Replication2

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Replication(2)
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Some Interesting Observations

• Top 1 of all documents account for 20 - 35 of
  proxy requests
• Top 10 account for 45 - 55 of requests
• It takes 25 to 40 of all documents to account
  for 70 of requests
• It takes 70 to 80 of all documents to account
  for 90 of requests

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Web Caching

• As an example, we use the web to illustrate
  caching and other related issues

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Web Browser Caching

• Web browsers have their own caches. When a page
  is downloaded from a site the web page is put
  into the browser cache.
• This is especially useful in those cases when the
  back button is pressed.
• If a new copy is needed then a refresh can be
  done.
• No page stays permanently in the cache. There is
  limited room.
• A replacement algorithm is needed to determine
  which cached page should be purged.

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Web Browser Caching

• Client pull
• The server provides the content with instructions
  on when the client should ask for a refreshed
  copy of the content or if the content should be
  cached.
• Server push
• The server transmits page information to the
  screen.
• The browser application displays the information
  and leaves the connection to the server open.
• With an open connection, the server can continue
  to push updated pages for your screen to display
  on an ongoing basis. You can close the connection
  by closing the page.
• The server is in control
• Browser caches are different from proxy caches
  (discussed next).

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Web Caching

• Proxy caches (also called proxy server)
• Intercepts HTTP requests from client
• Serves object if in its cache
• If not goes to objects home server
• On behalf of user, gets the object and possibly
  deposits in its cache before returning to user
• Usually deployed at edges of a network
• Wide area bandwidth savings, improved response
  time and increased availability of static
  web-based objects
• A browser may have to be configured to point to
  the proxy server.
• Usually a web cache is purchased and installed by
  an ISP e.g., a university.

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Push-Based Approach

• Server tracks all proxies that have requested
  objects
• If a web page is modified, notify each proxy
• Notification types
• Indicate object has changed invalidate
• Send new version of object update
• How to decide between invalidate and updates?
• Pros and cons?
• One approach send updates for more frequently
  accessed objects, invalidate for rest

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Push-Based Approaches

• Advantages
• Provide tight consistency minimal stale data
• Proxies can be passive
• Disadvantages
• Need to maintain state at the server
• Recall that HTTP is stateless
• Need mechanisms beyond HTTP
• State may need to be maintained indefinitely
• Not resilient to server crashes
• The disadvantage is the reason why push-based
  approaches are not used

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Pull-Based Approaches

• The proxy is entirely responsible for maintaining
  consistency
• The proxy periodically polls the server to see if
  object has changed
• Use if-modified-since HTTP messages This type
  of message can be used by a proxy to tell a
  remote server to return a copy only if it has
  been modified.
• Key question When should a proxy poll?
• Server-assigned Time-to-Live (TTL) values
• No guarantee if the object will change in the
  interim

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Pull-Based Approach Intelligent Polling

• Proxy can dynamically determine the refresh
  interval
• Compute based on past observations
• Start with a conservative refresh interval
• Increase interval if object has not changed
  between two successive polls
• Decrease interval if object is updated between
  two polls
• Adaptive No prior knowledge of object
  characteristics needed

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Pull-Based Approach

• Advantages
• Server remains stateless
• Resilient to both server and proxy failures
• Disadvantages
• Weaker consistency guarantees (objects can change
  between two polls and proxy will contain stale
  data until next poll)
• High message overhead

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A Hybrid Approach Leases

• Lease Duration of time for which server agrees
  to notify proxy of modification
• Issue lease on first request, send notification
  until expiry
• Need to renew lease upon expiry
• Smooth tradeoff between state and messages
  exchanged
• Zero duration polling, Infinite leases
  server-push
• Efficiency depends on the lease duration
• Limited use

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Cooperative Caching

• Caching infrastructure can have multiple web
  proxies
• Proxies can be arranged in a hierarchy or other
  structures
• Proxies can cooperate with one another
• Answer client requests
• Propagate server notifications
• Uses a combination of HTTP and ICP (Internet
  Caching Protocol).
• ICP can be used by one cache to quickly ask
  another cache if it has an object.
• HTTP is used to actually retrieve the object.

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Problems

• Caching proxies serve only their parents and not
  all Internet users.
• Content providers (say, Web servers) cannot rely
  on existence and correct implementation of
  caching proxies.
• Accounting issues with caching proxies
• Example www.cnn.com needs to know the number of
  hits to the advertisements displayed on the web
  page.

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Content Distribution Networks (CDN)

• Business Model A content provider such as
  www.cnn.com or Yahoo pays a CDN company (such as
  Akamai) to get its content to the requesting
  users with short delays.
• A CDN provides a mechanism for
• Replicating content on multiple servers in the
  Internet
• Providing clients with a means to determine the
  servers that can deliver the content fastest.

