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Figure 1: XRP Ledger Elements
The XRP Ledger has a new ledger version every several seconds. When the network agrees on the contents of a ledger version, that ledger version is validated, and its contents can never change. The validated ledger versions that preceded it form the ledger history. Even the most recent validated ledger is part of history, as it represents the state of the network as of a short time ago. In the present, the network is evaluating transactions which may be applied and finalized in the next ledger version. While this evaluation is happening, the network has candidate ledger versions that are not yet validated.
Figure 2: XRP Ledger History
A ledger version has two identifiers. One identifier is its ledger index. Ledger versions are numbered incrementally. For example, if the current ledger version has ledger index of 100, the previous has ledger index 99 and the next has ledger index 101. The other identifier is a ledger hash, which is a digital fingerprint of the ledger's contents.
As servers propose transactions to apply to the ledger, they may create several candidate ledger versions with slightly different contents. These candidate ledger versions have the same ledger index but different ledger hashes. Of the many candidates, only one can become validated. All the other candidate ledger versions are discarded. Thus, there is exactly one validated ledger hash for each ledger index in history.
User level changes to the ledger are the results of transactions. Examples of transactions include payments, changes to account settings or trust lines, and offers to trade. Each transaction authorizes one or more changes to the ledger, and is cryptographically signed by an account owner. Transactions are the only way to authorize changes to an account, or to change anything else in the ledger.
Each ledger version also contains a set of transactions and metadata about those transactions. The transactions it includes are only the ones that have been applied to the previous ledger version to create the new ledger version. The metadata records the exact effects of the transaction on the ledger's state data.
Figure 3: Transactions Applied to Ledger Version
The set of transactions included in a ledger instance are recorded in that ledger and allow audits of the XRP Ledger history. If an account balance is different in ledger N+1 than it was in ledger N, then ledger N+1 contains the transaction(s) responsible for the change.
Transactions that appear in a validated ledger may have succeeded in changing the ledger, or may have been processed without doing the requested action. Successful transactions have the tesSUCCESS result code which indicates the requested changes are applied to the ledger. Failed transactions in the ledger have tec class result codes.1
All transactions included in a ledger destroy some XRP as a transaction cost, regardless of whether they had a tes or tec code. The exact amount of XRP to destroy is defined by the signed transaction instructions.
There are other classes of result codes besides tes and tec. Any other classes of result code indicate provisional failures returned by API calls. Only tes and tec codes appear in ledgers. Transactions that are not included in ledgers cannot have any effect on the ledger state (including XRP balances), but transitions that provisionally failed may still end up succeeding.
When working with XRP Ledger APIs, applications must distinguish between candidate transactions proposed for inclusion in a ledger versus validated transactions which are included in a validated ledger. Only transaction results found in a validated ledger are immutable. A candidate transaction may eventually be included in a validated ledger, or it may not.
Important: Some rippled APIs provide provisional results, based on candidate transactions 2. Applications should never rely on provisional results to determine the final outcome of a transaction. The only way to know with certainty that a transaction finally succeeded is to check the status of the transaction until it is both in a validated ledger and has result code tesSUCCESS. If the transaction is in a validated ledger with any other result code, it has failed. If the ledger specified in a transaction’s LastLedgerSequence has been validated, yet the transaction does not appear in that ledger or any before it, then that transaction has failed and can never appear in any ledger. An outcome is final only for transactions that appear in a validated ledger or can never appear because of LastLedgerSequence restrictions as explained later in this document
The XRP Ledger Protocol – Consensus and Validation
The XRP Ledger Protocol – Consensus and Validation
The peer-to-peer XRP Ledger network consists of many independent XRP Ledger servers (typically running rippled) that accept and process transactions. Client applications sign and send transactions to XRP Ledger servers, which relay these candidate transactions throughout the network for processing. Examples of client applications include mobile and web wallets, gateways to financial institutions, and electronic trading platforms.
The servers that receive, relay and process transactions may be either tracking servers or validators. The major functions of tracking servers include distributing transactions from clients and responding to queries about the ledger. Validating servers perform the same functions as tracking servers and also contribute to advancing the ledger history. 3.
While accepting transactions submitted by client applications, each tracking server uses the last validated ledger as a starting point. The accepted transactions are candidates. The servers relay their candidate transactions to their peers, allowing the candidate transactions to propagate throughout the network. Ideally, each candidate transaction would be known to all servers, allowing each to consider the same set of transactions to apply to the last validated ledger. As transactions take time to propagate however, the servers do not work with the same set of candidate transactions at all times. To account for this, the XRP Ledger uses a process called consensus to ensure that the same transactions are processed and validated ledgers are consistent across the peer-to-peer XRP Ledger networks
Consensus
Consensus
The servers on the network share information about candidate transactions. Through the consensus process, validators agree on a specific subset of the candidate transactions to be considered for the next ledger. Consensus is an iterative process in which servers relay proposals, or sets of candidate transactions. Servers communicate and update proposals until a supermajority 4 of chosen validators agree on the same set of candidate transactions.
The servers on the network share information about candidate transactions. Through the consensus process, validators agree on a specific subset of the candidate transactions to be considered for the next ledger. Consensus is an iterative process in which servers relay proposals, or sets of candidate transactions. Servers communicate and update proposals until a supermajority 4 of chosen validators agree on the same set of candidate transactions.
During consensus, each server evaluates proposals from a specific set of servers, known as that server's trusted validators, or Unique Node List (UNL).5 Trusted validators represent a subset of the network which, when taken collectively, is "trusted" not to collude in an attempt to defraud the server evaluating the proposals. This definition of "trust" does not require that each individual chosen validator is trusted. Rather, validators are chosen based on the expectation they will not collude in a coordinated effort to falsify data relayed to the network 6.
Round 2
Round 2
Validators Propose and Revise Transaction Sets
— At the start of consensus, validators may have different sets of transactions. In later rounds, servers modify their proposals to match what their trusted validators proposed. This process determines which transactions they should apply to the ledger version currently being discussed, and which they should postpone for later ledger versions.
Candidate transactions that are not included in the agreed-upon proposal remain candidate transactions. They may be considered again in for the next ledger version. Typically, a transaction which is omitted from one ledger version is included in the next ledger version.
In some circumstances, a transaction could fail to achieve consensus indefinitely. One such circumstance is if the network increases the required transaction cost to a value higher than the transaction provides. The transaction could potentially succeed if the fees are lowered at some point in the future. To ensure that a transaction either succeeds or fails within a limited amount of time, transactions can be set to expire if they are not processed by a certain ledger index. For more information, see Reliable Transaction Submission.
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