Data eviction

Data eviction in SableDB occurs in three primary scenarios:

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Expired Items

SableDB stores data primarily on disk, a cost-effective solution. To manage expired items efficiently, SableDB checks an item's expiration status only when it's accessed. If an item is found to be expired, it's deleted, and a null value is returned to the caller.

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Overwritten Composite Items

A common issue arises when a composite item (like a Hash) is overwritten by a different data type. For instance, if you have a Hash named OverwatchTanks containing multiple fields (e.g. tank_1, tank_2, and tank_3), and then execute SET OverwatchTanks "bla", the OverwatchTanks key becomes a String. However, as each Hash field is stored as a separate record in RocksDB, the original tank_1, tank_2, and tank_3 fields become "orphaned" and inaccessible.

SableDB addresses this with a background cron task. This task periodically compares the declared type of a composite item with its actual stored value. If a mismatch is detected (e.g., OverwatchTanks is now a String but still has associated Hash records), the cron job uses bookkeeping records to identify the original type and then deletes the orphaned records from the database.

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User-Triggered Cleanup

When a user initiates a FLUSHALL or FLUSHDB command, SableDB leverages RocksDB's delete_range method to efficiently purge the data.