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227 lines
9.0 KiB
Markdown
227 lines
9.0 KiB
Markdown
# Pebble [](https://github.com/cockroachdb/pebble/actions/workflows/ci.yaml) [](https://godoc.org/github.com/cockroachdb/pebble) <sup><sub><sub>[Coverage](https://storage.googleapis.com/crl-codecover-public/pebble/index.html)</sub></sub></sup>
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#### [Nightly benchmarks](https://cockroachdb.github.io/pebble/)
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Pebble is a LevelDB/RocksDB inspired key-value store focused on
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performance and internal usage by CockroachDB. Pebble inherits the
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RocksDB file formats and a few extensions such as range deletion
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tombstones, table-level bloom filters, and updates to the MANIFEST
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format.
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Pebble intentionally does not aspire to include every feature in RocksDB and
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specifically targets the use case and feature set needed by CockroachDB:
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* Block-based tables
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* Checkpoints
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* Indexed batches
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* Iterator options (lower/upper bound, table filter)
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* Level-based compaction
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* Manual compaction
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* Merge operator
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* Prefix bloom filters
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* Prefix iteration
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* Range deletion tombstones
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* Reverse iteration
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* SSTable ingestion
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* Single delete
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* Snapshots
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* Table-level bloom filters
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RocksDB has a large number of features that are not implemented in
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Pebble:
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* Backups
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* Column families
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* Delete files in range
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* FIFO compaction style
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* Forward iterator / tailing iterator
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* Hash table format
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* Memtable bloom filter
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* Persistent cache
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* Pin iterator key / value
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* Plain table format
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* SSTable ingest-behind
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* Sub-compactions
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* Transactions
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* Universal compaction style
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***WARNING***: Pebble may silently corrupt data or behave incorrectly if
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used with a RocksDB database that uses a feature Pebble doesn't
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support. Caveat emptor!
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## Production Ready
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Pebble was introduced as an alternative storage engine to RocksDB in
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CockroachDB v20.1 (released May 2020) and was used in production
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successfully at that time. Pebble was made the default storage engine
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in CockroachDB v20.2 (released Nov 2020). Pebble is being used in
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production by users of CockroachDB at scale and is considered stable
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and production ready.
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## Advantages
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Pebble offers several improvements over RocksDB:
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* Faster reverse iteration via backwards links in the memtable's
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skiplist.
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* Faster commit pipeline that achieves better concurrency.
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* Seamless merged iteration of indexed batches. The mutations in the
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batch conceptually occupy another memtable level.
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* L0 sublevels and flush splitting for concurrent compactions out of L0 and
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reduced read-amplification during heavy write load.
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* Faster LSM edits in LSMs with large numbers of sstables through use of a
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copy-on-write B-tree to hold file metadata.
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* Delete-only compactions that drop whole sstables that fall within the bounds
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of a range deletion.
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* Block-property collectors and filters that enable iterators to skip tables,
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index blocks and data blocks that are irrelevant, according to user-defined
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properties over key-value pairs.
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* Range keys API, allowing KV pairs defined over a range of keyspace with
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user-defined semantics and interleaved during iteration.
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* Smaller, more approachable code base.
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See the [Pebble vs RocksDB: Implementation
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Differences](docs/rocksdb.md) doc for more details on implementation
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differences.
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## RocksDB Compatibility
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Pebble strives for forward compatibility with RocksDB 6.2.1 (the latest
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version of RocksDB used by CockroachDB). Forward compatibility means
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that a DB generated by RocksDB can be used by Pebble. Currently, Pebble
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provides bidirectional compatibility with RocksDB (a Pebble generated DB
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can be used by RocksDB) when using its FormatMostCompatible format. New
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functionality that is backwards incompatible is gated behind new format
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major versions. In general, Pebble only provides compatibility with the
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subset of functionality and configuration used by CockroachDB. The scope
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of RocksDB functionality and configuration is too large to adequately
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test and document all the incompatibilities. The list below contains
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known incompatibilities.
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* Pebble's use of WAL recycling is only compatible with RocksDB's
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`kTolerateCorruptedTailRecords` WAL recovery mode. Older versions of
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RocksDB would automatically map incompatible WAL recovery modes to
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`kTolerateCorruptedTailRecords`. New versions of RocksDB will
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disable WAL recycling.
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* Column families. Pebble does not support column families, nor does
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it attempt to detect their usage when opening a DB that may contain
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them.
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* Hash table format. Pebble does not support the hash table sstable
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format.
