# Cross-DC replication
The Cadence Global Domain feature provides clients with the capability to continue their workflow execution from another cluster in the event of a datacenter failover. Although you can configure a Global Domain to be replicated to any number of clusters, it is only considered active in a single cluster.
# Global Domains Architecture
Cadence has introduced a new top level entity, Global Domains, which provides support for replication of workflow execution across clusters. Client applications need to run workers polling on Activity/Decision tasks on all clusters. Cadence will only dispatch tasks on the current active cluster; workers on the standby cluster will sit idle until the Global Domain is failed over.
Because Cadence is a service that provides highly consistent semantics, we only allow external events like StartWorkflowExecution, SignalWorkflowExecution, etc. on an active cluster. Global Domains relies on light-weight transactions (paxos) on the local cluster (Local_Quorum) to update the workflow execution state and create replication tasks which are applied asynchronously to replicate state across clusters. If an application makes these API calls on a cluster where Global Domain is in standby mode, Cadence will reject those calls with DomainNotActiveError, which contains the name of the current active cluster. It is the responsibility of the application to forward the external event to the cluster that is currently active.
# New config for Global Domains
This config is used to distinguish domains local to the cluster from the global domain. It controls the creation of replication tasks on updates allowing the state to be replicated across clusters. This is a read-only setting that can only be set when the domain is provisioned.
A list of clusters where the domain can fail over to, including the current active cluster. This is also a read-only setting that can only be set when the domain is provisioned. A re-replication feature on the roadmap will allow updating this config to add/remove clusters in the future.
# Active Cluster Name
# Failover Version
Unique failover version which also represents the current active cluster for Global Domain. Cadence allows failover to be triggered from any cluster, so failover version is designed in a way to not allow conflicts if failover is mistakenly triggered simultaneously on two clusters.
# Conflict Resolution
Unlike local domains which provide at-most-once semantics for activity execution, Global Domains can only support at-least-once semantics. Cadence XDC relies on asynchronous replication of events across clusters, so in the event of a failover it is possible that activity gets dispatched again on the new active cluster due to a replication task lag. This also means that whenever workflow execution is updated after a failover by the new cluster, any previous replication tasks for that execution cannot be applied. This results in loss of some progress made by the workflow execution in the previous active cluster. During such conflict resolution, Cadence re-injects any external events like Signals to the new history before discarding replication tasks. Even though some progress could rollback during failovers, Cadence provides the guarantee that workflows won’t get stuck and will continue to make forward progress.
# Visibility API
All Visibility APIs are allowed on both active and standby clusters. This enables Cadence Web to work seamlessly for Global Domains as all visibility records for workflow executions can be queried from any cluster the domain is replicated to. Applications making API calls directly to the Cadence Visibility API will continue to work even if a Global Domain is in standby mode. However, they might see a lag due to replication delay when querying the workflow execution state from a standby cluster.
# Query Global Domain
The following command can be used to describe Global Domain metadata:
$ cadence --do cadence-canary-xdc d desc Name: cadence-canary-xdc Description: cadence canary cross dc testing domain OwnerEmail: email@example.com DomainData: Status: REGISTERED RetentionInDays: 7 EmitMetrics: true ActiveClusterName: dc1 Clusters: dc1, dc2
# Failover Global Domain
The following command can be used to failover Global Domain my-domain-global to the dc2 cluster:
$ cadence --do my-domain-global d up --ac dc2
# What happens to outstanding activities after failover?
Cadence does not forward activity completions across clusters. Any outstanding activity will eventually timeout based on the configuration. Your application should have retry logic in place so that the activity gets retried and dispatched again to a worker after the failover to the new DC. Handling this is pretty much the same as activity timeout caused by a worker restart even without Global Domains.
# What happens when a start or signal API call is made to a standby cluster?
Cadence will reject the call and return DomainNotActiveError. It is the responsibility of the application to forward the failed call to active cluster based on information provided in the error.
# What is the recommended pattern to send external events to an active cluster?
The recommendation at this point is to publish events to a Kafka topic if they can be generated in any DC. Then, have a consumer that consumes from the aggregated Kafka topic in the same DC and sends them to Cadence. Both the Kafka consumer and Global Domain need to be failed over together.