state/remote: Don't persist snapshot for unchanged state#21811
Merged
apparentlymart merged 2 commits intomasterfrom Jun 20, 2019
Merged
state/remote: Don't persist snapshot for unchanged state#21811apparentlymart merged 2 commits intomasterfrom
apparentlymart merged 2 commits intomasterfrom
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These statemgr interfaces are the new names for the older interfaces in the "state" package. These types alias each other so it doesn't really matter which we use, but the "state" package is deprecated and we intend to eventually remove it, so this is a further step in that direction.
Previously we would write to the backend for every call to PersistState, even if nothing changed since the last write, but update the serial only if the state had changed. The Terraform Cloud & Enterprise state storage have a simple safety check that any future write with an already-used lineage and serial must be byte-for-byte identical. StatesMarshalEqual is intended to detect that, but it only actually detects changes the state itself, and not changes to the snapshot metadata. Because we write the current Terraform version into the snapshot metadata during serialization, we'd previously have an issue where if the first state write after upgrading Terraform to a new version happened to change nothing about the state content then we'd write a new snapshot that differed only by Terraform version, and Terraform Cloud/Enterprise would then reject it. The snapshot header is discarded immediately after decoding, so we can't use information from it when deciding whether to increment the serial. The next best thing is to skip sending no-op snapshot updates to the state client in the first place. These writes are unnecessary anyway, and state storage owners have asked us in the past to elide these to avoid generating noise in their version logs, so we'll also finally meet those requests as a nice side-effect of this change. We didn't previously have tests for the full flow of retrieving and then successively updating persisted state snapshots, so this includes a test which covers that logic and includes an assertion that a no-op update does not get written to the state client.
mildwonkey
approved these changes
Jun 20, 2019
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@apparentlymart I believe it is going to be part of the next release. When is it planned to be released? |
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I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further. |
ghost
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Previously we would write to the backend for every call to
PersistState, even if nothing changed since the last write, but update the serial only if the state had changed.The Terraform Cloud & Enterprise state storage have a simple safety check that any future write with an already-used lineage and serial must be byte-for-byte identical.
StatesMarshalEqualis intended to detect that, but it only actually detects changes the state itself, and not changes to the snapshot metadata.Because we write the current Terraform version into the snapshot metadata during serialization, we'd previously have an issue where if the first state write after upgrading Terraform to a new version happened to change nothing about the state content then we'd write a new snapshot that differed only by Terraform version, and Terraform Cloud/Enterprise would then reject it.
The snapshot header is discarded immediately after decoding, so we can't use information from it when deciding whether to increment the serial. The next best thing is to skip sending no-op snapshot updates to the state client in the first place.
These writes are unnecessary anyway, and state storage owners have asked us in the past to elide these to avoid generating noise in their version logs, so we'll also finally meet those requests as a nice side-effect of this change.
We didn't previously have tests for the full flow of retrieving and then successively updating persisted state snapshots, so this includes a test which covers that logic and includes an assertion that a no-op update does not get written to the state client.
This fixes #21773.