:compat-mode: legacy
= Resource Constraints =
////
We prefer [[ch-constraints]], but older versions of asciidoc don't deal well
with that construct for chapter headings
////
anchor:ch-constraints[Chapter 7, Alerts]
indexterm:[Resource,Constraint]
== Scores ==
indexterm:[Resource,Score]
indexterm:[Node,Score]
Scores of all kinds are integral to how the cluster works.
Practically everything from moving a resource to deciding which
resource to stop in a degraded cluster is achieved by manipulating
scores in some way.
Scores are calculated per resource and node. Any node with a
negative score for a resource can't run that resource. The cluster
places a resource on the node with the highest score for it.
=== Infinity Math ===
Pacemaker implements +INFINITY+ (or equivalently, ++INFINITY+) internally as a
score of 1,000,000. Addition and subtraction with it follow these three basic
rules:
* Any value + +INFINITY+ = +INFINITY+
* Any value - +INFINITY+ = +-INFINITY+
* +INFINITY+ - +INFINITY+ = +-INFINITY+
[NOTE]
======
What if you want to use a score higher than 1,000,000? Typically this possibility
arises when someone wants to base the score on some external metric that might
go above 1,000,000.
The short answer is you can't.
The long answer is it is sometimes possible work around this limitation
creatively. You may be able to set the score to some computed value based on
the external metric rather than use the metric directly. For nodes, you can
store the metric as a node attribute, and query the attribute when computing
the score (possibly as part of a custom resource agent).
======
== Deciding Which Nodes a Resource Can Run On ==
indexterm:[Constraint,Location Constraint]
'Location constraints' tell the cluster which nodes a resource can run on.
There are two alternative strategies. One way is to say that, by default,
resources can run anywhere, and then the location constraints specify nodes
that are not allowed (an 'opt-out' cluster). The other way is to start with
nothing able to run anywhere, and use location constraints to selectively
enable allowed nodes (an 'opt-in' cluster).
Whether you should choose opt-in or opt-out depends on your
personal preference and the make-up of your cluster. If most of your
resources can run on most of the nodes, then an opt-out arrangement is
likely to result in a simpler configuration. On the other-hand, if
most resources can only run on a small subset of nodes, an opt-in
configuration might be simpler.
=== Location Properties ===
indexterm:[XML element,rsc_location element]
indexterm:[Constraint,Location Constraint,rsc_location element]
.Attributes of a rsc_location Element
[width="95%",cols="2m,1,<5",options="header",align="center"]
|=========================================================
|Attribute
|Default
|Description
|id
|
|A unique name for the constraint (required)
indexterm:[XML attribute,id attribute,rsc_location element]
indexterm:[XML element,rsc_location element,id attribute]
|rsc
|
|The name of the resource to which this constraint applies. A location
constraint must either have a +rsc+, have a +rsc-pattern+, or contain at least
one resource set.
indexterm:[XML attribute,rsc attribute,rsc_location element]
indexterm:[XML element,rsc_location element,rsc attribute]
|rsc-pattern
|
|A pattern matching the names of resources to which this constraint applies.
The syntax is the same as
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04[POSIX]
extended regular expressions, with the addition of an initial '!' indicating
that resources 'not' matching the pattern are selected. If the regular
expression contains submatches, and the constraint is governed by a
<<ch-rules,rule>>, the submatches can be referenced as +%0+ through +%9+ in
the rule's +score-attribute+ or a rule expression's +attribute+. A location
constraint must either have a +rsc+, have a +rsc-pattern+, or contain at least
one resource set.
indexterm:[XML attribute,rsc-pattern attribute,rsc_location element]
indexterm:[XML element,rsc_location element,rsc-pattern attribute]
|node
|
|The name of the node to which this constraint applies. A location constraint
must either have a +node+ and +score+, or contain at least one rule.
