Load Balancer

Abstract

---

• A Reverse Proxy (反向代理) that does Load Balancing
• 2 types Application Load Balancer and Network Load Balancer. The example above is an External Application Load Balancer

Benefits

---

Failover Capability

• If one server goes offline, all the traffic will be routed to other servers by Load Balancer. This prevents the website from going offline or the downtime incurred by spinning up a new server
• We will also add a new healthy web server to the server pool to balance the load
• Thus making the system Fault Tolerance (容错性)

Scalability

• If the traffic grows rapidly, and current set of servers are not enough to handle the traffic
• We only need to add more servers to the server pool, and the Load Balancer automatically starts to send requests to them

Load-balancing Strategies

---

Round-Robin Load-balancing

• Load Balancer distributes incoming requests equally among the available Server in a circular order. Simple and straightforward
• Usually the default load-balancing strategy, for example in AWS ALB

Weighted Round Robin Load-balancing

• Similar to Round-Robin Load-balancing, but Server are assigned different weights. Servers with higher weights receive more requests than those with lower weights
• This is commonly used in A/B Testing

Terminologies

---

Sticky Session

• Also known as session affinity or session persistence
• Refers to the practice of directing a Client’s requests to the same Server consistently for the duration of a session or user’s interaction

References

---

• Scale From Zero To Millions Of Users