# CubDB
`CubDB` is an embedded key-value database written in the Elixir language. It
runs locally, and is backed by a single file.
Both keys and values can be any Elixir (or Erlang) term, so no serialization
and de-serialization is necessary.
The `CubDB` database file uses an immutable data structure that provides several
guarantees:
- Robustness to data corruption, as entries are never changed in-place
- Atomic writes: write operations either entirely succeed or entirely fail
- Read operations run concurrently, and do not block nor are blocked by writes
- Ranges of entries are selected on immutable snapshots, giving always a
consistent view, even while write operations are being performed
## Usage
Start `CubDB` by specifying a directory for its database file (if not existing,
it will be created):
```elixir
{:ok, db} = CubDB.start_link("my/data/directory")
```
`get`, `put`, and `delete` operations work as you probably expect:
```elixir
CubDB.put(db, :foo, "some value")
#=> :ok
CubDB.get(db, :foo)
#=> "some value"
CubDB.delete(db, :foo)
#=> :ok
CubDB.get(db, :foo)
#=> nil
```
Range of keys are retrieved using `select`:
```elixir
for {key, value} <- [a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8] do
CubDB.put(db, key, value)
end
CubDB.select(db, min_key: :b, max_key: :e)
#=> {:ok, [b: 2, c: 3, d: 4, e: 5]}
```
But `select` can do much more than that. It can apply a pipeline of operations
(`map`, `filter`, `take`, `drop` and more) to the selected entries, it can
select the entries in normal or reverse order, and it can `reduce` the result
using an arbitrary function:
```elixir
# Take the sum of the last 3 even values:
CubDB.select(db,
reverse: true,
pipe: [
map: fn {_key, value} ->
value
end,
filter: fn value ->
is_integer(value) && Integer.is_even(value)
end,
take: 3
],
reduce: fn n, sum -> sum + n end
)
#=> {:ok, 18}
```
For more details, read the [API documentation](https://hexdocs.pm/cubdb/CubDB.html).
## Installation
`CubDB` can be installed by adding `cubdb` to your list of dependencies in
`mix.exs`:
```elixir
def deps do
[{:cubdb, "~> 0.3.0"}]
end
```
## Acknowledgement
The file data structure used by `CubDB` is partly inspired by
[CouchDB](http://couchdb.apache.org). A big thanks goes to the CouchDB
maintainers for several articles on the web explaining its design.