indexterm:[XML attribute,node attribute,rsc_location element]
indexterm:[XML element,rsc_location element,node attribute]
|score
|
|Positive values indicate a preference for running the affected resource(s) on
+node+ -- the higher the value, the stronger the preference. Negative values
indicate the resource(s) should avoid this node (a value of +-INFINITY+
changes "should" to "must"). A location constraint must either have a +node+
and +score+, or contain at least one rule.
indexterm:[XML attribute,score attribute,rsc_location element]
indexterm:[XML element,rsc_location element,score attribute]
|resource-discovery
|always
a|Whether Pacemaker should perform resource discovery (that is, check whether
the resource is already running) for this resource on this node. This should
normally be left as the default, so that rogue instances of a service can be
stopped when they are running where they are not supposed to be. However,
there are two situations where disabling resource discovery is a good idea:
when a service is not installed on a node, discovery might return an error
(properly written OCF agents will not, so this is usually only seen with other
agent types); and when Pacemaker Remote is used to scale a cluster to hundreds
of nodes, limiting resource discovery to allowed nodes can significantly boost
performance.
* +always:+ Always perform resource discovery for the specified resource on this node.
* +never:+ Never perform resource discovery for the specified resource on this node.
This option should generally be used with a -INFINITY score, although that is not strictly
required.
* +exclusive:+ Perform resource discovery for the specified resource only on
this node (and other nodes similarly marked as +exclusive+). Multiple location
constraints using +exclusive+ discovery for the same resource across
different nodes creates a subset of nodes resource-discovery is exclusive to.
If a resource is marked for +exclusive+ discovery on one or more nodes, that
resource is only allowed to be placed within that subset of nodes.
indexterm:[XML attribute,resource-discovery attribute,rsc_location element]
indexterm:[XML element,rsc_location element,resource-discovery attribute]
indexterm:[Constraint,Location Constraint,Resource Discovery]
|=========================================================
[WARNING]
=========
Setting resource-discovery to +never+ or +exclusive+ removes Pacemaker's
ability to detect and stop unwanted instances of a service running
where it's not supposed to be. It is up to the system administrator (you!)
to make sure that the service can 'never' be active on nodes without
resource-discovery (such as by leaving the relevant software uninstalled).
=========
=== Asymmetrical "Opt-In" Clusters ===
indexterm:[Asymmetrical Clusters]
indexterm:[Opt-In Clusters]
To create an opt-in cluster, start by preventing resources from
running anywhere by default:
----
# crm_attribute --name symmetric-cluster --update false
----
Then start enabling nodes. The following fragment says that the web
server prefers *sles-1*, the database prefers *sles-2* and both can
fail over to *sles-3* if their most preferred node fails.
.Opt-in location constraints for two resources
======
[source,XML]
-------
<constraints>
<rsc_location id="loc-1" rsc="Webserver" node="sles-1" score="200"/>
<rsc_location id="loc-2" rsc="Webserver" node="sles-3" score="0"/>
<rsc_location id="loc-3" rsc="Database" node="sles-2" score="200"/>
<rsc_location id="loc-4" rsc="Database" node="sles-3" score="0"/>
</constraints>
-------
======
=== Symmetrical "Opt-Out" Clusters ===
indexterm:[Symmetrical Clusters]
indexterm:[Opt-Out Clusters]
To create an opt-out cluster, start by allowing resources to run
anywhere by default:
----
# crm_attribute --name symmetric-cluster --update true
----
Then start disabling nodes. The following fragment is the equivalent
of the above opt-in configuration.
.Opt-out location constraints for two resources
======
[source,XML]
-------
<constraints>
<rsc_location id="loc-1" rsc="Webserver" node="sles-1" score="200"/>
<rsc_location id="loc-2-do-not-run" rsc="Webserver" node="sles-2" score="-INFINITY"/>
<rsc_location id="loc-3-do-not-run" rsc="Database" node="sles-1" score="-INFINITY"/>
<rsc_location id="loc-4" rsc="Database" node="sles-2" score="200"/>
</constraints>
-------
======
[[node-score-equal]]
=== What if Two Nodes Have the Same Score ===
If two nodes have the same score, then the cluster will choose one.
