Load Balancer

Categories

Layer 4 (L4) Load Balancer

• A.K.A Network Load Balancing
• Happens on Transport Layer
• Decision will be based on the TCP or UDP ports that packets use along with their source and destination IP addresses.
• Performs Network Address Translation (NAT) on the request packet.
• Maximizes the utilization and availability by distributing the traffic across IP addresses, switches, and routers.

Layer 7 (L7) Load Balancer

• A.K.A Application Load Balancer / HTTP(S) Load Balancer
• Happens on Application Layer
• Uses information such as HTTP header, cookies, uniform resource identifier, SSL session ID, and HTML form data to decide the routing request across the servers.

Global Server Load Balancing (GSLB)

• Distribute across servers globally
• DNS load balancing / CDN caching

Static Algorithms

Round Robin

• Distributed across the servers in a sequential or rotational manner
• Doesn’t consider the load already on a server so there is a risk that one of the servers receives a lot of requests and becomes overloaded

Weighted Round Robin

• Similar to round robin, plus each of the resources in a list is provided a weighted score
• Some of the servers get a bigger share of the overall request.

Source IP Hash

• Request is sent to the server based on the client’s IP address.

URL Hash

• Request is sent to the server based on the client’s URL.
• Cache the hash value and subsequent requests will map to the same server.

Randomized algorithm

• Maps requests to a random destination.
• Works well with server cluster with similar configurations.

Dynamic Algorithms

Least Connection

• Request will be directed to the server with the fewest number of requests or active connections.
• Load balancer needs to do some additional computing to identify the server with the least number of connections.
• Most useful when there is a huge number of persistent connections in the traffic unevenly distributed between the servers.

Weighted Least Connection

• Request will be directed to the server with the fewest number of requests or active connections and relative capacity of server.
• Server are weighted based on processing capabilities

Least Response Time

• Request is forwarded to the server with the fewest active connections and the least average response time.