Tales of a developer

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 ...

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 initializat...

Continuing with my  expiring records  module (see  my previous blog ), I've now switched it over to using  Mnesia , rather than a dictionary object stored in the state of my  gen_server  instance. It is still not a truly global, cross-node key/value store for expiring records, but it is getting there. I wanted to focus first on getting my tests to pass again with a RAM based table on one node. I just need to tweak the values passed in when creating the Mnesia table, I think, to make this work with a disc based table, replicated across nodes. Currently the table is created like this: prepare_table () -> case catch mnesia : table_info ( expiring_records , attributes ) of { 'EXIT' , _ } -> % % Table does not exist - create it erlang : display ( " Creating table " ), mnesia : create_table ( expiring_records , [ { attributes , record_info ( fields , record )}, ...

I'm continuing my experiments with Erlang - this time trying out  gen_server  with a simple key/value store with a twist - the values have an expiration date. As a first iteration I'm simply using a dictionary to store the values, and only expiring records when they are looked up. My plan is to extend this later on so that this can be a global key/value store across multiple Erlang nodes but for now I'm focusing on two things - get something going using  gen_server , and try out the  common_test  testing framework. The code is here:  https://github.com/snorristurluson/erl-expiring-records Let's first take a look at a couple of the test functions, to show the usage of this: get_non_expired_record ( Config ) -> Pid = ? config ( pid , Config ), Record = { " bingo " , " bongo " , erlang : system_time ( second ) + 3600 }, ok = gen_server : call ( Pid , { add , Record }), { ok , " bongo " } = gen_server : call...

I've been digging deeper into my experiments with Erlang and have improved the JSON parser I discussed in a  previous blog . The code lives at  https://github.com/snorristurluson/erl-simple-json  in case you want to take a look. The project is now set up to take advantage of  rebar3 , the Erlang build system, and tests are run automatically on  Travis CI . Thanks,  Lou Xun ! The parser now does proper tokenizing, rather than the simple string split operations I did in my first pass. The parser itself is surprisingly simple when it's working with tokens - here it is in its entirety: parse_json ( Input ) -> { Value , <<>>} = parse_json_value ( Input ), Value . parse_json_value ( Input ) -> { Token , Rest } = tokens : tokenize ( Input ), case Token of open_brace -> populate_object ( dict : new (), Rest ); open_square_bracket -> populate_list ([], Rest ); { ...