This choice may seem random and may not be what was intended, however
the cluster was not given enough information to know any better.
.Constraints where a resource prefers two nodes equally
======
[source,XML]
-------
<constraints>
<rsc_location id="loc-1" rsc="Webserver" node="sles-1" score="INFINITY"/>
<rsc_location id="loc-2" rsc="Webserver" node="sles-2" score="INFINITY"/>
<rsc_location id="loc-3" rsc="Database" node="sles-1" score="500"/>
<rsc_location id="loc-4" rsc="Database" node="sles-2" score="300"/>
<rsc_location id="loc-5" rsc="Database" node="sles-2" score="200"/>
</constraints>
-------
======
In the example above, assuming no other constraints and an inactive
cluster, +Webserver+ would probably be placed on +sles-1+ and +Database+ on
+sles-2+. It would likely have placed +Webserver+ based on the node's
uname and +Database+ based on the desire to spread the resource load
evenly across the cluster. However other factors can also be involved
in more complex configurations.
[[s-resource-ordering]]
== Specifying the Order in which Resources Should Start/Stop ==
indexterm:[Constraint,Ordering Constraint]
indexterm:[Resource,Start Order]
'Ordering constraints' tell the cluster the order in which certain
resource actions should occur.
[IMPORTANT]
====
Ordering constraints affect 'only' the ordering of resource actions;
they do 'not' require that the resources be placed on the
same node. If you want resources to be started on the same node
'and' in a specific order, you need both an ordering constraint 'and'
a colocation constraint (see <<s-resource-colocation>>), or
alternatively, a group (see <<group-resources>>).
====
=== Ordering Properties ===
indexterm:[XML element,rsc_order element]
indexterm:[Constraint,Ordering Constraint,rsc_order element]
.Attributes of a rsc_order Element
[width="95%",cols="1m,1,<4",options="header",align="center"]
|=========================================================
|Field
|Default
|Description
|id
|
|A unique name for the constraint
indexterm:[XML attribute,id attribute,rsc_order element]
indexterm:[XML element,rsc_order element,id attribute]
|first
|
|Name of the resource that the +then+ resource depends on
indexterm:[XML attribute,first attribute,rsc_order element]
indexterm:[XML element,rsc_order element,first attribute]
|then
|
|Name of the dependent resource
indexterm:[XML attribute,then attribute,rsc_order element]
indexterm:[XML element,rsc_order element,then attribute]
|first-action
|start
|The action that the +first+ resource must complete before +then-action+
can be initiated for the +then+ resource. Allowed values: +start+,
+stop+, +promote+, +demote+.
indexterm:[XML attribute,first-action attribute,rsc_order element]
indexterm:[XML element,rsc_order element,first-action attribute]
|then-action
|value of +first-action+
|The action that the +then+ resource can execute only after the
+first-action+ on the +first+ resource has completed. Allowed
values: +start+, +stop+, +promote+, +demote+.
indexterm:[XML attribute,then-action attribute,rsc_order element]
indexterm:[XML element,rsc_order element,then-action attribute]
|kind
|Mandatory
a|How to enforce the constraint. Allowed values:
* +Mandatory:+ +then-action+ will never be initiated for the +then+ resource
unless and until +first-action+ successfully completes for the +first+
resource.
* +Optional:+ The constraint applies only if both specified resource actions
are scheduled in the same transition (that is, in response to the same
cluster state). This means that +then-action+ is allowed on the +then+
resource regardless of the state of the +first+ resource, but if both actions
happen to be scheduled at the same time, they will be ordered.
