Behaviour under excessive load
In high traffic networks, a notary can receive a large amount of notarisation requests in a short time window. Once this number starts approaching the capacity the the notary can quickly handle, the notarisation response time will increase.
To avoid a notarisation request being lost in the event of a notary worker temporarily going down, retry functionality is built into Corda. If the node does not receive a response from the notary in a predefined period of time, the node will resend the notarisation request. Having this retry period fixed would mean that if the time to process a request starts starts to exceed the retry period, the notary will get inundated with requests. Hence to ensure the notary does not become overloaded the retry value is variable and based on the backpressure mechanism.
Flow Engine Behaviour
When a node makes a notarisation request it will receive back an estimated time to completion. This value represents an upper bound value for time that it should take to process the request, based on the current throughput of the cluster. Once the notarisation request has been processed the node will receive the response, however if no response has been received before the estimated processing time is up then the the node will retry the request.
This mechanism means that requests should always be processed in the event of a notary worker failure, whilst notaries under heavy load do not receive premature, redundant retry requests.
Artemis Messaging Layer Behaviour
A less common scenario that can occur is the notary workers get sent a very large amount of requests which causes the Artemis message broker to become overloaded. For example, if the notary worker receives messages at a greater rate than the maximum rate its Artemis message broker can consume messages from the queue then message processing delays can start to occur.
If running a HA notary cluster then scaling up the worker cluster size will help negate this problem. See Scaling A Notary Cluster for more information. The backpressure within the flow engine should also help to prevent this scenario.