View Source Mnesia System Information
The following topics are included:
- Database configuration data
- Core dumps
- Dumping tables
- Checkpoints
- Startup files, log file, and data files
- Loading tables at startup
- Recovery from communication failure
- Recovery of transactions
- Backup, restore, fallback, and disaster recovery
Database Configuration Data
The following two functions can be used to retrieve system information. For details, see the Reference Manual.
mnesia:table_info(Tab, Key) -> Info | exit({aborted,Reason}) returns information about one table, for example, the current size of the table and on which nodes it resides.
mnesia:system_info(Key) -> Info | exit({aborted, Reason}) returns information about the
Mnesia
system, for example, transaction statistics,db_nodes
, and configuration parameters.
Core Dumps
If Mnesia
malfunctions, system information is dumped to file
MnesiaCore.Node.When
. The type of system information contained in this file
can also be generated with the function mnesia_lib:coredump()
. If a Mnesia
system behaves strangely, it is recommended that a Mnesia
core dump file is
included in the bug report.
Dumping Tables
Tables of type ram_copies
are by definition stored in memory only. However,
these tables can be dumped to disc, either at regular intervals or before the
system is shut down. The function
mnesia:dump_tables(TabList) dumps all replicas of a
set of RAM tables to disc. The tables can be accessed while being dumped to
disc. To dump the tables to disc, all replicas must have the storage type
ram_copies
.
The table content is placed in a .DCD
file on the disc. When the Mnesia
system is started, the RAM table is initially loaded with data from its .DCD
file.
Checkpoints
A checkpoint is a transaction consistent state that spans over one or more
tables. When a checkpoint is activated, the system remembers the current content
of the set of tables. The checkpoint retains a transaction consistent state of
the tables, allowing the tables to be read and updated while the checkpoint is
active. A checkpoint is typically used to back up tables to external media, but
they are also used internally in Mnesia
for other purposes. Each checkpoint is
independent and a table can be involved in several checkpoints simultaneously.
Each table retains its old contents in a checkpoint retainer. For performance critical applications, it can be important to realize the processing overhead associated with checkpoints. In a worst case scenario, the checkpoint retainer consumes more memory than the table itself. Also, each update becomes slightly slower on those nodes where checkpoint retainers are attached to the tables.
For each table, it is possible to choose if there is to be one checkpoint retainer attached to all replicas of the table, or if it is enough to have only one checkpoint retainer attached to a single replica. With a single checkpoint retainer per table, the checkpoint consumes less memory, but it is vulnerable to node crashes. With several redundant checkpoint retainers, the checkpoint survives as long as there is at least one active checkpoint retainer attached to each table.
Checkpoints can be explicitly deactivated with the function
mnesia:deactivate_checkpoint(Name), where
Name
is the name of an active checkpoint. This function returns ok
if
successful or {error, Reason}
if there is an error. All tables in a checkpoint
must be attached to at least one checkpoint retainer. The checkpoint is
automatically deactivated by Mnesia
, when any table lacks a checkpoint
retainer. This can occur when a node goes down or when a replica is deleted. Use
arguments min
and max
(described in the following list) to control the
degree of checkpoint retainer redundancy.
Checkpoints are activated with the function
mnesia:activate_checkpoint(Args), where Args
is a list of the following tuples:
{name,Name}
, whereName
specifies a temporary name of the checkpoint. The name can be reused when the checkpoint has been deactivated. If no name is specified, a name is generated automatically.{max,MaxTabs}
, whereMaxTabs
is a list of tables that are to be included in the checkpoint. Default is[]
(empty list). For these tables, the redundancy is maximized. The old content of the table is retained in the checkpoint retainer when the main table is updated by the applications. The checkpoint is more fault tolerant if the tables have several replicas. When new replicas are added by the schema manipulation functionmnesia:add_table_copy/3
it also attaches a local checkpoint retainer.{min,MinTabs}
, whereMinTabs
is a list of tables that are to be included in the checkpoint. Default is[]
. For these tables, the redundancy is minimized, and there is to be single checkpoint retainer per table, preferably at the local node.{allow_remote,Bool}
, wherefalse
means that all checkpoint retainers must be local. If a table does not reside locally, the checkpoint cannot be activated.true
allows checkpoint retainers to be allocated on any node. Default istrue
.{ram_overrides_dump,Bool}
. This argument only applies to tables of typeram_copies
.Bool
specifies if the table state in RAM is to override the table state on disc.true
means that the latest committed records in RAM are included in the checkpoint retainer. These are the records that the application accesses.false
means that the records on the disc.DAT
file are included in the checkpoint retainer. These records are loaded on startup. Default isfalse
.
