path: root/lib/mnesia/doc/src/Mnesia_chap2.xmlsrc

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2018</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>Getting Started</title>
    <prepared>Claes Wikstr&ouml;m, Hans Nilsson and H&aring;kan Mattsson</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev>C</rev>
    <file>Mnesia_chap2.xml</file>
  </header>
    <marker id="getting_started"></marker>
  <p>This section introduces <c>Mnesia</c> with an example database.
    This example is referenced in the
    following sections, where the example is modified to
    illustrate various program constructs. This section illustrates
    the following mandatory procedures through examples:</p>
  <list type="bulleted">
    <item>Starting the Erlang session.
    </item>
    <item>Specifying the <c>Mnesia</c> directory where the database
      is to be stored.
    </item>
    <item>Initializing a new database schema with an attribute that
      specifies on which node, or nodes, that database is to operate.
    </item>
    <item>Starting <c>Mnesia</c>.
    </item>
    <item>Creating and populating the database tables.
    </item>
  </list>

  <section>
    <title>Starting Mnesia for the First Time</title>
    <p>This section provides a simplified demonstration of a <c>Mnesia</c>
      system startup. The dialogue from the Erlang shell is as follows:</p>
    <pre><![CDATA[
        unix>  erl -mnesia dir '"/tmp/funky"'
        Erlang (BEAM) emulator version 4.9
        
        Eshell V4.9  (abort with ^G)
        1> 
        1> mnesia:create_schema([node()]).
        ok
        2> mnesia:start().
        ok
        3> mnesia:create_table(funky, []).
        {atomic,ok}
        4> mnesia:info().
        ---> Processes holding locks <--- 
        ---> Processes waiting for locks <--- 
        ---> Pending (remote) transactions <--- 
        ---> Active (local) transactions <---
        ---> Uncertain transactions <--- 
        ---> Active tables <--- 
        funky          : with 0 records occupying 269 words of mem 
        schema         : with 2 records occupying 353 words of mem 
        ===> System info in version "1.0", debug level = none <===
        opt_disc. Directory "/tmp/funky" is used.
        use fall-back at restart = false
        running db nodes = [nonode@nohost]
        stopped db nodes = [] 
        remote           = []
        ram_copies       = [funky]
        disc_copies      = [schema]
        disc_only_copies = []
        [{nonode@nohost,disc_copies}] = [schema]
        [{nonode@nohost,ram_copies}] = [funky]
        1 transactions committed, 0 aborted, 0 restarted, 1 logged to disc
        0 held locks, 0 in queue; 0 local transactions, 0 remote
        0 transactions waits for other nodes: []
        ok      
    ]]></pre>
    <p>In this example, the following actions are performed:</p>
    <list type="bulleted">
      <item><em>Step 1:</em> The Erlang system is started from the UNIX
       prompt with a flag <c>-mnesia dir '"/tmp/funky"'</c>, which indicates
       in which directory to store the data.
      </item>
      <item><em>Step 2:</em> A new empty schema is initialized on the local
       node by evaluating
       <seealso marker="mnesia#create_schema/1">mnesia:create_schema([node()])</seealso>.
       The schema contains information about the database in general.
       This is explained in detail later.
      </item>
      <item><em>Step 3:</em> The DBMS is started by evaluating
       <seealso marker="mnesia#start/0">mnesia:start()</seealso>.
      </item>
      <item><em>Step 4:</em> A first table is created, called <c>funky</c>,
       by evaluating the expression <c>mnesia:create_table(funky, [])</c>.
       The table is given default properties.
      </item>
      <item><em>Step 5:</em> <seealso marker="mnesia#info/0">mnesia:info()</seealso>
       is evaluated to
       display information on the terminal about the status of the database.
      </item>
    </list>
  </section>

  <section>
    <title>Example</title>
    <p>A <c>Mnesia</c> database is organized as a set of tables.
      Each table is populated with instances (Erlang records).
      A table has also a number of properties, such as location and
      persistence.</p>

