+=head2 Plugin C<aggregation>
+
+The I<Aggregation plugin> makes it possible to aggregate several values into
+one using aggregation functions such as I<sum>, I<average>, I<min> and I<max>.
+This can be put to a wide variety of uses, e.g. average and total CPU
+statistics for your entire fleet.
+
+The grouping is powerful but, as with many powerful tools, may be a bit
+difficult to wrap your head around. The grouping will therefore be
+demonstrated using an example: The average and sum of the CPU usage across
+all CPUs of each host is to be calculated.
+
+To select all the affected values for our example, set C<Plugin cpu> and
+C<Type cpu>. The other values are left unspecified, meaning "all values". The
+I<Host>, I<Plugin>, I<PluginInstance>, I<Type> and I<TypeInstance> options
+work as if they were specified in the C<WHERE> clause of an C<SELECT> SQL
+statement.
+
+ Plugin "cpu"
+ Type "cpu"
+
+Although the I<Host>, I<PluginInstance> (CPU number, i.e. 0, 1, 2, ...) and
+I<TypeInstance> (idle, user, system, ...) fields are left unspecified in the
+example, the intention is to have a new value for each host / type instance
+pair. This is achieved by "grouping" the values using the C<GroupBy> option.
+It can be specified multiple times to group by more than one field.
+
+ GroupBy "Host"
+ GroupBy "TypeInstance"
+
+We do neither specify nor group by I<plugin instance> (the CPU number), so all
+metrics that differ in the CPU number only will be aggregated. Each
+aggregation needs I<at least one> such field, otherwise no aggregation would
+take place.
+
+The full example configuration looks like this:
+
+ <Plugin "aggregation">
+ <Aggregation>
+ Plugin "cpu"
+ Type "cpu"
+
+ GroupBy "Host"
+ GroupBy "TypeInstance"
+
+ CalculateSum true
+ CalculateAverage true
+ </Aggregation>
+ </Plugin>
+
+There are a couple of limitations you should be aware of:
+
+=over 4
+
+=item
+
+The I<Type> cannot be left unspecified, because it is not reasonable to add
+apples to oranges. Also, the internal lookup structure won't work if you try
+to group by type.
+
+=item
+
+There must be at least one unspecified, ungrouped field. Otherwise nothing
+will be aggregated.
+
+=back
+
+As you can see in the example above, each aggregation has its own
+B<Aggregation> block. You can have multiple aggregation blocks and aggregation
+blocks may match the same values, i.e. one value list can update multiple
+aggregations. The following options are valid inside B<Aggregation> blocks:
+
+=over 4
+
+=item B<Host> I<Host>
+
+=item B<Plugin> I<Plugin>
+
+=item B<PluginInstance> I<PluginInstance>
+
+=item B<Type> I<Type>
+
+=item B<TypeInstance> I<TypeInstance>
+
+Selects the value lists to be added to this aggregation. B<Type> must be a
+valid data set name, see L<types.db(5)> for details.
+
+If the string starts with and ends with a slash (C</>), the string is
+interpreted as a I<regular expression>. The regex flavor used are POSIX
+extended regular expressions as described in L<regex(7)>. Example usage:
+
+ Host "/^db[0-9]\\.example\\.com$/"
+
+=item B<GroupBy> B<Host>|B<Plugin>|B<PluginInstance>|B<TypeInstance>
+
+Group valued by the specified field. The B<GroupBy> option may be repeated to
+group by multiple fields.
+
+=item B<SetHost> I<Host>
+
+=item B<SetPlugin> I<Plugin>
+
+=item B<SetPluginInstance> I<PluginInstance>
+
+=item B<SetTypeInstance> I<TypeInstance>
+
+Sets the appropriate part of the identifier to the provided string.
+
+The I<PluginInstance> should include the placeholder C<%{aggregation}> which
+will be replaced with the aggregation function, e.g. "average". Not including
+the placeholder will result in duplication warnings and/or messed up values if
+more than one aggregation function are enabled.
