Caterpillar: A business process execution engine on the Ethereum blockchain

Abstract

Summary Blockchain platforms, such as Ethereum, allow a set of actors to maintain a ledger of transactions without relying on a central authority and to deploy programs, called smart contracts, that are executed whenever certain transactions occur. These features can be used as basic building blocks for executing collaborative business processes between mutually untrusting parties. However, implementing business processes using the low‐level primitives provided by blockchain platforms is cumbersome and error‐prone. In contrast, established business process management systems (BPMSs), such as those based on the standard Business Process Model and Notation (BPMN), provide convenient abstractions for rapid development of process‐oriented applications. This article demonstrates how to combine the advantages of a BPMS with those of a blockchain platform. The article introduces a blockchain‐based BPMN execution engine, named Caterpillar. Like any BPMN execution engine, Caterpillar supports the creation of instances of a process model and allows users to monitor the state of process instances and to execute tasks thereof. The specificity of Caterpillar is that the state of each process instance is maintained on the (Ethereum) blockchain and the workflow routing is performed by smart contracts generated by a BPMN‐to‐Solidity compiler. The Caterpillar compiler supports a large array of BPMN constructs, including subprocesses, multiple‐instance activities, and event handlers. The paper describes the architecture of Caterpillar and the interfaces it provides to support the monitoring of process instances, the allocation and execution of work items, and the execution of service tasks.

A Minimal Core Calculus for Solidity Contracts

“…Some of these language are domain-specific, e.g. for financial contracts [4,7] and for business processes [11,17], while some others are more abstract, modelling contracts as automata with guarded transitions [13,16]. Establishing the correctness of the compilation from these languages to Solidity would be one of the possible applications of a bare bone formal model, like our TinySol.…”

Interpreted Execution of Business Process Models on Blockchain

“…The proposed blockchain-based process execution system follows the same design principles as the compiled version of Caterpillar [3]. Specifically, the system is designed to enable a set of untrusting parties to develop, deploy and execute collaborative processes on blockchain in a tamper-proof manner.…”

“…1. To generate the IDATA structure we use the same annotations on the BPMN models defined in [3]. First, the process variables in lines 3-4 are copied from the global documentation of the model.…”

“…The input is the IDATA where the event is thrown, the event code (required for catching purposes) and its typeInfo. First, if the event is a message, a blockchain event will be written in the event log to notify that a certain point of the process execution was reached (lines [3][4]. Default and Message end events are propagated to the parent only if the execution in the current node (sub-process) is finished (lines 5-7).…”

“…Specifically, the system allows participants to maintain different variants of the same process model or to permanently modify a process model so that all running and future process instances follow the new version of the model. The proposed method has been implemented as an extension of Caterpillar [3], which now comprises two process execution engines: a compilation-based and an interpretationbased engine. An experimental evaluation assesses the costs of the interpreted execution approach compared to existing compiled approaches.…”