    <section>
      <title>Database</title>
    </section>
    <p>This example shows how to create a database called <c>Company</c>
      and the relationships shown in the following diagram:</p>
    <image file="company.gif">
      <icaption>Company Entity-Relation Diagram</icaption>
    </image>
    <p>The database model is as follows:</p>
    <list type="bulleted">
      <item>There are three entities: department, employee, and project.
      </item>
      <item>
        <p>There are three relationships between these entities:</p>
        <list type="ordered">
          <item>A department is managed by an employee,
            hence the <c>manager</c> relationship.
          </item>
          <item>An employee works at a department, hence the
            <c>at_dep</c> relationship.
          </item>
          <item>Each employee works on a number of projects,
            hence the <c>in_proj</c> relationship.
          </item>
        </list>
      </item>
    </list>

    <section>
      <title>Defining Structure and Content</title>
      <p>First the record definitions are entered into a text file
        named <c>company.hrl</c>. This file defines the following
        structure for the example database: </p>
      <codeinclude file="company.hrl" tag="%0" type="erl"></codeinclude>
      <p>The structure defines six tables in the database. In <c>Mnesia</c>,
        the function
        <seealso marker="mnesia#create_table/2">mnesia:create_table(Name, ArgList)</seealso>
        creates tables. <c>Name</c> is the table name.</p>
        <note><p>The current version of <c>Mnesia</c> does not require that
        the name of the table is the same as the record name, see
        <seealso marker="Mnesia_chap4#recordnames_tablenames">Record Names versus Table Names.</seealso>.</p></note>
      <p>For example, the table for employees is created with the
        function <c>mnesia:create_table(employee,
        [{attributes, record_info(fields, employee)}])</c>. The table
        name <c>employee</c> matches the name for records specified
        in <c>ArgList</c>. The expression
        <c>record_info(fields, RecordName)</c> is processed by the Erlang
        preprocessor and evaluates to a list containing the names of the
        different fields for a record.</p>
    </section>

    <section>
      <title>Program</title>
      <p>The following shell interaction starts <c>Mnesia</c> and
        initializes the schema for the <c>Company</c> database:</p>
      <pre>
        % <input>erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"'</input>
         Erlang (BEAM) emulator version 4.9
          
          Eshell V4.9  (abort with ^G)
          1> mnesia:create_schema([node()]).
          ok
          2> mnesia:start().
          ok</pre>
      <p>The following program module creates and populates previously
        defined tables:</p>
      <codeinclude file="company.erl" tag="%0" type="erl"></codeinclude>
    </section>

