{"version":1,"lectureId":"01M1DABYJK6ZWC90PVPVTQ9420","attempt":0,"publication":{"slug":"understanding-activity-and-workflow-task-event-histories-in-temporal","title":"Understanding Activity and Workflow Task Event Histories in Temporal","subject":"computer-science","summary":"Every event in a Temporal history is named after the state of the task it belongs to. Remove the leading ActivityTask or WorkflowTask from the name and what remains is Scheduled, Started, Completed, Failed or TimedOut: three steps of one lifecycle, in a fixed order. This lecture lays that lifecycle out, shows why a worker is the party that reports completion and failure while the Temporal cluster is the party that declares a timeout, and reads the Start-To-Close timeout straight off the picture as the window between step two and step three. The same reading then transfers to workflow task events, so an event history in the Web UI can be interpreted rather than decoded.","metaDescription":"Strip the prefix from a Temporal event name and it names the task state: the lifecycle, the closed states, and the Start-To-Close window.","transcript":"Open an event history in the Temporal Web UI and the names are long. Activity Task Scheduled. Activity Task Started. Activity Task Completed. There is a pattern in them, and it is worth a couple of minutes of your attention. Every activity event begins with the same two words. Cross those off, and what is left is a state: Scheduled, in this case. That is the whole convention. It is not a coincidence and it is not occasional. Every activity event in the history is named this way. So here are the five you will meet. Scheduled and Started at the top, then the three ways a task can end: Completed, Failed, and Timed Out. Five names, five states. And those five states are not five separate ideas. They are three steps in one lifecycle, so the next thing to do is lay that lifecycle out. Those five states are three steps, and an activity task always takes them in the same order. Step one is Scheduled: the Temporal cluster has put the task on a task queue. Nothing of yours has run yet. Step two is Started. A worker has polled that queue, taken the task off it, and begun executing your activity code. Step three is a closed state, and there are three of those. Completed, when the worker ran your code all the way through and returned a value. Failed, when running your code produced an error instead. And Timed Out, when the time allowed ran out before any result came back. So Scheduled is always first, Started is always second, and one of those three is always last. Anything else you see between them in the history belongs to the workflow, not to this task. Once you can see those three steps, the timeout names stop being jargon. Take the one called Start To Close. Its first half names step two, the moment the worker began. Its second half names step three, whichever of those closed states the task reaches. So the timeout is the longest gap allowed between those two moments, and nothing else. A Start To Close of ten seconds says exactly this: once a worker picks the task up, it has ten seconds to finish, to fail, or to be declared timed out. Read another one the same way. Schedule To Start spans step one to step two: how long the task may sit on the queue before a worker picks it up. Every timeout in Temporal is named this way: two points of that lifecycle, and the stretch of time between them. Now, which of those three closed states a task lands in is decided by two different parties, and the split between them is not arbitrary. Completed and Failed are the worker's call, and that is because the worker is the thing actually running your code. If your activity function returns a value, the worker reports the task as Completed. If it raises an error instead, the worker reports Failed. Same worker, same process, one difference in what your code did. Timed Out is the odd one out. That decision comes from the Temporal cluster, and not from the worker at all. So why is the cluster the one that decides a timeout? Think about what a timeout usually means in a distributed system. Here is the ordinary case. A worker has taken the task and started running your code, and the cluster is waiting for it to report the result back. Now the worker process dies. It cannot finish the task, so no Completed event is coming. It cannot hand you an error, so no Failed event is coming either. It cannot report anything at all. The only party left that can notice is the cluster. It knows when it handed the task out and how long the task was allowed, so when that time is up with nothing received, the cluster writes the timed out event itself. And that is the whole responsibility model. The worker decides Completed or Failed, because it is the only one that watched your code run. The cluster decides Timed Out, because it is the only one still there to notice. One more thing, and it is the reason any of this is worth learning. The same pattern runs straight through workflow task events. Here are the activity events again, with the state beside each one. And here are the workflow task events. Scheduled, when the cluster puts a workflow task on a queue. Started, when a worker picks it up and runs your workflow code. Then Completed, when the worker is done with it. Failed, when the code raised. And Timed Out, when the cluster stopped waiting. Two different kinds of task, one vocabulary. The state column is the same on both sides, and so is the division of labour: the worker reports completion or failure, and the cluster declares the timeout. So there is nothing here to memorize. Read the suffix on an event, and it tells you where that task had got to. Read the pair of names in a timeout, and it tells you which stretch of that lifecycle the clock was watching.","watch":{"version":1,"scenes":[{"title":"Reading the Event Names","start":0,"end":59.306354166666665,"objects":{"head":"a Heading that says \"Reading the Event Names\"","name":"a Math [text] that says \"$upright(\"ActivityTask\") upright(\"Scheduled\")$\"","names":"a Table [text] that says \"Event in the history What is left ActivityTaskScheduled Scheduled ActivityTaskStarted Started ActivityTaskCompleted Completed ActivityTaskFailed Failed ActivityTaskTimedOut TimedOut\" (rows=(('Event in the history', 'What is left'), ('ActivityTaskSchedu…, header=True)","rule":"a Panel that says \"Drop the leading $upright(\"ActivityTask\")$ from an activity event name. What is left is the state that task was in when the event was written.