Database Strategy for Microservices
Architectural decision record for database selection and data management in the FlowMart e-commerce platform
ADR-004: Database Strategy for Microservices in FlowMart E-commerce Platform
Status
Approved (2024-08-20)
Context
Our transition from a monolithic architecture to microservices necessitates a reassessment of our database strategy. The existing monolithic application uses a centralized Oracle database with hundreds of tables spanning multiple business domains. This approach has created several challenges:
- Schema Coupling: Changes to database schemas require careful coordination across teams to avoid breaking dependent services.
- Scalability Limitations: The relational database becomes a bottleneck during high-traffic periods, affecting all application features.
- One-Size-Fits-All Approach: Different data access patterns are all forced into the same relational model, leading to suboptimal performance.
- Operational Overhead: Database administration is complex due to competing requirements from different application components.
- Development Bottlenecks: Teams must coordinate database changes, slowing down development velocity.
- Cost Efficiency: The enterprise database licensing model is expensive and doesn’t scale cost-effectively with our varying workloads.
As we decompose our application into microservices, we need a database strategy that promotes service autonomy while ensuring data consistency, performance, and operational efficiency.
Decision
We will adopt a polyglot persistence strategy for our microservices architecture, following the principle of “right database for the right job.” Key aspects of this strategy include:
-
Database-per-Service Pattern:
- Each microservice will own and exclusively manage its database
- No direct database access from other services
- Services interact via well-defined APIs, not shared databases
-
Primary Database Technologies:
- PostgreSQL: Primary relational database for services with complex transactional requirements
- MongoDB: Document database for services with flexible schema requirements and read-heavy workloads
- Redis: In-memory data store for caching, session management, and real-time features
- Elasticsearch: Search engine for product catalog and content search
- Apache Cassandra: Wide-column store for high-volume time-series data and analytics
- Amazon DynamoDB: Managed NoSQL for highly scalable microservices with predictable access patterns
-
Data Consistency Patterns:
- Saga Pattern: For coordinating transactions across multiple services
- Event Sourcing: For critical business domains requiring complete audit trails
- CQRS: For services with asymmetric read/write loads
- Outbox Pattern: For reliable event publishing during state changes
-
Data Migration and Evolution:
- Versioned database schemas
- Backward-compatible schema changes
- Blue/green deployment for database changes
- Incremental data migration approach
-
Data Governance:
- Centralized data catalog and lineage tracking
- Common data models for shared business entities
- Standard approach to master data management
- Automated data quality monitoring
-
Operational Excellence:
- Infrastructure as Code (IaC) for all database resources
- Automated backup and recovery procedures
- Standardized monitoring and alerting
- Database reliability engineering practices
Database Selection Criteria by Domain
Domain | Database Type | Primary Factors | Secondary Considerations |
---|---|---|---|
Product Catalog | MongoDB + Elasticsearch | Schema flexibility, Search capabilities | High read-to-write ratio, Caching |
Order Management | PostgreSQL | ACID transactions, Complex queries | Event sourcing for audit trail |
Customer Profile | PostgreSQL | Data consistency, Relational integrity | Privacy compliance, Scalable reads |
Inventory | MongoDB | Frequent schema evolution, High write throughput | Eventual consistency model |
Cart & Checkout | Redis + PostgreSQL | Low latency, High availability | Transaction support for checkout |
Payment Processing | PostgreSQL | Strong consistency, Transaction support | Compliance requirements, Security |
Analytics | Cassandra | High write throughput, Time-series data | Analytical query patterns |
Recommendations | Redis + MongoDB | Low latency reads, Flexible data model | Machine learning feature support |
User Sessions | Redis | Ultra-low latency, Expiration support | High availability, Cross-region replication |
Content Management | MongoDB | Flexible schema, Document structure | Full-text search capability |
Consequences
Positive
-
Service Autonomy: Teams can independently select, optimize, and evolve their databases without cross-team coordination.
-
Optimized Performance: Each service can use a database technology optimized for its specific data access patterns.
-
Independent Scaling: Database resources can be scaled according to the specific needs of each service.
-
Improved Resilience: Database failures are isolated to specific services rather than affecting the entire system.
-
Fit-for-Purpose Solutions: Different data models (relational, document, key-value, etc.) can be applied where most appropriate.
-
Cost Optimization: Resources can be allocated based on the specific needs of each service.
