Alistair Coles 44a861787a Enable object server to return non-durable data ...

This patch improves EC GET response handling:

- The proxy no longer requires all object servers to have a
  durable file for the fragment archive that they return in
  response to a GET. The proxy will now be satisfied if just
  one object server has a durable file at the same timestamp
  as fragments from other object servers.

  This means that the proxy can now successfully GET an
  object that had missing durable files when it was PUT.

- The proxy will now ensure that it has a quorum of *unique*
  fragment indexes from object servers before considering a
  GET to be successful.

- The proxy is now able to fetch multiple fragment archives
  having different indexes from the same node. This enables
  the proxy to successfully GET an object that has some
  fragments that have landed on the same node, for example
  after a rebalance.

This new behavior is facilitated by an exchange of new
headers on a GET request and response between the proxy and
object servers.

An object server now includes with a GET (or HEAD) response:

- X-Backend-Fragments: the value of this describes all
  fragment archive indexes that the server has for the
  object by encoding a map of the form: timestamp -> <list
  of fragment indexes>

- X-Backend-Durable-Timestamp: the value of this is the
  internal form of the timestamp of the newest durable file
  that was found, if any.

- X-Backend-Data-Timestamp: the value of this is the
  internal form of the timestamp of the data file that was
  used to construct the diskfile.

A proxy server now includes with a GET request:

- X-Backend-Fragment-Preferences: the value of this
  describes the proxy's current preference with respect to
  those fragments that it would have object servers
  return. It encodes a list of timestamp, and for each
  timestamp a list of fragment indexes that the proxy does
  NOT require (because it already has them).

  The presence of a X-Backend-Fragment-Preferences header
  (even one with an empty list as its value) will cause the
  object server to search for the most appropriate fragment
  to return, disregarding the existence or not of any
  durable file. The object server assumes that the proxy
  knows best.

Closes-Bug: 1469094
Closes-Bug: 1484598

Change-Id: I2310981fd1c4622ff5d1a739cbcc59637ffe3fc3
Co-Authored-By: Paul Luse <paul.e.luse@intel.com>
Co-Authored-By: Clay Gerrard <clay.gerrard@gmail.com>

2016-09-16 11:40:14 +01:00

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