Round robin server setup - load-balancing

From what I understand, if you have multiple web servers, then you need some kind of load balancer that will split the traffic amongst your web servers.
Does this mean that the load balancer is the main connecting point on the network? ie. the load balancer has the IP address of the domain name?
If this is the case, it makes it really easy to add new hardware since you don't have to wait for any dns propogation right?

There are several solutions to this "problem".
You could round-robin at the DNS-level. I.e. have www.yourdomain.com point to several IP-addresses (well all your servers).
This doesn't give you any intelligence in the load balancing, but the load will be more or less randomly distributed, but you wouldn't be resilient to hardware failures as they would still require changes to DNS.
On the other hand you could use a proxy or a loadbalancing proxy that has a single IP but then distributes the traffic to several back-end boxes. This gives you a single point of failure (the proxy, you could of course have several proxies to defeat that problem) and would also give you the added bonus of being able to use some metric to divide the load more evenly and intelligently than with just round-robin dns.
This setup can also handle hardware failure in the back-end pretty seamlessly. The end user never sees the back-end, just the front-end.
There are other issues to think about as well, if your page uses sessions or other smart logic, you can run into synchronisation problems when your user (potentially) hits different servers on every access.

It does (in general). It depends on what server OS and software you are using, but in general, you'll hit the load balancer for each request, and the load balancer will then farm out the work according to the scheme you have in place (round robin, least busy, session controlled, application controlled, etc...)

andy has part of the answer, but for true load balancing and high availability you would want to use a pair of hardware load balancers like F5 bigips in an active passive configuration.
Yes your domain IP would be hosted on these devices and traffic would connect firstly to those devices. Bigips offer a lot of added functionality including multiple ways of load balancing and some great url rewriting, ssl acceleration, etc. It also allows you to run your web servers on a seperate non routable address scheme and even run multiple sites on different ports with the F5's handling the translations.
Once you introduce load balancing you may have some other considerations to take into account for your application(s) like sticky sessions and session state but that is a different subject

Related

Application load balancer vs network load balancer

I am new to AWS. I can't get a clear idea behind ALB vs NLB. Could anyone explain in a simple way?
There are some excellent answers out there already, let me pick out some key points that may help.
Network Load Balancer
As the name implies, this is for the network levels only. Typically layer 4.
It does not care, nor see, about anything regarding the application layer, such as cookies, headers, etc.
It is context-less, caring only about the network-layer information contained within the packets it is directing this way and that.
the 'balancing' done here is done so solely with IP addresses, port numbers, and other network variables.
Application Load Balancer
This takes into account multiple variables, from the application to the network. It can route its traffic based on this.
It is context-aware and can direct requests based on any single variable as easily as it can a combination of variables.
Key Differences
The network load balancer just forward requests whereas the application load balancer examines the contents of the HTTP request header to determine where to route the request
Network load balancing cannot assure availability of the application, where as Application load balancing can.
Some good sources from where I extracted this information are:
https://medium.com/awesome-cloud/aws-difference-between-application-load-balancer-and-network-load-balancer-cb8b6cd296a4
https://linuxacademy.com/community/show/22677-application-load-balancer-vs-network-load-balancer/
https://aws.amazon.com/elasticloadbalancing/features/#compare
In main response by #james above, network level has been defined multiple times and mentions about network layer information. However, I would like to mention, NLB yes operates on Layer4, but Layer4 is Transport Layer not Network Layer. NLB preserves source IP and thus Elastic IP can be used in case of NLB.

What is the difference between Reverse proxy and Load balancer?

