dotnet/orleans · error · Exception

This grain is already producing events

Error message

This grain is already producing events

What it means

Thrown by the Simple streaming sample's ProducerGrain.StartProducing when _timer is already non-null, i.e. the grain is already registered to emit periodic events. It guards against double-starting the producer timer on a single activation.

Source

Thrown at samples/Streaming/Simple/Grains/ProducerGrain.cs:26

public class ProducerGrain : Grain, IProducerGrain
{
    private readonly ILogger<IProducerGrain> _logger;

    private IAsyncStream<int>? _stream;
    private IGrainTimer? _timer;

    private int _counter = 0;

    public ProducerGrain(ILogger<IProducerGrain> logger)
    {
        _logger = logger;
    }

    public Task StartProducing(string ns, Guid key)
    {
        if (_timer is not null)
            throw new Exception("This grain is already producing events");

        // Get the stream
        var streamId = StreamId.Create(ns, key);
        _stream = this.GetStreamProvider(Constants.StreamProvider)
            .GetStream<int>(streamId);

        // Register a timer that produces an event every second
        var period = TimeSpan.FromSeconds(1);
        _timer = this.RegisterGrainTimer(TimerTick, new GrainTimerCreationOptions
        {
            DueTime = period,
            Period = period,
            Interleave = true
        });

        _logger.LogInformation("I will produce a new event every {Period}", period);

        return Task.CompletedTask;

View on GitHub (pinned to fca799fa70)

Solutions

1. Call StopProducing (which disposes/nulls _timer) before calling StartProducing again.
2. Make StartProducing idempotent: `if (_timer is not null) return Task.CompletedTask;` instead of throwing.
3. Ensure only one client owns the produce lifecycle for a given grain key.

Example fix

// before
if (_timer is not null) throw new Exception("This grain is already producing events");

// after: tolerate a repeat start
if (_timer is not null) return Task.CompletedTask;

// Make StartProducing idempotent at the call site
if (_timer is not null) return Task.CompletedTask;

null

null

When it happens

Trigger: A client calls StartProducing(ns, key) twice on the same producer grain activation without an intervening StopProducing (which nulls _timer). The second call sees _timer is not null and throws a base Exception.

Common situations: A UI/client with a retry button that re-calls StartProducing; multiple clients driving the same producer grain; forgetting that grain activations are single-threaded but state persists across calls within one activation.

Related errors

• Balance has not been written.
• Cannot find file {filename}
• This adapter only supports read
• Recovered {name} does not match. Written: {Serialize(written
• Missing value for {name}.

AI-assisted analysis of dotnet/orleans@fca799fa70 (2026-08-13). Data as JSON: /api/errors/51fb4e75f81fd61b. Report an issue: GitHub.