Ques 1. What Is Server Load Balancing? How does it work?
Server Load Balancing is a mechanism to distribute workload by providing availability and performance to software applications running across multiple servers. The objective is to offload the application load from single server and distribute among multiple physical servers via load balancing mechanism, which leads to improved application performance, smooth end user experience and scalable to handle increased requests.
Server Load Balancer works on the model of Virtual Servers and Real Servers. Virtual Servers have the IP address that is revealed to the clients over Internet or WAN locations. The Client sends the request to this Virtual IP address while hiding the real Servers. Real Servers are the application servers. One Virtual Server IP address on SLB is binded to TCP/UDP ports on multiple real Servers supporting the application. The client initiates the request for application on TCP/UDP port of the Virtual IP on the SLB.The SLB in response initiates the request to one of real Server (for that application) based on the configured algorithm across multiple servers. In its working, the SLB monitors the health of all the real servers and stops sending new requests to servers which are not available. Once the real server is available, SLB again includes real server in the list for accepting new connection requests. SLB maps one logical (virtual) server connection to multiple physical (real) servers. This allows a single IP address (virtual server IP address) can serve as the connection point for multiple TCP/UDP services such as HTTP, FTP or Telnet rather than each of the services requiring a different IP address for each service.
Ques 2. What is Global Load Balancing? How does it work?
Global Server Load Balancing (GSLB) is used for –
- Disaster Recovery functionality and
- Intelligent traffic redirection for optimal site selection.
It works as “Intelligent DNS” server, handling DNS resolutions based on algorithms, deciding where to resolve requesting traffic to.
Additionally, GSLB load balances traffic for application servers across Data Centers. The Internet traffic can be distributed among different data-centers located at different locations of the Globe. This technology avoids local downtimes and downtimes.
Ideally GSLB includes –
- Redirection of requests to other closest sites (DC) in case the usual DC or server does not respond.
- Forwarding the visitor requests to the site that is closer to the place from where the request is raised from a geographic point of view.
- In case a threshold is reached at a site, the requests are forwarded to other site which may be at a different geo-location. It algorithm would calculate the shortest distance from when the request has been raised.
GSLB keeps track of the location, performance, load, and availability of each data center and uses these factors to select the data center to which to send a client request.
Ques 3. What is Link Load Balancing? How does it work?
A link load balancer is also many times called a link balancer. It is a network appliance that distributes in-bound and out-bound traffic to and from multiple ISP links. Link load balancers are placed between Internet gateway routers and Internet firewalls.
A link load balancer uses traffic management algorithms to ensure that all leased ISP links are being used to their best advantage. The network administrator sets policies for the appliance so when it receives a request, it takes into account such things as client and end-user round-trip times and which ISP link has the most available bandwidth before completing the request.
Ques 4. What is NLB (Network Load Balancing)? How does it work?
The Network Load Balancing feature uses the TCP/IP networking protocol to route traffic to different hosts. NLB uses a proprietary Microsoft distribution algorithm to distribute network traffic among multiple servers or virtual machines within a cluster to avoid overloading any single host and also improve performance. The feature is available in Microsoft Windows Server 2008, 2008 R2, 2012 and 2012 R2 operating systems.
Ques 5. What are GSLB deployment models? Commonly used GSLB deployment models are –
- Active-Active deployment
- Active-Backup deployment
- Hybrid deployment
Ques 6. What are SLB deployment modes?
Server Load balancer solution is available in market by different vendors like F5, Radware ,Citrix Fortinet etc. and each vendor has some shades of deployment which is different from other.
However, to generalize we can have below deployment methods of SLB –
- 1-Arm Layer 2 mode (with SNAT)
- 1-Arm Layer 3 mode (with SNAT)
- 1-Arm Layer 3 mode (w/o SNAT)
- Inline Layer 2 mode
- Inline Layer 3 mode
Below link can provide detailed information on above 5 types of SLB deployment models – http://www.ipwithease.com/server-load-balancer-deployment-models/
Ques 7. What is health check in reference to Load balancing?
