bigchaindb/docs/source/topic-guides/models.md

The Transaction, Block and Vote Models

BigchainDB stores all its records in JSON documents.

The three main kinds of records are transactions, blocks and votes. Transactions are used to register, issue, create or transfer things (e.g. assets). Multiple transactions are combined with some other metadata to form blocks. Nodes append votes to blocks. This section is a reference on the details of transactions, blocks and votes.

Below we often refer to cryptographic hashes, keys and signatures. The details of those are covered in the section on cryptography.

Some Words of Caution

BigchainDB is still in the early stages of development. The data models described below may change substantially before BigchainDB reaches a production-ready state (i.e. version 1.0 and higher).

Transaction Concepts

Transactions are the most basic kind of record stored by BigchainDB. There are two kinds: creation transactions and transfer transactions.

A creation transaction can be used to register, issue, create or otherwise initiate the history of a single thing (or asset) in BigchainDB. For example, one might register an identity or a creative work. The things are often called "assets" but they might not be literal assets. A creation transaction also establishes the initial owner or owners of the asset. Only a federation node can create a valid creation transaction (but it's usually made based on a message from a client).

Currently, BigchainDB only supports indivisible assets. You can't split an asset apart into multiple assets, nor can you combine several assets together into one. Issue #129 is an enhancement proposal to support divisible assets.

A transfer transaction can transfer one or more assets to new owners.

BigchainDB works with the Interledger Protocol (ILP), a protocol for transferring assets between different ledgers, blockchains or payment systems.

The owner(s) of an asset can specifiy conditions (ILP crypto-conditions) which others must fulfill (satisfy) in order to become the new owner(s) of the asset. For example, a crypto-condition might require a signature from the owner, or from m-of-n owners (a threshold condition, e.g. 3-of-4).

When someone creates a transfer transaction with the goal of changing an asset's owners, they must fulfill the asset's current crypto-conditions (i.e. in a fulfillment), and they must provide new conditions (including the list of new owners).

Every create transaction contains exactly one fulfillment-condition pair. A transfer transaction can contain multiple fulfillment-condition pairs: one per asset transferred. Every fulfillment in a transfer transaction (input) must correspond to a condition (output) in a previous transaction. The diagram below illustrates some of these concepts: transactions are represented by light grey boxes, fulfillments have a label like f:0, and conditions have a label like c:0.

When a node is asked to check the validity of a transaction, it must do several things; the main things are:

• schema validation,
• double-spending checks (for transfer transactions),
• hash validation (i.e. is the calculated transaction hash equal to its id?), and
• validation of all fulfillments, including validation of cryptographic signatures if they’re among the conditions.

The full details of transaction validation can be found in the code for validate_transaction() in the BaseConsensusRules class of consensus.py (unless other validation rules are being used by a federation, in which case those should be consulted instead).

The Transaction Model

{
    "id": "<hash of transaction, excluding signatures (see explanation)>",
    "version": "<version number of the transaction model>",
    "transaction": {
        "fulfillments": ["<list of fulfillments>"],
        "conditions": ["<list of conditions>"],
        "operation": "<string>",
        "timestamp": "<timestamp from client>",
        "data": {
            "uuid": "<uuid4>",
            "payload": "<any JSON document>"
        }
    }
}

Here's some explanation of the contents of a transaction:

• id: The hash of everything inside the serialized transaction body (i.e. fulfillments, conditions, operation, timestamp and data; see below), with one wrinkle: for each fulfillment in fulfillments, fulfillment is set to null. The id is also the database primary key.
• version: Version number of the transaction model, so that software can support different transaction models.
• transaction:
  • fulfillments: List of fulfillments. Each fulfillment contains a pointer to an unspent asset and a crypto fulfillment that satisfies a spending condition set on the unspent asset. A fulfillment is usually a signature proving the ownership of the asset. See Conditions and Fulfillments below.
  • conditions: List of conditions. Each condition is a crypto-condition that needs to be fulfilled by a transfer transaction in order to transfer ownership to new owners. See Conditions and Fulfillments below.
  • operation: String representation of the operation being performed (currently either "CREATE" or "TRANSFER"). It determines how the transaction should be validated.
  • timestamp: The Unix time when the transaction was created. It's provided by the client. See the section on timestamps.
  • data:
    • uuid: UUID version 4 (random) converted to a string of hex digits in standard form.
    • payload: Can be any JSON document. It may be empty in the case of a transfer transaction.

