The following topics provide details about consensus in general, and PBFT in particular. When you are ready to start testing, four PBFT test cases are provided.

What is consensus?

Consensus is a method for validating the order of network requests, or transactions (deploy and invoke), on a blockchain network. The correct ordering of transactions is critical, because many types of network transactions have a dependency on one or more prior transactions (account debits often have a dependency on prior credits, for example). On a blockchain network, there is no single authority that determines the transaction order; instead, each blockchain node (or peer) has an equal say in establishing the order, by implementing the network consensus protocol. Consensus, therefore, ensures that a quorum of nodes agree on the order in which transactions are appended to the shared ledger. By resolving any discrepancies in the proposed transaction order, consensus guarantees that all network nodes are operating on an identical blockchain. In other words, consensus guarantees the integrity and consistency of blockchain network transactions.

What is PBFT?

Practical Byzantine Fault Tolerance (PBFT) is one of various consensus protocols which maintain the order of transactions on a blockchain network, despite threats to this order. One such threat is the arbitrary concurrent failure (a type of Byzantine fault) of multiple network nodes. Using PBFT, a blockchain network of (N) nodes can withstand (f) number of Byzantine nodes, where f = (N-1)/3. Put another way, PBFT requires that a minimum of 2*f + 1 nodes agree on the order of transaction requests before appending them, as a block, to the shared ledger. Deriving either formula reveals the rule that a PBFT network can withstand Byzantine faults on fewer than one-third of all network nodes.

Read more:
https://console.ng.bluemix.net/docs/services/blockchain/etn_pbft.html

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