Abstract
Despite the hype about blockchains and distributed ledgers, no formal abstraction of these objects has been proposed (This observation was also pointed out by Maurice Herlihy in his PODC2017 keynote talk). To face this issue, in this paper we provide a proper formulation of a distributed ledger object. In brief, we define a ledger object as a sequence of records, and we provide the operations and the properties that such an object should support. Implementation of a ledger object on top of multiple (possibly geographically dispersed) computing devices gives rise to the distributed ledger object. In contrast to the centralized object, distribution allows operations to be applied concurrently on the ledger, introducing challenges on the consistency of the ledger in each participant. We provide the definitions of three well known consistency guarantees in terms of the operations supported by the ledger object: (1) atomic consistency (linearizability), (2) sequential consistency, and (3) eventual consistency. We then provide implementations of distributed ledgers on asynchronous message passing crash-prone systems using an Atomic Broadcast service, and show that they provide eventual, sequential or atomic consistency semantics. We conclude with a variation of the ledger – the validated ledger – which requires that each record in the ledger satisfies a particular validation rule.
Partially supported by the Spanish Ministry of Science, Innovation and Universities grant DiscoEdge (TIN2017-88749-R), the Regional Government of Madrid (CM) grant Cloud4BigData (S2013/ICE-2894) co-funded by FSE & FEDER, the NSF of China grant 61520106005, and by funds for the promotion of research at the University of Cyprus. We would like to thank Paul Rimba and Neha Narula for helpful discussions.
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Notes
- 1.
We will use distributed ledger from now on, instead of blockchain.
- 2.
We define only one operation to access the value of the ledger for simplicity. In practice, other operations, like those to access individual records in the sequence, will also be available.
- 3.
We make explicit the exchange of request and responses between the process and the ledger to reveal the fact that the ledger is concurrent, i.e., accessed by several processes.
- 4.
Our formal definitions of linearizability and sequential consistency are adapted from [4].
- 5.
A sequence X extends a sequence Y when Y is a prefix of X.
- 6.
The atomic broadcast service used in the algorithms may internally have more restrictive requirements.
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Fernández Anta, A., Georgiou, C., Konwar, K., Nicolaou, N. (2019). Formalizing and Implementing Distributed Ledger Objects. In: Podelski, A., Taïani, F. (eds) Networked Systems. NETYS 2018. Lecture Notes in Computer Science(), vol 11028. Springer, Cham. https://doi.org/10.1007/978-3-030-05529-5_2
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