Distributed Ledger

A distributed ledger is a database that is replicated and synchronised across multiple independent computers without a central authority. Each computer (called a node) holds a full or partial copy of the ledger, and the network uses a consensus mechanism to ensure all copies agree on the state of accounts and transactions.

Contrast with centralised systems

Traditional databases — used by banks, governments, and most organisations — are centralised. A single entity owns the database, controls who can access it, and determines which transactions are valid.

A distributed ledger inverts this model. Instead of a single source of truth, there are many copies. Instead of a single entity deciding what is valid, the network reaches consensus. This eliminates the need for a trusted intermediary.

The trade-off is efficiency and simplicity. A centralised bank can process transactions far faster than a distributed ledger and can offer customer service (reversing errors, recovering lost passwords). A distributed ledger is slower but does not require trusting a bank.

Nodes and replication

In a distributed ledger, each participating node stores a copy of the ledger and validates incoming transactions. When a new transaction occurs, nodes gossip about it — forwarding it to other nodes — until all nodes have heard about it.

Some systems use full replication, where every node stores every transaction (e.g., Bitcoin). Others use partial replication, where nodes store a subset of data relevant to them. Partial replication reduces storage and bandwidth requirements but complicates auditing.

Consensus mechanisms

For a distributed ledger to function, all nodes must agree on the order and validity of transactions. This is the consensus problem. Various mechanisms exist:

• Proof-of-work — nodes compete to solve puzzles; the fastest wins the right to add the next block.
• Proof-of-stake — validators with locked collateral are randomly selected to propose blocks.
• Delegated proof-of-stake — token holders vote for a small number of validators.
• Practical Byzantine Fault Tolerance — a voting-based consensus used in some enterprise systems.

Each mechanism has trade-offs between decentralisation, scalability, and energy efficiency.

Immutability and fork resistance

Distributed ledgers are often called immutable because altering past records is extremely difficult. If a node tries to rewrite history, other nodes will reject it as invalid. An attacker would need to control a majority of nodes simultaneously, which is expensive on large networks.

However, no distributed ledger is perfectly immutable. With a majority attack (called a 51% attack), an attacker can rewrite history. The security of a distributed ledger depends on its consensus mechanism and the cost of controlling a majority of validators.

Use cases

Distributed ledgers are valuable when:

• Eliminating intermediaries. Payments can move directly between parties without a bank.
• Censorship resistance. No single entity can prevent transactions.
• Transparency. All participants can audit the ledger.
• Reduced fraud. Immutability makes altering records difficult.

Common use cases include:

• Cryptocurrencies. Bitcoin and Ethereum are distributed ledgers.
• Supply-chain tracking. Recording the journey of goods from manufacture to sale.
• Smart contracts. Programs that execute automatically on a distributed ledger.
• Land registries. Recording property ownership in jurisdictions without reliable government records.

Challenges and limitations

Distributed ledgers are slower than centralised systems. A bank’s database can process thousands of transactions per second; Bitcoin processes about seven. This latency is inherent — distributed consensus takes time.

They are also more expensive to operate. Each node maintains a copy of the ledger, consuming storage and bandwidth. Consensus mechanisms require computational work (puzzles or voting), consuming electricity.

Additionally, distributed ledgers are worse at reversing errors. If a transaction is sent to the wrong address by mistake, it cannot be undone like a bank transfer can be. This makes the user experience worse for genuine mistakes.

Variations and alternatives

Not all distributed ledgers use blockchain architecture. Some use a directed acyclic graph (DAG), where each transaction references multiple previous transactions, rather than grouping transactions into blocks. DAGs are potentially faster but more complex to implement.

Others use consensus algorithms that are less energy-intensive than proof-of-work, such as proof-of-stake or voting-based mechanisms.

See also