\""},"beats":[{"start":0,"say":"Open an event history in the Temporal Web UI and the names are long. Activity Task Scheduled. Activity Task Started. Activity Task Completed. There is a pattern in them, and it is worth a couple of minutes of your attention.","live":[],"does":[[0,"head is shown on the screen, written out."],[5.48,"name is shown on the screen, written out."]]},{"start":15.7045,"say":"Every activity event begins with the same two words. Cross those off, and what is left is a state: Scheduled, in this case.","live":["name","head"],"does":[[19.686999999999998,"name (the \"upright(\"ActivityTask\")\" part) is struck through — it is ruled out."],[22.694000000000003,"name (the \"upright(\"Scheduled\")\" part) is emphasized."]]},{"start":25.198,"say":"That is the whole convention. It is not a coincidence and it is not occasional. Every activity event in the history is named this way.","live":null,"does":[[25.198,"name (the \"upright(\"Scheduled\")\" part) is no longer emphasized."],[26.556,"rule is shown on the screen, written out."],[34.6715,"name is hidden from the screen — left the board."],[34.6715,"rule is hidden from the screen — left the board."]]},{"start":35.2715,"say":"So here are the five you will meet. Scheduled and Started at the top, then the three ways a task can end: Completed, Failed, and Timed Out.","live":["head"],"does":[[35.2715,"names is shown on the screen, written out."],[38.05800000000001,"names is shown on the screen, written out."],[38.66100000000001,"names is shown on the screen, written out."],[43.178000000000004,"names is shown on the screen, written out."],[44.246,"names is shown on the screen, written out."],[45.151,"names is shown on the screen, written out."]]},{"start":46.80800000000001,"say":"Five names, five states. And those five states are not five separate ideas. 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Completed, when the worker ran your code all the way through and returned a value.","live":["diagram","head","box_scheduled","label_scheduled","actor_cluster","box_started","label_started","actor_worker","arrow_dequeue"],"does":[[86.00135416666666,"box_completed is shown on the screen, written out."],[86.23449440321022,"label_completed is shown on the screen, written out."],[88.69535416666666,"arrow_completed is shown on the screen, written out."]]},{"start":91.21035416666666,"say":"Failed, when running your code produced an error instead. And Timed Out, when the time allowed ran out before any result came back.","live":["diagram","head","box_scheduled","label_scheduled","actor_cluster","box_started","label_started","actor_worker","arrow_dequeue","box_completed","label_completed","arrow_completed"],"does":[[91.55835416666667,"box_failed is shown on the screen, written out."],[91.88930343444909,"label_failed is shown on the screen, written out."],[93.78735416666667,"arrow_failed is shown on the screen, written out."],[95.78435416666666,"box_timed_out is shown on the screen, written out."],[96.01160312064155,"label_timed_out is shown on the screen, written out."],[98.58235416666666,"arrow_timed_out is shown on the screen, written out."]]},{"start":100.52985416666667,"say":"So Scheduled is always first, Started is always second, and one of those three is always last. 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Its first half names step two, the moment the worker began.","live":null,"does":[[121.10935416666666,"diagram moves to a new place on the board."],[121.10935416666666,"timeout_math is shown on the screen, written out."],[122.14335416666667,"drop_started is shown on the screen, written out."]]},{"start":125.62235416666667,"say":"Its second half names step three, whichever of those closed states the task reaches. 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Started, when a worker picks it up and runs your workflow code.","live":["label_left","head"],"does":[[287.40177083333333,"label_right is shown on the screen, written out."],[290.7217708333333,"workflow is shown on the screen, written out."],[294.80877083333337,"workflow is shown on the screen, written out."]]},{"start":298.9382708333333,"say":"Then Completed, when the worker is done with it. Failed, when the code raised. And Timed Out, when the cluster stopped waiting.","live":["label_left","label_right","head"],"does":[[299.59977083333337,"workflow is shown on the screen, written out."],[302.65277083333336,"workflow is shown on the screen, written out."],[305.59077083333335,"workflow is shown on the screen, written out."]]},{"start":309.08077083333336,"say":"Two different kinds of task, one vocabulary. The state column is the same on both sides, and so is the division of labour: the worker reports completion or failure, and the cluster declares the timeout.","live":null,"does":[[314.37477083333334,"activity (the \"column=2\" part) is emphasized."],[314.37477083333334,"workflow (the \"column=2\" part) is emphasized."],[316.63877083333335,"activity (the \"column=2\" part) is no longer emphasized."],[316.63877083333335,"workflow (the \"column=2\" part) is no longer emphasized."]]