# OCEL: A Standard for Object-Centric Event Logs

- Type: Conference paper
- URL: https://www.gyunam.com/publication/2021-simpda/
- Date: 2021-07-17
- Authors: Anahita Farhang Ghahfarokhi, Gyunam Park, Alessandro Berti, Wil M.P. van der Aalst
- Venue: European Conference on Advances in Databases and Information Systems
- DOI: https://doi.org/10.1007/978-3-030-85082-1_16
- PDF: https://www.gyunam.com/publication/2021-simpda/paper.pdf
- Research lines: [Object-centric process mining](https://www.gyunam.com/research-line/object-centric-process-mining/index.md)
- Tags: Conference Proceedings, Object-Centric Process Mining, SIMPDA2021

## Abstract

The application of process mining techniques to real-life information systems is often challenging. Considering a Purchase to Pay (P2P) process, several case notions such as order and item are involved, interacting with each other. Therefore, creating an event log where events need to relate to a single case (i.e., process instance) leads to convergence (i.e., the duplication of an event related to different cases) and divergence (i.e., the inability to separate events within the same case) problems. To avoid such problems, object-centric event logs have been proposed, where each event can be related to different objects. These can be exploited by a new set of process mining techniques. This paper describes OCEL (Object-Centric Event Log), a generic and scalable format for the storage of object-centric event logs. The implementation of the format can use either JSON or XML, and tool support is provided.

## Practical problem

Companies that want to analyze processes such as purchase-to-pay or order-to-cash from their ERP systems find that each event involves several interacting business objects, such as orders, items, packages and customers, rather than one case. Forcing such data into the standard single-case event log format (XES) duplicates events across cases (convergence) or blurs the order of events inside a case (divergence), distorting discovered models and performance figures. Earlier object-centric formats such as XOC replicate object attributes in every event and scale poorly with the number of events and objects, while tabular files lose typing. Analysts and tool builders need a common, compact, strongly typed exchange format for event data with multiple object types.

## Research questions

1. How should object-centric event logs be structured so that events relate to multiple typed objects without the convergence and divergence problems of single-case logs?
2. How can such a format be serialized and supported by tools so that it scales better than existing proposals such as XOC?

## Contributions

- The OCEL (Object-Centric Event Log) standard: a meta-model in which a log holds events and objects, each event references a set of objects of different types, and object attributes are stored once rather than per event.
- Support for strongly typed and nested elements (lists and maps) and global default elements for logs, events and objects.
- Serializations in JSON and XML with validation schemas, specified formally at ocel-standard.org.
- Tool support in Python (the ocel-support library on GitHub) and Java (`ProM` OCELStandard package) for import, export, schema validation and flattening to traditional event logs.
- Example object-centric event logs extracted from the SAP IDES system in both serializations.

## Key findings

- The paper is a standard proposal without experiments; it argues that decoupling object attributes from events avoids the replication that makes XOC logs grow badly with the number of events and objects.
- It demonstrates the format with a JSON-OCEL example of an order handling process with customer, item, order and package object types, and shows import, export and validation in Python and Java.
- Open limitations named by the paper: no consistency checks for advanced constraints such as the number of objects per event, no support for event streams, whole-log in-memory loading, and no real-world case studies yet.