# PersistentEts

Ets table backed by a persistence file

The table is persisted using the `:ets.file2tab/2` and `:ets.tab2file/3`
functions.

Table is to be created with `PersistentEts.new/3` in place of `:ets.new/2`.
After that all functions from `:ets` can be used like with any other table,
except `:ets.give_away/3` and `:ets.delete/1` - replacement functions are
provided in this module. The `:ets.setopts/2` function to change the heir
is not supported - the heir setting is leveraged by the persistence mechanism.

Like with regular ets table, the table is destroyed once the owning process
(the one that called `PersistentEts.new/3`) dies, but the table data is persisted
so it will be re-read when table is opened again.

## Example

```elixir
pid = spawn(fn -> 
  :foo = PersistentEts.new(:foo, "table.tab", [:named_table])
  :ets.insert(:foo, [a: 1])
end)
Process.exit(pid, :diediedie)
PersistentEts.new(:foo, "table.tab", [:named_table])
[a: 1] = :ets.tab2list(:foo)
```

## Why not Dets?

With Dets every operation (read or write) hits the disk. For many application such a performance penalty (compared to ets) is not acceptable. Furthermore Dets tables are limited to 2GB. Dets doesn't support the `ordered_set` table type either.

With PersistentEts, the table remains in memory, so all read and write operations have the same performance they would have with pure Ets. Only periodically the table state is saved to a file. There's also no file limit, besides the memory and disk limitations. Since it's a regular Ets table, unlike with Dets, all types are fully supported.

## Installation

If [available in Hex](https://hex.pm/docs/publish), the package can be installed
by adding `persistent_ets` to your list of dependencies in `mix.exs`:

```elixir
def deps do
  [{:persistent_ets, "~> 0.1.0"}]
end
```

Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
be found at [https://hexdocs.pm/persistent_ets](https://hexdocs.pm/persistent_ets).