Scalable Web Server Clustering Technologies

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Scalable Web Server Clustering Technologies

• Trevor Schroeder,Steve Goddard, and Byrav
  Ramamurthy University of Nebraska-Lincoln
• IEEE Netwwork ,volume 14 Issue 13 May-June 2000

2
Outline

• Introduction
• Terminology
• L4/2 Clustering
• L4/3 Clustering
• L7 Clustering
• Conclusions

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Introduction

• Background Growth of Internet, Dynamic content
  and increasing users force us to find faster
  server (Web).
• In the past, we replaced the web server with
  faster machine (processor).
• Drawback
• Short-term (Moore Law, the number of
  transistors per integrated circuit would double
  every 18 months)
• Expensive we need to replace almost the
  whole machine.
• Solution Add more processor or machine to the
  Web server. (It is commodity hardware and
  software, so that we can keep the past
  investment.)

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Introduction (cont.)

• Any server application may be clustered as long
  as it fulfills the following two properties.
• The application must maintain no state on the
  server
• Client/server transactions should be relatively
  short and hight in frequency

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Terminology

• L4/2 Layer 4 Switching with Layer 2 Packet
  Forwarding. The system has identical layer 3
  (Network) with unique MAC address.
• L4/3 Layer 4 Switching with Layer 3 Packet
  Forwarding. The system has identical layer 4
  (Transport, same services) with unique network
  address.
• Layer 7 Switch Make forwarding decision based on
  the content of client requests. It can employ
  L4/2 or L4/3.

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Terminology (cont.)

• Client-side Transparency The whole cluster
  servers appear to be a single host to clients
  because of the dispatcher.
• Server-side Transparency Each cluster server
  runs standard web-server designed for standalone
  server. It servers the requests forwarded from
  dispatcher just the same as the requests come
  directly from the clients.
•  
• Performance Index Connections per seconds or
  bits per seconds. (Cluster Maximum Utilization)

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L4/2 Clustering

• The clusters IP address (A) is shared by the
  dispatcher and servers through the use of primary
  and secondary IP addresses. (BK Each host can
  have several IP addresses.)
• The dispatchers primary IP address is A.
• The servers use A as secondary address.
• All packets whose destinations are A are
  forwarded to the dispatcher through the use of
  Address Resolution Protocol (ARP) in the nearest
  gateway/router.

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Technology Specification

• Load-Sharing Algorithm Round-Robin or other
  policies.
• Session Map When request is connection
  initiation, if it belongs to established
  connection in the map, forward it to the
  previously selected server, or select a server
  and save the connection in the map. If it doesnt
  contain a SYN, it maybe discarded or not.
• Backup method To avoid the down of the
  dispatcher and servers.

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L4/2 Traffic Flow
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L4/2 Traffic Flow (cont.)

• A client sends a request to A.
• The router sends the request to the dispatcher.
• Based on the load-sharing algorithm, the
  dispatcher selects actual server (2) to serve the
  client.
• Server 2 replies the client directly.

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Advantage vs. Disadvantage

• Advantage
• Servers reply clients directly, which avoid the
  dispatcher to be bottleneck.
• Dont need to recalculate the checksum because it
  operates on layer 2.
• Disadvantage
• There must be direct physical connection to all
  servers and the dispatcher.

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ONE-IP (Bell Lab, 1996)

• Load-Sharing Algorithm
• Routing-based Dispatching Hash the incoming
  clients address to get a number that indicates
  which server to service the request

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ONE-IP (cont.)

• Broadcast based dispatching Each server has a
  fixed and disjoint portion of the address space.

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ONE-IP (cont.)

• Drawback Cannot adapt to the condition that the
  client requests are disproportionately
  distributed.
• Backup Watchdog daemonm watchd
• Dispatcher fail The backup dispatcher will
  notice the missing heartbeat of the primary
  dispatcher and take over.
• Server fail Reconfigure the hash table or the
  address filters on other servers.