The function mnesia:activate_checkpoint(Args) returns one of the following values:
{ok, Name, Nodes}
{error, Reason}
Name
is the checkpoint name. Nodes
are the nodes where the checkpoint is
known.
A list of active checkpoints can be obtained with the following functions:
- mnesia:system_info(checkpoints) returns all active checkpoints on the current node.
- mnesia:table_info(Tab, checkpoints) returns active checkpoints on a specific table.
Startup Files, Log File, and Data Files
This section describes the internal files that are created and maintained by the
Mnesia
system. In particular, the workings of the Mnesia
log are described.
Startup Files
Start Mnesia states the following prerequisites
for starting Mnesia
:
- An Erlang session must be started and a
Mnesia
directory must be specified for the database. - A database schema must be initiated, using the function
mnesia:create_schema/1
.
The following example shows how these tasks are performed:
Step 1: Start an Erlang session and specify a Mnesia
directory for the
database:
% erl -sname klacke -mnesia dir '"/ldisc/scratch/klacke"'
Erlang (BEAM) emulator version 4.9
Eshell V4.9 (abort with ^G)
(klacke@gin)1> mnesia:create_schema([node()]).
ok
(klacke@gin)2>
^Z
Suspended
Step 2: You can inspect the Mnesia
directory to see what files have been
created:
% ls -l /ldisc/scratch/klacke
-rw-rw-r-- 1 klacke staff 247 Aug 12 15:06 FALLBACK.BUP
The response shows that the file FALLBACK.BUP
has been created. This is called
a backup file, and it contains an initial schema. If more than one node in the
function mnesia:create_schema/1
had been specified, identical backup files
would have been created on all nodes.
Step 3: Start Mnesia
:
(klacke@gin)3>mnesia:start( ).
ok
Step 4: You can see the following listing in the Mnesia
directory:
-rw-rw-r-- 1 klacke staff 86 May 26 19:03 LATEST.LOG
-rw-rw-r-- 1 klacke staff 34507 May 26 19:03 schema.DAT
The schema in the backup file FALLBACK.BUP
has been used to generate the file
schema.DAT
. Since there are no other disc resident tables than the schema, no
other data files were created. The file FALLBACK.BUP
was removed after the
successful "restoration". You also see some files that are for internal use by
Mnesia
.
Step 5: Create a table:
(klacke@gin)4> mnesia:create_table(foo,[{disc_copies, [node()]}]).
{atomic,ok}
Step 6: You can see the following listing in the Mnesia
directory:
% ls -l /ldisc/scratch/klacke
-rw-rw-r-- 1 klacke staff 86 May 26 19:07 LATEST.LOG
-rw-rw-r-- 1 klacke staff 94 May 26 19:07 foo.DCD
-rw-rw-r-- 1 klacke staff 6679 May 26 19:07 schema.DAT
The file foo.DCD
has been created. This file will eventually store all data
that is written into the foo
table.
Log File
When starting Mnesia
, a .LOG
file called LATEST.LOG
is created and placed
in the database directory. This file is used by Mnesia
to log disc-based
transactions. This includes all transactions that write at least one record in a
table that is of storage type disc_copies
or disc_only_copies
. The file also
includes all operations that manipulate the schema itself, such as creating new
tables. The log format can vary with different implementations of Mnesia
. The
Mnesia
log is currently implemented in the standard library module
disk_log
in Kernel
.
