View Source Build a Mnesia Database
This section describes the basic steps when designing a Mnesia
database and
the programming constructs that make different solutions available to the
programmer. The following topics are included:
- Define a schema
- Data model
- Start
Mnesia
- Create tables
Define a Schema
The configuration of a Mnesia
system is described in a schema. The schema is a
special table that includes information such as the table names and the storage
type of each table (that is, whether a table is to be stored in RAM, on disc, or
on both, as well as its location).
Unlike data tables, information in schema tables can only be accessed and modified by using the schema-related functions described in this section.
Mnesia
has various functions for defining the database schema. Tables can be
moved or deleted, and the table layout can be reconfigured.
An important aspect of these functions is that the system can access a table while it is being reconfigured. For example, it is possible to move a table and simultaneously perform write operations to the same table. This feature is essential for applications that require continuous service.
This section describes the functions available for schema management, all which return either of the following tuples:
{atomic, ok}
if successful{aborted, Reason}
if unsuccessful
Schema Functions
The schema functions are as follows:
mnesia:create_schema(NodeList)
initializes a new, empty schema. This is a mandatory requirement beforeMnesia
can be started.Mnesia
is a truly distributed DBMS and the schema is a system table that is replicated on all nodes in aMnesia
system. This function fails if a schema is already present on any of the nodes inNodeList
. The function requiresMnesia
to be stopped on the alldb_nodes
contained in parameterNodeList
. Applications call this function only once, as it is usually a one-time activity to initialize a new database.mnesia:delete_schema(DiscNodeList)
erases any old schemas on the nodes inDiscNodeList
. It also removes all old tables together with all data. This function requiresMnesia
to be stopped on alldb_nodes
.mnesia:delete_table(Tab)
permanently deletes all replicas of tableTab
.mnesia:clear_table(Tab)
permanently deletes all entries in tableTab
.mnesia:move_table_copy(Tab, From, To)
moves the copy of tableTab
from nodeFrom
to nodeTo
. The table storage type{type}
is preserved, so if a RAM table is moved from one node to another, it remains a RAM table on the new node. Other transactions can still perform read and write operation to the table while it is being moved.mnesia:add_table_copy(Tab, Node, Type)
creates a replica of tableTab
at nodeNode
. ArgumentType
must be either of the atomsram_copies
,disc_copies
, ordisc_only_copies
. If you add a copy of the system tableschema
to a node, you want theMnesia
schema to reside there as well. This action extends the set of nodes that comprise this particularMnesia
system.mnesia:del_table_copy(Tab, Node)
deletes the replica of tableTab
at nodeNode
. When the last replica of a table is removed, the table is deleted.mnesia:transform_table(Tab, Fun, NewAttributeList, NewRecordName)
changes the format on all records in tableTab
. It applies argumentFun
to all records in the table.Fun
must be a function that takes a record of the old type, and returns the record of the new type. The table key must not be changed.Example:
-record(old, {key, val}). -record(new, {key, val, extra}). Transformer = fun(X) when record(X, old) -> #new{key = X#old.key, val = X#old.val, extra = 42} end, {atomic, ok} = mnesia:transform_table(foo, Transformer, record_info(fields, new), new),
Argument
Fun
can also be the atomignore
, which indicates that only the metadata about the table is updated. Use ofignore
is not recommended (as it creates inconsistencies between the metadata and the actual data) but it is included as a possibility for the user do to an own (offline) transform.mnesia:change_table_copy_type(Tab, Node, ToType)
changes the storage type of a table. For example, a RAM table is changed to adisc_table
at the node specified asNode
.
Data Model
The data model employed by Mnesia
is an extended relational data model. Data
is organized as a set of tables and relations between different data records can
be modeled as more tables describing the relationships. Each table contains
instances of Erlang records. The records are represented as Erlang tuples.
Each Object Identifier (OID) is made up of a table name and a key. For example,
if an employee record is represented by the tuple
{employee, 104732, klacke, 7, male, 98108, {221, 015}}
, this record has an
OID, which is the tuple {employee, 104732}
.
Thus, each table is made up of records, where the first element is a record name
and the second element of the table is a key, which identifies the particular
record in that table. The combination of the table name and a key is an arity
two tuple {Tab, Key}
called the OID. For more information about the
relationship between the record name and the table name, see
Record Names versus Table Names.
What makes the Mnesia
data model an extended relational model is the ability
to store arbitrary Erlang terms in the attribute fields. One attribute value
can, for example, be a whole tree of OIDs leading to other terms in other
tables. This type of record is difficult to model in traditional relational
DBMSs.
Start Mnesia
Before starting Mnesia
, the following must be done:
- An empty schema must be initialized on all the participating nodes.
- The Erlang system must be started.
- Nodes with disc database schema must be defined and implemented with the
function
mnesia:create_schema(NodeList)
.
