A ledger represents the state of the DigitalBits universe at a given point in time. It contains the list of all the accounts and balances, all the orders in the distributed exchange, and any other data that persists.
The first ledger in the history of the network is called the genesis ledger.
Every DigitalBits Consensus Protocol (DCP) round, the network reaches consensus on which transaction set to apply to the last closed ledger; when the new set is applied, a new “last closed ledger” is defined.
Each ledger is cryptographically linked to a unique previous ledger, creating a historical ledger chain that goes back to the genesis ledger.
We define the sequence number of a ledger recursively:
- Genesis ledger has sequence number 1
- Ledger directly following a ledger with sequence number n has sequence number n+1
Every ledger has a ledger header. This header has references to the actual data within the ledger as well as a reference to the previous ledger. References here are cryptographic hashes of the content being referenced–the hashes behave like pointers in typical data structures but with added security guarantees.
You can think of the historical ledger chain as a linked list of ledger headers:
[Genesis] <---- [LedgerHeader_1] <----- … <---- [LedgerHeader_n]
See the protocol file for the object definitions.
Every ledger header has the following fields:
Version: Protocol version of this ledger.
Previous Ledger Hash: Hash of the previous ledger. Forms a chain of ledgers stretching back to the genesis ledger.
SCP value: During consensus all the nodes in the network run SCP and come to agreement about a pdeveloper.digitalbits.ioicular value. This value is stored here and in the next three fields (transaction set hash, close time, and upgrades).
Transaction set result hash: Hash of the results of applying the transaction set. This data is not, strictly speaking, necessary for validating the results of the transactions. However, this data makes it easier for entities to validate the result of a given transaction without having to apply the transaction set to the previous ledger.
Bucket list hash: Hash of all the objects in this ledger. The data structure that contains all the objects is called the bucket list.
Ledger sequence: The sequence number of this ledger.
Total coins: Total number of digitalbits in existence.
Fee pool: Number of digitalbits that have been paid in fees. This number will be added to the Algorithmic pool and reset to 0 the next time transfer between these pools. Note this is denominated in digitalbits, even though a transaction’s
feefield is in nibbs.
ID pool: The last used global ID. These IDs are used for generating objects.
Maximum Number of Transactions: The maximum number of transactions the validators have agreed to process in a given ledger. If more transactions are submitted than this number, the validators will include those with the highest fees.
Base reserve: The reserve the network uses when calculating an account’s minimum balance.
Skip list: Hashes of ledgers in the past. Allows you to jump back in time in the ledger chain without walking back ledger by ledger. There are 4 ledger hashes stored in the skip list. Each slot contains the oldest ledger that is mod of either 50 5000 50000 or 500000 depending on index skipList mod(50), skipList mod(5000), etc.
The ledger is a collection of entries. Currently there are 4 types of ledger entries. They’re specified in
This entry represents an account. In DigitalBits, everything is built around accounts: transactions are performed by accounts, and accounts control the access rights to balances.
Other entries are add-ons, owned by a main account entry. With every new entry attached to the account, the minimum balance in XDB goes up for the account. For details, see fees and minimum balance.
Trustlines are lines of credit the account has given a pdeveloper.digitalbits.ioicular issuer in a specific currency.
Trustline entries define the rules around the use of this currency. Rules can be defined by the user–e.g., setting a balance limit to limit risk–or by the issuer–e.g., an authorized flag.
Offers are entries that an account creates in the orderbook. They are a way to automate simple trading inside the DigitalBits network. For more on offers, refer to the distributed exchange documentation.
Data entries are key value pairs attached to an account. They allow account controllers to attach arbitrary data to their account. It provides a flexible extension point to add application specific data into the ledger.
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