    <section>
      <title>Program Explained</title>
      <p>The following commands and functions are used to initiate the
        <c>Company</c> database:</p>
      <list type="bulleted">
        <item><c>% erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"'</c>.
         This is a UNIX
         command-line entry that starts the Erlang system. The flag
         <c>-mnesia dir Dir</c> specifies the location of the
         database directory. The system responds and waits for
         further input with the prompt <c>1></c>.
        </item>
        <item>
        <seealso marker="mnesia#create_schema/1">mnesia:create_schema([node()])</seealso>.
         This function
         has the format <c>mnesia:create_schema(DiscNodeList)</c> and
         initiates a new schema. In this example, a non-distributed system
         using only one node is created. Schemas are fully explained in
         <seealso marker="Mnesia_chap3#def_schema">Define a Schema</seealso>.
        </item>
        <item><seealso marker="mnesia#start/0">mnesia:start()</seealso>.
         This function starts <c>Mnesia</c> and is fully explained in
         <seealso marker="Mnesia_chap3#start_mnesia">Start Mnesia</seealso>.
        </item>
      </list>
      <p>Continuing the dialogue with the Erlang shell produces the
        following:</p>
      <pre><![CDATA[
        3> company:init().
        {atomic,ok}
        4> mnesia:info().
        ---> Processes holding locks <--- 
        ---> Processes waiting for locks <--- 
        ---> Pending (remote) transactions <--- 
        ---> Active (local) transactions <---
        ---> Uncertain transactions <--- 
        ---> Active tables <--- 
        in_proj        : with 0 records occuping 269 words of mem 
        at_dep         : with 0 records occuping 269 words of mem 
        manager        : with 0 records occuping 269 words of mem 
        project        : with 0 records occuping 269 words of mem 
        dept           : with 0 records occuping 269 words of mem 
        employee       : with 0 records occuping 269 words of mem 
        schema         : with 7 records occuping 571 words of mem 
        ===> System info in version "1.0", debug level = none <===
        opt_disc. Directory "/ldisc/scratch/Mnesia.Company" is used.
        use fall-back at restart = false
        running db nodes = [nonode@nohost]
        stopped db nodes = [] 
        remote           = []
        ram_copies       =
            [at_dep,dept,employee,in_proj,manager,project]
        disc_copies      = [schema]
        disc_only_copies = []
        [{nonode@nohost,disc_copies}] = [schema]
        [{nonode@nohost,ram_copies}] =
            [employee,dept,project,manager,at_dep,in_proj]
        6 transactions committed, 0 aborted, 0 restarted, 6 logged to disc
        0 held locks, 0 in queue; 0 local transactions, 0 remote
        0 transactions waits for other nodes: []
        ok
      ]]></pre>
      <p>A set of tables is created. The function
        <seealso marker="mnesia#create_table/2">mnesia:create_table(Name, ArgList)</seealso>
        creates the required database tables. The
        options available with <c>ArgList</c> are explained in
        <seealso marker="Mnesia_chap3#create_tables">Create New Tables</seealso>.</p>
      <p>The function <c>company:init/0</c> creates the tables. Two tables
        are of type <c>bag</c>. This is the <c>manager</c> relation as well
        the <c>in_proj</c> relation. This is interpreted as: an
        employee can be manager over several departments, and an employee
        can participate in several projects. However, the <c>at_dep</c>
        relation is <c>set</c>, as an employee can only work in one department.
        In this data model, there are examples of relations that are 1-to-1
        (<c>set</c>) and  1-to-many (<c>bag</c>).</p>
      <p><seealso marker="mnesia#info/0">mnesia:info()</seealso>
        now indicates that a database has seven
        local tables, where six are the user-defined tables and one is
        the schema. Six transactions have been committed, as six successful
        transactions were run when creating the tables.</p>
      <p>To write a function that inserts an employee record into the
        database, there must be an <c>at_dep</c> record and a set of
        <c>in_proj</c> records inserted. Examine the following
        code used to complete this action:</p>
     <codeinclude file="company.erl" tag="%1" type="erl"></codeinclude>
      <list type="bulleted">
        <item>
          <p>The <c>insert_emp/3</c> arguments are as follows:</p>
          <list type="ordered">
            <item><c>Emp</c> is an employee record.
            </item>
            <item><c>DeptId</c> is the identity of the department where
             the employee works.
            </item>
            <item><c>ProjNames</c> is a list of the names of the projects
             where the employee works.</item>
          </list>
        </item>
      </list>
      <p>The function <c>insert_emp/3</c> creates a Functional Object (Fun).
        <c>Fun</c> is passed
        as a single argument to the function
        <seealso marker="mnesia#transaction/2">mnesia:transaction(Fun)</seealso>.
        This means that <c>Fun</c> is
        run as a transaction with the following properties:</p>
      <list type="bulleted">
        <item>A <c>Fun</c> either succeeds or fails.
        </item>
        <item>Code that manipulates the same data records can be
         run concurrently without the different processes interfering
         with each other. 
        </item>
      </list>
      <p>The function can be used as follows:</p>
      <code type="none">
          Emp  = #employee{emp_no= 104732,
                           name = klacke,
                           salary = 7,
                           sex = male,
                           phone = 98108,
                           room_no = {221, 015}},
        insert_emp(Emp, 'B/SFR', [Erlang, mnesia, otp]).</code>
      <note><p>For information about Funs, see "Fun Expressions" in
        section <c>Erlang Reference Manual</c> in System
        Documentation..</p>
      </note>
    </section>