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Terminology

• Content Any publicly accessible combination of
  text, images, applets, frames, MP3, video, flash,
  virtual reality objects, etc.
• Content Provider Any individual, organization,
  or company that has content that it wishes to
  make available to users.
• Origin Server Content providers server , where
  the content is first uploaded.
• Surrogate Server (sometimes called edge server)
  Content distributors server, where the
  replicated content is kept.

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Players of the game
Yahoo, MSNBC, CNN
Content Provider
Send content
Akamai, Digital Island, ATT
Content Distributor
Sells servers
Install servers
H/W and S/W Vendor

• Cisco,
• Lucent,
• Inktomi,
• CacheFlow

Hosting Provider
Exodus
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CDN Distribution

• The CDN company places hundreds of CDN servers in
  Internet hosting centers.
• The CDN replicates its customers content in the
  CDN servers. Whenever, a customer updates its
  content (e.g., web page), the CDN redistributes
  the fresh content to the CDN servers.
• The CDN provides a mechanism so that when a user
  requests content, the content is provided by the
  CDN server that can most rapidly deliver the
  content to the user.
• This can be the closest CDN server to the user
  (perhaps in the same ISP as the user) or may be a
  CDN server with a congestion-free path to the
  user.

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CDN Distribution
Origin server in North America
push content
Akamai CDN
CDN distribution node
push content
push content
CDN server in South America
push content
CDN server in Asia
CDN server in Europe
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CDN Functional Components

• Distribution Service
• Redirection Service
• Accounting and Billing system

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CDNDistribution Service

• The content provider determines which of its
  objects it wants the CDN to distribute.
• The content provider tags and then pushes this
  content to a CDN node, which in turn replicates
  and pushes the content to all its CDN servers.

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CDN Distribution Service

• When a browser in a users host is instructed to
  retrieve a specific object (specified using a
  URL), how does the browser determine whether it
  should retrieve the object from the origin server
  or from one of the CDN servers?
• As an example, suppose the hostname of the
  content provider is www.cnn.com
• Suppose the hostname of the CDN company is
  www.akamai.com

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CDN Redirection

• Users get an html document from www.cnn.com this
  could be index.html
• The file index.html uses a modified URL for
  content that has been replicated.
• Example If the gif files are what has been
  replicated then
  may be modified as follows
• af/x.gif
• The browser needs to resolve aXYZ.g.akamaitech.net
  hostname for replicated content.

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CDN Redirection

• DNS is configured so that all queries about
  g.akamaitech.net that arrive at a DNS server are
  sent to an authoritative DNS server for
  g.akamaitech.net. This is referred to as a Akamai
  DNS server (authoritative DNS server)
• When the Akamai DNS server receives the query, it
  extracts the IP address of the requesting
  browser.
• Based on the IP address and information that it
  has about the Internet (called a map), the IP
  address of an Akamai server(surrogate server) is
  returned to the requesting browser based on
  policy e.g., select the server that is the fewest
  hops away.

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CDN Redirection

• The Akamai DNS server IP address is now in the
  cache of the local DNS server.
• This implies that it is not always necessary to
  go to the root DNS server.
• The TTL associated with the IP address of an
  Akamai server(surrogate) is relatively small.
• This is done for performance reasons.
• Akamai content distribution servers are caches

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CDN Redirection

• What if content is not there?
• If the request content is not found then the
  surrogate will ask other surrogates within a
  specified region for information.
• If requested information is still not found or is
  stale, then a request is made to the original web
  site.

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CDN Redirection
Authoritative DNS server for cdn.com
CNN.com
PUT /images/.gif
64.236.24.28
Index.html
DNS query cdn.com ?
GET www.cnn.com/index.html
Index.html
64.236.24.28
... ...
GET /cnn/images/1.gif
1.gif
DNS query cdn.com ?
64.236.24.28
Client
Local DNS server
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CDN Selection

• The tricky issue is selecting which local content
  server to use for a particular request
• Want to spread load evenly
• Want minimal impact if server is added or
  removed.
• In Akamai, each surrogate server sends
  measurement results to the Network Operations
  Communications Center (NOCC).
• Measurement results include number of active TCP
  connections, HTTP request arrival rate, bandwidth
  availability, etc
• This information is used by the Akamai DNS
  server.

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Accounting Mechanism

• Accounting mechanisms collect and track
  information related to request routing,
  distribution and delivery.
• Information is gathered in real time and put into
  log files for each CDN component.
• This gets sent to the Network Operations
  Communications Center (NOCC).

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Full Site Delivery vs. Partial Site Delivery

• Full Site Delivery All the contents are
  delivered by the CDN (including HTML, images,
  and other objects).
• Partial Site delivery Only images, streaming
  media and other bandwidth intensive objects
  delivered by the CDN.

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CDNs and Content

• Content Suitable for CDNS
• Images
• Streaming media
• Java applets
• Static information
• Content not suitable
• Dynamic information
• Personalized information

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Current Akamai Customers
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Summary

• We have examined replication and issues related
  to the design and implementation of a replicated
  system.
• Many choices and tradeoffs to consider