  Blokchain is a promising technology to enable distributed and reliable data
sharing at the network edge. The high security in blockchain is undoubtedly a
critical factor for the network to handle important data item. On the other
hand, according to the dilemma in blockchain, an overemphasis on distributed
security will lead to poor transaction-processing capability, which limits the
application of blockchain in data sharing scenarios with high-throughput and
low-latency requirements. To enable demand-oriented distributed services, this
paper investigates the relationship between capability and security in
blockchain from the perspective of block propagation and forking problem.
First, a Markov chain is introduced to analyze the gossiping-based block
propagation among edge servers, which aims to derive block propagation delay
and forking probability. Then, we study the impact of forking on blockchain
capability and security metrics, in terms of transaction throughput,
confirmation delay, fault tolerance, and the probability of malicious
modification. The analytical results show that with the adjustment of block
generation time or block size, transaction throughput improves at the sacrifice
of fault tolerance, and vice versa. Meanwhile, the decline in security can be
offset by adjusting confirmation threshold, at the cost of increasing
confirmation delay. The analysis of capability-security trade-off can provide a
theoretical guideline to manage blockchain performance based on the
requirements of data sharing scenarios.