+
+The following example calculates the average usage of all "even" CPUs:
+
+ <Plugin "aggregation">
+ <Aggregation>
+ Plugin "cpu"
+ PluginInstance "/[0,2,4,6,8]$/"
+ Type "cpu"
+
+ SetPlugin "cpu"
+ SetPluginInstance "even-%{aggregation}"
+
+ GroupBy "Host"
+ GroupBy "TypeInstance"
+
+ CalculateAverage true
+ </Aggregation>
+ </Plugin>
+
+This will create the files:
+
+=over 4
+
+=item
+
+foo.example.com/cpu-even-average/cpu-idle
+
+=item
+
+foo.example.com/cpu-even-average/cpu-system
+
+=item
+
+foo.example.com/cpu-even-average/cpu-user
+
+=item
+
+...
+
+=back
+
+=item B<CalculateNum> B<true>|B<false>
+
+=item B<CalculateSum> B<true>|B<false>
+
+=item B<CalculateAverage> B<true>|B<false>
+
+=item B<CalculateMinimum> B<true>|B<false>
+
+=item B<CalculateMaximum> B<true>|B<false>
+
+=item B<CalculateStddev> B<true>|B<false>
+
+Boolean options for enabling calculation of the number of value lists, their
+sum, average, minimum, maximum andE<nbsp>/ or standard deviation. All options
+are disabled by default.
+
+=back
+
+=head2 Plugin C<amqp>
+
+The I<AMQMP plugin> can be used to communicate with other instances of
+I<collectd> or third party applications using an AMQP message broker. Values
+are sent to or received from the broker, which handles routing, queueing and
+possibly filtering or messages.
+
+ <Plugin "amqp">
+ # Send values to an AMQP broker
+ <Publish "some_name">
+ Host "localhost"
+ Port "5672"
+ VHost "/"
+ User "guest"
+ Password "guest"
+ Exchange "amq.fanout"
+ # ExchangeType "fanout"
+ # RoutingKey "collectd"
+ # Persistent false
+ # Format "command"
+ # StoreRates false
+ # GraphitePrefix "collectd."
+ # GraphiteEscapeChar "_"
+ # GraphiteSeparateInstances false
+ # GraphiteAlwaysAppendDS false
+ </Publish>
+
+ # Receive values from an AMQP broker
+ <Subscribe "some_name">
+ Host "localhost"
+ Port "5672"
+ VHost "/"
+ User "guest"
+ Password "guest"
+ Exchange "amq.fanout"
+ # ExchangeType "fanout"
+ # Queue "queue_name"
+ # RoutingKey "collectd.#"
+ </Subscribe>
+ </Plugin>
+
+The plugin's configuration consists of a number of I<Publish> and I<Subscribe>
+blocks, which configure sending and receiving of values respectively. The two
+blocks are very similar, so unless otherwise noted, an option can be used in
+either block. The name given in the blocks starting tag is only used for
+reporting messages, but may be used to support I<flushing> of certain
+I<Publish> blocks in the future.
+
+=over 4
+
+=item B<Host> I<Host>
+
+Hostname or IP-address of the AMQP broker. Defaults to the default behavior of
+the underlying communications library, I<rabbitmq-c>, which is "localhost".
+
+=item B<Port> I<Port>
+
+Service name or port number on which the AMQP broker accepts connections. This
+argument must be a string, even if the numeric form is used. Defaults to
+"5672".
+
+=item B<VHost> I<VHost>
+
+Name of the I<virtual host> on the AMQP broker to use. Defaults to "/".
+
+=item B<User> I<User>
+
+=item B<Password> I<Password>
+
+Credentials used to authenticate to the AMQP broker. By default "guest"/"guest"
+is used.
+
+=item B<Exchange> I<Exchange>
+
+In I<Publish> blocks, this option specifies the I<exchange> to send values to.
+By default, "amq.fanout" will be used.
+
+In I<Subscribe> blocks this option is optional. If given, a I<binding> between
+the given exchange and the I<queue> is created, using the I<routing key> if
+configured. See the B<Queue> and B<RoutingKey> options below.
+
+=item B<ExchangeType> I<Type>
+
+If given, the plugin will try to create the configured I<exchange> with this
+I<type> after connecting. When in a I<Subscribe> block, the I<queue> will then
+be bound to this exchange.