The log file grows continuously and must be dumped at regular intervals.
"Dumping the log file" means that Mnesia
performs all the operations listed in
the log and place the records in the corresponding .DAT
, .DCD
, and .DCL
data files. For example, if the operation "write record {foo, 4, elvis, 6}
" is
listed in the log, Mnesia
inserts the operation into the file foo.DCL
.
Later, when Mnesia
thinks that the .DCL
file is too large, the data is moved
to the .DCD
file. The dumping operation can be time consuming if the log is
large. Notice that the Mnesia
system continues to operate during log dumps.
By default Mnesia
either dumps the log whenever 1000 records have been written
in the log or when three minutes have passed. This is controlled by the two
application parameters -mnesia dump_log_write_threshold WriteOperations
and
-mnesia dump_log_time_threshold MilliSecs
.
Before the log is dumped, the file LATEST.LOG
is renamed to PREVIOUS.LOG
,
and a new LATEST.LOG
file is created. Once the log has been successfully
dumped, the file PREVIOUS.LOG
is deleted.
The log is also dumped at startup and whenever a schema operation is performed.
Data Files
The directory listing also contains one .DAT
file, which contains the schema
itself, contained in the schema.DAT
file. The DAT
files are indexed files,
and it is efficient to insert and search for records in these files with a
specific key. The .DAT
files are used for the schema and for
disc_only_copies
tables. The Mnesia
data files are currently implemented in
the standard library module dets
in STDLIB
.
All operations that can be performed on dets
files can also be performed on
the Mnesia
data files. For example, dets
contains the function
dets:traverse/2
, which can be used to view the contents of a Mnesia
DAT
file. However, this can only be done when Mnesia
is not running. So, to view
the schema file, do as follows;
{ok, N} = dets:open_file(schema, [{file, "./schema.DAT"},{repair,false},
{keypos, 2}]),
F = fun(X) -> io:format("~p~n", [X]), continue end,
dets:traverse(N, F),
dets:close(N).
Warning
The
DAT
files must always be opened with option{repair, false}
. This ensures that these files are not automatically repaired. Without this option, the database can become inconsistent, becauseMnesia
can believe that the files were properly closed. For information about configuration parameterauto_repair
, see the Reference Manual.
Warning
It is recommended that the data files are not tampered with while
Mnesia
is running. While not prohibited, the behavior ofMnesia
is unpredictable.
The disc_copies
tables are stored on disk with .DCL
and .DCD
files, which
are standard disk_log
files.
Loading Tables at Startup
At startup, Mnesia
loads tables to make them accessible for its applications.
Sometimes Mnesia
decides to load all tables that reside locally, and sometimes
the tables are not accessible until Mnesia
brings a copy of the table from
another node.
To understand the behavior of Mnesia
at startup, it is essential to understand
how Mnesia
reacts when it loses contact with Mnesia
on another node. At this
stage, Mnesia
cannot distinguish between a communication failure and a
"normal" node-down. When this occurs, Mnesia
assumes that the other node is no
longer running, whereas, in reality, the communication between the nodes has
failed.
To overcome this situation, try to restart the ongoing transactions that are
accessing tables on the failing node, and write a mnesia_down
entry to a log
file.
At startup, notice that all tables residing on nodes without a mnesia_down
entry can have fresher replicas. Their replicas can have been updated after the
termination of Mnesia
on the current node. To catch up with the latest
updates, transfer a copy of the table from one of these other "fresh" nodes. If
you are unlucky, other nodes can be down and you must wait for the table to be
loaded on one of these nodes before receiving a fresh copy of the table.
Before an application makes its first access to a table,
mnesia:wait_for_tables(TabList, Timeout) is to be
executed to ensure that the table is accessible from the local node. If the
function times out, the application can choose to force a load of the local
replica with mnesia:force_load_table(Tab) and
deliberately lose all updates that can have been performed on the other nodes
while the local node was down. If Mnesia
has loaded the table on another node
already, or intends to do so, copy the table from that node to avoid unnecessary
inconsistency.