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* Plain table format. Pebble does not support the plain table sstable
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format.
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* SSTable format version 3 and 4. Pebble does not support version 3
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and version 4 format sstables. The sstable format version is
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controlled by the `BlockBasedTableOptions::format_version` option.
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See [#97](https://github.com/cockroachdb/pebble/issues/97).
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## Format major versions
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Over time Pebble has introduced new physical file formats. Backwards
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incompatible changes are made through the introduction of 'format major
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versions'. By default, when Pebble opens a database, it defaults to
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`FormatMostCompatible`. This version is bi-directionally compatible with RocksDB
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6.2.1 (with the caveats described above).
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To opt into new formats, a user may set `FormatMajorVersion` on the
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[`Options`](https://pkg.go.dev/github.com/cockroachdb/pebble#Options)
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supplied to
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[`Open`](https://pkg.go.dev/github.com/cockroachdb/pebble#Open), or
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upgrade the format major version at runtime using
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[`DB.RatchetFormatMajorVersion`](https://pkg.go.dev/github.com/cockroachdb/pebble#DB.RatchetFormatMajorVersion).
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Format major version upgrades are permanent; There is no option to
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return to an earlier format.
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The table below outlines the history of format major versions:
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| Name | Value | Migration |
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|------------------------------------|-------|------------|
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| FormatMostCompatible | 1 | No |
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| FormatVersioned | 3 | No |
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| FormatSetWithDelete | 4 | No |
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| FormatBlockPropertyCollector | 5 | No |
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| FormatSplitUserKeysMarked | 6 | Background |
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| FormatSplitUserKeysMarkedCompacted | 7 | Blocking |
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| FormatRangeKeys | 8 | No |
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| FormatMinTableFormatPebblev1 | 9 | No |
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| FormatPrePebblev1Marked | 10 | Background |
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| FormatSSTableValueBlocks | 12 | No |
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| FormatFlushableIngest | 13 | No |
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| FormatPrePebblev1MarkedCompacted | 14 | Blocking |
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| FormatDeleteSizedAndObsolete | 15 | No |
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| FormatVirtualSSTables | 16 | No |
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Upgrading to a format major version with 'Background' in the migration
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column may trigger background activity to rewrite physical file
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formats, typically through compactions. Upgrading to a format major
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version with 'Blocking' in the migration column will block until a
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migration is complete. The database may continue to serve reads and
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writes if upgrading a live database through
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`RatchetFormatMajorVersion`, but the method call will not return until
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the migration is complete.
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For reference, the table below lists the range of supported Pebble format major
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versions for CockroachDB releases.
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| CockroachDB release | Earliest supported | Latest supported |
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|---------------------|------------------------------------|---------------------------|
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| 20.1 through 21.1 | FormatMostCompatible | FormatMostCompatible |
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| 21.2 | FormatMostCompatible | FormatSetWithDelete |
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| 21.2 | FormatMostCompatible | FormatSetWithDelete |
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| 22.1 | FormatMostCompatible | FormatSplitUserKeysMarked |
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| 22.2 | FormatMostCompatible | FormatPrePebblev1Marked |
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| 23.1 | FormatSplitUserKeysMarkedCompacted | FormatFlushableIngest |
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| 23.2 | FormatSplitUserKeysMarkedCompacted | FormatVirtualSSTables |
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| 24.1 plan | FormatSSTableValueBlocks | |
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## Pedigree
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Pebble is based on the incomplete Go version of LevelDB:
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https://github.com/golang/leveldb
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The Go version of LevelDB is based on the C++ original:
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https://github.com/google/leveldb
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Optimizations and inspiration were drawn from RocksDB:
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https://github.com/facebook/rocksdb
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## Getting Started
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### Example Code
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```go
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package main
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import (
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"fmt"
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"log"
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"github.com/cockroachdb/pebble"
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)
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func main() {
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db, err := pebble.Open("demo", &pebble.Options{})
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if err != nil {
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log.Fatal(err)
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}
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key := []byte("hello")
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if err := db.Set(key, []byte("world"), pebble.Sync); err != nil {
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log.Fatal(err)
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}
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value, closer, err := db.Get(key)
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if err != nil {
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log.Fatal(err)
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}
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fmt.Printf("%s %s\n", key, value)
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if err := closer.Close(); err != nil {
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log.Fatal(err)
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}
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if err := db.Close(); err != nil {
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log.Fatal(err)
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}
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}
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```
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