Skip to main content

Mnesia queries

I've added search and trim to my expiring records module in Erlang. This started out as an in-memory key/value store, that I then migrated over to using Mnesia and eventually to a replicated Mnesia table.
The fetch/1 function is already doing a simple query, with match_object.
Result = mnesia:match_object(expiring_records, #record{key = Key, value = '_', expires_at = '_'}, read)
The three parameters there are the name of the table - expiring_records, the matching pattern and the lock type (read lock).
The fetch/1 function looks up the key as it was added to the table with store/3. If the key is a tuple, we can also do a partial match:
Result = mnesia:match_object(expiring_records, #record{key = {'_', "bongo"}, value = '_', expires_at = '_'}, read)
I've added a search/1 function the module that takes in a matching pattern and returns a list of items where the key matches the pattern. Here's the test for the search/1 function:
search_partial_key(_Config) ->
    ok = expiring_records:store({"bingo", "parlor"}, "bongo", erlang:system_time(second) + 3600),
    ok = expiring_records:store("bingo", "bongo", erlang:system_time(second) + 3600),
    ok = expiring_records:store({"smoking", "parlor"}, "bongo", erlang:system_time(second) + 3600),
    ok = expiring_records:store("smoking", "bongo", erlang:system_time(second) + 3600),
    ok = expiring_records:store("parlor", "bongo", erlang:system_time(second) + 3600),
    ok = expiring_records:store({"reading", "parlor"}, "bongo", erlang:system_time(second) - 1),
    [A, B] = expiring_records:search({'_', "parlor"}),
    {"bingo", "parlor"} = A#record.key,
    {"smoking", "parlor"} = B#record.key.
For more complex queries we can use select. This function takes in a match specification that goes beyond the pattern matching done by match_object. The trim/0 function finds records where the expiration time has passed:
handle_call(trim, _From, State) ->
    Trans = fun() ->
        Now = erlang:system_time(second),
        MatchHead = #record{key='$1', expires_at = '$2', _='_'},
        Guard = {'>', Now, '$2'},
        Result = '$1',
        ExpiredKeys = mnesia:select(expiring_records, [{MatchHead, [Guard], [Result]}]),
        delete_records(ExpiredKeys)
    end,
    {atomic, ok} = mnesia:transaction(Trans),
    {reply, ok, State};
The MatchHead specifies the things we care about in the record and gives them labels that we can refer in the other parts of the match specification. The key is labelled $1 and is referred to in the Result. The expires_at field is labelled $2 and is referred to in the Guard. This guard expression is quite simple - Now should be larger than the expiration time of the record. This select call returns a list of keys for records that have expired, that will in turn get deleted.
I need to experiment with the performance of these sort of queries. Mnesia tables can have secondary indices that ought to help, but I'm sure queries end up being a sequential scan of all entries, applying the pattern matching or guard expression to each entry in turn.

Popular posts from this blog

Waiting for an answer

I want to describe my first iteration of exsim, the core server for the large scale simulation I described in my last blog post. A Listener module opens a socket for listening to incoming connections. Once a connection is made, a process is spawned for handling the login and the listener continues listening for new connections. Once logged in, a Player is created, and a Solarsystem is started (if it hasn't already). The solar system also starts a PhysicsProxy, and the player starts a Ship. These are all GenServer processes. The source for this is up on GitHub: https://github.com/snorristurluson/exsim Player The player takes ownership of the TCP connection and handles communication with the game client (or bot). Incoming messages are parsed in handle_info/2 and handled by the player or routed to the ship, as appropriate. The player creates the ship in its init/1 function. The state for the player holds the ship and the name of the player. Ship The ship holds the state of the ship - …

Replicated Mnesia

I'm still working on my expiring records module in Erlang (see here and here for my previous posts on this). Previously, I had started using Mnesia, but only a RAM based table. I've now switched it over to a replicated disc based table. That was easy enough, but it took a while to figure out how to do, nonetheless. I had assumed that simply adding ... {disc_copies, [node()]} ... to the arguments to mnesia:create_table would be enough. This resulted in an error: {app_test,init_per_testcase, {{badmatch, {aborted, {bad_type,expiring_records,disc_copies,nonode@nohost}}}, ... After some head-scratching and lots of Googling I realized that I was missing a call to mnesia:create_schema to allow it to create disc based tables. My tests for this module are done with common_test so I set up a per suite initialization function like this: init_per_suite(Config) ->mnesia:create_schema([node()]), mnesia:start(…