Virtual Web Cluster

7/27/2019 Virtual Web Cluster

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Virtual web clusterIn the virtual web cluster there is no switch

that is the (first) target for all requests.

Since there is no one that can distribute the

traffic we need to make all servers to

receive all traffic and then the servers

should investigate the packet and exactly

one should say yes I do it! All the othersshould skip the packet.

Two ways to get all messages toall servers

1. Unicast MAC address

2. Multicast MAC address


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Unicast MAC address

R

SVIP MAC

SVIP MAC

SVIP MAC All servers have the

same VIP address

and the same MAC

address!

Unicast MAC address (2)

Note:

It must be possible to change the MACaddress in the adapter card

It must be possible to install the MACaddress in the ARP cache of the router (it

may not ARP!) The servers must be on the same network

The servers cannot talk to each other!!!!!


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Add a new network for internal

communication

R

SVIP MAC

SVIP MAC

SVIP MAC

IP1

IP2

IPn

MAC1

MAC2

MACn

Multicast MAC address

R

SVIP MMAC

SVIP MMAC

SVIP MMAC

IP1

IP2

IPn

MAC1

MAC2

MACn


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Multicast MAC address (2) Keep private IP address and private MAC address

Set all server adapters to listen to a specifiedMulticast MAC address (MMAC)

Set the router ARP to send all VIP packets toMMAC (No ARP allowed!) This is not possiblefor all routers.

The ordinary IP addresses and MAC addresses are

there. Therefore the servers can talk to each otherand you can talk to an individual server fromoutside if needed.

Selection mechanisms in allservers

In both cases all servers are getting all

requests. Therefore we need a selection

mechanism in them which assures that

exactly one server is taking the request and

all follow on packets!


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Distributed web systemsIn distributed web systems the clients can see

the different IP-addresses that the servers

have.

The selection of which server to use is done

either on the DNS level or on the server

level.

Distributed web systems, DNSrouting

(p273)

If we can control the authoritative DNS server

we translate the first DNS request to server-

1, the next one to server-2 etc.

This will make the clients to ask different

servers but


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Distributed web systems, DNS

(2) DNS caching makes the method less

efficient.

Small TTLs are needed, which howeverincreases the load on the network.

The clients cache translations anyhow,independent of TTL.

We dont need to have the servers at onephysical location.

Distributed web systems, serverrouting

We can allow the original request to arrive toany server and then this server will, ifneeded, try to get the communication to betransferred to the correct server:

Methods

1. Triangulation using tunneling2. HTTP redirection

3. URL rewriting


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1. Triangulation using tunnelingThe first server gets the request and tunnels it

to the correct server.

The correct server creates an answer and

sends it directly to the client. We have got a

triangle.

The next interaction will continue with the

first server again etc.

2. HTTP redirection

The first web server responds with a status

code 301 or 302 to instruct the client to

resubmit the request to another server.(p 284)

+ the rerouting can be done on a medium

granularity level and can be content aware.- We will get extra long distance traffic.


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3. URL rewritingThe server which responds to the first file request

dynamically changes all URL to point to the

server which should do the work in the future.

- May cause extra DNS traffic and cause extra work

on the redirecting web server for interpretation of

links.

+ This may be implemented totally on user level.(p 286)

Dispatching

Up to now we have mostly been concerned with the

technology to get the request to be served by a

certain server.

Now we will discuss the decision issue: Which

server should do the job and when decided it will

be implemented by the methods described earlier.

This is calledDispatchingof the request.(p 288, 289)


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Content blind

Static dispatchingStatic dispatching is one of the alternatives for

switches working on level 4:

Random assignment

Round robin assignment

Both methods can easily be adapted to the

situation when servers have different

capacities.

Server state aware dispatching

The idea is to gather information from the

servers about the current load situation and

dispatch new request to the least loaded

server.

If no of request being processed is the

measure we need to adjust according to thepower of the server.


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Content aware dispatchingContent aware dispatching can be used for

different purposes:

Improve reference locality to make servercaches more efficient (cache affinity)

Allow specialized servers (for streamingmedia for example)

Better load sharing between the servers(p 291)

Documents

Virtual Web Cluster