Warning
Only one table is loaded by mnesia:force_load_table(Tab). Since committed transactions can have caused updates in several tables, the tables can become inconsistent because of the forced load.
The allowed AccessMode
of a table can be defined to be read_only
or
read_write
. It can be toggled with the function
mnesia:change_table_access_mode(Tab, AccessMode)
in runtime. read_only
tables and local_content
tables are always loaded
locally, as there is no need for copying the table from other nodes. Other
tables are primarily loaded remotely from active replicas on other nodes if the
table has been loaded there already, or if the running Mnesia
has decided to
load the table there already.
At startup, Mnesia
assumes that its local replica is the most recent version
and loads the table from disc if either of the following situations is detected:
mnesia_down
is returned from all other nodes that hold a disc resident replica of the table.- All replicas are
ram_copies
.
This is normally a wise decision, but it can be disastrous if the nodes have
been disconnected because of a communication failure, as the Mnesia
normal
table load mechanism does not cope with communication failures.
When Mnesia
loads many tables, the default load order is used. However, the
load order can be affected, by explicitly changing property load_order
for the
tables, with the function
mnesia:change_table_load_order(Tab, LoadOrder).
LoadOrder
is by default 0
for all tables, but it can be set to any integer.
The table with the highest load_order
is loaded first. Changing the load order
is especially useful for applications that need to ensure early availability of
fundamental tables. Large peripheral tables are to have a low load order value,
perhaps less than 0
Recovery from Communication Failure
There are several occasions when Mnesia
can detect that the network has been
partitioned because of a communication failure, for example:
Mnesia
is operational already and the Erlang nodes gain contact again. ThenMnesia
tries to contactMnesia
on the other node to see if it also thinks that the network has been partitioned for a while. IfMnesia
on both nodes has loggedmnesia_down
entries from each other,Mnesia
generates a system event, called{inconsistent_database, running_partitioned_network, Node}
, which is sent to theMnesia
event handler and other possible subscribers. The default event handler reports an error to the error logger.- If
Mnesia
detects at startup that both the local node and another node receivedmnesia_down
from each other,Mnesia
generates an{inconsistent_database, starting_partitioned_network, Node}
system event and acts as described in the previous item.
If the application detects that there has been a communication failure that can have caused an inconsistent database, it can use the function mnesia:set_master_nodes(Tab, Nodes) to pinpoint from which nodes each table can be loaded.
At startup, the Mnesia
normal table load algorithm is bypassed and the table
is loaded from one of the master nodes defined for the table, regardless of
potential mnesia_down
entries in the log. Nodes
can only contain nodes where
the table has a replica. If Nodes
is empty, the master node recovery mechanism
for the particular table is reset and the normal load mechanism is used at the
next restart.
The function mnesia:set_master_nodes(Nodes) sets
master nodes for all tables. For each table it determines its replica nodes and
starts mnesia:set_master_nodes(Tab, TabNodes)
with those replica nodes that are included in the Nodes
list (that is,
TabNodes
is the intersection of Nodes
and the replica nodes of the table).
If the intersection is empty, the master node recovery mechanism for the
particular table is reset and the normal load mechanism is used at the next
restart.
The functions mnesia:system_info(master_node_tables) and mnesia:table_info(Tab, master_nodes) can be used to obtain information about the potential master nodes.
Determining what data to keep after a communication failure is outside the scope
of Mnesia
. One approach is to determine which "island" contains most of the
nodes. Using option {majority,true}
for critical tables can be a way to ensure
that nodes that are not part of a "majority island" cannot update those tables.
Notice that this constitutes a reduction in service on the minority nodes. This
would be a tradeoff in favor of higher consistency guarantees.
The function mnesia:force_load_table(Tab) can be used to force load the table regardless of which table load mechanism that is activated.
