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159 lines
8.3 KiB
Markdown
159 lines
8.3 KiB
Markdown
#### `Ethdb` package hold's bouquet of objects to access DB
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Words "KV" and "DB" have special meaning here:
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- KV - key-value-style API to access data: let developer manage transactions, stateful cursors.
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- DB - object-oriented-style API to access data: Get/Put/Delete/WalkOverTable/MultiPut, managing transactions internally.
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So, DB abstraction fits 95% times and leads to more maintainable code - because it looks stateless.
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About "key-value-style": Modern key-value databases don't provide Get/Put/Delete methods,
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because it's very hard-drive-unfriendly - it pushes developers do random-disk-access which is [order of magnitude slower than sequential read](https://www.seagate.com/sg/en/tech-insights/lies-damn-lies-and-ssd-benchmark-master-ti/).
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To enforce sequential-reads - introduced stateful cursors/iterators - they intentionally look as file-api: open_cursor/seek/write_data_from_current_position/move_to_end/step_back/step_forward/delete_key_on_current_position/append.
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## Class diagram:
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```asciiflow.com
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// This is not call graph, just show classes from low-level to high-level.
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// And show which classes satisfy which interfaces.
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+-----------------------------------+ +-----------------------------------+
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| github.com/erigontech/mdbx-go | | google.golang.org/grpc.ClientConn |
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| (app-agnostic MDBX go bindings) | | (app-agnostic RPC and streaming) |
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+-----------------------------------+ +-----------------------------------+
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| |
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v v
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+-----------------------------------+ +-----------------------------------+
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| ethdb/kv_mdbx.go | | ethdb/kv_remote.go |
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| (tg-specific MDBX implementaion) | | (tg-specific remote DB access) |
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+-----------------------------------+ +-----------------------------------+
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v v
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+----------------------------------------------------------------------------------------------+
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| ethdb/kv_abstract.go |
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| (Common KV interface. DB-friendly, disk-friendly, cpu-cache-friendly. |
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| Same app code can work with local or remote database. |
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| Allows experiment with another database implementations. |
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| Supports context.Context for cancelation. Any operation can return error) |
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+----------------------------------------------------------------------------------------------+
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v v v
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+-----------------------------------+ +-----------------------------------+ +-----------------------------------+
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| ethdb/object_db.go | | ethdb/tx_db.go | | ethdb/remote/remotedbserver |
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| (thread-safe, stateless, | | (non-thread-safe, more performant | | (grpc server, using kv_abstract, |
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| opens/close short transactions | | than object_db, method Begin | | kv_remote call this server, 1 |
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| internally when need) | | DOESN'T create new TxDb object) | | transaction maps on 1 grpc stream |
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+-----------------------------------+ +-----------------------------------+ +-----------------------------------+
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v v
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+-----------------------------------------------------------------------------------------------+
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| ethdb/interface.go |
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| (Common DB interfaces. ethdb.Database and ethdb.DbWithPendingMutations are widely used) |
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+-----------------------------------------------------------------------------------------------+
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v
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+--------------------------------------------------+
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| ethdb/mutation.go |
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| (also known as "batch", recording all writes and |
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| them flush to DB in sorted way only when call |
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| .Commit(), use it to avoid random-writes. |
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| It use and satisfy ethdb.Database in same time |
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+--------------------------------------------------+
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```
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## ethdb.AbstractKV design:
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- InMemory, ReadOnly: `NewMDBX().Flags(mdbx.ReadOnly).InMem().Open()`
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- MultipleDatabases, Customization: `NewMDBX().Path(path).WithBucketsConfig(config).Open()`
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- 1 Transaction object can be used only within 1 goroutine.
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- Only 1 write transaction can be active at a time (other will wait).
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- Unlimited read transactions can be active concurrently (not blocked by write transaction).
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- Methods db.Update, db.View - can be used to open and close short transaction.
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- Methods Begin/Commit/Rollback - for long transaction.
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- it's safe to call .Rollback() after .Commit(), multiple rollbacks are also safe. Common transaction pattern:
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```
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tx, err := db.Begin(true, ethdb.RW)
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if err != nil {
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return err
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}
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defer tx.Rollback() // important to avoid transactions leak at panic or early return
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// ... code which uses database in transaction
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err := tx.Commit()
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if err != nil {
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return err
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}
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```
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- No internal copies/allocations. It means: 1. app must copy keys/values before put to database. 2. Data after read from db - valid only during current transaction - copy it if plan use data after transaction Commit/Rollback.
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- Methods .Bucket() and .Cursor(), can’t return nil, can't return error.
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- Bucket and Cursor - are interfaces - means different classes can satisfy it: for example `MdbxCursor` and `MdbxDupSortCursor` classes satisfy it.
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If your are not familiar with "DupSort" concept, please read [dupsort.md](../docs/programmers_guide/dupsort.md) first.
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- If Cursor returns err!=nil then key SHOULD be != nil (can be []byte{} for example).
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Then traversal code 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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- Move cursor: `cursor.Seek(key)`
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## ethdb.Database design:
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- Allows pass multiple implementations
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- Allows traversal tables by `db.Walk`
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## ethdb.TxDb design:
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- holds inside 1 long-running transaction and 1 cursor per table
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- method Begin DOESN'T create new TxDb object, it means this object can be passed into other objects by pointer,
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and high-level app code can start/commit transactions when it needs without re-creating all objects which holds
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TxDb pointer.
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- This is the reason why txDb.CommitAndBegin() method works: inside it creating new transaction object, pointer to TxDb stays valid.
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## How to dump/load table
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Install all database tools: `make db-tools`
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```
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./build/bin/mdbx_dump -a <datadir>/erigon/chaindata | lz4 > dump.lz4
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lz4 -d < dump.lz4 | ./build/bin/mdbx_load -an <datadir>/erigon/chaindata
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```
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## How to get table checksum
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```
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./build/bin/mdbx_dump -s table_name <datadir>/erigon/chaindata | tail -n +4 | sha256sum # tail here is for excluding header
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Header example:
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VERSION=3
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geometry=l268435456,c268435456,u25769803776,s268435456,g268435456
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mapsize=756375552
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maxreaders=120
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format=bytevalue
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database=TBL0001
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type=btree
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db_pagesize=4096
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duplicates=1
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dupsort=1
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HEADER=END
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```
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