github.com/decred/dcrd/rpcclient (go): Package rpcclient implements a websocket-enabled Decred JSON-RPC client.  This client provides a robust and easy to use client for interfacing with a Decred RPC server that uses a mostly btcd/bitcoin core style Decred JSON-RPC API.  This client has been tested with dcrd (https://github.com/decred/dcrd) and dcrwallet (https://github.com/decred/dcrwallet).  In addition to the compatible standard HTTP POST JSON-RPC API, dcrd and dcrwallet provide a websocket interface that is more efficient than the standard HTTP POST method of accessing RPC.  The section below discusses the differences between HTTP POST and websockets.  By default, this client assumes the RPC server supports websockets and has TLS enabled.  In practice, this currently means it assumes you are talking to dcrd or dcrwallet by default.  However, configuration options are provided to fall back to HTTP POST and disable TLS to support talking with inferior bitcoin core style RPC servers.  In HTTP POST-based JSON-RPC, every request creates a new HTTP connection, issues the call, waits for the response, and closes the connection.  This adds quite a bit of overhead to every call and lacks flexibility for features such as notifications.  In contrast, the websocket-based JSON-RPC interface provided by dcrd and dcrwallet only uses a single connection that remains open and allows asynchronous bi-directional communication.  The websocket interface supports all of the same commands as HTTP POST, but they can be invoked without having to go through a connect/disconnect cycle for every call.  In addition, the websocket interface provides other nice features such as the ability to register for asynchronous notifications of various events.  The client provides both a synchronous (blocking) and asynchronous API.  The synchronous (blocking) API is typically sufficient for most use cases.  It works by issuing the RPC and blocking until the response is received.  This allows  straightforward code where you have the response as soon as the function returns.  The asynchronous API works on the concept of futures.  When you invoke the async version of a command, it will quickly return an instance of a type that promises to provide the result of the RPC at some future time.  In the background, the RPC call is issued and the result is stored in the returned instance.  Invoking the Receive method on the returned instance will either return the result immediately if it has already arrived, or block until it has.  This is useful since it provides the caller with greater control over concurrency.  The first important part of notifications is to realize that they will only work when connected via websockets.  This should intuitively make sense because HTTP POST mode does not keep a connection open!  All notifications provided by dcrd require registration to opt-in.  For example, if you want to be notified when funds are received by a set of addresses, you register the addresses via the NotifyReceived (or NotifyReceivedAsync) function.  Notifications are exposed by the client through the use of callback handlers which are setup via a NotificationHandlers instance that is specified by the caller when creating the client.  It is important that these notification handlers complete quickly since they are intentionally in the main read loop and will block further reads until they complete.  This provides the caller with the flexibility to decide what to do when notifications are coming in faster than they are being handled.  In particular this means issuing a blocking RPC call from a callback handler will cause a deadlock as more server responses won't be read until the callback returns, but the callback would be waiting for a response.   Thus, any additional RPCs must be issued an a completely decoupled manner.  By default, when running in websockets mode, this client will automatically keep trying to reconnect to the RPC server should the connection be lost.  There is a back-off in between each connection attempt until it reaches one try per minute.  Once a connection is re-established, all previously registered notifications are automatically re-registered and any in-flight commands are re-issued.  This means from the caller's perspective, the request simply takes longer to complete.  The caller may invoke the Shutdown method on the client to force the client to cease reconnect attempts and return ErrClientShutdown for all outstanding commands.  The automatic reconnection can be disabled by setting the DisableAutoReconnect flag to true in the connection config when creating the client.  Minor RPC Server Differences and Chain/Wallet Separation  Some of the commands are extensions specific to a particular RPC server.  For example, the DebugLevel call is an extension only provided by dcrd (and dcrwallet passthrough).  Therefore if you call one of these commands against an RPC server that doesn't provide them, you will get an unimplemented error from the server.  An effort has been made to call out which commmands are extensions in their documentation.  Also, it is important to realize that dcrd intentionally separates the wallet functionality into a separate process named dcrwallet.  This means if you are connected to the dcrd RPC server directly, only the RPCs which are related to chain services will be available.  Depending on your application, you might only need chain-related RPCs.  In contrast, dcrwallet provides pass through treatment for chain-related RPCs, so it supports them in addition to wallet-related RPCs.  There are 3 categories of errors that will be returned throughout this package:  The first category of errors are typically one of ErrInvalidAuth, ErrInvalidEndpoint, ErrClientDisconnect, or ErrClientShutdown.  NOTE: The ErrClientDisconnect will not be returned unless the DisableAutoReconnect flag is set since the client automatically handles reconnect by default as previously described.  The second category of errors typically indicates a programmer error and as such the type can vary, but usually will be best handled by simply showing/logging it.  The third category of errors, that is errors returned by the server, can be detected by type asserting the error in a *dcrjson.RPCError.  For example, to detect if a command is unimplemented by the remote RPC server:  The following full-blown client examples are in the examples directory:
Keywords: blockchain|cryptocurrency|dcrd|decred|decred-daemon|decred-nodes|go|golang|p2p|peer-to-peer