¿Qué es un equilibrador de carga?

Los usuarios y clientes dependen de una capacidad casi en tiempo real para encontrar información y realizar transacciones. Los tiempos de espera o las respuestas poco fiables e incoherentes —incluso en los momentos de mayor demanda y uso— pueden alejar a un cliente para siempre. Y los picos elevados de necesidad de computación pueden causar estragos en un servidor interno o en un sistema de servidores si la demanda entrante —o «carga»— es demasiado alta para gestionarse con facilidad.

Advantages of using a load balancer include:

• Application availability: Users both internal and external need to be able to rely on application availability. If an application or function is down, lagging, or frozen, precious time is lost—and a potential source of friction is introduced that might drive a customer to a competitor.
• Application scalability: Imagine you run a ticketing company, and tickets for a popular performance are announced to be available at a certain date and time. There could be thousands or even more people trying to access your site to buy tickets. Without a load balancer, your site would be limited to whatever your single/first server can accommodate—which likely won’t be much with that much demand. Instead, you can plan for this big spike in traffic by having a load balancer to direct requests and traffic to other available compute surfaces. And that means more customers can get their desired tickets.
• Application security: Load balancing also lets organizations scale their security solutions. One of the primary ways is by distributing traffic across multiple backend systems, which helps to minimize the attack surface and makes it more difficult to exhaust resources and saturate links. Load balancers can also redirect traffic to other systems if one system is vulnerable or compromised. In addition, load balancers can offer an extra layer of protection against DDoS attacks by rerouting traffic between servers if a particular server becomes vulnerable.
• Application performance: By doing all of the above, a load balancer boosts application performance. By increasing security, by optimizing uptime, and by enabling scalability through spikes in demand, load balancers keep your applications working as designed—and the way you, and your customers, want them to.

Existen dos tipos de algoritmos de equilibrado de carga según su funcionamiento: estáticos y dinámicos. El equilibrado de carga estático mide la carga entrante utilizando algoritmos que cuentan con información sobre la capacidad de rendimiento de los servidores en la red distribuida. Por su parte, el equilibrado de carga dinámico identifica de manera dinámica, durante el tiempo de ejecución, la cantidad de carga que debe redistribuirse y el sistema que debe asumirla. Está diseñado para entornos con grandes fluctuaciones en la carga entrante.

Los siguientes son algunos de los tipos comunes de algoritmos de equilibrio de carga.

• Round robin: This algorithm sends traffic to a list of servers in rotation using the Domain Name System (DNS). (Note: DNS load balancing can also be a dynamic solution.)
• Threshold: This algorithm distributes tasks based on a threshold value that is set by the administrator.
• Random with two choices: The “power of two” algorithm selects two servers at random and sends the request to the one that is selected by then applying the Least Connections algorithm or the Least Time algorithm, if so configured.
• Least connections: A new request is sent to the server with the fewest current connections to clients. The relative computing capacity of each server is factored into determining which one has the least connections or which is using the least amount of bandwidth or resources.
• Least time: In this algorithm, a request is sent to the server selected by a formula that combines the fastest response time and fewest active connections. 
• URL hash: This algorithm generates a hash value based on the URL present in client requests. The requests are forwarded to servers based on the hash value. The load balancer caches the hashed value of the URL, so subsequent requests that use the same URL result in a cache hit and are forwarded to the same server.
• Source IP hash: This algorithm uses the client’s source and destination IP addresses to generate a unique hash key to tie the client to a particular server. As the key can be regenerated if the session disconnects, this allows reconnection requests to get redirected to the same server used previously.
• Consistent hashing: This algorithm maps both clients and servers onto a ring structure, with each server assigned multiple points on the ring based on its capacity. When a client request comes in, it is hashed to a point on the ring, and is then dynamically routed clockwise to the next available server.

El equilibrio de carga responde estática o dinámicamente a la solicitud de un usuario y la distribuye a uno de los servidores back-end capaces de satisfacerla. Si uno de los servidores se cae, el equilibrador de carga redirige el tráfico al resto de servidores en línea.

An example of static load balancing: A company hosts a website with content that is largely static. This scenario would be ideal for a static load balancer because the traffic needs are predictable and consistent. The company can use two (or more) identical web servers across which the static load balancer can distribute traffic.

An example of dynamic load balancing: A company experiences surges, spikes, and drops in traffic. Some are predictable and some are not. These organizations would benefit from dynamic load balancing. Such companies might include an e-commerce retailer announcing Black Friday hours and dates; a healthcare company which has just announced it can schedule online appointments for a seasonal vaccine; a government unemployment agency which requires unemployment insurance recipients to file a weekly claim on a certain day of the week; a relief organization that may need to respond quickly online to a natural disaster. Some of these surges and spikes in traffic and demand can be planned for, but some cannot. In these scenarios, a dynamic load balancing algorithm will help ensure access to apps and resources when customers and users need them most.

Existen otros tipos de soluciones de equilibradores de carga, que pueden utilizarse solos o en una red con equilibradores de carga nativos de la nube. A continuación se indican algunos tipos destacados.

Hardware Load Balancer: A hardware load balancer is a physical device with a specialized operating system that can be programmed to distribute web traffic across several application servers, usually on-premises. 

Software Load Balancer: A software load balancer operates like a physical load balancer, but it runs on software programs. The software keeps apps available through all kinds of traffic demands, using both static and dynamic load balancing to eliminate single points of failure.

Virtual Load Balancer: A type of load balancer that combines hardware and software load balancers is a virtual load balancer. It uses application delivery controller software that helps to distribute network traffic load among hardware backend servers.