Address and Namespace Design¶
Hyperledger Sawtooth stores data within a Merkle Tree. Data is stored in leaf nodes, and each node is accessed using an addressing scheme that is composed of 35 bytes, represented as 70 hex characters. The recommended way to construct an address is to use the hex-encoded hash values of the string or strings that make up the address elements. However, the encoding of the address is completely up to the transaction family defining the namespace, and does not need to involve hashing. Hashing is a useful way to deterministically generate likely non-colliding byte arrays of a fixed length.
An address begins with a namespace prefix of six hex characters representing three bytes. The rest of the address, 32 bytes represented as 64 hex characters, can be calculated in various ways. However, certain guidelines should be followed when creating addresses, and specific requirements must be met.
The address must be deterministic: that is, any validator or client that needs to calculate the address must be able to calculate the same address, every time, when given the same inputs.
All data under a namespace prefix follows a consistent address and data encoding/serialization scheme that is determined by the transaction family which defines the namespace.
The namespace prefix consists of six hex characters, or three bytes. An example namespace prefix that utilizes the string making up the transaction family namespace name to calculate the prefix is demonstrated by the following Python code:
prefix = hashlib.sha256("example_txn_family_namespace".encode('utf-8')).hexdigest()[:6]
Alternatively, a namespace prefix can utilize an arbitrary scheme. The current Settings transaction family uses a prefix of ‘000000’, for example.
The rest of the address, or remaining 32 bytes (64 hex characters), must be calculated using a defined deterministic encoding format. Each address within a namespace should be unique, or the namespace consumers must be able to deal with collisions in a deterministic way.
The addressing schema can be as simple or as complex as necessary, based on the requirements of the transaction family.
Simple Example - IntegerKey¶
For a description of the IntegerKey Transaction family, see IntegerKey Transaction Family.
The transaction family prefix is:
This resolves to ‘1cf126’.
To store a value in the entry Name, the address would be calculated like this:
address = "1cf126" + hashlib.sha512('name'.encode('utf-8')).hexdigest()[-64:]
A value could then be stored at this address, by constructing and sending a transaction to a validator, where the transaction will be processed and included in a block.
This address would also be used to retrieve the data.
More Complex Addressing Schemes¶
For a more complex example, let’s use a hypothetical transaction family which stores information on different object types for a widget. The data on each object type is keyed to a unique object identifier.
- prefix = “my-transaction-family-namespace-example”
- object-type = “widget-type”
- unique-object-identifier = ”unique-widget-identifier”
>>> hashlib.sha256("my-transaction-family-namespace-example".encode('utf-8')).hexdigest()[:6] + hashlib.sha256("widget-type".encode('utf-8')).hexdigest()[:4] + hashlib.sha256("unique-widget-identifier".encode('utf-8')).hexdigest()[:60] '4ae1df0ad3ac05fdc7342c50d909d2331e296badb661416896f727131207db276a908e'
In this case, the address is composed partly of a hexdigest made of the widget-type, and partly made up of the unique-widget-identifier. This encoding scheme choice prevents collisions between data objects that have identical identifiers, but which have different object types.
Since the addressing scheme is not mandated beyond the basic requirements, there is a lot of flexibility. The example above is just an example. Your own addressing schema should be designed with your transaction family’s requirements in mind.