I am trying to understand how reverse proxy and load balancing are different from each other. When its useful to use reverse proxy over load balancing.
Both promise to improve efficiency and sits in between client and server. They nearly look the same when we try to understand them, but still their functionality differs.
Load balancing: Is hardware or a software unit that distributes the total load on a website by distributing it to multiple servers.
The algorithms used by load balancing should be chosen as such it makes the best use of each servers’ capacity and can provide the result as fast as possible.
Load balancers are of three categories: DNS Round Robin, L3/L4 Load Balancer [ works on IP and TCP layer ], and L7 Load Balancer [ works on application layer].
The different kinds of algorithms used by load balancer for distributing load are IP Hash, Least connection, Round robin, Least traffic, etc.
Reverse Proxy: They act as a face of website or we can say they serve as a gateway that web traffic has to pass. The main role of a reverse proxy is:
Security: They act as a wall to your backend server. Protecting the backend from direct interactions and thus improving the security of the overall system.
Web acceleration: It also provides features like caching, SSL encryption, and Compression to reduce the time to provide responses to clients.
Flexibility: The changes in backend architecture become more flexible as the client can only access the reverse proxy.
A reverse proxy can even be relevant even when there is only one server in your system. In such cases there is no requirement of load balancers but still the reverse proxy can be useful providing security, flexibility and web acceleration.
According to this link,
A reverse proxy accepts a request from a client, forwards it to a server that can fulfill it, and returns the server’s response to the client. In other words, Reverse proxies act as such for HTTP traffic and application programming interfaces.
A load balancer distributes incoming client requests among a group of servers, in each case returning the response from the selected server to the appropriate client. Load balancers can deal with multiple protocols — HTTP as well as Domain Name System protocol, Simple Message Transfer Protocol and Internet Message Access Protocol. A load balancer receives and routes client requests for application, text, image or video data to any server in a pool that is capable of fulfilling them and then returns the server’s response to the client.

Load balancing: when should a user always connect to the same server?

I came across the "source" load balancing algorithm in HAProxy, which ensures that a user will connect to the same server, by choosing server based on a hash of the source IP.
Why and when is it important for a user to connect to the same server? I cannot think of a reason, assuming that all candidate servers serve identical content.
Furthermore, if there was the need for a user to always connect to the same server, then wouldn't load balancing be completely irrelevant for this user?
It is important for a user to connect to the same server if we want to achieve session persistence.
For example, when talking about a HTTP session, there are information/variables (think about a shopping cart) specific to the session in question.
This dynamic information is not shared by the candidate servers in case they are not configured to do so and it is simpler to deal with it at the load-balancing level.
The preferred way to deal with this in HAProxy is by using cookies, but this only works in HTTP mode. HAProxy offers the source load balancing algorithm, in case cookies can't be used. This can be used in TCP mode or with HTTP clients that refuse cookies.
Load balancing will be irrelevant for the user in question right, until the cookie expires. But we generally need load balancing when dealing with many users so that they can be served by multiple servers with each user sticking with one of them.

high availability websites

what's the best way to achieve high availability for a dynamic website? If I create a second copy on another server and do not wish to use a load balancer since it will mess up user sessions, what are the best alternatives?
You can store session data in a database instead, which gets around that problem, then you can round-robin the requests to the application servers.
(Good) Load Balancers can be configured to be "sticky" which means they send requests from the same IP to the same server each time.
Even if you have a load balancer sitting infront of two backend webservers, you just move the single point of failure onto the load balancer instead of the webserver. So your application would still not be highly available.
I highly recommend using a load balancer and at least a pair of web servers. At work, we use HA Proxy, which is fully capable of ensuring sessions are 'sticky', and are sent to the same web server unless it goes down, where it will fail over.
To make your load balancer highly available, you can set up two load balancing servers which are a mirror image of each other. Assign a single virtual IP to both of your load balancers. Write a script that will poll the other server to check if it's down; if it's down, have that script pick up that virtual IP address. The script should be running on both servers.
This link describes one way of managing a virtual IP address. Similar articles have been written for a large number of linux distros, but they are all based on the same method.
Loadbalancers. They should be configured in such a way that they can handle the sessions. Maybe by sending the same ip to the same backend every time. Or store them inside a database, or some shared memory if it needs to be really fast for some reason i haven't thought of.

Does each cloud instance have its own IP

For a cloud instance that runs Apache, I'm guessing the cloud has an IP address.
One of the benefits of using a cloud is scaling, but I'm not sure how that scaling happens. I thought that new instances are created automatically to accommodate rise in traffic. IF that's correct (correct me if I'm wrong), then does that mean that each new instance would have its own IP or what? because if that's the case, it would complicate matters a lot when pointing a domain to a cloud.
The cloud sits behind a load balancer which is able to redirect traffic to different spawned instances of Apache servers. In that since you can grow and shrink to any number of servers based on how much traffic you are receiving.