Cancelling invocations #56
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slinkydeveloper
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This text is meant to be open, so that SDKs can choose how to implement the handling of the cancellation signal.
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slinkydeveloper
commented
Dec 5, 2023
| @@ -41,7 +41,8 @@ Depending on the specific syscall, the Restate runtime generates as response eit | |||
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| Each syscall defines a priori whether it replies with an ack or a completion, or doesn't reply at all. | |||
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| There are a couple of special message streams for initializing and closing the invocation. | |||
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This text was unclear/wrong
slinkydeveloper
commented
Dec 5, 2023
| @@ -41,7 +41,8 @@ Depending on the specific syscall, the Restate runtime generates as response eit | |||
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| Each syscall defines a priori whether it replies with an ack or a completion, or doesn't reply at all. | |||
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| There are a couple of special message streams for initializing and closing the invocation. | |||
| Depending on the programming language and/or specific SDK, the SDKs expose a way to await completions using some form of | |||
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This clarification helps understand a bit the rest of the text i believe.
slinkydeveloper
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Dec 5, 2023
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| ### Cancelling an invocation | ||
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| The Restate runtime has a feature to send a signal to "cancel" an invocation, in order to allow the user to eventually | ||
| execute operations to revert/compensate its progress. | ||
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| The idea is that the SDK receives the signal at the beginning of the invocation, surfaces it in the user space and at | ||
| the same time records in the journal where it surfaced. | ||
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| To start the cancellation process, the runtime invokes the SDK with the `StartMessage.cancelled` set to `true`. It's up | ||
| to the SDKs to decide how and when to surface the cancellation signal to the user code, be it an exception, or a | ||
| saga-like library, or a mix of both. During the handling of the cancellation, the SDK MUST be able to suspend. | ||
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| The SDK must guarantee that the cancellation signal is fired in the user code deterministically, meaning that on a | ||
| subsequent replay of the invocation, e.g. due to a suspension or a network error, the signal should be fired again on | ||
| the same point. For this purpose the SDKs can record, when needed, the point where the cancellation signal was fired by | ||
| using the `CancelledEntryMessage`. | ||
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| When the SDK doesn't implement cancellation, the runtime cancellation signal has no effect on the target invocation. | ||
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Given the SDK has more or less freedom regarding how to implement the cancellation, I kept this text very generic, as from the protocol POV how the sdk exposes the cancellation signal and guarantees its determinism is not important.
Perhaps if we wanna describe these implementation details of the SDKs, we need some other form of text, like SDK features specs or similar.
tillrohrmann
reviewed
Dec 8, 2023
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Fix #52