When running a distributed system with two or more participating nodes, the
function mnesia:start()
must be executed on each
participating node. This would typically be part of the boot script in an
embedded environment. In a test environment or an interactive environment,
mnesia:start()
can also be used either from the Erlang shell or another
program.
Initialize a Schema and Start Mnesia
Let us use the example database Company
, described in
Getting Started to illustrate how to run a
database on two separate nodes, called a@gin
and b@skeppet
. Each of these
nodes must have a Mnesia
directory and an initialized schema before Mnesia
can be started. There are two ways to specify the Mnesia
directory to be used:
Specify the
Mnesia
directory by providing an application parameter either when starting the Erlang shell or in the application script. Previously, the following example was used to create the directory for theCompany
database:% erl -mnesia dir '"/ldisc/scratch/Mnesia.Company"'
If no command-line flag is entered, the
Mnesia
directory becomes the current working directory on the node where the Erlang shell is started.
To start the Company
database and get it running on the two specified nodes,
enter the following commands:
- On the node
a@gin
:
gin % erl -sname a -mnesia dir '"/ldisc/scratch/Mnesia.company"'
- On the node
b@skeppet
:
skeppet % erl -sname b -mnesia dir '"/ldisc/scratch/Mnesia.company"'
- On one of the two nodes:
(a@gin)1> mnesia:create_schema([a@gin, b@skeppet]).
- The function
mnesia:start()
is called on both nodes. - To initialize the database, execute the following code on one of the two nodes:
dist_init() ->
mnesia:create_table(employee,
[{ram_copies, [a@gin, b@skeppet]},
{attributes, record_info(fields,
employee)}]),
mnesia:create_table(dept,
[{ram_copies, [a@gin, b@skeppet]},
{attributes, record_info(fields, dept)}]),
mnesia:create_table(project,
[{ram_copies, [a@gin, b@skeppet]},
{attributes, record_info(fields, project)}]),
mnesia:create_table(manager, [{type, bag},
{ram_copies, [a@gin, b@skeppet]},
{attributes, record_info(fields,
manager)}]),
mnesia:create_table(at_dep,
[{ram_copies, [a@gin, b@skeppet]},
{attributes, record_info(fields, at_dep)}]),
mnesia:create_table(in_proj,
[{type, bag},
{ram_copies, [a@gin, b@skeppet]},
{attributes, record_info(fields, in_proj)}]).
As illustrated, the two directories reside on different nodes, because
/ldisc/scratch
(the "local" disc) exists on the two different nodes.
By executing these commands, two Erlang nodes are configured to run the
Company
database, and therefore, initialize the database. This is required
only once when setting up. The next time the system is started,
mnesia:start()
is called on both nodes, to initialize the
system from disc.
In a system of Mnesia
nodes, every node is aware of the current location of
all tables. In this example, data is replicated on both nodes and functions that
manipulate the data in the tables can be executed on either of the two nodes.
Code that manipulate Mnesia
data behaves identically regardless of where the
data resides.
The function mnesia:stop()
stops Mnesia
on the node where
the function is executed. The functions mnesia:start/0
and mnesia:stop/0
work on the "local" Mnesia
system. No functions start or stop a set of nodes.
Startup Procedure
Start Mnesia
by calling the following function:
mnesia:start().
This function initiates the DBMS locally.
The choice of configuration alters the location and load order of the tables. The alternatives are as follows:
- Tables that are only stored locally are initialized from the local
Mnesia
directory. - Replicated tables that reside locally as well as somewhere else are either
initiated from disc or by copying the entire table from the other node,
depending on which of the different replicas are the most recent.
Mnesia
determines which of the tables are the most recent. - Tables that reside on remote nodes are available to other nodes as soon as they are loaded.
Table initialization is asynchronous. The function call
mnesia:start()
returns the atom ok
and then starts to
initialize the different tables. Depending on the size of the database, this can
take some time, and the application programmer must wait for the tables that the
application needs before they can be used. This is achieved by using the
function mnesia:wait_for_tables(TabList, Timeout)
,
which suspends the caller until all tables specified in TabList
are properly
initiated.
A problem can arise if a replicated table on one node is initiated, but Mnesia
deduces that another (remote) replica is more recent than the replica existing
on the local node, and the initialization procedure does not proceed. In this
situation, a call to mnesia:wait_for_tables/2
, suspends the caller until the
remote node has initialized the table from its local disc and the node has
copied the table over the network to the local node.
However, this procedure can be time-consuming, the shortcut function
mnesia:force_load_table(Tab)
loads all the tables
from disc at a faster rate. The function forces tables to be loaded from disc
regardless of the network situation.
Thus, it can be assumed that if an application wants to use tables a
and b
,
the application must perform some action similar to following before it can use
the tables:
case mnesia:wait_for_tables([a, b], 20000) of
{timeout, RemainingTabs} ->
panic(RemainingTabs);
ok ->
synced
end.
Warning
When tables are forcefully loaded from the local disc, all operations that were performed on the replicated table while the local node was down, and the remote replica was alive, are lost. This can cause the database to become inconsistent.
