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• maximum number of loans per user is limited to 10 by automated patron blocks configuration
• a user has no open loans at the moment
• user checks out an item, but ITEM_CHECKED_OUT event does NOT reach mod-patron-blocks (which keeps count of loans for every user)
• over the next few months user checks out 10 more items, each time a corresponding event reaches mod-patron-blocks successfully
• library notices that user has 11 open loans, while the the limit is 10
• library reports a bug in mod-patron-blocks - the most likely culprit from user's perspective
• during investigation a developers discovers that the block was not imposed because of a failed event delivery which took place months ago

Consequences of the Push mechanism while Data Import

The existing PubSub is a Push mechanism. Source Record Manager would place large numbers of messages (one per record) into the queue during a large import job. Mod-pubsub would then push these into the callback function provided by mod-inventory. There was no means for mod-inventory to say “enough already”, it would get overloaded and crash. This was discussed with Folijet previously, and no viable solution was found.

The proposed scheme of modules interaction through Direct Kafka

In the case of Direct Kafka approach, OkApi and PubSub are no longer required, modules A and B interact directly with Kafka:

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• Guaranteed delivery provided by Kafka allows addressing reliability concern
• Improved data consistency since Kafka does not deliver newer messages until older ones are acknowledged
• Better performance by eliminating the overhead of multiple HTTP calls per event dispatch
• Enabling good HA since every new Event Consumer instance connects Kafka within a consumer group, so that the load is distributed evenly
• Improved manageability because of easier investigation capabilities, less data inconsistency, and following fail-fast approach
• the Pull mechanism provided by the Direct Kafka (as implemented in Data Import) - this implementation places the consumer code in mod-inventor,y and it will pull message from Kafka when it has capacity

Known challenges:

• Configuration (including Kafka, topics, group consumer, authorization) is more complicated than with PubSub
• While Kafka supports exactly-once delivery, the at-least-once implementation is simpler and more manageable. In turn, at-least-once means that the Event Consumer must be prepared to handle potential duplicate events

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Below is the list of modules participating in Circulation where refactoring will be required:

Module name	Owning team	Is it a Producer or Consumer of events?
mod-circulation	Vega	Producer (in a number of flows), Consumer (for events from mod-feesfines)
mod-feesfines	Vega	Producer
mod-patron-blocks	Vega	Consumer
mod-audit	Firebird	Consumer
mod-remote-storage	Firebird	Consumer

Wouldn't it make sense to tune the PubSub instead of switching to Direct Kafka?

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