Designing an email delivery system with high availability requires thinking about several key areas including system architecture, data replication, load balancing, failover strategies and monitoring. Here’s a general design process:
1. **System Architecture**:
We should adopt a distributed microservice-based architecture. In this design, distinct components of the service, such as the SMTP servers, email processing services, and frontend servers, can all be scaled out separately and updated independently without affecting the entire system.
2. **Data Replication and Redundancy**:
To ensure availability, you need to protect against single points of failure. Use redundant storage and data replication strategies to ensure there are always multiple copies of your data. A common strategy is to use an eventually consistent data store like Apache Cassandra, which provides tunable consistency and high availability, or Google Cloud’s Spanner, which provides strong global consistency.
3. **Load Balancing**:
Load balancing helps your system to manage traffic efficiently. Load balancers can distribute the incoming emails across multiple SMTP servers to prevent any single server from becoming a bottleneck. There are many types of load balancing algorithms available such as Round Robin, Weighted Round Robin, Least Connection, etc.
4. **Failover Strategies and Recovery Procedures**:
In case of server failure, the system should switch to a backup system. This is typically achieved by running multiple replicas of your servers and using health checks and heartbeats to detect failures. A common practice here is to use automated rollout and rollback processes for releasing changes in the systems with minimal impact on the running systems.
5. **Monitoring**:
Continuous system monitoring helps to ensure system availability. Building real-time monitoring and alerting into the system allows for prompt reaction when issues occur. This could include tools for log management and system metrics.
Hence, A sample architecture diagram for the system can be as follows:
graph TB
A[Internet] -- SMTP/Data --> B[Load Balancer]
B -- SMTP/Data --> C[Server 1]
B -- SMTP/Data --> D[Server 2]
C -- Sync --> D
D -- Sync --> C
C -- Write --> E[Database 1]
D -- Write --> F[Database 2]
E -- Replicate --> F
F -- Replicate --> EIn all, designing an email delivery system for high availability requires ensuring that the infrastructure can handle high traffic and is not prone to single points of failure by deploying several strategies including redundant and distributed systems, load balancing, data replication, regular health checks, and robust monitoring and alerting systems.