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GDNSD-PLUGIN-WEIGHTED(8) gdnsd GDNSD-PLUGIN-WEIGHTED(8)
NAME
gdnsd-plugin-weighted - gdnsd plugin implementing "weighted" records
SYNOPSIS
Example plugin config:
plugins => {
weighted => {
multi = false # default
service_types = none
up_thresh => 0.5 # default
corpwww => {
lb01 = [ lb01.example.com., 99 ]
lb02 = [ lb02.example.com., 15 ]
lb03 = [ lb03, 1 ]
}
frontwww6 => {
service_types = up
multi = true
wwwhost01 = [ 2001:db8::123, 4 ]
wwwhost02 = [ 2001:db8::456, 1 ]
wwwhost03 = [ 2001:db8::789, 2 ]
}
pubwww => {
service_types = [ web_check, foo ]
up_thresh => 0.01,
pubhost01 = [ 192.0.2.1, 44 ]
pubhost02 = [ 192.0.2.2, 11 ]
pubhost02 = [ 192.0.2.3, 11 ]
pubhost02 = [ 192.0.2.4, 11 ]
}
cdnwww => {
service_types = web_check
datacenter1 => {
d1-lb1 = [ 127.0.0.1, 2 ]
d1-lb2 = [ 127.0.0.2, 2 ]
}
datacenter2 => {
d2-lb1 = [ 127.0.0.3, 2 ]
d2-lb2 = [ 127.0.0.4, 2 ]
d2-lb3 = [ 127.0.0.5, 1 ]
}
}
mixed => {
multi => false,
addrs_v4 => {
lb1 = [ 127.0.0.3, 2 ]
lb2 = [ 127.0.0.4, 2 ]
}
addrs_v6 => {
multi => true
www6set1 = {
lb01 => [ 2001:db8::123, 4 ]
lb02 => [ 2001:db8::456, 1 ]
}
www6set2 = {
lb01 => [ 2001:db8::789, 4 ]
lb02 => [ 2001:db8::ABC, 1 ]
}
}
}
cn => {
cnames => {
foo = [ lb01.example.com., 99 ]
bar = [ lb02.example.com., 15 ]
}
}
}
}
Zonefile RRs referencing the above:
www.corp 300 DYNC weighted!corpwww
www6.front 300 DYNA weighted!frontwww6
www 300 DYNA weighted!pubwww
cdn 300 DYNA weighted!cdnwww
mixed-a 300 DYNA weighted!mixed
cnames 300 DYNC weighted!cn
DESCRIPTION
gdnsd-plugin-weighted can be used to return one (or a subset) of
several address records, or one of several CNAME records based on
dynamic-weighted probabilities.
CONFIGURATION - TOP LEVEL
At the top level, there are three special parameter keys:
"service_types", "up_thresh", and "multi". These only affect the
behavior of address ("DYNA") lookups. They are ignored for CNAME
("DYNC") lookups. All of these keys are inherited and override-able at
the per-resource and per-address-family levels.
The top-level default "service_types" is "default", which is a built-in
service type provided by gdnsd. For more information about configuring
non-default service type's, see the main gdnsd.config(5) documentation.
The default polls the "/" URL at each IP address on port 80 for a "200
OK" response every 10 seconds, and has some anti-flap parameters that
stabilize the state against isolated, intermittent failures.
"multi" is a boolean that can be "true" or "false", and defaults to
"false". "multi" controls the behavior of the algorithm for selecting
result addresses, discussed in detail later.
"up_thresh" defines a floating point fraction of summed address weights
in the range "(0.0 - 1.0]", defaulting to 0.5, and is used to influence
failure/failover behavior.
Other than those three, the rest of the top level keys are the names of
your resources, and their values are the configuration of each
resource.
CONFIGURATION - PER-RESOURCE
Inside a given resource's configuration hash, again the three address-
related parameters "services_types", "multi", and "up_thresh" may be
specified to override their settings per-resource.
There are two basic configuration modes within a resource:
1) Explicit per-family/type sub-stanzas. In this mode, the resource
contains one or more of the keys "addrs_v4", "addrs_v6", and "cnames".
"addrs_v4" and/or "addrs_v6" can both be specified in the same
resource, but "cnames" cannot co-exist with either of the other two.
The contents of each stanza configure response RRs of the given type
for this resource, and in the addrs_v4/addrs_v6 cases, the 3 behavioral
parameters "service_types", "multi", and "up_thresh" can be overridden
per-address-family as well.