Recovery of Transactions
A Mnesia
table can reside on one or more nodes. When a table is updated,
Mnesia
ensures that the updates are replicated to all nodes where the table
resides. If a replica is inaccessible (for example, because of a temporary
node-down), Mnesia
performs the replication later.
On the node where the application is started, there is a transaction coordinator process. If the transaction is distributed, there is also a transaction participant process on all the other nodes where commit-work needs to be performed.
Internally Mnesia
uses several commit protocols. The selected protocol depends
on which table that has been updated in the transaction. If all the involved
tables are symmetrically replicated (that is, they all have the same
ram_nodes
, disc_nodes
, and disc_only_nodes
currently accessible from the
coordinator node), a lightweight transaction commit protocol is used.
The number of messages that the transaction coordinator and its participants
need to exchange is few, as the Mnesia
table load mechanism takes care of the
transaction recovery if the commit protocol gets interrupted. Since all involved
tables are replicated symmetrically, the transaction is automatically recovered
by loading the involved tables from the same node at startup of a failing node.
It does not matter if the transaction was committed or terminated as long as the
ACID properties can be ensured. The lightweight commit protocol is non-blocking,
that is, the surviving participants and their coordinator finish the
transaction, even if any node crashes in the middle of the commit protocol.
If a node goes down in the middle of a dirty operation, the table load mechanism ensures that the update is performed on all replicas, or none. Both asynchronous dirty updates and synchronous dirty updates use the same recovery principle as lightweight transactions.
If a transaction involves updates of asymmetrically replicated tables or updates of the schema table, a heavyweight commit protocol is used. This protocol can finish the transaction regardless of how the tables are replicated. The typical use of a heavyweight transaction is when a replica is to be moved from one node to another. Then ensure that the replica either is entirely moved or left as it was. Do never end up in a situation with replicas on both nodes, or on no node at all. Even if a node crashes in the middle of the commit protocol, the transaction must be guaranteed to be atomic. The heavyweight commit protocol involves more messages between the transaction coordinator and its participants than a lightweight protocol, and it performs recovery work at startup to finish the terminating or commit work.
The heavyweight commit protocol is also non-blocking, which allows the surviving
participants and their coordinator to finish the transaction regardless (even if
a node crashes in the middle of the commit protocol). When a node fails at
startup, Mnesia
determines the outcome of the transaction and recovers it.
Lightweight protocols, heavyweight protocols, and dirty updates, are dependent
on other nodes to be operational to make the correct heavyweight transaction
recovery decision.
If Mnesia
has not started on some of the nodes that are involved in the
transaction and neither the local node nor any of the already running nodes
know the outcome of the transaction, Mnesia
waits for one, by default. In the
worst case scenario, all other involved nodes must start before Mnesia
can
make the correct decision about the transaction and finish its startup.
Thus, Mnesia
(on one node) can hang if a double fault occurs, that is, when
two nodes crash simultaneously and one attempts to start when the other refuses
to start, for example, because of a hardware error.
The maximum time that Mnesia
waits for other nodes to respond with a
transaction recovery decision can be specified. The configuration parameter
max_wait_for_decision
defaults to infinity
, which can cause the indefinite
hanging as mentioned earlier. However, if the parameter is set to a definite
time period (for example, three minutes), Mnesia
then enforces a transaction
recovery decision, if needed, to allow Mnesia
to continue with its startup
procedure.
The downside of an enforced transaction recovery decision is that the decision
can be incorrect, because of insufficient information about the recovery
decisions from the other nodes. This can result in an inconsistent database
where Mnesia
has committed the transaction on some nodes but terminated it on
others.
In fortunate cases, the inconsistency is only visible in tables belonging to a specific application. However, if a schema transaction is inconsistently recovered because of the enforced transaction recovery decision, the effects of the inconsistency can be fatal. However, if the higher priority is availability rather than consistency, it can be worth the risk.