* +Serialize:+ Ensure that the specified actions are never performed
concurrently for the specified resources. +First-action+ and +then-action+
can be executed in either order, but one must complete before the other can
be initiated. An example use case is when resource start-up puts a high load
on the host.
indexterm:[XML attribute,kind attribute,rsc_order element]
indexterm:[XML element,rsc_order element,kind attribute]
|symmetrical
|TRUE for +Mandatory+ and +Optional+ kinds. FALSE for +Serialize+ kind.
|If true, the reverse of the constraint applies for the opposite action (for
example, if B starts after A starts, then B stops before A stops).
+Serialize+ orders cannot be symmetrical.
indexterm:[XML attribute,symmetrical attribute,rsc_order element]
indexterm:[XML element,rsc_order element,symmetrical attribute]
|=========================================================
+Promote+ and +demote+ apply to the master role of
<<s-resource-promotable,promotable>> resources.
=== Optional and mandatory ordering ===
Here is an example of ordering constraints where +Database+ 'must' start before
+Webserver+, and +IP+ 'should' start before +Webserver+ if they both need to be
started:
.Optional and mandatory ordering constraints
======
[source,XML]
-------
<constraints>
<rsc_order id="order-1" first="IP" then="Webserver" kind="Optional"/>
<rsc_order id="order-2" first="Database" then="Webserver" kind="Mandatory" />
</constraints>
-------
======
Because the above example lets +symmetrical+ default to TRUE,
+Webserver+ must be stopped before +Database+ can be stopped,
and +Webserver+ should be stopped before +IP+
if they both need to be stopped.
[[s-resource-colocation]]
== Placing Resources Relative to other Resources ==
indexterm:[Constraint,Colocation Constraint]
indexterm:[Resource,Location Relative to Other Resources]
'Colocation constraints' tell the cluster that the location of one resource
depends on the location of another one.
Colocation has an important side-effect: it affects the order in which
resources are assigned to a node. Think about it: You can't place A relative to
B unless you know where B is.
footnote:[
While the human brain is sophisticated enough to read the constraint
in any order and choose the correct one depending on the situation,
the cluster is not quite so smart. Yet.
]
So when you are creating colocation constraints, it is important to
consider whether you should colocate A with B, or B with A.
Another thing to keep in mind is that, assuming A is colocated with
B, the cluster will take into account A's preferences when
deciding which node to choose for B.
For a detailed look at exactly how this occurs, see
http://clusterlabs.org/doc/Colocation_Explained.pdf[Colocation Explained].
[IMPORTANT]
====
Colocation constraints affect 'only' the placement of resources; they do 'not'
require that the resources be started in a particular order. If you want
resources to be started on the same node 'and' in a specific order, you need
both an ordering constraint (see <<s-resource-ordering>>) 'and' a colocation
constraint, or alternatively, a group (see <<group-resources>>).
====
=== Colocation Properties ===
indexterm:[XML element,rsc_colocation element]
indexterm:[Constraint,Colocation Constraint,rsc_colocation element]
.Attributes of a rsc_colocation Constraint
[width="95%",cols="1m,1,<4",options="header",align="center"]
|=========================================================
|Field
|Default
|Description
|id
|
|A unique name for the constraint (required).
indexterm:[XML attribute,id attribute,rsc_colocation element]
indexterm:[XML element,rsc_colocation element,id attribute]
|rsc
|
|The name of a resource that should be located relative to +with-rsc+ (required).
indexterm:[XML attribute,rsc attribute,rsc_colocation element]
indexterm:[XML element,rsc_colocation element,rsc attribute]
|with-rsc
|
|The name of the resource used as the colocation target. The cluster will
decide where to put this resource first and then decide where to put +rsc+ (required).
indexterm:[XML attribute,with-rsc attribute,rsc_colocation element]
indexterm:[XML element,rsc_colocation element,with-rsc attribute]
|node-attribute
|#uname
|The node attribute that must be the same on the node running +rsc+ and the
node running +with-rsc+ for the constraint to be satisfied. (For details,
see <<s-coloc-attribute>>.)