    <section>
      <title>Initial Database Content</title>
      <p>After the insertion of the  employee named <c>klacke</c>,
        the database has the following records:</p>
      <marker id="table2_1"></marker>
      <table>
        <row>
          <cell align="left" valign="middle">emp_no</cell>
          <cell align="left" valign="middle">name</cell>
          <cell align="left" valign="middle">salary</cell>
          <cell align="left" valign="middle">sex</cell>
          <cell align="left" valign="middle">phone</cell>
          <cell align="left" valign="middle">room_no</cell>
        </row>
        <row>
          <cell align="left" valign="middle">104732</cell>
          <cell align="left" valign="middle">klacke</cell>
          <cell align="left" valign="middle">7</cell>
          <cell align="left" valign="middle">male</cell>
          <cell align="left" valign="middle">98108</cell>
          <cell align="left" valign="middle">{221, 015}</cell>
        </row>
        <tcaption>employee Database Record</tcaption>
      </table>
      <p>This <c>employee</c> record has the Erlang record/tuple
        representation
        <c>{employee, 104732, klacke, 7, male, 98108, {221, 015}}</c>.</p>
      <marker id="table2_2"></marker>
      <table>
        <row>
          <cell align="left" valign="middle">emp</cell>
          <cell align="left" valign="middle">dept_name</cell>
        </row>
        <row>
          <cell align="left" valign="middle">klacke</cell>
          <cell align="left" valign="middle">B/SFR</cell>
        </row>
        <tcaption>at_dep Database Record</tcaption>
      </table>
      <p>This <c>at_dep</c> record has the Erlang tuple representation
        <c>{at_dep, klacke, 'B/SFR'}</c>.</p>
      <marker id="table3_3"></marker>
      <table>
        <row>
          <cell align="left" valign="middle">emp</cell>
          <cell align="left" valign="middle">proj_name</cell>
        </row>
        <row>
          <cell align="left" valign="middle">klacke</cell>
          <cell align="left" valign="middle">Erlang</cell>
        </row>
        <row>
          <cell align="left" valign="middle">klacke</cell>
          <cell align="left" valign="middle">otp</cell>
        </row>
        <row>
          <cell align="left" valign="middle">klacke</cell>
          <cell align="left" valign="middle">mnesia</cell>
        </row>
        <tcaption>in_proj Database Record</tcaption>
      </table>
      <p>This <c>in_proj</c> record has the Erlang tuple representation
        <c>{in_proj, klacke, 'Erlang', klacke, 'otp', klacke,
        'mnesia'}</c>.</p>
      <p>There is no difference between rows in a table and <c>Mnesia</c>
        records. Both concepts are the same and are used
        interchangeably throughout this User's Guide.</p>
      <p>A <c>Mnesia</c> table is populated by <c>Mnesia</c> records. For
        example, the tuple <c>{boss, klacke, bjarne}</c> is a record. The
        second element in this tuple is the key. To identify a table
        uniquely, both the key and the table name is needed.
        The term Object Identifier (OID) is
        sometimes used for the arity two tuple {Tab, Key}. The OID for
        the record <c>{boss, klacke, bjarne}</c> is the arity two
        tuple <c>{boss, klacke}</c>. The first element of the tuple is
        the type of the record and the second element is the key. An
        OID can lead to zero, one, or more records depending on
        whether the table type is <c>set</c> or <c>bag</c>.</p>
      <p>The record <c>{boss, klacke, bjarne}</c> can also be inserted.
        This record contains an implicit reference to
        another employee that does not yet exist in the
        database. <c>Mnesia</c> does not enforce this.</p>
    </section>

