diff --git a/doc/Pacemaker_Explained/en-US/Ch-Multi-site-Clusters.txt b/doc/Pacemaker_Explained/en-US/Ch-Multi-site-Clusters.txt index bc81d96813..dd5e9b8744 100644 --- a/doc/Pacemaker_Explained/en-US/Ch-Multi-site-Clusters.txt +++ b/doc/Pacemaker_Explained/en-US/Ch-Multi-site-Clusters.txt @@ -1,376 +1,323 @@ = Multi-Site Clusters and Tickets = -[[Multisite]] -== Abstract == 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)'. == 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. == 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 (Cluster Ticket Registry)` -mechanism. It guarantees that the cluster resources will be highly +The overlay cluster can be managed by a CTR in order to +guarantee that the cluster resources will be highly available across different cluster sites. 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. +=== Ticket === -=== Components and Concepts === - -==== Ticket ==== - -"Tickets" are, essentially, cluster-wide attributes. A ticket grants the +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 need to be stopped. +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.) +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+. -==== Dead Man Dependency ==== +=== 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 the 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. +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. +=== Cluster Ticket Registry === -==== CTR (Cluster Ticket Registry) ==== - -This is for those scenarios where the tickets management is supposed to -be automatic (instead of the administrator revoking the ticket somewhere, +A CTR is a network daemon 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). -A `CTR` is a network daemon that handles granting, -revoking, and timing out "tickets". The participating clusters would run -the daemons that would connect to each other, exchange information on -their connectivity details, and vote on which site gets which ticket(s). +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. +=== Configuration Replication === -==== Configuration Replication ==== - -As usual, the CIB is synchronized within each cluster, but it is not synchronized +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. [[s-ticket-constraints]] == 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 master mode to slave mode. .Constraint that fences node if +ticketA+ is revoked ==== [source,XML] ------- ------- ==== 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 multi-state resource (i.e. it could run in master or slave mode), you might want to configure that only master mode depends on +ticketA+. With the following configuration, +rsc1+ will be demoted to slave mode if +ticketA+ is revoked: +.Constraint that demotes +rsc1+ if +ticketA+ is revoked +==== [source,XML] ------- ------- +==== -You can create more `rsc_ticket` constraints to let multiple resources -depend on the same ticket. - -`rsc_ticket` also supports resource sets. So one can easily list all the -resources in one `rsc_ticket` constraint. For example: +You can create multiple `rsc_ticket` constraints to let multiple resources +depend on the same ticket. However, `rsc_ticket` also supports resource sets, +so one can easily list all the resources in one `rsc_ticket` constraint instead. +.Ticket constraint for multiple resources +==== [source,XML] ------- - - - - - - - - - - + + + + + + + + + + ------- +==== -In the example, there are two resource sets for listing the resources with -different `roles` in one `rsc_ticket` constraint. There's no dependency -between the two resource sets. And there's no dependency among the +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+. == Managing Multi-Site Clusters == === 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. ==== === Granting and Revoking Tickets via a Cluster Ticket Registry === We will use https://github.com/ClusterLabs/booth[Booth] here as an example of software that can be used with pacemaker as a Cluster Ticket Registry. Booth implements the http://en.wikipedia.org/wiki/Paxos_%28computer_science%29['Paxos'] lease 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. Nodes belonging to the same cluster site should be synchronized via NTP. However, time synchronization is not required between the individual cluster sites. `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 `booth client` command. -After you have initially granted a ticket to a site, the booth mechanism +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 client` command line tool can be used to grant, list, or -revoke tickets. The `booth client` commands work on any machine where -the booth daemon is running. - -If you are managing tickets via `Booth`, only use `booth client` for manual -intervention instead of `crm_ticket`. That can make sure the same ticket +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. ==== -Booth includes an implementation of -http://en.wikipedia.org/wiki/Paxos_algorithm['Paxos'] and 'Paxos Lease' -algorithm, which guarantees the distributed consensus among different -cluster sites. - -[NOTE] -==== -`Arbitrator` - -Each site runs one booth instance that is responsible for communicating -with the other sites. 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. As all booth instances -communicate with each other, arbitrators help to make more reliable -decisions about granting or revoking tickets. - -An arbitrator is especially important for a two-site scenario: For example, -if site `A` can no longer communicate with site `B`, there are two possible -causes for that: - -- `A` network failure between `A` and `B`. - -- Site `B` is down. - -However, if site `C` (the arbitrator) can still communicate with site `B`, -site `B` must still be up and running. - -==== - -===== 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. - -The most common 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. - -Nodes belonging to the same cluster site should be synchronized via NTP. However, -time synchronization is not required between the individual cluster sites. - - === 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 ------- == For more information == * http://doc.opensuse.org/products/draft/SLE-HA/SLE-ha-guide_sd_draft/cha.ha.geo.html[SUSE's Multi-site Clusters guide] * https://github.com/ClusterLabs/booth[Booth]