Domain HA

A domain is fully available only when all of its subcomponents are available. However, transaction processing can still run over the domain even if only the mediator and the sequencer are available. The domain services handle new connections to domains, and the topology manager handles the changes to the topology state; unavailability of these two components affects only the services they handle. As all of these components can run in separate processes, we handle the HA of each component separately.

Sequencer HA

The HA properties of the Sequencer depend on the chosen implementation. When the sequencer is based on a HA ledger, such as Hyperledger Fabric, the sequencer automatically becomes HA. The domain service can return multiple sequenced endpoints, any of which can be used to interact with the underlying ledger.

For the database sequencer, we use an active-active setup over a shared database. The setup relies on the database for both HA and consistency.

Database Sequencer HA

The database Sequencer uses the database itself to ensure that events are sequenced with a consistent order. Many Sequencer nodes can be deployed where each node has a Sequencer reader and writer component, all of these components can concurrently read and write to the same database. A load balancer can be used to evenly distribute requests between these nodes. The canton health endpoint can be used to halt sending requests to a node that reports itself as unhealthy.

Sequencers nodes are statically configured with the total number of possible Sequencer nodes and each node is assigned a distinct index from this range. This index is used to partition available event timestamps to ensure two sequencer node will never use the same event id/timestamp.

Events are written to the events table and can be read in ascending timestamp order. To provide a continuous monotonic stream of events, readers need to know the point at which events can be read without the risk of an earlier event being inserted by a writer process. To do this writers regularly update a watermark table where they publish their latest event timestamp. Readers take the minimum timestamp from this table as the point they can safely query events for.

If a Sequencer node was to fail, it would stop updating its watermark value and when it becomes the minimum timestamp this will cause all readers to effectively pause at this point (at they cannot read beyond this point). Other Sequencers writers when updating their own watermark also check that the other sequencer watermarks are being updated in a timely manner. If it is noticed that a Sequencer node has not updated its watermark within a configurable interval then it will be marked as offline and this watermark will no longer be included in the query for the minimum event timestamp. This causes future events from the offline Sequencer to be ignored after this timestamp. For this process to operate optimally the clocks of the hosts of the Sequencer nodes are expected to be synchronized - this is considered reasonable for where all Sequencer hosts are co-located and NTP is used.

If the failed Sequencer has recovered and would like to resume operation, it should delete all events past its last know watermark to avoid incorrectly re-inserting them into the events the readers will see, as readers may have read subsequent events by this time. This is safe to do without effecting events that have been read as any events written by the offline Sequencer after it is marked offline are ignored by readers. It should then replace its old watermark with a new timestamp for events it will start inserting then resume normal operation, ensuring that this is greater than any existing value.

When a Sequencer fails and resumes operation there will be short pause in reading from other Sequencers due to updates to the watermark table. However requests to the other Sequencer nodes should continue successfully, and any events written during this period will be available to read as soon as the pause has completed. Any send requests that were being processed by the failed Sequencer process will likely be lost, but can be safely retried once their max-sequencing-time has been exceeded without the risk of creating duplicate events.

Mediator HA

The approach for mediator node HA follows the same principles as outlined for participant HA in Replicated Participant Node Architecture. Namely a mediator node is replicated and only one replica is active. All replicas of the same mediator node share the same database, both for sharing the state as well as to coordinate the active mediator node replica.