73 lines
3.2 KiB
Markdown
73 lines
3.2 KiB
Markdown
# Key-Value Database
|
|
|
|
Databases supporting mappings of arbitrary byte sequences.
|
|
|
|
## Interfaces
|
|
|
|
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.
|
|
|
|
### `DBConnection`
|
|
|
|
This interface represents a connection to a versioned key-value database. All versioning operations are performed using methods on this type.
|
|
|
|
* The `Versions` method returns a `VersionSet` which represents an immutable view of the version history at the current state.
|
|
* Version history is modified via the `{Save,Delete}Version` methods.
|
|
* Operations on version history do not modify any database contents.
|
|
|
|
### `DBReader`, `DBWriter`, and `DBReadWriter`
|
|
|
|
These types represent transactions on the database contents. Their methods provide CRUD operations as well as iteration.
|
|
|
|
* Writeable transactions call `Commit` flushes operations to the source DB.
|
|
* All open transactions must be closed with `Discard` or `Commit` before a new version can be saved on the source DB.
|
|
* The maximum number of safely concurrent transactions is dependent on the backend implementation.
|
|
* A single transaction object is not safe for concurrent use.
|
|
* Write conflicts on concurrent transactions will cause an error at commit time (optimistic concurrency control).
|
|
|
|
#### `Iterator`
|
|
|
|
* An iterator is invalidated by any writes within its `Domain` to the source transaction while it is open.
|
|
* An iterator must call `Close` before its source transaction is closed.
|
|
|
|
### `VersionSet`
|
|
|
|
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.
|
|
|
|
## Implementations
|
|
|
|
### In-memory DB
|
|
|
|
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.
|
|
|
|
* 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.
|
|
|
|
### BadgerDB
|
|
|
|
A [BadgerDB](https://pkg.go.dev/github.com/dgraph-io/badger/v3)-based backend. Internally, this uses BadgerDB's ["managed" mode](https://pkg.go.dev/github.com/dgraph-io/badger/v3#OpenManaged) for version management.
|
|
Note that Badger only recognizes write conflicts for rows that are read _after_ a conflicting transaction was opened. In other words, the following will raise an error:
|
|
|
|
```go
|
|
tx1, tx2 := db.Writer(), db.ReadWriter()
|
|
key := []byte("key")
|
|
tx2.Get(key)
|
|
tx1.Set(key, []byte("a"))
|
|
tx2.Set(key, []byte("b"))
|
|
tx1.Commit() // ok
|
|
err := tx2.Commit() // err is non-nil
|
|
```
|
|
|
|
But this will not:
|
|
|
|
```go
|
|
tx1, tx2 := db.Writer(), db.ReadWriter()
|
|
key := []byte("key")
|
|
tx1.Set(key, []byte("a"))
|
|
tx2.Set(key, []byte("b"))
|
|
tx1.Commit() // ok
|
|
tx2.Commit() // ok
|
|
```
|
|
|
|
### RocksDB
|
|
|
|
A [RocksDB](https://github.com/facebook/rocksdb)-based backend. Internally this uses [`OptimisticTransactionDB`](https://github.com/facebook/rocksdb/wiki/Transactions#optimistictransactiondb) to allow concurrent transactions with write conflict detection. Historical versioning is internally implemented with [Checkpoints](https://github.com/facebook/rocksdb/wiki/Checkpoints).
|