    <section>
      <title>Adding Records and Relationships to Database</title>
      <p>After adding more records to the <c>Company</c> database, the
        result can be the following records:</p>
      <p><c>employees</c>:</p>
      <code type="none">
        {employee, 104465, "Johnson Torbjorn",   1, male,  99184, {242,038}}.
        {employee, 107912, "Carlsson Tuula",     2, female,94556, {242,056}}.
        {employee, 114872, "Dacker Bjarne",      3, male,  99415, {221,035}}.
        {employee, 104531, "Nilsson Hans",       3, male,  99495, {222,026}}.
        {employee, 104659, "Tornkvist Torbjorn", 2, male,  99514, {222,022}}.
        {employee, 104732, "Wikstrom Claes",     2, male,  99586, {221,015}}.
        {employee, 117716, "Fedoriw Anna",       1, female,99143, {221,031}}.
        {employee, 115018, "Mattsson Hakan",     3, male,  99251, {203,348}}.</code>
      <p><c>dept</c>:</p>
      <code type="none">
        {dept, 'B/SF',  "Open Telecom Platform"}.
        {dept, 'B/SFP', "OTP - Product Development"}.
        {dept, 'B/SFR', "Computer Science Laboratory"}.</code>
      <p><c>projects</c>:</p>
      <code type="none">
        %% projects
        {project, erlang, 1}.
        {project, otp, 2}.
        {project, beam, 3}.
        {project, mnesia, 5}.
        {project, wolf, 6}.
        {project, documentation, 7}.
        {project, www, 8}.</code>
      <p>These three tables, <c>employees</c>, <c>dept</c>, and
        <c>projects</c>, are
        made up of real records. The following database content is
        stored in the tables and is built on
        relationships. These tables are <c>manager</c>,
        <c>at_dep</c>, and <c>in_proj</c>.</p>
      <p><c>manager</c>:</p>
      <code type="none">
        {manager, 104465, 'B/SF'}.
        {manager, 104465, 'B/SFP'}.
        {manager, 114872, 'B/SFR'}.</code>
      <p><c>at_dep</c>:</p>
      <code type="none">
        {at_dep, 104465, 'B/SF'}.
        {at_dep, 107912, 'B/SF'}.
        {at_dep, 114872, 'B/SFR'}.
        {at_dep, 104531, 'B/SFR'}.
        {at_dep, 104659, 'B/SFR'}.
        {at_dep, 104732, 'B/SFR'}.
        {at_dep, 117716, 'B/SFP'}.
        {at_dep, 115018, 'B/SFP'}.</code>
      <p><c>in_proj</c>:</p>
      <code type="none">
        {in_proj, 104465, otp}.
        {in_proj, 107912, otp}.
        {in_proj, 114872, otp}.
        {in_proj, 104531, otp}.
        {in_proj, 104531, mnesia}.
        {in_proj, 104545, wolf}.
        {in_proj, 104659, otp}.
        {in_proj, 104659, wolf}.
        {in_proj, 104732, otp}.
        {in_proj, 104732, mnesia}.
        {in_proj, 104732, erlang}.
        {in_proj, 117716, otp}.
        {in_proj, 117716, documentation}.
        {in_proj, 115018, otp}.
        {in_proj, 115018, mnesia}.</code>
      <p>The room number is an attribute of the employee
        record. This is a structured attribute that consists of a
        tuple. The first element of the tuple identifies a corridor,
        and the second element identifies the room in that
        corridor. An alternative is to represent this as a record
        <c>-record(room, {corr, no}).</c> instead of an anonymous
        tuple representation.</p>
      <p>The <c>Company</c> database is now initialized and contains
        data.</p>
    </section>

    <section>
      <title>Writing Queries</title>
      <p>Retrieving data from DBMS is usually to be done with the
        functions
        <seealso marker="mnesia#read/3">mnesia:read/3</seealso> or
        <seealso marker="mnesia#read/2">mnesia:read/1</seealso>.
        The following function raises the salary:</p>
      <codeinclude file="company.erl" tag="%5" type="erl"></codeinclude>
      <p>Since it is desired to update the record using the function
        <seealso marker="mnesia#write/1">mnesia:write/1</seealso>
        after the salary has been increased, a write
        lock (third argument to <c>read</c>) is acquired when the record from
        the table is read.</p>
      <p>To read the values from the table directly is not always possible.
        It can be needed to search one or more tables to get the
        wanted data, and this is done by writing database queries. Queries
        are always more expensive operations than direct lookups done with
        <c>mnesia:read</c>. Therefore, avoid queries in
        performance-critical code.</p>
      <p>Two methods are available for writing database queries:</p>
      <list type="bulleted">
        <item><c>Mnesia</c> functions</item>
        <item>QLC</item>
      </list>

