Distributed Systems 19. Content Delivery Networks (CDN) Paul Krzyzanowski

Transcription

1 Distributed Systems 19. Content Delivery Networks (CDN) Paul Krzyzanowski 1

2 Motivation Serving web content from one location presents problems Scalability Reliability Performance Flash crowd problem What if everyone comes to you at once? Cache content and serve requests from multiple servers at the network edge (close to the user) Reduce demand on site s infrastructure Provide faster service to users Content comes from nearby servers 2

3 Focus on Content Computing is still done by the site host s server(s) Offload the static parts Images Video CSS files Static pages 3

4 Serving & Consuming Web server Internet 4

5 Caching Proxy Caching Proxy Caching Proxies Web server Internet 5

6 Load Balancer Load Balancing Web server Web server Web server Internet 6

7 Internet End-to-End Packet Delivery Network edge: applications & hosts Web server Local ISP Tier 2 ISP Tier 1 ISP Tier 1 ISP Local ISP Tier 2 ISP Network core: routers 7

8 Multihoming Get links from multiple ISPs 1 IP address but multiple links Announce address to upstream routers via BGP Local ISP Web server Tier 2 ISP Tier 1 ISP Tier 1 ISP Local ISP Tier 2 ISP 8

9 copy Mirroring Synchronize multiple servers Web server Local ISP Tier 2 ISP Tier 1 ISP Tier 1 ISP Web server Local ISP Tier 2 ISP 9

10 Improving scalability, availability, & performance Scalability Load balance among multiple servers Multiple ISPs if network congestion is a concern Availability Replicate servers Multiple data centers & ISPs Performance Replicate servers for load balancing Cache content and serve requests from multiple servers at the network edge (close to the user) Reduce demand on site s infrastructure Provide faster service to users Content comes from nearby servers 10

11 Akamai Distributed Caching Company evolved from MIT research "Invent a better way to deliver Internet content" Tackle the "flash crowd" problem Akamai runs on 95,000 servers in 1,900 networks across 71 countries 11

12 Traditional approaches to web scaling Local balancing Data center or ISP can fail Multihoming IP protocols (BGP) are often not quick to find new routes Mirroring at multiple sites Synchronization can be difficult Proxy servers Typically a client-side solution Low cache hit rates All require extra capacity and extra capital costs 12

13 Akamai s goal Akamai tries to serve clients from servers likely to have the content Nearest: lowest round-trip time Available: server that is not too loaded Likely: server that is likely to have the data 13

14 Akamai Mapping Akamai does this via Dynamic DNS Customer modifies URLs (akamizes) Direct requests to the right content servers Resolve a host name based on: service requested (e.g., QuickTime, HTML, Windows Media) content requested (is server likely to have it? based on hash) server health server load user location network status load balancing 14

15 DNS Servers Remember how DNS queries work? Referrals & caching Set short cache time Client DNS queries will get referred to Akamai DNS servers 15

16 DNS Resolution Resolve a7.g.akamai.net NS resolver contacts root server Root server sends a referral to a name server responsible for.net Resolver queries.net name server Returns a referral for.akamai.net This is the top-level Akamai server Resolver queries a top-level Akamai server Returns a referral for.g.akamai.net Low-level Akamai server (TTL approx 1 hour) Low-level servers are in the same location as edge servers closest to user Resolver queries a low-level Akamai server Returns IP addresses of servers available to satisfy the request Short TTL (several seconds to 1 minute) 16

17 Akamai needs data to do this Map network topology Based on BGP and traceroute information Estimate hops and transit time Content servers report their load to a monitoring application Monitoring app publishes load reports to a local DNS server DNS server determines which IP addresses to return when resolving names Load shedding: If servers get too loaded, the DNS server will not respond with those addresses 17

18 Populating the cache If an Akamai server does not have the content Get it from another Akamai server Or get it from the site 18

19 Types of content Static content Cached depending on original site's requirements (never to forever) Dynamic content Caching proxies cannot do this Akamai uses Edge Side Includes technology ( Assembles dynamic content on edge servers Similar to server-side includes Page is broken into fragments with independent caching properties Assembled on demand Streaming media Live stream is sent to an entry-point server in the Akamai network Stream is delivered from the entry-point server to multiple edge servers Edge servers serve content to end users. 19

20 Other Things CDNs Do 20

21 Signed URLs Example: Amazon CloudFront Control access to content via a signed URL URL contains: policy or a reference to a policy Signature = encrypted hash URL cannot be modified Policies include: Validity: start time & expiration time Range of IP addresses that are allowed to access the object 21

22 LimeLight Reach Video Universal URL Publish Web site Transcoder Content server Content server Content server f(player, device, encoding parameters) 22

23 Server-side Video Ad Insertion Example, Limelight Reach Ads Ad Server Ad selection Ad call Web site Ad insertion Request context CDN Media with ad Request content 23

24 The End 24