rel_erl is a lightweight relational in-memory database for Erlang.
The process dictionary is officially considered evil, but sometimes it helps. rel_erl gets the best out of process dictionary.
Your machine must have the Erlang runtime in order to run rel_erl.erl.
Erlang can be downloaded from here:
Running rel_erl.erl from command line:
escript rel_erl.erl <schema file> <output dir>
escript rel_erl.erl example.txt .
The rel_erl schema is an ASCII-encoded plain text file.
The schema file contains:
There must be at least one table in the schema.
The syntax of the schema file is based on indentation. Index and table headers should not have any whitespace at the start of line.
Field lines must be indented with either tabs or spaces.
A table has one or more fields. One of the fields must be the primary key of the table.
table person id pk first_name second_name
In this example, the person table has three fields: id, first_name and second_name. id is the primary key of the table. Records can be quickly retrieved using the primary key.
A table may have indexes. An index specifies that one or more columns in the table are unique. Indexes fulfill two functions:
The following snipped defines an index named person_by_names on the person table. This index makes sure that the combination of first_name and second_name in the person table is unique for all rows.
index person_by_names person first_name second_name
Primary key fields and indexed fields are read-only.
A simple link models one-to-one or one-to-many relationship.
It is implemented as a field with the primary key of another record.  (empty list) means a null reference. Links can reference records in a different or in the same table. Records that are pointed to by simple links have a reference counter. When a new value is assigned to a simple link field, these actions take place:
It is impossible to delete a record that has a non-zero reference count.
table profession prof_id pk name table person id pk first_name second_name prof -> profession
Here, a person can have a profession. The system makes sure that:
The primary key field can be a simple link reference.
A collection link is a two-way one-to-many link:
 (empty list) means a null reference. The one side field (the list of references) cannot contains nulls.
The system ensures that:
Collection links make it easy to do joins and enumerate over the list of records linked to the given one.
table department dep_id pk persons  table person id pk first_name second_name department -> department.persons
In this example, a department has a list of person's that belong to it.
The collection field on the one side cannot point to records of several tables.
The reference field on the many side can be the primary key.
Please note that a spaces are necessary near  and ->
table Fun name # an ordinary field, read-write code pk # the primary key field, read-only foo -> moo # a simple link field (references "moo" table), read-write table Better bar -> cool pk # a simple link which is also the primary key, read-only hot -> moo.bar # a collection link ("many" side, references field "bar" # of "moo" table), read-write some  # a collection link ("one" side), read-only
The insert function creates a new record.
The caller must supply the primary key and values for all indexed fields.
If an indexed field is a reference, the key of the referenced field should be supplied.
table person id pk first_name second_name phone index person_by_names person first_name second_name
The usage of the generated insert function:
insert_person(201, "John", "Smith")
insert enforces referential and unique constraints.
The default value of a field is  (empty list).
The delete function deletes a record.
The caller must supply the primary key of the deleted record.
delete enforces referential integrity. If the deleted record is referenced, the function returns error, and the data remains intact.
Setters set values to fields.
The setter accepts the primary key and the new value of the field and returns the old value.
It is impossible to set a new value to an indexed or primary key field.
Setters enforce referential integrity. They don't allow fields to point to non-existent records.
Setters make sure that the record being modified actually exists.
The collection field on the one side of a one-to-many link is read-only. The only way to change it is set a value to the corresponding field on the many side.
Fetch functions perform fast retrieval of records. A separate fetch function is generated for each index and for the primary key.
Fetch functions take the values of the indexed fields and return the records. If the record does not exist, the fetch function returns an empty tuple.
Person = find_person_by_pk(201), Person2 = find_person_by_names("John", "Smith")
Getters return values stored in fields. A getter takes the record as the parameter, not the primary key.
Person2 = find_person_by_names("John", "Smith"), FirstName = get_person_first_name(Person2)
The module name and the output source code filename are based on the schema filename.
rel_erl is written in DRAKON-Erlang using DRAKON Editor.
The real source file is rel_erl.drn.
rel_erl.erl is autogenerated from rel_erl.drn.
rel_erl is Public Domain.
April 8, 2012
Author: Stepan Mitkin firstname.lastname@example.org