Every load balancer is a reverse proxy, but not every reverse proxy has to be a load balancer. Question: What does the OSI model look like?Question: What are the requirements for a load balancer?Answer → To create a fault tolerant system so that when a client makes a request to the load balancer, it can communicate with one or more backends. Question: Explain how a layer 4 load balancer works?Answer → A layer 4 load balancer works as follows: (1) Handshake → The Layer 4 load balancer first establishes a TCP connection with the backend server. (2) Warm-up → The load balancer can establish multiple TCP-based connections with the backend and then keep those connections active. This is to ensure that whenever a client connects to the load balancer, it does not have to go through the connection warm-up process every time. (3) Client Connection → When a client connects to a Layer 4 load balancer, the connection will have a state at the load balancer level and will be mapped to any one of the connections to the backend servers. Hence, Layer 4 load balancers are stateful.
Question: Does a layer 4 load balancer also act as a NAT layer?Answer → Yes, a layer 4 load balancer acts as a NAT layer/gateway/router for the clients.
Question: Can you give an example of HTTP-based calls (Restful API calls) from the client to the backend?Answer → Imagine a client sends an Http call to a load balancer. Also imagine that this particular request is split into 2 segments. Step #1) The request arrives at the load balancer, which then chooses to send all the segments of this request to a certain backend server. Step #2) Now, suppose the client sends another segment (segment #3) and then writes it back to the same connection.
Step #3) Now, suppose the same client initiates another request on the same connection, then that request will also be read first and then written back to the backend server. Step #4) Now, suppose a different client makes another request, then depending on the load balancing algorithm, the request may be forwarded to another backend server. Question: What companies provide L4 load balancers?Answer → Companies like Radware and F5-BigIp provide L4 load balancers. Question: What are the pros and cons of L4 load balancers?(1) The following are the advantages of L4 LB:
(2) The following are the disadvantages of L4 LB:
Question: Explain how a layer 7 load balancer works?Answer → A layer 7 load balancer works as follows: (1) Handshake → The Layer 7 load balancer also establishes a TCP connection with the backend server. (2) Warm-up → The load balancer can establish multiple TCP-based connections with the backend and then keep those connections active. This is to ensure that whenever a client connects to the load balancer, it does not have to go through the connection warm-up process every time. (3) Client Connection → When a client connects to a Layer 7 load balancer, the connection becomes protocol specific.
NOTE: If you want your L7 LB to face external clients, then:
Question: How does an L7 load balancer handle HTTP based requests?Step #1) First, it receives the request and sends it to the backend server 1. Step #2) Next, suppose the same client sends another request, then L7 LB can actually forward it to other backend servers as well since L7 LB is stateless. Question: What are the pros and cons of L7 load balancers?(1) The following are the advantages of L4 LB:
(2) The following are the disadvantages of L4 LB:
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