indexterm:[XML attribute,node-attribute attribute,rsc_colocation element]
indexterm:[XML element,rsc_colocation element,node-attribute attribute]
|score
|
|Positive values indicate the resources should run on the same
node. Negative values indicate the resources should run on
different nodes. Values of \+/- +INFINITY+ change "should" to "must".
indexterm:[XML attribute,score attribute,rsc_colocation element]
indexterm:[XML element,rsc_colocation element,score attribute]
|=========================================================
=== Mandatory Placement ===
Mandatory placement occurs when the constraint's score is
++INFINITY+ or +-INFINITY+. In such cases, if the constraint can't be
satisfied, then the +rsc+ resource is not permitted to run. For
+score=INFINITY+, this includes cases where the +with-rsc+ resource is
not active.
If you need resource +A+ to always run on the same machine as
resource +B+, you would add the following constraint:
.Mandatory colocation constraint for two resources
====
[source,XML]
<rsc_colocation id="colocate" rsc="A" with-rsc="B" score="INFINITY"/>
====
Remember, because +INFINITY+ was used, if +B+ can't run on any
of the cluster nodes (for whatever reason) then +A+ will not
be allowed to run. Whether +A+ is running or not has no effect on +B+.
Alternatively, you may want the opposite -- that +A+ 'cannot'
run on the same machine as +B+. In this case, use
+score="-INFINITY"+.
.Mandatory anti-colocation constraint for two resources
====
[source,XML]
<rsc_colocation id="anti-colocate" rsc="A" with-rsc="B" score="-INFINITY"/>
====
Again, by specifying +-INFINITY+, the constraint is binding. So if the
only place left to run is where +B+ already is, then
+A+ may not run anywhere.
As with +INFINITY+, +B+ can run even if +A+ is stopped.
However, in this case +A+ also can run if +B+ is stopped, because it still
meets the constraint of +A+ and +B+ not running on the same node.
=== Advisory Placement ===
If mandatory placement is about "must" and "must not", then advisory
placement is the "I'd prefer if" alternative. For constraints with
scores greater than +-INFINITY+ and less than +INFINITY+, the cluster
will try to accommodate your wishes but may ignore them if the
alternative is to stop some of the cluster resources.
As in life, where if enough people prefer something it effectively
becomes mandatory, advisory colocation constraints can combine with
other elements of the configuration to behave as if they were
mandatory.
.Advisory colocation constraint for two resources
====
[source,XML]
<rsc_colocation id="colocate-maybe" rsc="A" with-rsc="B" score="500"/>
====
[[s-coloc-attribute]]
=== Colocation by Node Attribute ===
The +node-attribute+ property of a colocation constraints allows you to express
the requirement, "these resources must be on similar nodes".
As an example, imagine that you have two Storage Area Networks (SANs) that are
not controlled by the cluster, and each node is connected to one or the other.
You may have two resources +r1+ and +r2+ such that +r2+ needs to use the same
SAN as +r1+, but doesn't necessarily have to be on the same exact node.
In such a case, you could define a <<s-node-attributes,node attribute>> named
+san+, with the value +san1+ or +san2+ on each node as appropriate. Then, you
could colocate +r2+ with +r1+ using +node-attribute+ set to +san+.
[[s-resource-sets]]
== Resource Sets ==
'Resource sets' allow multiple resources to be affected by a single constraint.
indexterm:[Constraint,Resource Set]
indexterm:[Resource,Resource Set]
.A set of 3 resources
====
[source,XML]
----
<resource_set id="resource-set-example">
<resource_ref id="A"/>
<resource_ref id="B"/>
<resource_ref id="C"/>
</resource_set>
----
====
Resource sets are valid inside +rsc_location+,
+rsc_order+ (see <<s-resource-sets-ordering>>),
+rsc_colocation+ (see <<s-resource-sets-colocation>>),
and +rsc_ticket+ (see <<s-ticket-constraints>>) constraints.
