Art of Codeartofcode
Lesson 7 of 16

26. CDNs

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The picture to keep in your head
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26.1 Why use a CDN?
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26.2 What are edge locations (PoPs)?
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26.3 Pull vs push CDN: what's the difference?
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26.4 How do cache keys and headers work?
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26.5 How does purging work?
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26.6 What is an origin shield?
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26.7 Can the CDN handle dynamic content?
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26.8 How do CDNs deliver video (HLS, DASH)?
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26.9 What happens when the CDN fails?
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Memory card and where the analogy breaks
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Practice: five levels
13
Interview questions
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Solutions
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Sources
16
Code lab
26. CDNs

26.6 What is an origin shield?

3 min read+10 XP

In the warehouse: A new biscuit launches. Fifty warehouses each find an empty shelf and each phone the factory. The factory gets fifty calls for the same thing. Put a regional depot in between: warehouses call the depot, the depot calls the factory once, then supplies all fifty.

In the system: An origin shield is a designated cache layer (one PoP, or a regional tier) that sits between all the edges and your origin. Edge misses go to the shield; only shield misses go to the origin.

without a shield, 50 PoP misses become 50 origin fetches; with one, a single fetch.

Figure: the shield turns many edge misses into one origin request.

  • Fewer origin requests for new or expired content: in the chapter's code, a new cover requested at 50 PoPs means 50 origin fetches without a shield, 1 with it.
  • Higher overall hit ratio, because the shield is shared by all edges.
  • Combined with request collapsing (the shield sends one request for many simultaneous misses on the same object), it protects the origin from stampedes, much like single-flight in Chapter 13.

Cost: an extra hop on misses, and the shield itself must be well connected to the origin; pick a shield location close to your origin (Mumbai for the bookstore). Most CDNs offer it as a setting (for example, CloudFront Origin Shield, or Fastly's shielding).

Memory code: "Warehouses call the depot; only the depot calls the factory."