Load balancers (SLB, LLB and GSLB) use the concept of health monitoring to verify end to end availability of service being provisioned. Once health monitoring diagnosis a failure of service (like application server etc.) the traffic is redirected to other available servers or applications or links.
ICMP is one of the most common health monitoring probe used by Load balancers, though other methods are also available like –
- TCP/UDP ports (HTTP, TCP and HTTPS etc.)
- Response time
- Delay
Ques 8. What are commonly used Load balancing mechanisms in SLB?
Though multitude of Algorithms have been devised in SLB products, some of commonly used load balancing algorithms include –
- Round robin
- Weighted round robin
- Least connections
- Least response time
- Least traffic
- Source IP hash
Ques 9. What are commonly used Load balancing mechanisms in GSLB?
Commonly used Load Balancing mechanisms in GSLB involve following –
- Round Robin
- Least Response Time
- Least Connections:
- Least Bandwidth
- Least Packets
- Source IP Hash
- Custom Load
- Round Trip Time (RTT)
- Static Proximity
Ques 10. Give the names of some vendors/company products which provide SLB solutions?
Below are name of some vendors/companies who provide SLB Solutions –
- F5
- Citrix
- Kemp Technologies
- Radware
- Barracuda
- Fortinet
- Resonate
- Zen
- AWS
Ques 11. What is Reverse Proxy Cache?
The Reverse Proxy can cache requests, “files” once, and serves it out to many clients without bothering the web server. When used just for static files like images or CSS, reverse Proxy cache will be a great help. The key benefits come when you can cache html page requests so the PHP and database layers are also offloaded from intensive work. The proxy server will register the expire times of each request and clear it from the cache so a new version will be fetched from the web server the next time a client requests it.
Ques 12. Can SLB be used for site level redundancy?
Server Load balancing solution is tailor made Local to Data Center (or Site) for Distributing user requests to Servers for improved response and performance. SLB can’t provide services for application across 2 Site or locations.
Ques 13. Explain the term server farm / server pool?
Server farm refers to collection of real servers that contain the same content and that typically reside in the same physical location in a data center. The cluster/group of servers are generally used for Web Hosting services. This setup mitigates dependency on single server availability and provides smooth end user experience.
Ques 14. Explain the term Session Persistence?
Session persistence refers to directing a client’s requests to the same backend web or application server for the duration of a “session” or the time it takes to complete a task or transaction.
For instance – A user engaged in a transaction requiring multiple steps such purchasing a product or upgrading his account. As the client and server exchange data, they need to store state information about the session (Sometimes referred to as context) for the transaction to proceed smoothly. In this case, session persistence makes it much easier for transactions to complete successfully.
Ques 15. Customer A has 5 Web servers in his Data Center and want to load balance traffic coming from Internet users across these 5 servers to equally distribute the load. Which type of Load Balancing solution will the customer use?
The Load Balancer solution which meets this requirement is Server Load Balancer. The requests to VIP of Server Load balancer will be distributed (based on algorithm configured) to all the 5 servers.
Ques 16. Customer B has 2 Web servers one each Data Center and DR site. Requirement is to make sure that when Data Center server or whole of Data Center goes down, the traffic is redirected DR site web server. Which Load Balancing solution addresses this requirement?
GSLB solution will address this type of scenario. The GSLB service over Cloud or dedicated appliance will continuously monitor Web Servers over Internet on probes like ICMP, TCP or HTTP. Let’s say the Data Center goes down due to a catastrophe, The GSLB will get the
information of non-availability of DC servers (due to failed response to health check probes) and provide the IP address (to DNS query) of DR site Web Servers.
Ques 17. What is reverse proxy? How is it related to Load balancing?
