ceremonyclient/pebble/README.md

# Pebble [![Build Status](https://github.com/cockroachdb/pebble/actions/workflows/ci.yaml/badge.svg?branch=master)](https://github.com/cockroachdb/pebble/actions/workflows/ci.yaml) [![GoDoc](https://godoc.org/github.com/cockroachdb/pebble?status.svg)](https://godoc.org/github.com/cockroachdb/pebble) <sup><sub><sub>[Coverage](https://storage.googleapis.com/crl-codecover-public/pebble/index.html)</sub></sub></sup>

#### [Nightly benchmarks](https://cockroachdb.github.io/pebble/)

Pebble is a LevelDB/RocksDB inspired key-value store focused on
performance and internal usage by CockroachDB. Pebble inherits the
RocksDB file formats and a few extensions such as range deletion
tombstones, table-level bloom filters, and updates to the MANIFEST
format.

Pebble intentionally does not aspire to include every feature in RocksDB and
specifically targets the use case and feature set needed by CockroachDB:

* Block-based tables
* Checkpoints
* Indexed batches
* Iterator options (lower/upper bound, table filter)
* Level-based compaction
* Manual compaction
* Merge operator
* Prefix bloom filters
* Prefix iteration
* Range deletion tombstones
* Reverse iteration
* SSTable ingestion
* Single delete
* Snapshots
* Table-level bloom filters

RocksDB has a large number of features that are not implemented in
Pebble:

* Backups
* Column families
* Delete files in range
* FIFO compaction style
* Forward iterator / tailing iterator
* Hash table format
* Memtable bloom filter
* Persistent cache
* Pin iterator key / value
* Plain table format
* SSTable ingest-behind
* Sub-compactions
* Transactions
* Universal compaction style

***WARNING***: Pebble may silently corrupt data or behave incorrectly if
used with a RocksDB database that uses a feature Pebble doesn't
support. Caveat emptor!

## Production Ready

Pebble was introduced as an alternative storage engine to RocksDB in
CockroachDB v20.1 (released May 2020) and was used in production
successfully at that time. Pebble was made the default storage engine
in CockroachDB v20.2 (released Nov 2020). Pebble is being used in
production by users of CockroachDB at scale and is considered stable
and production ready.

## Advantages

Pebble offers several improvements over RocksDB:

* Faster reverse iteration via backwards links in the memtable's
  skiplist.
* Faster commit pipeline that achieves better concurrency.
* Seamless merged iteration of indexed batches. The mutations in the
  batch conceptually occupy another memtable level.
* L0 sublevels and flush splitting for concurrent compactions out of L0 and
  reduced read-amplification during heavy write load.
* Faster LSM edits in LSMs with large numbers of sstables through use of a
  copy-on-write B-tree to hold file metadata.
* Delete-only compactions that drop whole sstables that fall within the bounds
  of a range deletion.
* Block-property collectors and filters that enable iterators to skip tables,
  index blocks and data blocks that are irrelevant, according to user-defined
  properties over key-value pairs.
* Range keys API, allowing KV pairs defined over a range of keyspace with
  user-defined semantics and interleaved during iteration.
* Smaller, more approachable code base.

See the [Pebble vs RocksDB: Implementation
Differences](docs/rocksdb.md) doc for more details on implementation
differences.

## RocksDB Compatibility

Pebble strives for forward compatibility with RocksDB 6.2.1 (the latest
version of RocksDB used by CockroachDB). Forward compatibility means
that a DB generated by RocksDB can be used by Pebble. Currently, Pebble
provides bidirectional compatibility with RocksDB (a Pebble generated DB
can be used by RocksDB) when using its FormatMostCompatible format. New
functionality that is backwards incompatible is gated behind new format
major versions. In general, Pebble only provides compatibility with the
subset of functionality and configuration used by CockroachDB. The scope
of RocksDB functionality and configuration is too large to adequately
test and document all the incompatibilities. The list below contains
known incompatibilities.

* Pebble's use of WAL recycling is only compatible with RocksDB's
  `kTolerateCorruptedTailRecords` WAL recovery mode. Older versions of
  RocksDB would automatically map incompatible WAL recovery modes to
  `kTolerateCorruptedTailRecords`. New versions of RocksDB will
  disable WAL recycling.
* Column families. Pebble does not support column families, nor does
  it attempt to detect their usage when opening a DB that may contain
  them.
* Hash table format. Pebble does not support the hash table sstable
  format.
* Plain table format. Pebble does not support the plain table sstable
  format.
* SSTable format version 3 and 4. Pebble does not support version 3
  and version 4 format sstables. The sstable format version is
  controlled by the `BlockBasedTableOptions::format_version` option.
  See [#97](https://github.com/cockroachdb/pebble/issues/97).

