  Nowadays, blockchain is becoming a synonym for distributed ledger technology.
However, blockchain is only one of the specializations in the field and is
currently well-covered in existing literature, but mostly from a cryptographic
point of view. Besides blockchain technology, a new paradigm is gaining
momentum: directed acyclic graphs. The contribution presented in this paper is
twofold. Firstly, the paper analyzes distributed ledger technology with an
emphasis on the features relevant to distributed systems. Secondly, the paper
analyses the usage of directed acyclic graph paradigm in the context of
distributed ledgers, and compares it with the blockchain-based solutions. The
two paradigms are compared using representative implementations: Bitcoin,
Ethereum and Nano. We examine representative solutions in terms of the applied
data structures for maintaining the ledger, consensus mechanisms, transaction
confirmation confidence, ledger size, and scalability.