A Reverse Proxy is a type of proxy server that retrieves resources on behalf of a client from one or more servers. These resources are then returned to the client as though they originated from the proxy server itself.
In other words, a reverse proxy is usually an Internet-facing proxy used as a front-end to control and protect access from outside Network to a server on a private network. A reverse proxy commonly also performs tasks such as load-balancing, authentication, decryption or caching.
A reverse proxy accepts requests from external clients on behalf of servers stationed behind it just like what the figure below illustrates.
To the client it is the reverse proxy that is providing file transfer services. The client is unknown to the file transfer servers behind the proxy, which are actually providing those services. In effect, whereas a forward proxy hides the identities of clients, a reverse proxy hides the identities of servers.
Ques 18. What is meant by SSL Offload?
SSL offloading takes all the processing of SSL encryption and decryption off the main Web server and moves it to a separate device (Server Load Balancer). This offloads Web server from overhead of handling SSL traffic and benefits with faster processing and better end user experience.
Below are 2 diagrams depicting SSL Service running (1) Web Server (2) Server Load Balancer. In 1st Scenario, Web Server itself is running SSL Service for protection against hacker attack, however this takes away a lot of CPU and Memory resources of Server which may decline the actual service Web Server application is providing to end users.
In 2nd Scenario, Server Load balancer takes responsibility to run SSL Service on behalf of Web Server and therefore offloads Web server to perform its primary task of providing service to end users thereby improving the customer experience. In this scenario SLB is performing what we call as SSL Offload.
Ques 19. Which are 4 essential factors considered while selecting a Server Load balancer?
Key factors considered while selecting a Server Load Balancer are as below –
- Throughput
- Concurrent L4/L7 connection
- SSL Connections
- High availability (like Dual Power Supply, Fail to wire, Field replaceable units etc.)
Ques 20. What are benefits of SLB?
Below are the benefits of using SLB –
- Load Distribution – Distributes application workload across multiple servers
- Health monitoring – Server and application level Health Monitoring
- Performance – Increases application performance and Scalability
- Management – Improves the management and administration
- SSL Offload – Offloads the web server of encrypting/decrypting SSL traffic.
- Persistence – Maintenance of session persistence in application which manage client state on the server side.
- Scalability – Increases scalability by allowing new virtual and/or physical server addition transparently without disruption.
Ques 21. What are benefits of GSLB/GTM?
Enlisted below are the benefits we can garner from GSLB/GTM –
- Provides Data Center failover and Business continuity
- Provides Optimized multi-site deployments
- Ensures end user’s experience is the best.
- Includes geographic and network proximity policy metrics
Ques 22. What are benefits of LLB?
Below are the key benefits of having LLB in the network –
- Link Aggregation – Link Balancer automatically aggregates Internet bandwidth from multiple connections. Administrators can choose multiple connections to the same or different ISPs for the purposes of consolidating access to affordable Internet bandwidth
- Inbound Link Balancing – Link Load balancer can perform functionality of Authoritative DNS and manipulate incoming traffic (like Web hosted Servers etc.) to use multiple links in Active-Active scenario.
- Link Redundancy – During a link failure, LLB moves the traffic over working link. Also it regularly checks the health of a given connection, assuring fast reconnection when Internet service is restored.
- Bandwidth management through QOS – The LLB provides administrators with tools to automatically prioritize critical Internet applications. For example, Web browsing and email can be guaranteed bandwidth while peer-to-peer applications and media streaming can be assigned a lower priority.
- Network Services – Some LLBs also render additional services like DHCP Server, DNS caching, and VPN etc.
- Acceleration – Offers integrated acceleration and optimization for boosting application performance.
Ques 23. Which layer does SLB work on?
SLB works upto Layer 7 (like handling HTTPS requests and response). Layer 7 load balancers base their routing decisions on various characteristics of the HTTP header and on the actual contents of the message, such as the URL, the type of data (text, video, graphics), or information in a cookies.