A resource set has a number of properties that can be set,
though not all have an effect in all contexts.
.Attributes of a resource_set Element
[width="95%",cols="2m,1,<5",options="header",align="center"]
|=========================================================
|Field
|Default
|Description
|id
|
|A unique name for the set
indexterm:[XML attribute,id attribute,resource_set element]
indexterm:[XML element,resource_set element,id attribute]
|sequential
|true
|Whether the members of the set must be acted on in order.
Meaningful within +rsc_order+ and +rsc_colocation+.
indexterm:[XML attribute,sequential attribute,resource_set element]
indexterm:[XML element,resource_set element,sequential attribute]
|require-all
|true
|Whether all members of the set must be active before continuing.
With the current implementation, the cluster may continue even if only one
member of the set is started, but if more than one member of the set is
starting at the same time, the cluster will still wait until all of those have
started before continuing (this may change in future versions).
Meaningful within +rsc_order+.
indexterm:[XML attribute,require-all attribute,resource_set element]
indexterm:[XML element,resource_set element,require-all attribute]
|role
|
|Limit the effect of the constraint to the specified role.
Meaningful within +rsc_location+, +rsc_colocation+ and +rsc_ticket+.
indexterm:[XML attribute,role attribute,resource_set element]
indexterm:[XML element,resource_set element,role attribute]
|action
|
|Limit the effect of the constraint to the specified action.
Meaningful within +rsc_order+.
indexterm:[XML attribute,action attribute,resource_set element]
indexterm:[XML element,resource_set element,action attribute]
|score
|
|'Advanced use only.' Use a specific score for this set within the constraint.
indexterm:[XML attribute,score attribute,resource_set element]
indexterm:[XML element,resource_set element,score attribute]
|=========================================================
[[s-resource-sets-ordering]]
== Ordering Sets of Resources ==
A common situation is for an administrator to create a chain of
ordered resources, such as:
.A chain of ordered resources
======
[source,XML]
-------
<constraints>
<rsc_order id="order-1" first="A" then="B" />
<rsc_order id="order-2" first="B" then="C" />
<rsc_order id="order-3" first="C" then="D" />
</constraints>
-------
======
.Visual representation of the four resources' start order for the above constraints
image::images/resource-set.png["Ordered set",width="16cm",height="2.5cm",align="center"]
=== Ordered Set ===
To simplify this situation, resource sets (see <<s-resource-sets>>) can be used
within ordering constraints:
.A chain of ordered resources expressed as a set
======
[source,XML]
-------
<constraints>
<rsc_order id="order-1">
<resource_set id="ordered-set-example" sequential="true">
<resource_ref id="A"/>
<resource_ref id="B"/>
<resource_ref id="C"/>
<resource_ref id="D"/>
</resource_set>
</rsc_order>
</constraints>
-------
======
While the set-based format is not less verbose, it is significantly
easier to get right and maintain.
[IMPORTANT]
=========
If you use a higher-level tool, pay attention to how it exposes this
functionality. Depending on the tool, creating a set +A B+ may be equivalent to
+A then B+, or +B then A+.
=========
=== Ordering Multiple Sets ===
The syntax can be expanded to allow sets of resources to be ordered relative to
each other, where the members of each individual set may be ordered or
unordered (controlled by the +sequential+ property). In the example below, +A+
and +B+ can both start in parallel, as can +C+ and +D+, however +C+ and +D+ can
only start once _both_ +A+ _and_ +B+ are active.
