Page 110 - Handbook of Deep Learning in Biomedical Engineering Techniques and Applications

98 Chapter 4 A critical review on using blockchain technology in education domain

3.3 Directed acyclic graph approaches
In a BC, blocks can be added sequentially to a single linear chain,
after a waiting time. Directed acyclic graph (DAG) technology adds
single transaction (not blocks) to multiple branches of a tree struc-
ture in parallel. A DAG is a tree that links a transaction to multiple
transactions. DAG is a public permissioned noneBC-based distrib-
uted ledger technology. IOTA and Byteball use this approach. The
main advantage of DAGs is its speed and scalability. The possibility
of double spending in BCs is eliminated as transactions are
validated based on the previous transactions linked to it.
Challenges include centralization due to preselection of validators
and forming of parasite chains on invalid transactions. BC
consensus requires validation of blocks (with multiple transac-
tions). In DAG, individual nodes act as both miners and validators,
as one transaction validates another [14]. Table 4.3 compares the
features of linear BC with tree-based DAG for digital ledgers.
Some of the advantages of DAG-based approach include the
following:
• Overcoming double spending as DAG validates a particular
transaction based on the previous number of transactions
• Reducing transaction size by linking DAG to existing graphs

Table 4.3 Comparison of BC-based DLT and DAG-based DLT approaches.

BC-based DLT DAG-based DLT

Linear chain of blocks (set of transactions) Tree of transactions
Validation of blocks Validation of transactions
Public/private/consortium Public permissioned
Medium volume of transactions Large volumes of transactions (micro/nanotransactions)
Transparency Less transparent
Immutable Immutable
High latency Low latency
High energy consumption (PoW) Low energy consumption
Higher user fees Avoids user fees
Scalable Highly scalable
Tends to become centralized (power More decentralized
concentration)
Not so Susceptible to security lapse if volume of transactions is
reduced
BC, blockchain; DAG, directed acyclic graph; DLT, digital ledger technology; PoW, proof of work.