lucaong/cubdb

CubDB is an embedded key-value database for the Elixir language. It is designed for robustness, and for minimal need of resources.

Head to the API reference for usage details, or read the Frequently Asked Questions and the How To section for more information.

Features

• Both keys and values can be any Elixir (or Erlang) term.

• Basic get, put, and delete operations, selection of ranges of entries sorted by key with select.

• Atomic, Consistent, Isolated, Durable (ACID) transactions.

• Multi version concurrency control (MVCC) allowing concurrent read operations, that do not block nor are blocked by writes.

• Unexpected shutdowns or crashes won't corrupt the database or break atomicity of transactions.

• Manual or automatic compaction to reclaim disk space.

To ensure consistency, performance, and robustness to data corruption, CubDB database file uses an append-only, immutable B-tree data structure. Entries are never changed in-place, and read operations are performed on zero cost immutable snapshots.

Usage

Start CubDB by specifying a directory for its database file (if not existing, it will be created):

{:ok, db} = CubDB.start_link(data_dir: "my/data/directory")

Important {: .warning}

Avoid starting multiple CubDB processes on the same data directory. Only one CubDB process should use a specific data directory at any time.

get, put, and delete operations work as you probably expect:

CubDB.put(db, :foo, "some value")
#=> :ok

CubDB.get(db, :foo)
#=> "some value"

CubDB.delete(db, :foo)
#=> :ok

CubDB.get(db, :foo)
#=> nil

Multiple operations can be performed atomically with the transaction function and the CubDB.Tx module:

# Swapping `:a` and `:b` atomically:
CubDB.transaction(db, fn tx ->
  a = CubDB.Tx.get(tx, :a)
  b = CubDB.Tx.get(tx, :b)

  tx = CubDB.Tx.put(tx, :a, b)
  tx = CubDB.Tx.put(tx, :b, a)

  {:commit, tx, :ok}
end)
#=> :ok

Alternatively, it is possible to use put_multi, delete_multi, and the other [...]_multi functions, which also guarantee atomicity:

CubDB.put_multi(db, [a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8])
#=> :ok

Range of entries sorted by key are retrieved using select:

CubDB.select(db, min_key: :b, max_key: :e) |> Enum.to_list()
#=> [b: 2, c: 3, d: 4, e: 5]

The select function can select entries in normal or reverse order, and returns a lazy stream, so one can use functions in the Stream and Enum modules to map, filter, and transform the result, only fetching from the database the relevant entries:

# Take the sum of the last 3 even values:
CubDB.select(db, reverse: true) # select entries in reverse order
|> Stream.map(fn {_key, value} -> value end) # discard the key and keep only the value
|> Stream.filter(fn value -> is_integer(value) && Integer.is_even(value) end) # filter only even integers
|> Stream.take(3) # take the first 3 values
|> Enum.sum() # sum the values
#=> 18

Read-only snapshots are useful when one needs to perform several reads or selects, ensuring isolation from concurrent writes, but without blocking them. When nothing needs to be written, using a snapshot is preferable to using a transaction, because it will not block writes.

Snapshots come at no cost: nothing is actually copied or written on disk or in memory, apart from some small internal bookkeeping. After obtaining a snapshot with with_snapshot, one can read from it using the functions in the CubDB.Snapshot module:

# the key of y depends on the value of x, so we ensure consistency by getting
# both entries from the same snapshot, isolating from the effects of concurrent
# writes
{x, y} = CubDB.with_snapshot(db, fn snap ->
  x = CubDB.Snapshot.get(snap, :x)
  y = CubDB.Snapshot.get(snap, x)

  {x, y}
end)

The functions that read multiple entries like get_multi, select, etc. are internally using a snapshot, so they always ensure consistency and isolation from concurrent writes, implementing multi version concurrency control (MVCC).

For more details, read the API documentation.

Installation

CubDB can be installed by adding :cubdb to your list of dependencies in mix.exs:

def deps do
  [
    {:cubdb, "~> 2.0.2"}
  ]
end

Acknowledgement

The file data structure used by CubDB is inspired by CouchDB. A big thanks goes to the CouchDB maintainers for the readable codebase and extensive documentation.

Copyright and License

Copyright 2022 Luca Ongaro

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.