persisting ETS tables to disk with DETS
about
decide on table name
# use the same table name for both ETS and DETS
table_name = :my_datadecide on DETS file location
dets_dir = "tmp"
File.mkdir(dets_dir)
dets_path = Path.join([dets_dir, "#{table_name}.dets"])create a new in-memory ETS table
- data structures
- set
- ordered_set
- bag
- duplicate_bag
- access level
- public
- Any process can read or write to the table
- protected
- The owner process can read and write to the table. Other processes can only read the table. This is the default setting for the access rights
- private
- Only the owner process can read or write to the table
- public
https://www.erlang.org/doc/man/ets.html#new-2
init_ets = fn ->
:ets.new(table_name, [
:named_table,
:bag,
:public
])
end
init_ets.()create a new DETS table file for persistence
-
table type must be the same as the DETS table's
-
options
- auto_save
- type
- file
https://www.erlang.org/doc/man/dets.html#open_file-2
:dets.open_file(table_name,
type: :bag,
file: dets_path |> String.to_charlist(),
auto_save: 60_000
):ets.to_dets/2 and :dets.to_ets/2
:ets.to_dets/2
- https://www.erlang.org/doc/man/ets.html#to_dets-2
- Fills an already created/opened DETS table with the objects in the already opened ETS table
:dets.to_ets/2
- https://www.erlang.org/doc/man/dets.html#to_ets-2
- Inserts the objects of DETS table into ETS table
load DETS data into ETS
:dets.to_ets(table_name, table_name)
# check the contents of the ETS table
:ets.tab2list(table_name)delete entries by key
:ets.delete(table_name, :event_a)
:ets.tab2list(table_name)delete all entries
:ets.delete_all_objects(table_name)
:ets.tab2list(table_name)insert entries into ETS
entries = [
{:event_a, %{id: 1}},
{:event_a, %{id: 2}},
{:event_b, %{id: 3}}
]
for entry <- entries do
true = :ets.insert(table_name, entry)
end
# check the contents of the ETS table
:ets.tab2list(table_name)save the ETS table to the DETS file
persist_data = fn table_name ->
with :ok <- :dets.from_ets(table_name, table_name),
:ok <- :dets.sync(table_name) do
:ok
else
{:error, reason} ->
Logger.error("Unable to sync DETS #{table_name}, #{inspect(reason)}")
end
end
persist_data.(table_name)recreate the ETS table from scratch
:ets.delete(table_name)
init_ets.()
# now the ETS table is empty
[] = :ets.tab2list(table_name)restore all the ETS data from the DETS table
:dets.to_ets(table_name, table_name)
# now the ETS table should be filled with data from the backup
:ets.tab2list(table_name)