If Mnesia
detects an inconsistent transaction decision, an
{inconsistent_database, bad_decision, Node}
system event is generated to give
the application a chance to install a fallback or other appropriate measures to
resolve the inconsistency. The default behavior of the Mnesia
event handler is
the same as if the database became inconsistent as a result of partitioned
network (as described earlier).
Backup, Restore, Fallback, and Disaster Recovery
The following functions are used to back up data, to install a backup as fallback, and for disaster recovery:
- [mnesia:backup_checkpoint(Name, Opaque, [Mod])](
mnesia:backup_checkpoint/2
) performs a backup of the tables included in the checkpoint. - [mnesia:backup(Opaque, [Mod])](
mnesia:backup/1
) activates a new checkpoint that covers allMnesia
tables and performs a backup. It is performed with maximum degree of redundancy (see also the function mnesia:activate_checkpoint(Args),{max, MaxTabs} and {min, MinTabs})
. - [mnesia:traverse_backup(Source, [SourceMod,] Target, [TargetMod,] Fun,
Acc)](
mnesia:traverse_backup/4
) can be used to read an existing backup, create a backup from an existing one, or to copy a backup from one type media to another. - mnesia:uninstall_fallback() removes previously installed fallback files.
- mnesia:restore(Opaque, Args) restores a set of tables from a previous backup.
- [mnesia:install_fallback(Opaque, [Mod])](
mnesia:install_fallback/1
) can be configured to restartMnesia
and the reload data tables, and possibly the schema tables, from an existing backup. This function is typically used for disaster recovery purposes, when data or schema tables are corrupted.
These functions are explained in the following sections. See also Checkpoints, which describes the two functions used to activate and deactivate checkpoints.
Backup
Backup operation are performed with the following functions:
- [mnesia:backup_checkpoint(Name, Opaque, [Mod])](
mnesia:backup_checkpoint/2
) - [mnesia:backup(Opaque, [Mod])](
mnesia:backup/1
) - [mnesia:traverse_backup(Source, [SourceMod,] Target, [TargetMod,] Fun,
Acc)](
mnesia:traverse_backup/4
)
By default, the actual access to the backup media is performed through module
mnesia_backup
for both read and write. Currently mnesia_backup
is
implemented with the standard library module disc_log
. However, you can write
your own module with the same interface as mnesia_backup
and configure
Mnesia
so that the alternative module performs the actual accesses to the
backup media. The user can therefore put the backup on a media that Mnesia
does not know about, possibly on hosts where Erlang is not running. Use
configuration parameter -mnesia backup_module <module>
for this purpose.
The source for a backup is an activated checkpoint. The backup function
[mnesia:backup_checkpoint(Name, Opaque,[Mod])](mnesia:backup_checkpoint/2
) is
most commonly used and returns ok
or {error,Reason}
. It has the following
arguments:
Name
is the name of an activated checkpoint. For details on how to include table names in checkpoints, see the functionmnesia:activate_checkpoint(ArgList)
in Checkpoints.Opaque
.Mnesia
does not interpret this argument, but it is forwarded to the backup module. TheMnesia
default backup modulemnesia_backup
interprets this argument as a local filename.Mod
is the name of an alternative backup module.
The function [mnesia:backup(Opaque [,Mod])](mnesia:backup/1
) activates a new
checkpoint that covers all Mnesia
tables with maximum degree of redundancy and
performs a backup. Maximum redundancy means that each table replica has a
checkpoint retainer. Tables with property local_contents
are backed up as they
look on the current node.
You can iterate over a backup, either to transform it into a new backup, or only
read it. The function [mnesia:traverse_backup(Source, [SourceMod,] Target,
[TargetMod,] Fun, Acc)](mnesia:traverse_backup/4
), which normally returns
{ok, LastAcc}
, is used for both of these purposes.