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L4/3 Clustering

• The dispatcher appears as a single host to
  clients while as a gateway to the servers (IP
  address A).
• Each server has its own IP address that can be
  globally unique or locally unique (IP addresses
  B1, B2, , Bn).
• Load sharing algorithm Round robin or other
  algorithms
• Keep a session map table.

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L4/3 Clustering (cont.)
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L4/3 Clustering (cont.)

• A client sends request with A as the destination
• The packet comes to the dispatcher
• Based on the load sharing algorithm and session
  table, select the server, rewrite the destination
  IP address, recalculate the checksums, forward it
  to the server
• The server replies the request through the
  dispatcher (gateway) address A as the destination
  address.
• The dispatcher rewrite the source IP address of
  reply as A, recalculate the checksums, forward it
  to the client.
• Disadvantage
• Recalculate twice the checksums. (IP and TCP)
• All traffic flow through the dispatcher.
  (Bottleneck)

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Magicrouter

• University of California at Berkeley, 1996
• Fast Packet Interposing and modifications of
  kernel
• Load sharing Algorithms
• Round robin
• Random
• Incremental Load
• Backup
• Dispatcher primary backup model.
• Server Use ARP to map server IP addresses to MAC
  addresses to detect the fail of servers.

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LocalDirector (Cisco, 1996)

• Load sharing Algorithm
• Least connections choose the server with fewest
  connections
• Fastest Response choose the server that response
  the request first.
• Round-Robin Strictly RR policy.
• Backup
• Dispatcher extra LocalDirector unit that linked
  to the primary one with special failover cable
• Server Contact servers periodically, when fail,
  remove it, continue to contact, when up, add to
  the server pool
•  

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LSNAT

• University of Nebraska-Lincoln
• RFC2391 Load Sharing using IP Network Address
  Translation (LSNAT)
•  
• Backup
• Dispatcher select one server as new dispatcher.
  Distributed State Reconstruction Mechanism to
  rebuild the map of existing connections.
• Server Exclude from active servers pool. When
  up, include it again.

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L7 Clustering

• Make dispatch decision based on the content.
  (Application Layer)
• Content-based dispatching

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LARD

• Locality-Aware Request Distribution, Rice
  University
• It uses TCP handoff protocol with the modified
  kernel.
• Different server processes different kind of
  requests, which can make use of specialized
  server.

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Web Accelerator (IBM)

• The accelerator can now perform content-based
  routing in which it makes intelligent decisions
  about where to route requests based on the URL.
• L7 based on L4/2
• Web page caching
• The dispatcher services as a gateway/router.
• All traffic flows through the dispatcher.

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ArrowPoint

• Content-based dispatching policy
• Caching mechanism is similar to Web Accelerator
• Sticky connection
• Hot standby of the dispatcher and server node
  fail detection mechanism.

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Conclusion(1/4)

• L4/2 Clustering
• Bottleneck power of dispatcher to process
  incoming request
• Advantage Sustainable request rate.
• L4/3 Clustering
• Bottleneck recalculation of checksums.
•  
• L7 Clustering
• Bottleneck complexity of content-based
  dispatching algorithm
• Advantage Localizing request space and caching
  request results.

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Conclusion (2/4)

• Qualitative comparison
• Client-based approach
• Advantage Reduce the load on web server by
  implementing route service in client side.
• Disadvantage It is not general applicability and
  it need the server-side cooperation.
• Dispatcher-based approach
• Advantage Full control of client requests to
  gain good load balancing. Easy to implementation.
• Disadvantage Risk of dispatcher bottleneck.

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Conclusion (3/4)

• DNS-based approach
• Advantage High Scalability. No risk of
  bottleneck.
• Disadvantage
• Due to the address caching mechanisms, need
  sophisticated algorithms to gain load balancing.
• Less than 32 web servers for each public URL
  because of the limitation of UDP packet size.
• Server-based approach
• Advantage No risk of single-point failure and
  bottleneck.
• Disadvantage Redirection will increase the
  latency time for clients.

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Conclusion (4/4)