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Add documentation for recalc option

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Martin Sumner 2020-03-24 20:21:44 +00:00
parent 8a9db9e75e
commit 20a7a22571
3 changed files with 11 additions and 7 deletions
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2
docs/DESIGN.md
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@ -120,7 +120,7 @@ Three potential recovery strategies are supported to provide some flexibility fo
- retain - on compaction KeyDeltas are retained in the Journal, only values are removed.
- recalc (not yet implemented) - the compaction rules assume that on recovery the key changes will be recalculated by comparing the change with the current database state. In recovery the key changes will be recalculated by comparing the change with the current database state.
- recalc - the compaction rules assume that on recovery the key changes will be recalculated by comparing the change with the current database state. In recovery the key changes will be recalculated by comparing the change with the current database state. A user-defined function should be passed in at startup to achieve this recalculation (to override `leveled_head:diff_indexspeacs/3`).
### Hot Backups

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docs/STARTUP_OPTIONS.md
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@ -77,11 +77,11 @@ However, what if the Ledger had been erased? This could happen due to some corr
The are three potential strategies:
- `skip` - don't worry about this scenario, require the Ledger to be backed up;
- `recovr` - don't worry about this scenario, require the Ledger to be backed up;
- `retain` - discard the object itself on compaction but keep the key changes;
- `recalc` - recalculate the indexes on reload by comparing the information on the object with the current state of the Ledger (as would be required by the PUT process when comparing IndexSpecs at PUT time).
There is no code for `recalc` at present it is simply a logical possibility. So to set a reload strategy there should be an entry like `{reload_strategy, [{TagName, skip|retain}]}`. By default tags are pre-set to `retain`. If there is no need to handle a corrupted Ledger, then all tags could be set to `skip`.
To set a reload strategy requires a list of tuples to match tag names to strategy `{reload_strategy, [{TagName, recovr|retain|recalc}]}`. By default tags are pre-set to `retain`. If there is no need to handle a corrupted Ledger, then all tags could be set to `recovr` - this assumes that either the ledger files are protected by some other means from corruption, or an external anti-entropy mechanism will recover the lost data.
## Compression Method