2) Automatic top-level detection of just one family/type. In this
mode, you can configure the top-level of a resource with valid contents
for one of the stanzas "addrs_v4", "addrs_v6", or "cnames", and the
family/type will be autodetected, and be the only type available in
this resource.
CONFIGURATION - CNAMES
When configuring cnames, the value of each item should be "[ CNAME,
WEIGHT ]", and the resource will be useful for "DYNC" zonefile records,
resolving to a weighted CNAME record in responses. The weighting
decision will return a single CNAME answer, with the probability of
each choice being "weight / total_weight".
If the CNAMEs are not fully-qualified (do not end in "."), the current
$ORIGIN value for the zonefile RR being queried will be appended to
complete the name, much as you would expect if the same not-fully-
qualified name were substituted into the zonefiles everywhere the
relevant DYNC record exists.
CONFIGURATION - ADDRESSES
With the exception that "addrs_v4" and "addrs_v6" must contain only
addresses of the correct family (or in the top-level auto-detect case,
the top level entries must all be of the same family), the two stanzas
behave identically. When both are present, they are both used in every
"DYNA" response (as gdnsd always includes opposite-family records in
the Additional section of A/AAAA queries).
Within either address family type, there are two different binary
dimensions (multi -> true/false, and grouped-vs-ungrouped) upon which
the configuration and behavior hinge, leading to four different
possible cases: ungrouped-single, ungrouped-multi, grouped-single, and
grouped-multi. Each will be discussed in detail below:
THE UNGROUPED SINGLE CASE
This is the simplest case. The code detects this case when it sees
that "multi" is false (the default), and that the values of the keys
are arrays rather than sub-hashes. Each hash key is an address label,
and each value is an array of "[ IPADDR, WEIGHT ]".
When answering a query in this case, first the weights are converted to
dynamic weights. The dynamic weight of an address is its configured
weight if the monitored state is "UP" or "DANGER", or zero if the
monitored state is "DOWN". The dynamic weights are summed to produce a
dynamic weight total, and then a single address to respond with is
chosen from the set, with each address having the odds
"addr_dynamic_weight / total_dynamic_weight".
However, if the "total_dynamic_weight" is less than "ceil(up_thresh *
total_configured_weight)", then the dynamic weights are all reset to
their configured full values so that the response odds are the same as
if all were "UP", and resource-level failure is signalled to any upper-
layer meta-plugin (e.g. metafo or geoip) when applicable.
Example (X could be a whole resource, or an addrs_v4 stanza):
X => {
multi => false # default
# odds below assume no addresses are down:
lb01 => [ 192.0.2.1, 45 ] # 25% chance (45/180)
lb02 => [ 192.0.2.1, 60 ] # 33% chance (60/180)
lb03 => [ 192.0.2.1, 75 ] # 42% chance (75/180)
}
THE UNGROUPED MULTI CASE
This case is detected when, (as above) the values of the keys are
arrays of "[ IPADDR, WEIGHT]", but the parameter "multi" is true. The
change from the above behavior is primarily that multiple addresses
from the weighted set can be returned in each response. The "maximum",
rather than the sum, of the dynamic weights (again, zero for down
addresses, configured-weight otherwise), is found, and the odds of each
address's inclusion in the response set is "addr_dyanmic_weight /
max_dynamic_weight".
This means all non-"DOWN" addresses which share the group's maximum
dynamic weight value will always be included, whereas others will be
optionally included depending on the odds. At least one address is
always returned (because logically, at least one address has the
maximum weight, giving it a 100% chance), and sometimes the full
non-"DOWN" set will be returned.
"up_thresh" behaves as in the previous case: If the sum of the dynamic
weight values is less than "ceil(up_thresh * total_configured_weight)",
then the dynamic weights are all set to their configured values and the
result set is calculated as if all were "UP", while signalling
resource-level failure to upstream meta-plugins (geoip or metafo).
Example:
X => {
multi => true
# odds below assume no addresses are down:
lb01 => [ 192.0.2.1, 45 ] # 75% chance (45/60)
lb02 => [ 192.0.2.1, 60 ] # 100% chance (60/60)
lb03 => [ 192.0.2.1, 60 ] # 100% chance (60/60)
# overall possible result-sets:
# lb01,lb02,lb03 -> 75%
# lb02,lb03 -> 25%
}
THE GROUPED SINGLE CASE
The grouped cases are detected when the keys' values are sub-hashes at
the outer level rather than arrays of "[ IPADDR, WEIGHT]". In the
grouped case, first the set is divided into named groups, and then
within each group individual addresses are configured as "addrlabel =>
[ IPADDR, WEIGHT ]".
