Erlang Term Storage

Erlang Term Storage, commonly referred to as ETS, is a powerful storage engine built into OTP. Every entry in an ETS table is a tuple (or corresponding record), and one piece of the tuple is designated the key.

ETS can hold four kinds of collections (Laurent 2017, 141):

  1. Sets (set): Can contain only one entry with a given key.
  2. Ordered Sets (ordered_set): Same as a set, but also maintains a traversal order based on the keys. Great for anything you want to keep in alphabetic or numeric order.
  3. Bags (bag): Lets you store more than one entry with a given key. However, if you have multiple entries that have identical values, they get combined into a single entry.
  4. Duplicate Bags (duplicate_bag): Not only lets you store more than one entry with a given key, but also lets you store multiple entries with identical values.

Creating and populating a table

  • The ets:new/2 function lets you create a table.
  • If you do specify named_table, processes can reach the table as long as they know the name, without needing access to that return value.

  • Using the keypos tuple lets you specify which record value should be the key. 

        rd(food, {name, calories, price, group}).
    ets:new(food, [ordered_set, {keypos,#food.name}, named_table]).
    %% or dets:open_file, if you want to use dts

  • To dispose of a table: 

        ets:delete(TableId)
    % or
    dets:close(TableId)

Simple Queries

  • ets:lookup/2: Which is defined for both ETS and DTS.

References:

Laurent, Simon St. 2017. Introducing Erlang: Getting Started in Functional Programming. O’Reilly Media, Inc.

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