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[engine] Only record a resource's chosen alias. #9288

Merged
merged 2 commits into from Mar 28, 2022
Merged

[engine] Only record a resource's chosen alias. #9288

merged 2 commits into from Mar 28, 2022

Commits on Mar 25, 2022

  1. [engine] Only record a resource's chosen alias. 

    As we discovered when removing aliases from the state entirely, the
snapshotter needs to be alias-aware so that it can fix up references to
resources that were aliased. After a resource operation finishes, the
snapshotter needs to write out a new copy of the snapshot. However, at
the time we write the snapshot, there may be resources that have not yet
been registered that refer to the just-registered resources by a
different URN due to aliasing. Those references need to be fixed up
prior to writing the snapshot in order to preserve the snapshot's
integrity (in particular, the property that all URNs refer to resources
that exist in the snapshot).

For example, consider the following simple dependency graph: A <-- B.
When that graph is serialized, B will contain a reference to A in its
dependency list. Let the next run of the program produces the graph A'
<-- B where A' is aliased to A. After A' is registered, the snapshotter
needs to write a snapshot that contains its state, but B must also be
updated so it references A' instead of A, which will no longer be in the
snapshot.

These changes take advantage of the fact that although a resource can
provide multiple aliases, it can only ever resolve those aliases to a
single resource in the existing state. Therefore, at the time the
statefile is fixed up, each resource in the statefile could only have
been aliased to a single old resource, and it is sufficient to store
only the URN of the chosen resource rather than all possible aliases. In
addition to preserving the ability to fix up references to aliased
resources, retaining the chosen alias allows the history of a logical
resource to be followed across aliases.
    @pgavlin
    pgavlin committed Mar 25, 2022
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  2. PR feedback

    @pgavlin
    pgavlin committed Mar 25, 2022
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    0890634 View commit details
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