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Some notes on Zeebe (A scalable process orchestrator)

Zeebe is a horizontally scalable, language agnostic, highly available, process orchestration engine that can execute a BPMN 2.0 workflow.

Key aspects of Scalability

  • No central database
  • Event sources (append only logs). Logs are written to disks
  • Log replication (Atomix) and Compaction (delete event after all exporters have processed an event).
  • RocksDb for snapshots / Projections.
  • Support for many exporters like Elasticsearch, Kafka and Hazelcast.
  • One thread per log means no locking contention.

Scale1

  • Peer to peer clusters.
  • Requires 2 nodes initially (main and contact point), then GOSSIP (that’s how EPIDEMICS spread).

Scale2

  • Replication using RAFT consensus algorithm with ATOMIX as the implementation. http://thesecretlivesofdata.com/raft/

  • Leaders can only write, followers are for replicas.

  • Zeebe is a CP system in the CAP theorem.

  • To achieve consistency, QUORUM must be obtained.

        [ quorum ≥(replication group size / 2) + 1 ]
  • QUOROM avoids split-brain phenomena.

Scale3

  • Partitions for throughout and multi-threading:
  • Each partition is a separate physical append only log.
  • Each partition has its own leader that gets write access.
  • All workflow events of a single flow go to the same partition for single write access
  • Use of gRPC for remote communication.

Scale4

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