+
+=item B<Queue> I<Queue> (Subscribe only)
+
+Configures the I<queue> name to subscribe to. If no queue name was configures
+explicitly, a unique queue name will be created by the broker.
+
+=item B<RoutingKey> I<Key>
+
+In I<Publish> blocks, this configures the routing key to set on all outgoing
+messages. If not given, the routing key will be computed from the I<identifier>
+of the value. The host, plugin, type and the two instances are concatenated
+together using dots as the separator and all containing dots replaced with
+slashes. For example "collectd.host/example/com.cpu.0.cpu.user". This makes it
+possible to receive only specific values using a "topic" exchange.
+
+In I<Subscribe> blocks, configures the I<routing key> used when creating a
+I<binding> between an I<exchange> and the I<queue>. The usual wildcards can be
+used to filter messages when using a "topic" exchange. If you're only
+interested in CPU statistics, you could use the routing key "collectd.*.cpu.#"
+for example.
+
+=item B<Persistent> B<true>|B<false> (Publish only)
+
+Selects the I<delivery method> to use. If set to B<true>, the I<persistent>
+mode will be used, i.e. delivery is guaranteed. If set to B<false> (the
+default), the I<transient> delivery mode will be used, i.e. messages may be
+lost due to high load, overflowing queues or similar issues.
+
+=item B<Format> B<Command>|B<JSON>|B<Graphite> (Publish only)
+
+Selects the format in which messages are sent to the broker. If set to
+B<Command> (the default), values are sent as C<PUTVAL> commands which are
+identical to the syntax used by the I<Exec> and I<UnixSock plugins>. In this
+case, the C<Content-Type> header field will be set to C<text/collectd>.
+
+If set to B<JSON>, the values are encoded in the I<JavaScript Object Notation>,
+an easy and straight forward exchange format. The C<Content-Type> header field
+will be set to C<application/json>.
+
+If set to B<Graphite>, values are encoded in the I<Graphite> format, which is
+"<metric> <value> <timestamp>\n". The C<Content-Type> header field will be set to
+C<text/graphite>.
+
+A subscribing client I<should> use the C<Content-Type> header field to
+determine how to decode the values. Currently, the I<AMQP plugin> itself can
+only decode the B<Command> format.
+
+=item B<StoreRates> B<true>|B<false> (Publish only)
+
+Determines whether or not C<COUNTER>, C<DERIVE> and C<ABSOLUTE> data sources
+are converted to a I<rate> (i.e. a C<GAUGE> value). If set to B<false> (the
+default), no conversion is performed. Otherwise the conversion is performed
+using the internal value cache.
+
+Please note that currently this option is only used if the B<Format> option has
+been set to B<JSON>.
+
+=item B<GraphitePrefix> (Publish and B<Format>=I<Graphite> only)
+
+A prefix can be added in the metric name when outputting in the I<Graphite> format.
+It's added before the I<Host> name.
+Metric name will be "<prefix><host><postfix><plugin><type><name>"
+
+=item B<GraphitePostfix> (Publish and B<Format>=I<Graphite> only)
+
+A postfix can be added in the metric name when outputting in the I<Graphite> format.
+It's added after the I<Host> name.
+Metric name will be "<prefix><host><postfix><plugin><type><name>"
+
+=item B<GraphiteEscapeChar> (Publish and B<Format>=I<Graphite> only)
+
+Specify a character to replace dots (.) in the host part of the metric name.
+In I<Graphite> metric name, dots are used as separators between different
+metric parts (host, plugin, type).
+Default is "_" (I<Underscore>).
+
+=item B<GraphiteSeparateInstances> B<true>|B<false>
+
+If set to B<true>, the plugin instance and type instance will be in their own
+path component, for example C<host.cpu.0.cpu.idle>. If set to B<false> (the
+default), the plugin and plugin instance (and likewise the type and type
+instance) are put into one component, for example C<host.cpu-0.cpu-idle>.
+
+=item B<GraphiteAlwaysAppendDS> B<true>|B<false>
+
+If set to B<true>, append the name of the I<Data Source> (DS) to the "metric"
+identifier. If set to B<false> (the default), this is only done when there is
+more than one DS.
+
+=back
+