We have two apache servers for load balancing. Whenever I upload a file on one server. Using load balancing concepts, will it get copied into other server.
Do these two server maintain replica of each other?
If not, how to do that? How to maintain the replica of one another servers?
If Yes, what configuration is required.
Thanks for help.
Load balancing balances the requests that are sent to a load balancer to the server that actually answers them.
Handling files that are uploaded to one server is on the application level - it must be handled by your application - e.g. through storing it in a location that all nodes can access (filesystem, database).
There's nothing that tomcat or an appserver can do for you - because they don't know what needs to be replicated and what doesn't. They don't know if something that you uploaded will be processed and can be forgotten, or if it will be stored for later download.
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Hi I'm little confused about load balancer concept
I've read some articles about loadbalancer in nginx and from what I've understand is that the load balancer spread the request into multiple servers !
But i thought if one server is down another one is up and running (not simultaneously all server together)
and another thing is when request spread between servers what happen to static data like sessions and InMemory Database like RedisDB
I think i'm confused and missunderstood the loadbalancer mechanism
and from what I've understand is that the load balancer spread the request into multiple servers ! But i thought if one server is down another one is up and running (not simultaneously all server together)
As it comes from the name the goal of load balancer (LB) is to balance the load. As per wiki definition for example:
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.
To perform this task load balancer obviously need to have some monitoring over the resources, including liveness checks (so it can bring out of the rotation the failing servers/nodes). Ideally LB should work with stateless services (i.e. request could be routed to any of the servers supporting handling such request type) but that is not always the case due to multiple reasons, for example in ASP.NET in case of non-distributed session requests should have been routed to servers which handled the previous request from the session, which could have been handled with so called sticky session/cookie.
and another thing is when request spread between servers what happen to static data like sessions and InMemory Database like RedisDB
It is not very clear what is the question here. As I mentioned before ideally you will want to have stateless services which will use some shared datastore (s) to handle the requests, so if request comes for any server/node it can load all the needed data to handle it.
So in short when request comes to LB it selects one of the servers based on some algorithm (round robin, resource based, sharding, response time based, etc.) and send this request to this server so in theory based on the used approach sequential requests from the same source can hit different nodes/servers (so basically this is one of the ways to horizontally scale your application).
I actually found my answer in nginx doc page
Short answer is IP-Hash mechanism
Nginx doc word :
Please note that with round-robin or least-connected load balancing, each subsequent client’s request can be potentially distributed to a different server. There is no guarantee that the same client will be always directed to the same server.
If there is the need to tie a client to a particular application server — in other words, make the client’s session “sticky” or “persistent” in terms of always trying to select a particular server — the ip-hash load balancing mechanism can be used.
With ip-hash, the client’s IP address is used as a hashing key to determine what server in a server group should be selected for the client’s requests. This method ensures that the requests from the same client will always be directed to the same server except when this server is unavailable.
To configure ip-hash load balancing, just add the ip_hash directive to the server (upstream) group configuration:
upstream myapp1 {
ip_hash;
server srv1.example.com;
server srv2.example.com;
server srv3.example.com;
}
http://nginx.org/en/docs/http/load_balancing.html
I am currently evaluating CrateDB and came up this question. I know that CrateDB proudly claims itself to be a no-config database. So, is there automatic load balancing? Or is it possible to add a load balancer before the dabase?
After browsing the web for a bit I found this howto indicating there is no automatic load balancing in CrateDB. Is that correct?
yes, load balancing happens automatically:
the client libraries can be configured with multiple nodes and then do round-robin scheduling
if you pass a query to a node - the execution of the query will happen distributed in the cluster anyhow.
Considering a three-tier app ( web server, app server and database).
[Apache Web server -> Tomcat app server -> database]
How to build an app stack (leaving out the database part) that has no single point of failure and is fault tolerant ?
IMHO, this is quite an open-ended question. How specific is a single point of failure - single app server, single physical server, single data centre, network?
A starting point would be to run your Tomcat and Apache servers in clusters. Alternatively, you can run separate instances, fronted with a load balancer such as HAProxy - except, to avoid a single point of failure, you will need redundancy on the load balancer as well. I recently worked on a project where we had two instances of a load balancer, fronted with a virtual IP (VIP). The load balancers communicated with two different app server instances, using a round-robin approach. Clients connected to the VIP in order to use the application, and they were completely oblivious to the fact that there were multiple servers behind it.
As an additional comment, you may also want to look at space-based architecture - https://en.wikipedia.org/wiki/Space-based_architecture.
The load balancing process indicated in this link :
https://httpd.apache.org/docs/2.4/mod/mod_proxy_balancer.html
Is requiring two servers to work.
How can I adapt the described process to make it work with only one server ?
The entire point of load balancing is to equally split traffic over a number of servers. Implementing load balancing with one server would be completely missing the point and entirely useless.
I have a Glassfish v2u2 cluster with two instances and I want to to fail-over between them. Every document that I read on this subject says that I should use a load balancer in front of Glassfish, like Apache httpd. In this scenario failover works, but I again have a single point of failure.
Is Glassfish able to do that fail-over without a load balancer in front?
The we solved this is that we have two IP addresses which both respond to the URL. The DNS provider (DNS Made Easy) will round robin between the two. Setting the timeout low will ensure that if one server fails the other will answer. When one server stops responding, DNS Made Easy will only send the other host as the server to respond to this URL. You will have to trust the DNS provider, but you can buy service with extremely high availability of the DNS lookup
As for high availability, you can have cluster setup which allows for session replication so that the user won't loose more than potentially one request which fails.
Hmm.. JBoss can do failover without a load balancer according to the docs (http://docs.jboss.org/jbossas/jboss4guide/r4/html/cluster.chapt.html) Chapter 16.1.2.1. Client-side interceptor.
As far as I know glassfish the cluster provides in-memory session replication between nodes. If I use Suns Glassfish Enterprise Application Server I can use HADB which promisses 99.999% of availability.
No, you can't do it at the application level.
Your options are:
Round-robin DNS - expose both your servers to the internet and let the client do the load-balancing - this is quite attractive as it will definitely enable fail-over.
Use a different layer 3 load balancing system - such as "Windows network load balancing" , "Linux Network Load balancing" or the one I wrote called "Fluffy Linux cluster"
Use a separate load-balancer that has a failover hot spare
In any of these cases you still need to ensure that your database and session data etc, are available and in sync between the members of your cluster, which in practice is much harder.