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Data Consistency

Data in this context is a set of records that producers send to the partition leader. The leader is responsible for receiving messages from the producer, sending messages to the consumer, and replicating them to followers. There can be only one leader per partition at any point in time. Depending on the cluster and topic configurations, there can be many or zero followers.

Messages get the order that the leader observes, and the reordering is proscribed. Records order inside the message is kept and cannot be changed. Each record gets assigned to a unique monotonically increased number called offset.

After a record gets accepted by the leader, it can be in one of two states: COMMITTED or UNCOMMITTED. COMMITTED denotes that all followers have acknowledged the record. If there are no followers in the partition, the state is always COMMITTED once a record gets acknowledged. Records are UNCOMMITTED in all other cases. See more details about Synchronization Algorithm.

Neither the leader nor the follower waits for data persistence (fsync) before sending an acknowledgment of the record. It means that uncommitted records may be lost if Leader crashes.

The leader does not uphold an atomicity guarantee for the entire message. Records are processed one by one. If an error occurs, the operation aborts, response with an error message is returned, but Fluvio does not roll back previous records from the batch.

What records state to use is a configurable option for both producers and consumers.

 

Producer Isolation

Isolation is a configuration parameter of Producer that has two values:

  1. ReadCommitted - Leader waits for records to get committed before sending acknowledgement to Producer.
$ fluvio produce greetings --isolation read_committed
  1. ReadUncommitted - Leader does not wait for records to get committed before sending acknowledgement to Producer.
$ fluvio produce greetings --isolation read_uncommitted

ReadUncommitted isolation gives lower latency but has weaker guarantees.

If not specified, ReadUncommitted isolation is used by default.

 

Consumer Isolation

Isolation is a configuration parameter of Consumer that has two values:

  1. ReadCommitted - Read COMMITTED records only. Leader doesn’t send UNCOMMITTED records to Consumer.
$ fluvio consume greetings --isolation read_committed
  1. ReadUncommitted - Read all records regardless of the state.
$ fluvio consume greetings --isolation read_uncommitted

If not specified, ReadUncommitted isolation is used by default.