diff options
| -rw-r--r-- | lib/mnesia/doc/src/Mnesia_chap7.xmlsrc | 7 | ||||
| -rw-r--r-- | lib/mnesia/doc/src/mnesia.xml | 32 |
2 files changed, 38 insertions, 1 deletions
diff --git a/lib/mnesia/doc/src/Mnesia_chap7.xmlsrc b/lib/mnesia/doc/src/Mnesia_chap7.xmlsrc index 7078499fbf..21174340d1 100644 --- a/lib/mnesia/doc/src/Mnesia_chap7.xmlsrc +++ b/lib/mnesia/doc/src/Mnesia_chap7.xmlsrc @@ -473,6 +473,13 @@ dets:close(N). </pre> <c>mnesia:table_info(Tab, master_nodes)</c> may be used to obtain information about the potential master nodes. </p> + <p>Determining which data to keep after communication failure is outside + the scope of Mnesia. One approach would be to determine which "island" + contains a majority of the nodes. Using the <c>{majority,true}</c> option + for critical tables can be a way of ensuring that nodes that are not part + of a "majority island" are not able to update those tables. Note that this + constitutes a reduction in service on the minority nodes. This would be + a tradeoff in favour of higher consistency guarantees.</p> <p>The function <c>mnesia:force_load_table(Tab)</c> may be used to force load the table regardless of which table load mechanism is activated. diff --git a/lib/mnesia/doc/src/mnesia.xml b/lib/mnesia/doc/src/mnesia.xml index 16e78ea0af..2a2c7d3a9f 100644 --- a/lib/mnesia/doc/src/mnesia.xml +++ b/lib/mnesia/doc/src/mnesia.xml @@ -161,6 +161,14 @@ If a new item is inserted with the same key as </p> </item> <item> + <p><c>majority</c> This attribute can be either <c>true</c> or + <c>false</c> (default is <c>false</c>). When <c>true</c>, a majority + of the table replicas must be available for an update to succeed. + Majority checking can be enabled on tables with mission-critical data, + where it is vital to avoid inconsistencies due to network splits. + </p> + </item> + <item> <p><c>snmp</c> Each (set based) Mnesia table can be automatically turned into an SNMP ordered table as well. This property specifies the types of the SNMP keys. @@ -650,6 +658,17 @@ mnesia:change_table_copy_type(person, node(), disc_copies) </desc> </func> <func> + <name>change_table_majority(Tab, Majority) -> {aborted, R} | {atomic, ok}</name> + <fsummary>Change the majority check setting for the table.</fsummary> + <desc> + <p><c>Majority</c> must be a boolean; the default is <c>false</c>. + When <c>true</c>, a majority of the table's replicas must be available + for an update to succeed. When used on fragmented tables, <c>Tab</c> + must be the name base table. Directly changing the majority setting on + individual fragments is not allowed.</p> + </desc> + </func> + <func> <name>clear_table(Tab) -> {aborted, R} | {atomic, ok}</name> <fsummary>Deletes all entries in a table.</fsummary> <desc> @@ -753,6 +772,14 @@ mnesia:change_table_copy_type(person, node(), disc_copies) priority will be loaded first at startup. </p> </item> + <item> + <p><c>{majority, Flag}</c>, where <c>Flag</c> must be a boolean. + If <c>true</c>, any (non-dirty) update to the table will abort unless + a majority of the table's replicas are available for the commit. + When used on a fragmented table, all fragments will be given + the same majority setting. + </p> + </item> <item> <p><c>{ram_copies, Nodelist}</c>, where <c>Nodelist</c> is a list of the nodes where this table @@ -1737,7 +1764,10 @@ mnesia:create_table(person, <c>write</c> and <c>sticky_write</c> are supported. </p> <p>If the user wants to update the record it is more efficient to - use <c>write/sticky_write</c> as the LockKind. + use <c>write/sticky_write</c> as the LockKind. If majority checking + is active on the table, it will be checked as soon as a write lock is + attempted. This can be used to quickly abort if the majority condition + isn't met. </p> </desc> </func> |