Before the traversal starts, the source backup media is opened with
SourceMod:open_read(Source)
, and the target backup media is opened with
TargetMod:open_write(Target)
. The arguments are as follows:
SourceMod
andTargetMod
are module names.Source
andTarget
are opaque data used exclusively by the modulesSourceMod
andTargetMod
for initializing the backup media.Acc
is an initial accumulator value.Fun(BackupItems, Acc)
is applied to each item in the backup. The Fun must return a tuple{ValGoodBackupItems, NewAcc}
, whereValidBackupItems
is a list of valid backup items.NewAcc
is a new accumulator value. TheValidBackupItems
are written to the target backup with the functionTargetMod:write/2
.LastAcc
is the last accumulator value, that is, the lastNewAcc
value that was returned byFun
.
Also, a read-only traversal of the source backup can be performed without
updating a target backup. If TargetMod==read_only
, no target backup is
accessed.
By setting SourceMod
and TargetMod
to different modules, a backup can be
copied from one backup media to another.
Valid BackupItems
are the following tuples:
{schema, Tab}
specifies a table to be deleted.{schema, Tab, CreateList}
specifies a table to be created. For more information aboutCreateList
, seemnesia:create_table/2
.{Tab, Key}
specifies the full identity of a record to be deleted.{Record}
specifies a record to be inserted. It can be a tuple withTab
as first field. Notice that the record name is set to the table name regardless of whatrecord_name
is set to.
The backup data is divided into two sections. The first section contains information related to the schema. All schema-related items are tuples where the first field equals the atom schema. The second section is the record section. Schema records cannot be mixed with other records and all schema records must be located first in the backup.
The schema itself is a table and is possibly included in the backup. Each node
where the schema table resides is regarded as a db_node
.
The following example shows how
mnesia:traverse_backup can be used to rename a
db_node
in a backup file:
change_node_name(Mod, From, To, Source, Target) ->
Switch =
fun(Node) when Node == From -> To;
(Node) when Node == To -> throw({error, already_exists});
(Node) -> Node
end,
Convert =
fun({schema, db_nodes, Nodes}, Acc) ->
{[{schema, db_nodes, lists:map(Switch,Nodes)}], Acc};
({schema, version, Version}, Acc) ->
{[{schema, version, Version}], Acc};
({schema, cookie, Cookie}, Acc) ->
{[{schema, cookie, Cookie}], Acc};
({schema, Tab, CreateList}, Acc) ->
Keys = [ram_copies, disc_copies, disc_only_copies],
OptSwitch =
fun({Key, Val}) ->
case lists:member(Key, Keys) of
true -> {Key, lists:map(Switch, Val)};
false-> {Key, Val}
end
end,
{[{schema, Tab, lists:map(OptSwitch, CreateList)}], Acc};
(Other, Acc) ->
{[Other], Acc}
end,
mnesia:traverse_backup(Source, Mod, Target, Mod, Convert, switched).
view(Source, Mod) ->
View = fun(Item, Acc) ->
io:format("~p.~n",[Item]),
{[Item], Acc + 1}
end,
mnesia:traverse_backup(Source, Mod, dummy, read_only, View, 0).
Restore
Tables can be restored online from a backup without restarting Mnesia
. A
restore is performed with the function
mnesia:restore(Opaque, Args), where Args
can contain the
following tuples:
{module,Mod}
. The backup moduleMod
is used to access the backup media. If omitted, the default backup module is used.{skip_tables, TableList}
, whereTableList
is a list of tables, which is not to be read from the backup.{clear_tables, TableList}
, whereTableList
is a list of tables, which is to be cleared before the records from the backup are inserted. That is, all records in the tables are deleted before the tables are restored. Schema information about the tables is not cleared or read from the backup.{keep_tables, TableList}
, whereTableList
is a list of tables, which is not to be cleared before the records from the backup are inserted. That is, the records in the backup are added to the records in the table. Schema information about the tables is not cleared or read from the backup.{recreate_tables, TableList}
, whereTableList
is a list of tables, which is to be recreated before the records from the backup are inserted. The tables are first deleted and then created with the schema information from the backup. All the nodes in the backup need to be operational.{default_op, Operation}
, whereOperation
is one of the operationsskip_tables
,clear_tables
,keep_tables
, orrecreate_tables
. The default operation specifies which operation is to be used on tables from the backup that are not specified in any of the previous lists. If omitted, the operationclear_tables
is used.
