In computing, load balancing is the process of distributing a set of tasks over a set of resources (computing units), with the aim of making their overall processing more efficient. Load balancing can optimize the response time and avoid unevenly overloading some compute nodes while other compute nodes are left idle.
Load balancing is the subject of research in the field of parallel computers. Two main approaches exist: static algorithms, which do not take into account the state of the different machines, and dynamic algorithms, which are usually more general and more efficient but require exchanges of information between the different computing units, at the risk of a loss of efficiency.
The job of the load balancer is to fairly divide all clients’ requests among the pool of available servers. Load balancers perform this job to avoid overloading or crashing servers.
Load balancers provide the following capabilities:
- Scalability: By adding servers, the capacity of the application/service can be increased seamlessly. Load balancers make such upscaling or downscaling transparent to the end users.
- Availability: Even if some servers go down or suffer a fault, the system still remains available. One of the jobs of the load balancers is to hide faults and failures of servers.
- Performance: Load balancers can forward requests to servers with a lesser load so the user can get a quicker response time. This not only improves performance but also improves resource utilization.
Here’s an abstract depiction of how load balancers work:
Simplified working of a load balancer
Services offered by load balancers
LBs not only enable services to be scalable, available, and highly performant, they offer some key services like the following:
- Health checking: LBs use the heartbeat protocol to monitor the health and, therefore, reliability of end-servers. Another advantage of health checking is the improved user experience.
- TLS termination: LBs reduce the burden on end-servers by handling TLS termination with the client.
- Predictive analytics: LBs can predict traffic patterns through analytics performed over traffic passing through them or using statistics of traffic obtained over time.
- Reduced human intervention: Because of LB automation, reduced system administration efforts are required in handling failures.
- Service discovery: An advantage of LBs is that the clients’ requests are forwarded to appropriate hosting servers by inquiring about the service registry.
- Security: LBs may also improve security by mitigating attacks like denial-of-service (DoS) at different layers of the OSI model (layers 3, 4, and 7).
As a whole, load balancers provide flexibility, reliability, redundancy, and efficiency to the overall design of the system.
Load balancers are usually deployed in pairs as a means of disaster recovery.
If one load balancer fails, and there’s nothing to failover to, the overall service will go down.
Generally, to maintain high availability, enterprises use clusters of load balancers that use heartbeat communication to check the health of load balancers at all times.
On failure of primary LB, the backup can take over. But, if the entire cluster fails, manual rerouting can also be performed in case of emergencies.
Types of load balancers
Depending on the requirements, load balancing can be performed at the network/transport and application layer of the open systems interconnection (OSI) layers.
- Layer 4 load balancers: Layer 4 refers to the load balancing performed on the basis of transport protocols like TCP and UDP. These types of LBs maintain connection/session with the clients and ensure that the same (TCP/UDP) communication ends up being forwarded to the same back-end server. Even though TLS termination is performed at layer 7 LBs, some layer 4 LBs also support it.
- Layer 7 load balancers: Layer 7 load balancers are based on the data of application layer protocols. It’s possible to make application-aware forwarding decisions based on HTTP headers, URLs, cookies, and other application-specific data—for example, user ID. Apart from performing TLS termination, these LBs can take responsibilities like rate limiting users, HTTP routing, and header rewriting.
Note: Layer 7 load balancers are smart in terms of inspection. However layer 4 load balancers are faster in terms of processing.
