IPLD Store

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InterPlanetary Linked Data (IPLD) is a set of libraries which allow for the interoperability of content-addressed data structures across different distributed systems and protocols. It provides a fundamental ‘common language’ to primitive cryptographic hashing, enabling data structures to be verifiably referenced and retrieved between two independent protocols. For example, a user can reference an IPFS directory in an Ethereum transaction or smart contract.

The IPLD Store of a Filecoin Node is local storage for hash-linked data.

IPLD is fundamentally comprised of three layers:

• the Block Layer, which focuses on block formats and addressing, how blocks can advertise or self-describe their codec
• the Data Model Layer, which defines a set of required types that need to be included in any implementation - discussed in more detail below.
• the Schema Layer, which allows for extension of the Data Model to interact with more complex structures without the need for custom translation abstractions.

Further details about IPLD can be found in its specification.

The Data Model

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At its core, IPLD defines a Data Model for representing data. The Data Model is designed for practical implementation across a wide variety of programming languages, while maintaining usability for content-addressed data and a broad range of generalized tools that interact with that data.

The Data Model includes a range of standard primitive types (or “kinds”), such as booleans, integers, strings, nulls and byte arrays, as well as two recursive types: lists and maps. Because IPLD is designed for content-addressed data, it also includes a “link” primitive in its Data Model. In practice, links use the CID specification. IPLD data is organized into “blocks”, where a block is represented by the raw, encoded data and its content-address, or CID. Every content-addressable chunk of data can be represented as a block, and together, blocks can form a coherent graph, or Merkle DAG.

Applications interact with IPLD via the Data Model, and IPLD handles marshalling and unmarshalling via a suite of codecs. IPLD codecs may support the complete Data Model or part of the Data Model. Two codecs that support the complete Data Model are DAG-CBOR and DAG-JSON. These codecs are respectively based on the CBOR and JSON serialization formats but include formalizations that allow them to encapsulate the IPLD Data Model (including its link type) and additional rules that create a strict mapping between any set of data and it’s respective content address (or hash digest). These rules include the mandating of particular ordering of keys when encoding maps, or the sizing of integer types when stored.

IPLD in Filecoin

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IPLD is used in two ways in the Filecoin network:

• All system datastructures are stored using DAG-CBOR (an IPLD codec). DAG-CBOR is a more strict subset of CBOR with a predefined tagging scheme, designed for storage, retrieval and traversal of hash-linked data DAGs. As compared to CBOR, DAG-CBOR can guarantee determinism.
• Files and data stored on the Filecoin network are also stored using various IPLD codecs (not necessarily DAG-CBOR).

IPLD provides a consistent and coherent abstraction above data that allows Filecoin to build and interact with complex, multi-block data structures, such as HAMT and AMT. Filecoin uses the DAG-CBOR codec for the serialization and deserialization of its data structures and interacts with that data using the IPLD Data Model, upon which various tools are built. IPLD Selectors can also be used to address specific nodes within a linked data structure.

IpldStores

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The Filecoin network relies primarily on two distinct IPLD GraphStores:

• One ChainStore which stores the blockchain, including block headers, associated messages, etc.
• One StateStore which stores the payload state from a given blockchain, or the stateTree resulting from all block messages in a given chain being applied to the genesis state by the Filecoin VM.

The ChainStore is downloaded by a node from their peers during the bootstrapping phase of Chain Sync and is stored by the node thereafter. It is updated on every new block reception, or if the node syncs to a new best chain.

The StateStore is computed through the execution of all block messages in a given ChainStore and is stored by the node thereafter. It is updated with every new incoming block’s processing by the VM Interpreter, and referenced accordingly by new blocks produced atop it in the block header’s ParentState field.