The argument Opaque
is forwarded to the backup module. It returns
{atomic, TabList}
if successful, or the tuple {aborted, Reason}
if there is
an error. TabList
is a list of the restored tables. Tables that are restored
are write-locked during the restore operation. However, regardless of any lock
conflict caused by this, applications can continue to do their work during the
restore operation.
The restoration is performed as a single transaction. If the database is large, it cannot always be restored online. The old database must then be restored by installing a fallback, followed by a restart.
Fallback
The function [mnesia:install_fallback(Opaque,
[Mod])](mnesia:install_fallback/2
) installs a backup as fallback. It uses the
backup module Mod
, or the default backup module, to access the backup media.
The function returns ok
if successful, or {error, Reason}
if there is an
error.
Installing a fallback is a distributed operation, which is only performed on
all db_nodes
. The fallback restores the database the next time the system is
started. If a Mnesia
node with a fallback installed detects that Mnesia
on
another node has died, it unconditionally terminates itself.
A fallback is typically used when a system upgrade is performed. A system
typically involves the installation of new software versions, and Mnesia
tables are often transformed into new layouts. If the system crashes during an
upgrade, it is highly probable that reinstallation of the old applications is
required, and restoration of the database to its previous state. This can be
done if a backup is performed and installed as a fallback before the system
upgrade begins.
If the system upgrade fails, Mnesia
must be restarted on all db_nodes
to
restore the old database. The fallback is automatically deinstalled after a
successful startup. The function
mnesia:uninstall_fallback() can also be used to
deinstall the fallback after a successful system upgrade. Again, this is a
distributed operation that is either performed on all db_nodes
or none. Both
the installation and deinstallation of fallbacks require Erlang to be
operational on all db_nodes
, but it does not matter if Mnesia
is running or
not.
Disaster Recovery
The system can become inconsistent as a result of a power failure. The UNIX
feature fsck
can possibly repair the file system, but there is no guarantee
that the file content is consistent.
If Mnesia
detects that a file has not been properly closed, possibly as a
result of a power failure, it tries to repair the bad file in a similar manner.
Data can be lost, but Mnesia
can be restarted even if the data is
inconsistent. Configuration parameter -mnesia auto_repair <bool>
can be used
to control the behavior of Mnesia
at startup. If <bool>
has the value
true
, Mnesia
tries to repair the file. If <bool>
has the value false
,
Mnesia
does not restart if it detects a suspect file. This configuration
parameter affects the repair behavior of log files, DAT
files, and the default
backup media.
Configuration parameter -mnesia dump_log_update_in_place <bool>
controls the
safety level of the function mnesia:dump_log() By
default, Mnesia
dumps the transaction log directly into the DAT
files. If a
power failure occurs during the dump, this can cause the randomly accessed DAT
files to become corrupt. If the parameter is set to false
, Mnesia
copies the
DAT
files and target the dump to the new temporary files. If the dump is
successful, the temporary files are renamed to their normal DAT
suffixes. The
possibility for unrecoverable inconsistencies in the data files becomes much
smaller with this strategy. However, the actual dumping of the transaction log
becomes considerably slower. The system designer must decide whether speed or
safety is the higher priority.
Replicas of type disc_only_copies
are only affected by this parameter during
the initial dump of the log file at startup. When designing applications with
very high requirements, it can be appropriate not to use disc_only_copies
tables at all. The reason for this is the random access nature of normal
operating system files. If a node goes down for a reason such as a power
failure, these files can be corrupted because they are not properly closed. The
DAT
files for disc_only_copies
are updated on a per transaction basis.
If a disaster occurs and the Mnesia
database is corrupted, it can be
reconstructed from a backup. Regard this as a last resort, as the backup
contains old data. The data is hopefully consistent, but data is definitely lost
when an old backup is used to restore the database.