16. Multi-Site Clusters and Tickets¶
Apart from local clusters, Pacemaker also supports multi-site clusters. That means you can have multiple, geographically dispersed sites, each with a local cluster. Failover between these clusters can be coordinated manually by the administrator, or automatically by a higher-level entity called a Cluster Ticket Registry (CTR).
16.1. Challenges for Multi-Site Clusters¶
Typically, multi-site environments are too far apart to support synchronous communication and data replication between the sites. That leads to significant challenges:
- How do we make sure that a cluster site is up and running?
- How do we make sure that resources are only started once?
- How do we make sure that quorum can be reached between the different sites and a split-brain scenario avoided?
- How do we manage failover between sites?
- How do we deal with high latency in case of resources that need to be stopped?
In the following sections, learn how to meet these challenges.
16.2. Conceptual Overview¶
Multi-site clusters can be considered as “overlay” clusters where each cluster site corresponds to a cluster node in a traditional cluster. The overlay cluster can be managed by a CTR in order to guarantee that any cluster resource will be active on no more than one cluster site. This is achieved by using tickets that are treated as failover domain between cluster sites, in case a site should be down.
The following sections explain the individual components and mechanisms that were introduced for multi-site clusters in more detail.
16.2.1. Ticket¶
Tickets are, essentially, cluster-wide attributes. A ticket grants the
right to run certain resources on a specific cluster site. Resources can
be bound to a certain ticket by rsc_ticket constraints. Only if the
ticket is available at a site can the respective resources be started there.
Vice versa, if the ticket is revoked, the resources depending on that
ticket must be stopped.
The ticket thus is similar to a site quorum, i.e. the permission to
manage/own resources associated with that site. (One can also think of the
current have-quorum flag as a special, cluster-wide ticket that is
granted in case of node majority.)
Tickets can be granted and revoked either manually by administrators (which could be the default for classic enterprise clusters), or via the automated CTR mechanism described below.
A ticket can only be owned by one site at a time. Initially, none of the sites has a ticket. Each ticket must be granted once by the cluster administrator.
The presence or absence of tickets for a site is stored in the CIB as a
cluster status. With regards to a certain ticket, there are only two states
for a site: true (the site has the ticket) or false (the site does
not have the ticket). The absence of a certain ticket (during the initial
state of the multi-site cluster) is the same as the value false.
16.2.2. Dead Man Dependency¶
A site can only activate resources safely if it can be sure that the other site has deactivated them. However after a ticket is revoked, it can take a long time until all resources depending on that ticket are stopped “cleanly”, especially in case of cascaded resources. To cut that process short, the concept of a Dead Man Dependency was introduced.
If a dead man dependency is in force, if a ticket is revoked from a site, the nodes that are hosting dependent resources are fenced. This considerably speeds up the recovery process of the cluster and makes sure that resources can be migrated more quickly.
This can be configured by specifying a loss-policy="fence" in
rsc_ticket constraints.
16.2.3. Cluster Ticket Registry¶
A CTR is a coordinated group of network daemons that automatically handles granting, revoking, and timing out tickets (instead of the administrator revoking the ticket somewhere, waiting for everything to stop, and then granting it on the desired site).
Pacemaker does not implement its own CTR, but interoperates with external software designed for that purpose (similar to how resource and fencing agents are not directly part of pacemaker).
Participating clusters run the CTR daemons, which connect to each other, exchange information about their connectivity, and vote on which sites gets which tickets.
A ticket is granted to a site only once the CTR is sure that the ticket has been relinquished by the previous owner, implemented via a timer in most scenarios. If a site loses connection to its peers, its tickets time out and recovery occurs. After the connection timeout plus the recovery timeout has passed, the other sites are allowed to re-acquire the ticket and start the resources again.
This can also be thought of as a “quorum server”, except that it is not a single quorum ticket, but several.
16.2.4. Configuration Replication¶
As usual, the CIB is synchronized within each cluster, but it is not synchronized across cluster sites of a multi-site cluster. You have to configure the resources that will be highly available across the multi-site cluster for every site accordingly.
16.3. Configuring Ticket Dependencies¶
The rsc_ticket constraint lets you specify the resources depending on a certain ticket. Together with the constraint, you can set a loss-policy that defines what should happen to the respective resources if the ticket is revoked.
The attribute loss-policy can have the following values:
fence:Fence the nodes that are running the relevant resources.stop:Stop the relevant resources.freeze:Do nothing to the relevant resources.demote:Demote relevant resources that are running in the promoted role.
Constraint that fences node if ticketA is revoked
<rsc_ticket id="rsc1-req-ticketA" rsc="rsc1" ticket="ticketA" loss-policy="fence"/>
The example above creates a constraint with the ID rsc1-req-ticketA. It
defines that the resource rsc1 depends on ticketA and that the node running
the resource should be fenced if ticketA is revoked.
