access

Access Node

The access node provides a single point of contact to interact with the Flow network. It implements the Access API

It is a GRPC server which also connects to a collection node and an execution node via GRPC.

At a high level it does the following:

1. Forwards transaction received from the client via the SendTransaction call to the collection node.
2. Forwards all Script related calls (ExecuteScriptAtLatestBlock, ExecuteScriptAtBlockID and ExecuteScriptAtBlockHeight) to one of the execution node
3. Follows updates to the blockchain and locally caches transactions, collections, and sealed blocks.
4. Replies to client API calls for information such as GetBlockByID, GetCollectionByID, GetTransaction etc.

NOTE: The Access node does not participate in the Flow protocol

Table of Contents generated with DocToc

• Terminology
• Processes
  • Transaction Lifecycle
• Engines
  • Follower Engine
  • Ingestion
  • Requester
  • RPC
  • Ping

Terminology

• Transaction - a transaction represents a unit of computation that is submitted to the Flow network.
• Collection - a set of transactions proposed by a cluster of collection nodes.
• Header, also Block Header - a data structure containing the meta-data for a block, including the Merkle root hash for the payload as well as the relevant consensus node signatures.
• Block - the combination of a block header with block contents, representing all the data necessary to construct and validate the entirety of the block.

Processes

Transaction Lifecycle

1. Transactions are received by the access node via the SendTransaction API call.
2. The access node forwards the transaction to one of the Collection node in the Collection node cluster to which this transaction belongs to and stores it locally as well.
3. If a GetTransaction request is received, the transaction is read from local storage and returned if found.
4. If a GetTransactionResult request is received, an execution node is requested for events for the transaction and the transaction status is derived as follows:
   1. If the collection containing the transaction and the block containing that collection is found locally, but the transaction has expired then its status is returned as expired.
   2. If either the collection or the block is not found locally, but the transaction has not expired, then its status is returned as pending
   3. If the transaction has neither expired nor is it pending, but the execution node has not yet executed the transaction, then the status of the transaction is returned as finalized.
   4. If the execution node has executed the transaction, then if the height of the block containing the transaction is greater than the highest sealed block, then the status of the transaction is returned as executed else it is returned as sealed.
   5. If the collection, block, or chain state lookup failed then the status is returned as unknown.

Engines

Engines are units of application logic that are generally responsible for a well-isolated process that is part of the bigger system. They receive messages from the network on selected channels and submit messages to the network on the same channels.

Follower Engine

The Follower engine follows the consensus progress and notifies the ingestion engine of any new finalized block.

Ingestion

The ingestion engine receives finalized blocks from the follower engine and request all the collections for the block via the requester engine. As the collections arrive, it persists the collections and the transactions within the collection in the local storage.

Requester

The requester engine requests collections from the collection nodes on behalf of the ingestion engine.

RPC

The rpc engine is the GRPC server which responds to the Access API requests from clients. It also supports GRPCWebproxy requests.

REST

The rest engine is the HTTP server that implements the OpenAPI schema and handles requests from clients. The API docuemntation is available here.

Ping

The ping engine pings all the other nodes specified in the identity list via a libp2p ping and reports via metrics if the node is reachable or not. This helps identify nodes in the system which are unreachable.

Access node sequence diagram