Whereas the parent directory "states" contains the models that represent
state in memory, this package's responsibility is in serializing a subset
of that data to a JSON-based file format and then reloading that data
back into memory later.
For reading, this package supports state file formats going back to
version 1, using lightly-adapted versions of the migration code previously
used in the "terraform" package. State data is upgraded to the latest
version step by step and then transformed into the in-memory state
representation, which is distinct from any of the file format structs in
this package to enable these to evolve separately.
For writing, only the latest version (4) is supported, which is a new
format that is a slightly-flattened version of the new in-memory state
models introduced in the prior commit. This format retains the outputs
from only the root module and it flattens out the module and instance
parts of the hierarchy by including the identifiers for these inside
the child object. The loader then reconstructs the multi-layer structure
we use for more convenient access in memory.
For now, the only testing in this package is of round-tripping different
versions of state through a read and a write, ensuring the output is
as desired. This exercises all of the reading, upgrading, and writing
functions but should be augmented in later commits to improve coverage
and introduce more focused tests for specific parts of the functionality.
Our previous state models in the "terraform" package had a few limitations
that are addressed here:
- Instance attributes were stored as map[string]string with dot-separated
keys representing traversals through a data structure. Now that we have
a full type system, it's preferable to store it as a real data
structure.
- The existing state structures skipped over the "resource" concept and
went straight to resource instance, requiring heuristics to decide
whether a particular resource should appear as a single object or as
a list of objects when used in configuration expressions.
- Related to the previous point, the state models also used incorrect
terminology where "ResourceState" was really a resource instance state
and "InstanceState" was really the state of a particular remote object
associated with an instance. These new models use the correct names for
each of these, introducing the idea of a "ResourceInstanceObject" as
the local record of a remote object associated with an instance.
This is a first pass at fleshing out a new model for state. Undoubtedly
there will be further iterations of this as we work on integrating these
new models into the "terraform" package.
These new model types no longer serve double-duty as a description of the
JSON state file format, since they are for in-memory use only. A
subsequent commit will introduce a separate package that deals with
persisting state to files and reloading those files later.
Martin Atkins