.Ordered sets of unordered resources
======
[source,XML]
-------
<constraints>
<rsc_order id="order-1">
<resource_set id="ordered-set-1" sequential="false">
<resource_ref id="A"/>
<resource_ref id="B"/>
</resource_set>
<resource_set id="ordered-set-2" sequential="false">
<resource_ref id="C"/>
<resource_ref id="D"/>
</resource_set>
</rsc_order>
</constraints>
-------
======
.Visual representation of the start order for two ordered sets of unordered resources
image::images/two-sets.png["Two ordered sets",width="13cm",height="7.5cm",align="center"]
Of course either set -- or both sets -- of resources can also be
internally ordered (by setting +sequential="true"+) and there is no
limit to the number of sets that can be specified.
.Advanced use of set ordering - Three ordered sets, two of which are internally unordered
======
[source,XML]
-------
<constraints>
<rsc_order id="order-1">
<resource_set id="ordered-set-1" sequential="false">
<resource_ref id="A"/>
<resource_ref id="B"/>
</resource_set>
<resource_set id="ordered-set-2" sequential="true">
<resource_ref id="C"/>
<resource_ref id="D"/>
</resource_set>
<resource_set id="ordered-set-3" sequential="false">
<resource_ref id="E"/>
<resource_ref id="F"/>
</resource_set>
</rsc_order>
</constraints>
-------
======
.Visual representation of the start order for the three sets defined above
image::images/three-sets.png["Three ordered sets",width="16cm",height="7.5cm",align="center"]
[IMPORTANT]
====
An ordered set with +sequential=false+ makes sense only if there is another
set in the constraint. Otherwise, the constraint has no effect.
====
=== Resource Set OR Logic ===
The unordered set logic discussed so far has all been "AND" logic.
To illustrate this take the 3 resource set figure in the previous section.
Those sets can be expressed, +(A and B) then \(C) then (D) then (E and F)+.
Say for example we want to change the first set, +(A and B)+, to use "OR" logic
so the sets look like this: +(A or B) then \(C) then (D) then (E and F)+.
This functionality can be achieved through the use of the +require-all+
option. This option defaults to TRUE which is why the
"AND" logic is used by default. Setting +require-all=false+ means only one
resource in the set needs to be started before continuing on to the next set.
.Resource Set "OR" logic: Three ordered sets, where the first set is internally unordered with "OR" logic
======
[source,XML]
-------
<constraints>
<rsc_order id="order-1">
<resource_set id="ordered-set-1" sequential="false" require-all="false">
<resource_ref id="A"/>
<resource_ref id="B"/>
</resource_set>
<resource_set id="ordered-set-2" sequential="true">
<resource_ref id="C"/>
<resource_ref id="D"/>
</resource_set>
<resource_set id="ordered-set-3" sequential="false">
<resource_ref id="E"/>
<resource_ref id="F"/>
</resource_set>
</rsc_order>
</constraints>
-------
======
[IMPORTANT]
====
An ordered set with +require-all=false+ makes sense only in conjunction with
+sequential=false+. Think of it like this: +sequential=false+ modifies the set
to be an unordered set using "AND" logic by default, and adding
+require-all=false+ flips the unordered set's "AND" logic to "OR" logic.
====
[[s-resource-sets-colocation]]
== Colocating Sets of Resources ==
Another common situation is for an administrator to create a set of
colocated resources.
The simplest way to do this is to define a resource group (see
<<group-resources>>), but that cannot always accurately express the desired
relationships. For example, maybe the resources do not need to be ordered.
Another way would be to define each relationship as an individual constraint,
but that causes a difficult-to-follow constraint explosion as the number of
resources and combinations grow.
.Colocation chain as individual constraints, where A is placed first, then B, then C, then D
======
[source,XML]
-------
<constraints>
<rsc_colocation id="coloc-1" rsc="D" with-rsc="C" score="INFINITY"/>
<rsc_colocation id="coloc-2" rsc="C" with-rsc="B" score="INFINITY"/>
<rsc_colocation id="coloc-3" rsc="B" with-rsc="A" score="INFINITY"/>
</constraints>
-------
======
To express complicated relationships with a simplified syntax
footnote:[which is not the same as saying easy to follow],
<<s-resource-sets,resource sets>> can be used within colocation constraints.