},{"start":323.22977083333336,"say":"So there is nothing here to memorize. Read the suffix on an event, and it tells you where that task had got to. Read the pair of names in a timeout, and it tells you which stretch of that lifecycle the clock was watching.","live":null,"does":[[326.44577083333337,"activity (the \"column=2\" part) is indicated — a transient flash."],[326.44577083333337,"workflow (the \"column=2\" part) is indicated — a transient flash."],[336.6694791666667,"activity is hidden from the screen — left the board."],[336.6694791666667,"head is hidden from the screen — left the board."],[336.6694791666667,"label_left is hidden from the screen — left the board."],[336.6694791666667,"label_right is hidden from the screen — left the board."],[336.6694791666667,"workflow is hidden from the screen — left the board."]]}]}]},"durationSeconds":338,"chapters":[{"title":"Reading the Event Names","startSeconds":0,"narration":"Open an event history in the Temporal Web UI and the names are long. Activity Task Scheduled. Activity Task Started. Activity Task Completed. There is a pattern in them, and it is worth a couple of minutes of your attention. Every activity event begins with the same two words. Cross those off, and what is left is a state: Scheduled, in this case. That is the whole convention. It is not a coincidence and it is not occasional. Every activity event in the history is named this way. So here are the five you will meet. Scheduled and Started at the top, then the three ways a task can end: Completed, Failed, and Timed Out. Five names, five states. And those five states are not five separate ideas. They are three steps in one lifecycle, so the next thing to do is lay that lifecycle out."},{"title":"Three Steps, and the Window Between Them","startSeconds":59.306354166666665,"narration":"Those five states are three steps, and an activity task always takes them in the same order. Step one is Scheduled: the Temporal cluster has put the task on a task queue. Nothing of yours has run yet. Step two is Started. A worker has polled that queue, taken the task off it, and begun executing your activity code. Step three is a closed state, and there are three of those. Completed, when the worker ran your code all the way through and returned a value. Failed, when running your code produced an error instead. And Timed Out, when the time allowed ran out before any result came back. So Scheduled is always first, Started is always second, and one of those three is always last. Anything else you see between them in the history belongs to the workflow, not to this task. Once you can see those three steps, the timeout names stop being jargon. Take the one called Start To Close. Its first half names step two, the moment the worker began. Its second half names step three, whichever of those closed states the task reaches. So the timeout is the longest gap allowed between those two moments, and nothing else. A Start To Close of ten seconds says exactly this: once a worker picks the task up, it has ten seconds to finish, to fail, or to be declared timed out. Read another one the same way. Schedule To Start spans step one to step two: how long the task may sit on the queue before a worker picks it up. Every timeout in Temporal is named this way: two points of that lifecycle, and the stretch of time between them."},{"title":"Who Decides How It Ends","startSeconds":168.65925000000004,"narration":"Now, which of those three closed states a task lands in is decided by two different parties, and the split between them is not arbitrary. Completed and Failed are the worker's call, and that is because the worker is the thing actually running your code. If your activity function returns a value, the worker reports the task as Completed. If it raises an error instead, the worker reports Failed. Same worker, same process, one difference in what your code did. Timed Out is the odd one out. That decision comes from the Temporal cluster, and not from the worker at all. So why is the cluster the one that decides a timeout? Think about what a timeout usually means in a distributed system. Here is the ordinary case. A worker has taken the task and started running your code, and the cluster is waiting for it to report the result back. Now the worker process dies. It cannot finish the task, so no Completed event is coming. It cannot hand you an error, so no Failed event is coming either. It cannot report anything at all. The only party left that can notice is the cluster. It knows when it handed the task out and how long the task was allowed, so when that time is up with nothing received, the cluster writes the timed out event itself. And that is the whole responsibility model. The worker decides Completed or Failed, because it is the only one that watched your code run. The cluster decides Timed Out, because it is the only one still there to notice."},{"title":"The Same Pattern for Workflow Tasks","startSeconds":274.77377083333334,"narration":"One more thing, and it is the reason any of this is worth learning. The same pattern runs straight through workflow task events. Here are the activity events again, with the state beside each one. And here are the workflow task events. Scheduled, when the cluster puts a workflow task on a queue. Started, when a worker picks it up and runs your workflow code. Then Completed, when the worker is done with it. Failed, when the code raised. And Timed Out, when the cluster stopped waiting. Two different kinds of task, one vocabulary. The state column is the same on both sides, and so is the division of labour: the worker reports completion or failure, and the cluster declares the timeout. So there is nothing here to memorize. Read the suffix on an event, and it tells you where that task had got to. Read the pair of names in a timeout, and it tells you which stretch of that lifecycle the clock was watching."}]}}