From 497f4b2215e2fc1b6a10c3f96c71b90dbb529eae Mon Sep 17 00:00:00 2001
From: Ulf Wiger <ulf.wiger@erlang-solutions.com>
Date: Sun, 30 Jan 2011 20:19:48 +0100
Subject: Add documentation text about majority checking
---
lib/mnesia/doc/src/Mnesia_chap7.xmlsrc | 7 +++++++
lib/mnesia/doc/src/mnesia.xml | 32 +++++++++++++++++++++++++++++++-
2 files changed, 38 insertions(+), 1 deletion(-)
(limited to 'lib/mnesia')
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
@@ -160,6 +160,14 @@ If a new item is inserted with the same key as
behavior. The default is <c>false</c>.
</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.
@@ -649,6 +657,17 @@ mnesia:change_table_copy_type(person, node(), disc_copies)
<c>LoadOrder</c> priority will be loaded first at startup.</p>
</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>
@@ -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>
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