Later, when we get to the models for the block and the vote, we'll see that both include a signature (from the node which created it). You may wonder why transactions don't have signatures... The answer is that they do! They're just hidden inside the fulfillment string of each fulfillment. A creation transaction is signed by the node that created it. A transfer transaction is signed by whoever currently controls or owns it.

What gets signed? For each fulfillment in the transaction, the "fullfillment message" that gets signed includes the operation, timestamp, data, version, id, corresponding condition, and the fulfillment itself, except with its fulfillment string set to null. The computed signature goes into creating the fulfillment string of the fulfillment.

One other note: Currently, transactions contain only the public keys of asset-owners (i.e. who own an asset or who owned an asset in the past), inside the conditions and fulfillments. A transaction does not contain the public key of the client (computer) which generated and sent it to a BigchainDB node. In fact, there's no need for a client to have a public/private keypair. In the future, each client may also have a keypair, and it may have to sign each sent transaction (using its private key); see Issue #347 on GitHub. In practice, a person might think of their keypair as being both their "ownership-keypair" and their "client-keypair," but there is a difference, just like there's a difference between Joe and Joe's computer.

Conditions and Fulfillments

To create a transaction that transfers an asset to new owners, one must fulfill the asset’s current conditions (crypto-conditions). The most basic kinds of conditions are:

• A hashlock condition: One can fulfill a hashlock condition by providing the correct “preimage” (similar to a password or secret phrase)
• A simple signature condition: One can fulfill a simple signature condition by a providing a valid cryptographic signature (i.e. corresponding to the public key of an owner, usually)
• A timeout condition: Anyone can fulfill a timeout condition before the condition’s expiry time. After the expiry time, nobody can fulfill the condition. Another way to say this is that a timeout condition’s fulfillment is valid (TRUE) before the expiry time and invalid (FALSE) after the expiry time. Note: at the time of writing, timeout conditions are BigchainDB-specific (i.e. not part of the Interledger specs).

A more complex condition can be composed by using n of the above conditions as inputs to an m-of-n threshold condition (a logic gate which outputs TRUE iff m or more inputs are TRUE). If there are n inputs to a threshold condition:

• 1-of-n is the same as a logical OR of all the inputs
• n-of-n is the same as a logical AND of all the inputs

For example, one could create a condition requiring that m (of n) owners provide signatures before their asset can be transferred to new owners.

One can also put different weights on the inputs to threshold condition, along with a threshold that the weighted-sum-of-inputs must pass for the output to be TRUE. Weights could be used, for example, to express the number of shares that someone owns in an asset.

The (single) output of a threshold condition can be used as one of the inputs of other threshold conditions. This means that one can combine threshold conditions to build complex logical expressions, e.g. (x OR y) AND (u OR v).

Aside: In BigchainDB, the output of an m-of-n threshold condition can be inverted on the way out, so an output that would have been TRUE would get changed to FALSE (and vice versa). This enables the creation of NOT, NOR and NAND gates. At the time of writing, this “inverted threshold condition” is BigchainDB-specific (i.e. not part of the Interledger specs). It should only be used in combination with a timeout condition.

When one creates a condition, one can calculate its fulfillment length (e.g. 96). The more complex the condition, the larger its fulfillment length will be. A BigchainDB federation can put an upper limit on the allowed fulfillment length, as a way of capping the complexity of conditions (and the computing time required to validate them).

If someone tries to make a condition where the output of a threshold condition feeds into the input of another “earlier” threshold condition (i.e. in a closed logical circuit), then their computer will take forever to calculate the (infinite) “condition URI”, at least in theory. In practice, their computer will run out of memory or their client software will timeout after a while.

Aside: In what follows, the list of new_owners (in a condition) is always who owned the asset at the time the transaction completed, but before the next transaction started. The list of current_owners (in a fulfillment) is always equal to the list of new_owners in that asset's previous transaction.

Conditions

One New Owner

If there is only one new owner, the condition will be a simple signature condition (i.e. only one signature is required).

{
    "cid": "<condition index>",
    "condition": {
        "details": {
            "bitmask": "<base16 int>",
            "public_key": "<new owner public key>",
            "signature": null,
            "type": "fulfillment",
            "type_id": "<base16 int>"
        },
        "uri": "<string>"
    },
    "new_owners": ["<new owner public key>"]
}

• Condition header:
  • cid: Condition index so that we can reference this output as an input to another transaction. It also matches the input fid, making this the condition to fulfill in order to spend the asset used as input with fid.
  • new_owners: A list containing one item: the public key of the new owner.
• Condition body:
  • bitmask: A set of bits representing the features required by the condition type.
  • public_key: The new_owner's public key.
  • type_id: The fulfillment type ID; see the ILP spec.
  • uri: Binary representation of the condition using only URL-safe characters.

