40 lines
2.1 KiB
Markdown
40 lines
2.1 KiB
Markdown
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# Key-Value Database
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Databases supporting mappings of arbitrary byte sequences.
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## Interfaces
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The database interface types consist of objects to encapsulate the singular connection to the DB, transactions being made to it, historical version state, and iteration.
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### `DBConnection`
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This interface represents a connection to a versioned key-value database. All versioning operations are performed using methods on this type.
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* The `Versions` method returns a `VersionSet` which represents an immutable view of the version history at the current state.
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* Version history is modified via the `{Save,Delete}Version` methods.
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* Operations on version history do not modify any database contents.
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### `DBReader`, `DBWriter`, and `DBReadWriter`
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These types represent transactions on the database contents. Their methods provide CRUD operations as well as iteration.
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* Writeable transactions call `Commit` flushes operations to the source DB.
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* All open transactions must be closed with `Discard` or `Commit` before a new version can be saved on the source DB.
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* The maximum number of safely concurrent transactions is dependent on the backend implementation.
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* A single transaction object is not safe for concurrent use.
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* Write conflicts on concurrent transactions will cause an error at commit time (optimistic concurrency control).
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#### `Iterator`
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* An iterator is invalidated by any writes within its `Domain` to the source transaction while it is open.
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* An iterator must call `Close` before its source transaction is closed.
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### `VersionSet`
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This represents a self-contained and immutable view of a database's version history state. It is therefore safe to retain and conccurently access any instance of this object.
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## Implementations
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### In-memory DB
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The in-memory DB in the `db/memdb` package cannot be persisted to disk. It is implemented using the Google [btree](https://pkg.go.dev/github.com/google/btree) library.
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* This currently does not perform write conflict detection, so it only supports a single open write-transaction at a time. Multiple and concurrent read-transactions are supported.
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