-
Incremental Adoption: New database technologies can be introduced for new services without migrating legacy data.
Negative
-
Increased Operational Complexity: Managing multiple database technologies requires broader expertise and more sophisticated tooling.
-
Data Consistency Challenges: Maintaining consistency across service boundaries requires careful design and implementation.
-
Learning Curve: Development teams need to learn multiple database technologies and data access patterns.
-
Distributed Transactions: Business processes spanning multiple services require more complex transaction management.
-
Increased Infrastructure Costs: Running and maintaining multiple database systems may increase overall infrastructure costs.
-
Data Duplication: Some data may need to be duplicated across services, creating synchronization challenges.
-
Monitoring and Troubleshooting Complexity: Different databases require different monitoring approaches and expertise.
Mitigation Strategies
-
Platform Database Service:
- Create an internal platform team that provides database-as-a-service capabilities
- Standardize on a limited set of supported database technologies
- Provide automated provisioning, backup, and monitoring
-
Data Access Layer Pattern:
- Create standardized libraries for common database access patterns
- Abstract database-specific details behind consistent interfaces
- Implement retry logic, circuit breakers, and observability
-
Data Synchronization Framework:
- Implement a standardized approach to CDC (Change Data Capture)
- Create reusable components for the Outbox Pattern
- Develop standard data synchronization workflows
-
Database Reliability Engineering:
- Establish dedicated database reliability engineers
- Create runbooks for common database operations
- Implement chaos engineering practices for database failure testing
-
Developer Training and Support:
- Comprehensive training program for supported database technologies
- Internal knowledge base and best practice documentation
- Database design review process for new services
Implementation Details
Phase 1: Foundation (Q3-Q4 2024)
- Establish database platform team and core services
- Set up standard PostgreSQL and MongoDB offerings
- Implement database CI/CD pipelines
- Develop initial monitoring and alerting
- Create database provisioning automation
Phase 2: Expansion (Q1-Q2 2025)
- Add Redis and Elasticsearch to supported offerings
- Implement data synchronization framework
- Develop CQRS and Event Sourcing patterns
- Create data governance tooling
- Expand monitoring capabilities
Phase 3: Advanced Capabilities (Q3-Q4 2025)
- Add Cassandra and specialized databases
- Implement advanced high availability features
- Develop cross-region replication capabilities
- Create advanced analytics integration
- Implement predictive scaling and optimization
Considered Alternatives
1. Shared Database Approach
Pros: Simplicity, familiar model, transactional integrity, easier reporting
Cons: Tight coupling, scalability limitations, schema coordination challenges
This approach would minimize short-term changes but would recreate many of the current issues in our new architecture.
2. Single Database Technology (PostgreSQL Only)
Pros: Operational simplicity, consistent expertise, familiar tooling
Cons: Not optimal for all workloads, potential performance compromises
While PostgreSQL is versatile, it isn’t the best choice for all our diverse data requirements.
3. Database-as-a-Service Only (e.g., AWS RDS/DynamoDB)
Pros: Reduced operational overhead, managed scaling, built-in high availability
Cons: Vendor lock-in, potential cost concerns, less flexibility
While we’ll use managed services where appropriate, we need the flexibility to run certain databases on our own infrastructure.
4. Fully Centralized Data Lake Approach
Pros: Centralized analytics, simplified data governance, consistent data model
Cons: Complex real-time synchronization, potential performance issues, development overhead
This approach works well for analytics but isn’t suitable for operational data needs.
References
- Kleppmann, Martin. “Designing Data-Intensive Applications” (O’Reilly Media)
- Vernon, Vaughn. “Implementing Domain-Driven Design” (Addison-Wesley)
- Fowler, Martin. “PolyglotPersistence” martinfowler.com
- Richardson, Chris. “Microservices Patterns” (Manning Publications)
- Database per Service Pattern
- Saga Pattern
Decision Record History
Date | Version | Description | Author |
---|---|---|---|
2024-07-20 | 0.1 | Initial draft | Robert Kim |
2024-08-02 | 0.2 | Added domain-specific database recommendations | Maria Garcia |
2024-08-15 | 0.3 | Incorporated feedback from architecture review | David Boyne |
2024-08-20 | 1.0 | Approved by Architecture Board | Architecture Board |
Appendix A: Database Architecture Overview
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Appendix B: Data Consistency Patterns by Service Type
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Appendix C: Database Migration Strategy
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