If the startup procedure fails, the function mnesia:start()
returns the cryptic tuple
{error,{shutdown, {mnesia_sup,start_link,[normal,[]]}}}
. To get more
information about the start failure, use command-line arguments
-boot start_sasl
as argument to the erl
script.
Create Tables
The function mnesia:create_table(Name, ArgList)
creates tables. When executing this function, it returns one of the following
responses:
{atomic, ok}
if the function executes successfully{aborted, Reason}
if the function fails
The function arguments are as follows:
Name
is the name of the table. It is usually the same name as the name of the records that constitute the table. For details, seerecord_name
.ArgList
is a list of{Key,Value}
tuples. The following arguments are valid:{type, Type}
, whereType
must be either of the atomsset
,ordered_set
, orbag
. Default isset
.Notice that currently
ordered_set
is not supported fordisc_only_copies
tables.A table of type
set
orordered_set
has either zero or one record per key, whereas a table of typebag
can have an arbitrary number of records per key. The key for each record is always the first attribute of the record.The following example illustrates the difference between type
set
andbag
:f() -> F = fun() -> mnesia:write({foo, 1, 2}), mnesia:write({foo, 1, 3}), mnesia:read({foo, 1}) end, mnesia:transaction(F).
This transaction returns the list
[{foo,1,3}]
if tablefoo
is of typeset
. However, the list[{foo,1,2}, {foo,1,3}]
is returned if the table is of typebag
.Mnesia
tables can never contain duplicates of the same record in the same table. Duplicate records have attributes with the same contents and key.{disc_copies, NodeList}
, whereNodeList
is a list of the nodes where this table is to reside on disc.Write operations to a table replica of type
disc_copies
write data to the disc copy and to the RAM copy of the table.It is possible to have a replicated table of type
disc_copies
on one node, and the same table stored as a different type on another node. Default is[]
. This arrangement is desirable if the following operational characteristics are required:- Read operations must be fast and performed in RAM.
- All write operations must be written to persistent storage.
A write operation on a
disc_copies
table replica is performed in two steps. First the write operation is appended to a log file, then the actual operation is performed in RAM.{ram_copies, NodeList}
, whereNodeList
is a list of the nodes where this table is stored in RAM. Default is[node()]
. If the default value is used to create a table, it is located on the local node only.Table replicas of type
ram_copies
can be dumped to disc with the functionmnesia:dump_tables(TabList)
.{disc_only_copies, NodeList}
. These table replicas are stored on disc only and are therefore slower to access. However, a disc-only replica consumes less memory than a table replica of the other two storage types.{index, AttributeNameList}
, whereAttributeNameList
is a list of atoms specifying the names of the attributesMnesia
is to build and maintain. An index table exists for every element in the list. The first field of aMnesia
record is the key and thus need no extra index.The first field of a record is the second element of the tuple, which is the representation of the record.
{snmp, SnmpStruct}
.SnmpStruct
is described in the SNMP User's Guide. Basically, if this attribute is present inArgList
ofmnesia:create_table/2
, the table is immediately accessible the SNMP.It is easy to design applications that use SNMP to manipulate and control the system.
Mnesia
provides a direct mapping between the logical tables that make up an SNMP control application and the physical data that makes up aMnesia
table. The default value is[]
.{local_content, true}
. When an application needs a table whose contents is to be locally unique on each node,local_content
tables can be used. The name of the table is known to allMnesia
nodes, but its contents is unique for each node. Access to this type of table must be done locally.{attributes, AtomList}
is a list of the attribute names for the records that are supposed to populate the table. Default is the list[key, val]
. The table must at least have one extra attribute besides the key. When accessing single attributes in a record, it is not recommended to hard code the attribute names as atoms. Use the constructrecord_info(fields, record_name)
instead.The expression
record_info(fields, record_name)
is processed by the Erlang preprocessor and returns a list of the record field names. With the record definition-record(foo, {x,y,z}).
, the expressionrecord_info(fields,foo)
is expanded to the list[x,y,z]
. It is therefore possible for you to provide the attribute names or to use therecord_info/2
notation.It is recommended to use the
record_info/2
notation, as it becomes easier to maintain the program and the program becomes more robust with regards to future record changes.{record_name, Atom}
specifies the common name of all records stored in the table. All records stored in the table must have this name as their first element.record_name
defaults to the name of the table. For more information, see Record Names versus Table Names.
As an example, consider the following record definition:
-record(funky, {x, y}).
The following call would create a table that is replicated on two nodes, has an
extra index on attribute y
, and is of type bag
.
mnesia:create_table(funky, [{disc_copies, [N1, N2]}, {index, [y]},
{type, bag}, {attributes, record_info(fields, funky)}]).
Whereas a call to the following default code values would return a table with a
RAM copy on the local node, no extra indexes, and the attributes defaulted to
the list [key,val]
.
mnesia:create_table(stuff, [])