Example:
X => {
group1 => {
lb01 => [ 192.0.2.1, 10 ]
lb02 => [ 192.0.2.1, 20 ]
lb03 => [ 192.0.2.1, 30 ]
}
group2 => {
lb01 => [ 192.0.2.7, 10 ]
lb02 => [ 192.0.2.8, 20 ]
lb03 => [ 192.0.2.9, 30 ]
}
}
The grouped single case, of course, occurs when the configuration
layout is as shown above, and the "multi" parameter is "false" (the
default).
In grouped-single mode, essentially the groups are weighted against
each other similarly to the single case for ungrouped addresses,
resulting in the choice of a single group from the set of groups. Then
the addresses within the chosen group are weighted against each other
in multi-style, returning potentially more than one address from the
chosen group.
Specifically, each group's odds of being the single group chosen is
"group_dyn_weight / total_dyn_weight", where the group's dynamic weight
is the sum of the dynamic weights within it ("DOWN" addresses are
zero), and the total dynamic weight is the dynamic sum of all groups.
Then within each group, the odds of each address being included in the
multi-response set is "addr_dyn_weight / group_max_dyn_weight".
"up_thresh" operates on all groups as a whole, and if the non-"DOWN"
sum of all weights in all groups fails to meet the standard of
"ceil(up_thresh * total_sum_configured_weight)", then all addresses
will be treated as if they are "UP" for selection purposes, and
resource-level failure will be signalled upstream.
THE GROUPED MULTI CASE
You can probably infer this one's behavior from reading about the
previous three cases. The difference from the previous grouped-single
case is that the multi-vs-single behaviors are reversed. Multiple
groups are chosen based on the dynamic maximum weight between the
groups, and a single weighted address is returned from the subset
within each chosen group. All of the details above logically apply in
the way you would expect, as all of these four cases internally share
the same code and logic, they just apply different bits of it to
different subsets of the problem.
GENERAL NOTES ON ADDRESS MODE CASES ABOVE
Note that any time multi-selection is in effect at a layer (the top
layer when multi is true, or within a group when when multi is false),
the minimum count of chosen items will be the count of items that share
the maximum weight within the set. e.g. a set of items with weights
"30, 30, 30, 20, 20" will always choose at least 3/5 items (because the
first three have 100% odds of inclusion), and the total response set
will range as high as all 5 items with some probability.
A practical use-case example for grouped-single:
Splitting groups on subnet boundaries in grouped-single mode gives the
result that a single response packet never mixes subnets. This would
enable your DNS-based balancing to defeat certain forms of client-level
Destination Address Selection interference, while still returning
multiple addresses per response (all from one subnet).
A practical use-case for grouped-multi:
Suppose you have a large set of addresses which can be logically
grouped into subsets that have some shared failure risk (e.g.
subpartitions of a datacenter which share infrastructure). With
grouped-multi behavior, clients will get up to N (count of groups)
addresses in a round-robin response, but a given response set will
never contain two addresses from the same group/subset. This maximizes
the chance that the client can successfully fail over to another
address in the list when its primary selection fails, since the total
set in each response does not share any per-subset failure mode.
LIMITS
All weights must be positive integer values greater than zero and less
than 2^20 (1048576).
There is a limit of 64 addresses, address-groups, or cnames at the top
level of a resource (or per address family in the addrs_v4/addrs_v6
cases), and a limit of 64 addresses within each address group in the
grouped modes.
SEE ALSO
gdnsd.config(5), gdnsd.zonefile(5), gdnsd(8)
The gdnsd manual.
COPYRIGHT AND LICENSE
Copyright (c) 2012 Anton Tolchanov <me@knyar.net>, Brandon L Black
<blblack@gmail.com> and Jay Reitz <jreitz@gmail.com>
This file is part of gdnsd-plugin-weighted.
gdnsd-plugin-weighted is free software: you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation, either version 3 of the
License, or (at your option) any later version.
gdnsd-plugin-weighted is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License along
with gdnsd-plugin-weighted. If not, see
<http://www.gnu.org/licenses/>.
gdnsd 1.11.4 2014-07-18 GDNSD-PLUGIN-WEIGHTED(8)