Do Akamai edge servers share cached content, or go to origin?

If an Akamai edge server has cached an url, will it share that content with other edge servers, or will edge servers that don't have the content cached locally go back to the origin to get the content?

I'd love to have an official, Akamai document for this, but will of course appreciate any input!

Note that I have tried this out, and see what I expect is the answer - that edge servers will go back to origin at least some of the time to get the content, even if it resides on another edge server.

For example, I left a curl running all weekend to request a resource that caches for 7 days, and see that I got 3 different cached responses (different by response headers), and can see that the origin must've been accessed at least 3 times,

$ cat /t/akamai_dump_requests_all_weekend.txt | grep x-rate-limit-reset| sort | uniq -c
259 < x-rate-limit-reset: 1489776484
  1 < x-rate-limit-reset: 1489779291
 12 < x-rate-limit-reset: 1489781137

and I see alot of different edge servers listed in my dumps as well, though this is normal, i think.

 66 a80-12-96-140.deploy.akamaitechnologies.com
 65 a204-237-142-14.deploy.akamaitechnologies.com
 51 a204-237-142-44.deploy.akamaitechnologies.com
 38 a80-12-96-230.deploy.akamaitechnologies.com
 8 a65-158-180-197.deploy.akamaitechnologies.com
 6 a23-58-92-92.deploy.akamaitechnologies.com
 6 a23-58-92-39.deploy.akamaitechnologies.com
 5 a65-158-180-190.deploy.akamaitechnologies.com
 5 a64-145-68-25.deploy.akamaitechnologies.com
 5 a64-145-68-15.deploy.akamaitechnologies.com
 4 a65-158-180-180.deploy.akamaitechnologies.com
 4 a204-141-247-173.deploy.akamaitechnologies.com
 4 a204-141-247-143.deploy.akamaitechnologies.com
 2 a66-110-100-23.deploy.akamaitechnologies.com
 1 a72-37-154-53.deploy.akamaitechnologies.com
 1 a23-61-206-205.deploy.akamaitechnologies.com
 1 a205-185-195-182.deploy.akamaitechnologies.com

I didnt get an answer here, so I posted this same question on the Akamai Community Forums, and got the following response from Neil Jedrzejewski, a Senior Solutions Architect at Akamai. Thanks Neil !

First of all, which edge server answers a request will vary over time based on
our low-level mapping system and load in a specific network region. Don't read
too much into the fact that you get many different edge servers - swapping them
in and out is part and parcel of how we give our customers scale and
availability.

To answer your question about shared caching, a high level explanation goes like
this.

When a client makes a request our mapping system will return the IP address(es)
of an edge server best located to honour the request. These edge servers are
grouped together in what we call network "regions". If a specific edge server
receives a request and cannot fulfil it from it's own cache, it will send out
a broadcast message (ICP) on it's back-plane asking if any other edge machine
peers in the same region has the object. The timeout for a response to this
request is very short (as the request is local) and if a peer has it, it will
pass the request to the peer and served the response before caching it
locally.

If no local peer is able to satisfy the request, the edge machine will them go
forward to it's cache parent as a new client request and the parent will attempt
to satisfy the request (again, checking with it's own ICP peers), serving the
object out of cache to the edge machine. The edge machine will then serve it
back to the client and cache it locally for next time. If the object is
unavailable or invalid (read: TTL expired) along the entire cache hierarchy
chain, then yes, it will go back to origin to re-validate or reacquire the
object.

An important point to remember is that caching is "best effort" only. Although
your TTL for an object was 7 days, that is simply an instruction to the cache on
how long to consider the content "fresh" and a valid response for a request.
However it is not a guarantee that the object will remain in a servers cache.
Objects can and will drop out of cache if they are infrequently requested or due
to other operational factors. This is where ICP and parent caches help because
they help consolidate requests. So although an object may drop out of a specific
edge cache, it may well still be in the cache parent as many edges are passing
requests through it thus giving a high cache-hit ratio.

So in short, our caching system.  Will use different edge machines to respond to
a request over time based on our mapping systems insight into which machine will
best serve the client request.  Will ask a local peer if it has a copy of an
object if it cannot satisfy the request itself.  Will forward the request to a
cache parent if necessary to fulfil the request.  Will go back to origin for the
object if the object is "stale" or if itself, a peer or parent cannot satisfy
the request.

Hope that helps.

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