mirror of
https://gitlab.com/pulsechaincom/erigon-pulse.git
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107 lines
3.1 KiB
Markdown
107 lines
3.1 KiB
Markdown
## Target:
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To build 1 key-value abstraction on top of Bolt, LMDB and RemoteDB (our own read-only TCP protocol for key-value databases).
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## Design principles:
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- No internal copies/allocations - all must be delegated to user. It means app must copy keys/values before put to database.
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Make it part of contract - written clearly in docs, because it's unsafe (unsafe to put slice to DB and then change it).
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Known problems: mutation.Put does copy internally.
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- Low-level API: as close to original LMDB as possible.
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- Expose concept of transaction - app-level code can .Rollback() or .Commit() at once.
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## Result interface:
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```
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type KV interface {
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View(ctx context.Context, f func(tx Tx) error) (err error)
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Update(ctx context.Context, f func(tx Tx) error) (err error)
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Close() error
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Begin(ctx context.Context, writable bool) (Tx, error)
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}
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type Tx interface {
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Bucket(name []byte) Bucket
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Commit(ctx context.Context) error
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Rollback() // doesn't return err to be defer friendly
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}
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type Bucket interface {
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Get(key []byte) (val []byte, err error)
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Put(key []byte, value []byte) error
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Delete(key []byte) error
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Cursor() Cursor
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}
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type Cursor interface {
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Prefix(v []byte) Cursor
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MatchBits(uint) Cursor
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Prefetch(v uint) Cursor
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NoValues() NoValuesCursor
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First() ([]byte, []byte, error)
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Seek(seek []byte) ([]byte, []byte, error)
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Next() ([]byte, []byte, error)
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Walk(walker func(k, v []byte) (bool, error)) error
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}
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type NoValuesCursor interface {
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First() ([]byte, uint32, error)
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Seek(seek []byte) ([]byte, uint32, error)
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Next() ([]byte, uint32, error)
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Walk(walker func(k []byte, vSize uint32) (bool, error)) error
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}
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```
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## Rationale and Features list:
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#### Buckets concept:
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- Bucket is an interface, can’t be nil, can't return error
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#### InMemory and ReadOnly modes:
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- `NewLMDB().InMem().ReadOnly().Open(ctx)`
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#### Context:
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- For transactions - yes
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- For .First() and .Next() methods - no
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#### Cursor/Iterator:
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- Cursor is an interface, can’t be nil, can't return error
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- `cursor.Prefix(prefix)` filtering keys by given prefix. RemoteDb - to support server side filtering.
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- `cursor.Prefetch(1000)` - useful for Remote
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- Methods .First, .Next, .Seek - can return error. If err!=nil then key SHOULD be !=nil (can be []byte{} for example). Then looping code will look as:
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```go
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for k, v, err := c.First(); k != nil; k, v, err = c.Next() {
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if err != nil {
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return err
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}
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// logic
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}
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```
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#### Concept of Item:
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- No Lazy values, but can disable fetching values by: `.Cursor().PrefetchValues(false).FirstKey()`
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#### Managed/un-managed transactions
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- Tx is an interface
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- db.Update, db.View - yes
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- db.Batch - no
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- all keys and values returned by all method are valid until end of transaction
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- transaction object can be used only withing 1 goroutine
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#### Errors:
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- Lib-Errors must be properly wrapped to project: for example ethdb.ErrKeyNotFound
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#### i.SeekTo vs i.Rewind: TBD
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#### in-memory LRU cache: TBD
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- Reverse Iterator
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## Not covered by Abstractions:
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- DB stats, bucket.Stats(), item.EstimatedSize()
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- buckets stats, buckets list
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- TTL of keys
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- Monotonic int DB.GetSequence
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- Nested Buckets
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- Backups, tx.WriteTo
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