JSON RPC

Overview​

The JSON RPC module implements the JSON RPC API layer, something that dApp developers use to interact with the blockchain.

It includes support for standard json-rpc endpoints, as well as websocket endpoints.

Blockchain Interface​

The Polygon Edge uses the blockchain interface to define all the methods that the JSON RPC module needs to use, in order to deliver its endpoints.

The blockchain interface is implemented by the Minimal server. It is the base implementation that's passed into the JSON RPC layer.

jsonrpc/blockchain.go

type blockchainInterface interface {
    // Header returns the current header of the chain (genesis if empty)
    Header() *types.Header

    // GetReceiptsByHash returns the receipts for a hash
    GetReceiptsByHash(hash types.Hash) ([]*types.Receipt, error)

    // Subscribe subscribes for chain head events
    SubscribeEvents() blockchain.Subscription

    // GetHeaderByNumber returns the header by number
    GetHeaderByNumber(block uint64) (*types.Header, bool)

    // GetAvgGasPrice returns the average gas price
    GetAvgGasPrice() *big.Int

    // AddTx adds a new transaction to the tx pool
    AddTx(tx *types.Transaction) error

    // State returns a reference to the state
    State() state.State

    // BeginTxn starts a transition object
    BeginTxn(parentRoot types.Hash, header *types.Header) (*state.Transition, error)

    // GetBlockByHash gets a block using the provided hash
    GetBlockByHash(hash types.Hash, full bool) (*types.Block, bool)

    // ApplyTxn applies a transaction object to the blockchain
    ApplyTxn(header *types.Header, txn *types.Transaction) ([]byte, bool, error)

    stateHelperInterface
}

ETH Endpoints​

All the standard JSON RPC endpoints are implemented in:

jsonrpc/eth_endpoint.go

Filter Manager​

The Filter Manager is a service that runs alongside the JSON RPC server.

It provides support for filtering blocks on the blockchain. Specifically, it includes both a log and a block level filter.

The Filter Manager relies heavily on Subscription Events, mentioned in the Blockchain section

jsonrpc/filter_manager.go

type Filter struct {
    id string

    // block filter
    block *headElem

    // log cache
    logs []*Log

    // log filter
    logFilter *LogFilter

    // index of the filter in the timer array
    index int

    // next time to timeout
    timestamp time.Time

    // websocket connection
    ws wsConn
}


type FilterManager struct {
    logger hclog.Logger

    store   blockchainInterface
    closeCh chan struct{}

    subscription blockchain.Subscription

    filters map[string]*Filter
    lock    sync.Mutex

    updateCh chan struct{}
    timer    timeHeapImpl
    timeout  time.Duration

    blockStream *blockStream
}

Filter Manager events get dispatched in the Run method:

jsonrpc/filter_manager.go

func (f *FilterManager) Run() {

    // watch for new events in the blockchain
    watchCh := make(chan *blockchain.Event)
    go func() {
        for {
            evnt := f.subscription.GetEvent()
            if evnt == nil {
                return
            }
            watchCh <- evnt
        }
    }()

    var timeoutCh <-chan time.Time
    for {
        // check for the next filter to be removed
        filter := f.nextTimeoutFilter()
        if filter == nil {
            timeoutCh = nil
        } else {
            timeoutCh = time.After(filter.timestamp.Sub(time.Now()))
        }

        select {
        case evnt := <-watchCh:
            // new blockchain event
            if err := f.dispatchEvent(evnt); err != nil {
                f.logger.Error("failed to dispatch event", "err", err)
            }

        case <-timeoutCh:
            // timeout for filter
            if !f.Uninstall(filter.id) {
                f.logger.Error("failed to uninstall filter", "id", filter.id)
            }

        case <-f.updateCh:
            // there is a new filter, reset the loop to start the timeout timer

        case <-f.closeCh:
            // stop the filter manager
            return
        }
    }
}

📜 Resources​

• Ethereum JSON-RPC