eventstore_client 0.7.1

A community developed gRPC client library for EventStoreDB. EventStoreDB is an open-source, functional database with Complex Event Processing capabilities.

A community developed gRPC client library for EventStoreDB.

EventStoreDB is an open-source, functional database with Complex Event Processing capabilities. Documentation for EventStoreDB can be found here.

This package is still in active development and not yet reached a stable version. You should expect breaking changes that will require refactoring both code and behaviors of code depending on this package. When stable 1.0.0 is released, we pledge to keep the package backwards compatible within each major version.

Getting started

• Create a cluster in EventStore Cloud (or deploy your own)
• Create an appropriate connection string
• Create an instance of EventStoreStreamsClient with given connection string

A simple usage example of reading from a self-hosted single-node:

import 'package:eventstore_client/eventstore_client.dart';

void main() async {
  // Create a client instance
  final client = EventStoreStreamsClient(
    EventStoreClientSettings.parse(
      'esdb://127.0.0.1:2113',
    ),
  );

  // Fetch all events in EventStore
  final result = await client.readFromAll(
    forward: true,
    resolveLinks: true,
    position: LogPosition.start,
  );

  // Only print if read was successful
  if (result.isOK) {
    await for (var event in result.stream) {
      print(event);
    }
  }
}

Usage and documentation

This client is not yet properly documented, but we have made some example usages, and published the API reference. For additional documentation about general usage, see the official documentation for gRPC clients on eventstore.com.

Note on gRPC-web support

EventStoreDB does not support the gRPC-web protocol directly (in-process proxy). The EventStore team could add in-process proxy support using Grpc.AspNetCore.Web, but I doubt they will prioritize this because gRCP-web have limited streaming support (details below). If you plan to connect directly to EventStoreDB from a browser, you need to configure a gRCP-web proxy in front of EventStoreDB, that translates between native gRPC and gRPC-web protocols automatically (see official Google implementation of grpc-web).

IMPORTANT!

gRPC-web have limited streaming support. Especially bidirectional streams (both RPC requests and responses are sent and received using streams) which EventStoreDB use for Append-operations, are not supported by the gRPC-web protocol. The following EventStore commands are not supported in the browser as a consequence of these limitations:

// persistent.proto
service PersistentSubscriptions {
  // Client streaming of requests is not supported by grpc-web
  rpc Read (stream ReadReq) returns (stream ReadResp);
}

// streams.proto
service Streams {
  // Client streaming of requests is not supported by grpc-web
  rpc Append (stream AppendReq) returns (AppendResp);
  // Client streaming of requests is not supported by grpc-web
  rpc BatchAppend (stream BatchAppendReq) returns (stream BatchAppendResp);
}

When browser-support for Streams is high enough (see global coverage), the gRCP-web protocol will become optional and web clients will also be able to use the native gRPC protocol.

Choices if you need web-support

You have two principally different options to choose from when connecting to EventStoreDB from a browser with EventStore client.

If you only need to read events, then one of these options is sufficient

1. use Envoy as bridge between gprc-web and native grpc protocols (official Google solution)
2. implement an in-process proxy in EventStoreDB yourself

If you also need to support appending events from the browser, then you need to make a custom gRPC server between the gRCP-web proxy (Envoy or in-process) and EventStoreDB. This custom gRPC server must translate the bi-directional Append command exposed by EventStoreDB to a command exposed by the custom gRCP server that is supported by the gRPC-web protocol. You could do this by implementing an Append command with an unary request (all events in one request) that returns a stream of responses (same as the Append command exposed by EventStoreDB does). This command is supported by gRCP-web and should work (in theory, I have not tested it myself).

My advice

If you need Append-support from the browser, I think the better solution is to wait for the official support of native gRPC that use Streams. I don't know when this will happen, but the global coverage is already high. See the roadmap for full streaming support for official information about web-support.

Multi-platform coding tips

When working with Dart code that only runs on specific platforms, you need organize the code such that it only loads on these platforms. You can do this by organizing platform specific code in files that are conditionally imported and exported. See constants.dart for a simple example and client.dart for a more complicated one. Both operate using the following principle:

// my_library.dart

// Conditional usage based on dart:html support
// NOTE: Conditional could be any other core dart feature
// IMPORTANT! The actual code in each library must have the 
// same contract for this to work. E.g same constant, field, 
// function, class or mixin name that this file uses. 
import 'dep_io.dart' if(dart.library.html) 'dep_web.dart';

// Conditional export of elements in dep_io.dart and dep_web.dart
// NOTE: This is only needed if consumers of my_library need to 
// use any elements in dep_io.dart and dep_web.dart 
// (invoke, create instance, subclass or override).
export 'dep_io.dart' if(dart.library.html) 'dep_web.dart';

// Some code that depends on 'Dep' class with same public 
// interface in both dep_io.dart and dep_web.dart
class MyLibrary extends Dep{}

We have used this method to conditionally set

• the MAX constant to safe maximum values (2^53-1 for js, 2^64-1 for all others)
• set EventStoreClientBase.isGrpcWeb - used to gracefully fail unsupported operations
• add guards that prevents use of EventStore commands that is unsupported on web

Features and bugs

This package is in active development and breaking changes should still be expected. It currently supports the following EventStoreDB client APIs:

• ✅ Streams API
• ✅ Projections Management API
• ✅ Persistent Subscriptions Management API
• ✅ Gossip API (open source version of EventstoreDB)
• ✅ User Management API
• ✅ Operations API
• ✅ Monitoring API (statistics only from v21.x)

We are working on the following APIs:

• ⏱ Elections API

Please file feature requests and bugs at the issue tracker.

Community

There is a community discussion space at Event Store Discuss. If you prefer Discord, there is also an #event-store-db channel. Click to join the community and start asking questions. The maintainers of this community developed client will do their best to answer any questions in both these channels.

Contributing

Development is done on the master branch. As for the official clients, we also attempt to do our best to ensure that the history remains clean and to do so, we generally ask contributors to squash their commits into a set or single logical commit. For additional information about contributing, see CONTRIBUTION.md.