      <section>
        <title>Using Mnesia Functions</title>
        <p>The following function extracts the names of the female employees
          stored in the database:</p>
        <pre>
mnesia:select(employee, [{#employee{sex = female, name = '$1', _ = '_'},[], ['$1']}]).</pre>
        <p><c>select</c> must always run within an activity, such as a
          transaction. The following function can be constructed to call
          from the shell:</p>
        <codeinclude file="company.erl" tag="%20" type="erl"></codeinclude>
        <p>The <c>select</c> expression matches all entries in table
          employee with the field <c>sex</c> set to <c>female</c>.</p>
        <p>This function can be called from the shell as follows:</p>
        <pre>
          (klacke@gin)1> <input>company:all_females().</input>
          {atomic,  ["Carlsson Tuula", "Fedoriw Anna"]}</pre>
        <p>For a description of <c>select</c> and its syntax, see
          <seealso marker="Mnesia_chap4#matching">Pattern Matching</seealso>.
	</p>
      </section>

      <section>
        <title>Using QLC </title>
        <p>This section contains simple introductory examples only. For
          a full description of the QLC query language, see the
          <seealso marker="stdlib:qlc">qlc</seealso> manual page in
          <c>STDLIB</c>.</p>
        <p>Using QLC can be more expensive than using <c>Mnesia</c>
          functions directly but offers a nice syntax.</p>
        <p>The following function extracts a list of female employees
          from the database:</p>
        <pre>
          Q = qlc:q([E#employee.name || E <![CDATA[<-]]> mnesia:table(employee),
                                E#employee.sex == female]),
          qlc:e(Q),</pre>
        <p>Accessing <c>Mnesia</c> tables from a QLC list comprehension must
          always be done within a transaction. Consider the following
          function:</p>
        <codeinclude file="company.erl" tag="%2" type="erl"></codeinclude>
        <p>This function can be called from the shell as follows:</p>
        <pre>
          (klacke@gin)1> <input>company:females().</input>
          {atomic, ["Carlsson Tuula", "Fedoriw Anna"]}</pre>
        <p>In traditional relational database terminology, this
          operation is called a selection, followed by a projection.</p>
        <p>The previous list comprehension expression contains a
          number of syntactical elements:</p>
        <list type="bulleted">
          <item>The first <c>[</c> bracket is read as "build the
           list".
          </item>
          <item>The <c>||</c> "such that" and the arrow <c><![CDATA[<-]]></c>
           is read as "taken from".
          </item>
        </list>
        <p>Hence, the previous list comprehension demonstrates the
          formation of the list <c>E#employee.name</c> such that <c>E</c> is
          taken from the table of employees, and attribute <c>sex</c>
          of each record is equal to the atom <c>female</c>.</p>
        <p>The whole list comprehension must be given to the function
          <c>qlc:q/1</c>.</p>
        <p>List comprehensions with low-level <c>Mnesia</c> functions
          can be combined in the same transaction. To raise the
          salary of all female employees, execute the following:</p>
        <codeinclude file="company.erl" tag="%4" type="erl"></codeinclude>
        <p>The function <c>raise_females/1</c> returns the tuple
          <c>{atomic, Number}</c>, where <c>Number</c> is the number of
          female employees who received a salary increase. If an error
          occurs, the value <c>{aborted, Reason}</c> is returned, and
          <c>Mnesia</c> guarantees that the salary is not
          raised for any employee.</p>
        <p><em>Example:</em></p>
        <pre>
          33><input>company:raise_females(33).</input>
          {atomic,2}</pre>
      </section>
    </section>
  </section>
</chapter>