## Format major versions

Over time Pebble has introduced new physical file formats.  Backwards
incompatible changes are made through the introduction of 'format major
versions'. By default, when Pebble opens a database, it defaults to
`FormatMostCompatible`. This version is bi-directionally compatible with RocksDB
6.2.1 (with the caveats described above).

To opt into new formats, a user may set `FormatMajorVersion` on the
[`Options`](https://pkg.go.dev/github.com/cockroachdb/pebble#Options)
supplied to
[`Open`](https://pkg.go.dev/github.com/cockroachdb/pebble#Open), or
upgrade the format major version at runtime using
[`DB.RatchetFormatMajorVersion`](https://pkg.go.dev/github.com/cockroachdb/pebble#DB.RatchetFormatMajorVersion).
Format major version upgrades are permanent; There is no option to
return to an earlier format.

The table below outlines the history of format major versions:

| Name                               | Value | Migration  |
|------------------------------------|-------|------------|
| FormatMostCompatible               |   1   | No         |
| FormatVersioned                    |   3   | No         |
| FormatSetWithDelete                |   4   | No         |
| FormatBlockPropertyCollector       |   5   | No         |
| FormatSplitUserKeysMarked          |   6   | Background |
| FormatSplitUserKeysMarkedCompacted |   7   | Blocking   |
| FormatRangeKeys                    |   8   | No         |
| FormatMinTableFormatPebblev1       |   9   | No         |
| FormatPrePebblev1Marked            |  10   | Background |
| FormatSSTableValueBlocks           |  12   | No         |
| FormatFlushableIngest              |  13   | No         |
| FormatPrePebblev1MarkedCompacted   |  14   | Blocking   |
| FormatDeleteSizedAndObsolete       |  15   | No         |
| FormatVirtualSSTables              |  16   | No         |

Upgrading to a format major version with 'Background' in the migration
column may trigger background activity to rewrite physical file
formats, typically through compactions. Upgrading to a format major
version with 'Blocking' in the migration column will block until a
migration is complete. The database may continue to serve reads and
writes if upgrading a live database through
`RatchetFormatMajorVersion`, but the method call will not return until
the migration is complete.

For reference, the table below lists the range of supported Pebble format major
versions for CockroachDB releases.

| CockroachDB release | Earliest supported                 | Latest supported          |
|---------------------|------------------------------------|---------------------------|
| 20.1 through 21.1   | FormatMostCompatible               | FormatMostCompatible      |
| 21.2                | FormatMostCompatible               | FormatSetWithDelete       |
| 21.2                | FormatMostCompatible               | FormatSetWithDelete       |
| 22.1                | FormatMostCompatible               | FormatSplitUserKeysMarked |
| 22.2                | FormatMostCompatible               | FormatPrePebblev1Marked   |
| 23.1                | FormatSplitUserKeysMarkedCompacted | FormatFlushableIngest     |
| 23.2                | FormatSplitUserKeysMarkedCompacted | FormatVirtualSSTables     |
| 24.1 plan           | FormatSSTableValueBlocks           |                           |

## Pedigree

Pebble is based on the incomplete Go version of LevelDB:

https://github.com/golang/leveldb

The Go version of LevelDB is based on the C++ original:

https://github.com/google/leveldb

Optimizations and inspiration were drawn from RocksDB:

https://github.com/facebook/rocksdb

## Getting Started

### Example Code

```go
package main

import (
	"fmt"
	"log"

	"github.com/cockroachdb/pebble"
)

func main() {
	db, err := pebble.Open("demo", &pebble.Options{})
	if err != nil {
		log.Fatal(err)
	}
	key := []byte("hello")
	if err := db.Set(key, []byte("world"), pebble.Sync); err != nil {
		log.Fatal(err)
	}
	value, closer, err := db.Get(key)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%s %s\n", key, value)
	if err := closer.Close(); err != nil {
		log.Fatal(err)
	}
	if err := db.Close(); err != nil {
		log.Fatal(err)
	}
}
```