Multiple New Owners

If there are multiple new owners, they can create a ThresholdCondition requiring a signature from each of them in order to spend the asset. For example:

{
    "cid": "<condition index>",
    "condition": {
        "details": {
            "bitmask": 41,
            "subfulfillments": [
                {
                    "bitmask": 32,
                    "public_key": "<new owner 1 public key>",
                    "signature": null,
                    "type": "fulfillment",
                    "type_id": 4,
                    "weight": 1
                },
                {
                    "bitmask": 32,
                    "public_key": "<new owner 2 public key>",
                    "signature": null,
                    "type": "fulfillment",
                    "type_id": 4,
                    "weight": 1
                }
            ],
            "threshold": 2,
            "type": "fulfillment",
            "type_id": 2
        },
        "uri": "cc:2:29:ytNK3X6-bZsbF-nCGDTuopUIMi1HCyCkyPewm6oLI3o:206"},
        "new_owners": [
            "<new owner 1 public key>",
            "<new owner 2 public key>"
        ]
}

• subfulfillments: a list of fulfillments
  • weight: integer weight for each subfulfillment's contribution to the threshold
• threshold: threshold to reach for the subfulfillments to reach a valid fulfillment

The weights and threshold could be adjusted. For example, if the threshold was changed to 1 above, then only one of the new owners would have to provide a signature to spend the asset.

Fulfillments

One Current Owner

If there is only one current owner, the fulfillment will be a simple signature fulfillment (i.e. containing just one signature).

{
    "current_owners": ["<public key of current owner>"],
    "fid": 0,
    "fulfillment": "cf:4:RxFzIE679tFBk8zwEgizhmTuciAylvTUwy6EL6ehddHFJOhK5F4IjwQ1xLu2oQK9iyRCZJdfWAefZVjTt3DeG5j2exqxpGliOPYseNkRAWEakqJ_UrCwgnj92dnFRAEE",
    "input": {
        "cid": 0,
        "txid": "11b3e7d893cc5fdfcf1a1706809c7def290a3b10b0bef6525d10b024649c42d3"
    }
}

• fid: Fulfillment index. It matches a cid in the conditions with a new crypto-condition that the new owner needs to fulfill to spend this asset.
• current_owners: A list of public keys of the current owners; in this case it has just one public key.
• fulfillment: A crypto-conditions URI that encodes the cryptographic fulfillments like signatures and others, see crypto-conditions.
• input: Pointer to the asset and condition of a previous transaction
  • cid: Condition index
  • txid: Transaction id

The Block Model

{
    "id": "<hash of block>",
    "block": {
        "timestamp": "<block-creation timestamp>",
        "transactions": ["<list of transactions>"],
        "node_pubkey": "<public key of the node creating the block>",
        "voters": ["<list of federation nodes public keys>"]
    },
    "signature": "<signature of block>",
    "votes": ["<list of votes>"]
}

• id: The hash of the serialized block (i.e. the timestamp, transactions, node_pubkey, and voters). This is also a database primary key; that's how we ensure that all blocks are unique.
• block:
  • timestamp: The Unix time when the block was created. It's provided by the node that created the block. See the section on timestamps.
  • transactions: A list of the transactions included in the block.
  • node_pubkey: The public key of the node that create the block.
  • voters: A list of public keys of federation nodes. Since the size of the federation may change over time, this will tell us how many nodes existed in the federation when the block was created, so that at a later point in time we can check that the block received the correct number of votes.
• signature: Signature of the block by the node that created the block. (To create the signature, the node serializes the block contents and signs that with its private key.)
• votes: Initially an empty list. New votes are appended as they come in from the nodes.

The Vote Model

Each node must generate a vote for each block, to be appended to that block's votes list. A vote has the following structure:

{
    "node_pubkey": "<the public key of the voting node>",
    "vote": {
        "voting_for_block": "<id of the block the node is voting for>",
        "previous_block": "<id of the block previous to this one>",
        "is_block_valid": "<true|false>",
        "invalid_reason": "<None|DOUBLE_SPEND|TRANSACTIONS_HASH_MISMATCH|NODES_PUBKEYS_MISMATCH",
        "timestamp": "<Unix time when the vote was generated, provided by the voting node>"
    },
    "signature": "<signature of vote>"
}

Note: The invalid_reason was not being used as of v0.1.3 and may be dropped in a future version of BigchainDB.