.Equivalent colocation chain expressed using +resource_set+
======
[source,XML]
-------
<constraints>
<rsc_colocation id="coloc-1" score="INFINITY" >
<resource_set id="colocated-set-example" sequential="true">
<resource_ref id="A"/>
<resource_ref id="B"/>
<resource_ref id="C"/>
<resource_ref id="D"/>
</resource_set>
</rsc_colocation>
</constraints>
-------
======
[NOTE]
====
Within a +resource_set+, the resources are listed in the order they are
_placed_, which is the reverse of the order in which they are _colocated_.
In the above example, resource +A+ is placed before resource +B+, which is
the same as saying resource +B+ is colocated with resource +A+.
====
As with individual constraints, a resource that can't be active prevents any
resource that must be colocated with it from being active. In both of the two
previous examples, if +B+ is unable to run, then both +C+ and by inference +D+
must remain stopped.
[IMPORTANT]
=========
If you use a higher-level tool, pay attention to how it exposes this
functionality. Depending on the tool, creating a set +A B+ may be equivalent to
+A with B+, or +B with A+.
=========
Resource sets can also be used to tell the cluster that entire _sets_ of
resources must be colocated relative to each other, while the individual
members within any one set may or may not be colocated relative to each other
(determined by the set's +sequential+ property).
In the following example, resources +B+, +C+, and +D+ will each be colocated
with +A+ (which will be placed first). +A+ must be able to run in order for any
of the resources to run, but any of +B+, +C+, or +D+ may be stopped without
affecting any of the others.
.Using colocated sets to specify a shared dependency
======
[source,XML]
-------
<constraints>
<rsc_colocation id="coloc-1" score="INFINITY" >
<resource_set id="colocated-set-2" sequential="false">
<resource_ref id="B"/>
<resource_ref id="C"/>
<resource_ref id="D"/>
</resource_set>
<resource_set id="colocated-set-1" sequential="true">
<resource_ref id="A"/>
</resource_set>
</rsc_colocation>
</constraints>
-------
======
[NOTE]
====
Pay close attention to the order in which resources and sets are listed.
While the members of any one sequential set are placed first to last (i.e., the
colocation dependency is last with first), multiple sets are placed last to
first (i.e. the colocation dependency is first with last).
====
[IMPORTANT]
====
A colocated set with +sequential="false"+ makes sense only if there is
another set in the constraint. Otherwise, the constraint has no effect.
====
There is no inherent limit to the number and size of the sets used.
The only thing that matters is that in order for any member of one set
in the constraint to be active, all members of sets listed after it must also
be active (and naturally on the same node); and if a set has +sequential="true"+,
then in order for one member of that set to be active, all members listed
before it must also be active.
If desired, you can restrict the dependency to instances of promotable clone
resources that are in a specific role, using the set's +role+ property.
.Colocation in which the members of the middle set have no interdependencies, and the last set listed applies only to instances in the master role
======
[source,XML]
-------
<constraints>
<rsc_colocation id="coloc-1" score="INFINITY" >
<resource_set id="colocated-set-1" sequential="true">
<resource_ref id="F"/>
<resource_ref id="G"/>
</resource_set>
<resource_set id="colocated-set-2" sequential="false">
<resource_ref id="C"/>
<resource_ref id="D"/>
<resource_ref id="E"/>
</resource_set>
<resource_set id="colocated-set-3" sequential="true" role="Master">
<resource_ref id="A"/>
<resource_ref id="B"/>
</resource_set>
</rsc_colocation>
</constraints>
-------
======
.Visual representation of the above example (resources are placed from left to right)
image::images/pcmk-colocated-sets.png["Colocation chain",width="960px",height="431px",align="center"]
[NOTE]
====
Unlike ordered sets, colocated sets do not use the +require-all+ option.
====