓ Works	✗ Does NOT Work
Pro Staging/Production	✓ Works	✓ Works
With Custom Config	✓ Works	✗ NOT with CACHE_CONFIGURATION
Pro Recommendation	✓ Enable	✓ Enable

How It Works

Read vs Write Operations

With Slave Connections Enabled

Read operations (SELECT): Sent to slave (read-only replica)
Write operations (INSERT, UPDATE, DELETE): Sent to master database

Performance Impact

By offloading read operations to a slave:

Master database has more capacity for write operations
Read operations are distributed across multiple nodes
Overall application response time improves
Database can handle higher traffic loads

Best Practices

Do's

Enable both MySQL and Redis on Pro accounts
Configure in stage → deploy section
Test performance improvements
Use default Redis config for slave connections
Monitor database performance after enabling
Enable in Staging before Production

Don'ts

Don't expect Redis slave in Integration
Don't use REDIS_USE_SLAVE_CONNECTION with custom CACHE_CONFIGURATION
Don't forget both default to false
Don't skip Pro recommendation
Don't configure outside stage → deploy
Don't assume it works without enabling

Troubleshooting

Redis Slave Not Working?

Check These Common Issues

Integration environment: Redis slave is NOT available in Integration
Custom CACHE_CONFIGURATION: Remove custom config or don't use slave connection
Wrong configuration section: Must be in stage → deploy, not global
Not enabled: Variable must be explicitly set to true

Exam Tips

Key Points to Remember

Two variables: MYSQL_USE_SLAVE_CONNECTION and REDIS_USE_SLAVE_CONNECTION
Default value: Both default to false
Configuration file: .magento.env.yaml
Configuration section: stage → deploy
Purpose: Enable read-only connections to speed up responses
Pro recommendation: Enable BOTH MySQL and Redis slave connections
MySQL availability: Works in all environments (Integration, Staging, Production)
Redis limitation 1: NOT available in Integration environment
Redis limitation 2: NOT available with custom CACHE_CONFIGURATION
Redis works in: Pro Staging and Production (with default config)
Performance benefit: Offloads read operations from master database
Operation split: Reads go to slave, writes go to master

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Slave Connections