If resource rsc1 were a promotable resource, you might want to configure
that only being in the promoted role depends on ticketA. With the following
configuration, rsc1 will be demoted if ticketA is revoked:
Constraint that demotes rsc1 if ticketA is revoked
<rsc_ticket id="rsc1-req-ticketA" rsc="rsc1" rsc-role="Promoted" ticket="ticketA" loss-policy="demote"/>
You can create multiple rsc_ticket constraints to let multiple resources depend on the same ticket. However, rsc_ticket also supports resource sets (see Resource Sets), so one can easily list all the resources in one rsc_ticket constraint instead.
Ticket constraint for multiple resources
<rsc_ticket id="resources-dep-ticketA" ticket="ticketA" loss-policy="fence">
<resource_set id="resources-dep-ticketA-0" role="Started">
<resource_ref id="rsc1"/>
<resource_ref id="group1"/>
<resource_ref id="clone1"/>
</resource_set>
<resource_set id="resources-dep-ticketA-1" role="Promoted">
<resource_ref id="ms1"/>
</resource_set>
</rsc_ticket>
In the example above, there are two resource sets, so we can list resources
with different roles in a single rsc_ticket constraint. There’s no dependency
between the two resource sets, and there’s no dependency among the
resources within a resource set. Each of the resources just depends on
ticketA.
Referencing resource templates in rsc_ticket constraints, and even
referencing them within resource sets, is also supported.
If you want other resources to depend on further tickets, create as many
constraints as necessary with rsc_ticket.
16.4. Managing Multi-Site Clusters¶
16.4.1. Granting and Revoking Tickets Manually¶
You can grant tickets to sites or revoke them from sites manually. If you want to re-distribute a ticket, you should wait for the dependent resources to stop cleanly at the previous site before you grant the ticket to the new site.
Use the crm_ticket command line tool to grant and revoke tickets.
To grant a ticket to this site:
# crm_ticket --ticket ticketA --grant
To revoke a ticket from this site:
# crm_ticket --ticket ticketA --revoke
Important
If you are managing tickets manually, use the crm_ticket command with great care, because it cannot check whether the same ticket is already granted elsewhere.
16.4.2. Granting and Revoking Tickets via a Cluster Ticket Registry¶
We will use Booth here as an example of software that can be used with pacemaker as a Cluster Ticket Registry. Booth implements the Raft algorithm to guarantee the distributed consensus among different cluster sites, and manages the ticket distribution (and thus the failover process between sites).
Each of the participating clusters and arbitrators runs the Booth daemon boothd.
An arbitrator is the multi-site equivalent of a quorum-only node in a local cluster. If you have a setup with an even number of sites, you need an additional instance to reach consensus about decisions such as failover of resources across sites. In this case, add one or more arbitrators running at additional sites. Arbitrators are single machines that run a booth instance in a special mode. An arbitrator is especially important for a two-site scenario, otherwise there is no way for one site to distinguish between a network failure between it and the other site, and a failure of the other site.
The most common multi-site scenario is probably a multi-site cluster with two sites and a single arbitrator on a third site. However, technically, there are no limitations with regards to the number of sites and the number of arbitrators involved.
Boothd at each site connects to its peers running at the other sites and exchanges connectivity details. Once a ticket is granted to a site, the booth mechanism will manage the ticket automatically: If the site which holds the ticket is out of service, the booth daemons will vote which of the other sites will get the ticket. To protect against brief connection failures, sites that lose the vote (either explicitly or implicitly by being disconnected from the voting body) need to relinquish the ticket after a time-out. Thus, it is made sure that a ticket will only be re-distributed after it has been relinquished by the previous site. The resources that depend on that ticket will fail over to the new site holding the ticket. The nodes that have run the resources before will be treated according to the loss-policy you set within the rsc_ticket constraint.
Before the booth can manage a certain ticket within the multi-site cluster, you initially need to grant it to a site manually via the booth command-line tool. After you have initially granted a ticket to a site, boothd will take over and manage the ticket automatically.
Important
The booth command-line tool can be used to grant, list, or revoke tickets and can be run on any machine where boothd is running. If you are managing tickets via Booth, use only booth for manual intervention, not crm_ticket. That ensures the same ticket will only be owned by one cluster site at a time.
16.4.2.1. Booth Requirements¶
- All clusters that will be part of the multi-site cluster must be based on Pacemaker.
- Booth must be installed on all cluster nodes and on all arbitrators that will be part of the multi-site cluster.
- Nodes belonging to the same cluster site should be synchronized via NTP. However, time synchronization is not required between the individual cluster sites.
16.4.3. General Management of Tickets¶
Display the information of tickets:
# crm_ticket --info
Or you can monitor them with:
# crm_mon --tickets
Display the rsc_ticket constraints that apply to a ticket:
# crm_ticket --ticket ticketA --constraints
When you want to do maintenance or manual switch-over of a ticket,
revoking the ticket would trigger the loss policies. If
loss-policy="fence", the dependent resources could not be gracefully
stopped/demoted, and other unrelated resources could even be affected.
The proper way is making the ticket standby first with:
# crm_ticket --ticket ticketA --standby
Then the dependent resources will be stopped or demoted gracefully without triggering the loss policies.
If you have finished the maintenance and want to activate the ticket again, you can run:
# crm_ticket --ticket ticketA --activate