{"record":{"id":"51fb4e75f81fd61b","repo":"dotnet/orleans","slug":"this-grain-is-already-producing-events","errorCode":null,"errorMessage":"This grain is already producing events","messagePattern":"This grain is already producing events","errorType":"validation","errorClass":"Exception","httpStatus":null,"severity":"error","filePath":"samples/Streaming/Simple/Grains/ProducerGrain.cs","lineNumber":26,"sourceCode":"\npublic class ProducerGrain : Grain, IProducerGrain\n{\n    private readonly ILogger<IProducerGrain> _logger;\n\n    private IAsyncStream<int>? _stream;\n    private IGrainTimer? _timer;\n\n    private int _counter = 0;\n\n    public ProducerGrain(ILogger<IProducerGrain> logger)\n    {\n        _logger = logger;\n    }\n\n    public Task StartProducing(string ns, Guid key)\n    {\n        if (_timer is not null)\n            throw new Exception(\"This grain is already producing events\");\n\n        // Get the stream\n        var streamId = StreamId.Create(ns, key);\n        _stream = this.GetStreamProvider(Constants.StreamProvider)\n            .GetStream<int>(streamId);\n\n        // Register a timer that produces an event every second\n        var period = TimeSpan.FromSeconds(1);\n        _timer = this.RegisterGrainTimer(TimerTick, new GrainTimerCreationOptions\n        {\n            DueTime = period,\n            Period = period,\n            Interleave = true\n        });\n\n        _logger.LogInformation(\"I will produce a new event every {Period}\", period);\n\n        return Task.CompletedTask;","sourceCodeStart":8,"sourceCodeEnd":44,"githubUrl":"https://github.com/dotnet/orleans/blob/fca799fa70ecb6ad975224271703ca43221f58de/samples/Streaming/Simple/Grains/ProducerGrain.cs#L8-L44","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","solutions":["Call StopProducing (which disposes/nulls _timer) before calling StartProducing again.","Make StartProducing idempotent: `if (_timer is not null) return Task.CompletedTask;` instead of throwing.","Ensure only one client owns the produce lifecycle for a given grain key."],"exampleFix":"// before\nif (_timer is not null) throw new Exception(\"This grain is already producing events\");\n\n// after: tolerate a repeat start\nif (_timer is not null) return Task.CompletedTask;","handlingStrategy":"validation","validationCode":"// Make StartProducing idempotent at the call site\nif (_timer is not null) return Task.CompletedTask;","typeGuard":"null","tryCatchPattern":"null","preventionTips":["Call StopProducing before starting again.","Make StartProducing idempotent rather than throwing on a repeat call.","Coordinate which client owns the producer lifecycle for a grain key."],"tags":["streaming","producer","lifecycle","timer","sample"],"backgroundTag":null,"analyzedSha":"fca799fa70ecb6ad975224271703ca43221f58de","analyzedAt":"2026-08-13T19:55:57.938Z","schemaVersion":2},"datasetVersion":"2026-08-14T00:17:13.853Z"}