# Optional outputs **URL:** \ **Category:** Cylc Support\ **Created:** [May 12, 2023, 9:48am UTC](https://cylc.discourse.group/t/optional-outputs/650 "2023-05-12T09:48:43Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![ukmo-ccbunney](https://yyz2.discourse-cdn.com/free1/user_avatar/cylc.discourse.group/ukmo-ccbunney/32/251_2.png) [@ukmo-ccbunney](https://cylc.discourse.group/u/ukmo-ccbunney)\ **Post date:** [May 12, 2023, 9:48am UTC](https://cylc.discourse.group/t/optional-outputs/650/1 "2023-05-12T09:48:43Z")
[cylc 8.1.4] I am using `?` to set optional output dependencies on a task, but got stuck for a while with the error: `GraphParseError: Output XXXXX:succeeded can't be both required and optional` Here’s a simple example of what I was doing: ```auto [scheduling] [[graph]] R1 = """ a => b b? | b:expired? => c """ ``` This produced the above error. I can only get the error to go away if I add `?` to the first dependency, i.e.: ```auto [scheduling] [[graph]] R1 = """ a => b? # <--- why? b? | b:expired? => c """ ``` but I am struggling to understand why I need to do this. This line is surely just setting the dependencies of `b` triggering based on `a`? It is not setting any dependencies on `b`’s output. Is this by design, or a bug? I can’t quite get my head around it!! The Cylc8 documentation has an example like my code above here: [Scheduling Configuration — Cylc 8.1.4 documentation](https://cylc.github.io/cylc-doc/stable/html/user-guide/writing-workflows/scheduling.html#basic-example), copied below for reference: ```auto foo => bar bar:fail? => recover bar? | recover => baz ``` Thanks! ---
**Author:** ![hilary.j.oliver](https://yyz2.discourse-cdn.com/free1/user_avatar/cylc.discourse.group/hilary.j.oliver/32/4_2.png) [@hilary.j.oliver](https://cylc.discourse.group/u/hilary.j.oliver)\ **Post date:** [May 12, 2023, 10:50am UTC](https://cylc.discourse.group/t/optional-outputs/650/2 "2023-05-12T10:50:55Z")
> [@ukmo-ccbunney](#): > > The Cylc8 documentation has an example like my code above here: [Scheduling Configuration — Cylc 8.1.4 documentation](https://cylc.github.io/cylc-doc/stable/html/user-guide/writing-workflows/scheduling.html#basic-example), copied below for reference: Unfortunately that example in the docs is wrong, it fails with the same error as your graph. We’ll fix it! ---
**Author:** ![ukmo-ccbunney](https://yyz2.discourse-cdn.com/free1/user_avatar/cylc.discourse.group/ukmo-ccbunney/32/251_2.png) [@ukmo-ccbunney](https://cylc.discourse.group/u/ukmo-ccbunney)\ **Post date:** [May 12, 2023, 11:02am UTC](https://cylc.discourse.group/t/optional-outputs/650/3 "2023-05-12T11:02:45Z")
OK - thanks @hilary.j.oliver . It wasn’t clear to me from the docs that the first dependency `a => b` needed an optional output on `b`. I know the docs say that if one graph entry has an dependency on an optional output then it must be declared for all graph entries, but I can’t quite understand what that means in the context of `a => b?`. `a` will always trigger `b`, regardless of whether it has optional outputs. Sorry if I am being a bit dim here !!! 😃 ---
**Author:** ![hilary.j.oliver](https://yyz2.discourse-cdn.com/free1/user_avatar/cylc.discourse.group/hilary.j.oliver/32/4_2.png) [@hilary.j.oliver](https://cylc.discourse.group/u/hilary.j.oliver)\ **Post date:** [May 12, 2023, 11:13am UTC](https://cylc.discourse.group/t/optional-outputs/650/4 "2023-05-12T11:13:36Z")
Not dim at all, it’s quite a change from Cylc 7! The arrows (rather obviously) set the dependence of the downstream task on a particular output of the upstream one. But additionally, in Cylc 8, every appearance of a task in the graph states - whether explicitly or implicitly - the optional or required nature of the relevant task output, and that goes for the downstream task as well. Every reference to a particular task output has to be marked optional if any are, to avoid ambiguity - so you can look at (say) `b` (or `b?`) anywhere in the graph and instantly see if that its success is required (or optional). Even if there is nothing downstream of a task, this matters. This (below) says, if `foo` succeeds, trigger `bar`. It also says **both** `foo` and `bar` are required to succeed (if they run at all) in order for the workflow to finish successfully: ```auto foo => bar ``` If `foo` fails, `bar` will not trigger, and `foo` will be marked as an incomplete task (that did not complete its required outputs), and the as a result the workflow will stall with a warning rather than shut down as finished if there is nothing else to do. This (below) also says, if `foo` fails `bar` will not trigger and `foo` will be marked as incomplete. BUT this time, if `foo` succeeds and `bar` runs, success of `bar` is optional (i.e. we expect that sometimes `bar` might fail and that is OK - either it doesn’t matter, or we have another path in the graph to handle the failure) and the workflow can shut down as complete successfully either way: ```auto foo => bar? ``` I hope that makes a bit more sense? ---
**Author:** ![ukmo-ccbunney](https://yyz2.discourse-cdn.com/free1/user_avatar/cylc.discourse.group/ukmo-ccbunney/32/251_2.png) [@ukmo-ccbunney](https://cylc.discourse.group/u/ukmo-ccbunney)\ **Post date:** [May 12, 2023, 11:55am UTC](https://cylc.discourse.group/t/optional-outputs/650/6 "2023-05-12T11:55:18Z")
Thanks @hilary.j.oliver for the detailed explanation. Yes - I can see how that makes sense now. The pertinent bit that I was not appreciating is that not having `b` as optional, even when it has no downstream tasks, was implying that `b` could not fail… I think I have this straight in my head now. Many thanks for taking the time to answer! 🙂 ---
**Author:** ![MetRonnie](https://yyz2.discourse-cdn.com/free1/user_avatar/cylc.discourse.group/metronnie/32/125_2.png) [@MetRonnie](https://cylc.discourse.group/u/MetRonnie)\ **Post date:** [May 15, 2023, 10:09am UTC](https://cylc.discourse.group/t/optional-outputs/650/7 "2023-05-15T10:09:27Z")
One thing to remember is that `foo => bar` is short for `foo:succeed => bar:succeed`, so Cylc expects `bar` to succeed and therefore failure is not an option, unless you add the question mark to `bar:succeed?` ---
**Author:** ![ukmo-ccbunney](https://yyz2.discourse-cdn.com/free1/user_avatar/cylc.discourse.group/ukmo-ccbunney/32/251_2.png) [@ukmo-ccbunney](https://cylc.discourse.group/u/ukmo-ccbunney)\ **Post date:** [May 16, 2023, 8:05am UTC](https://cylc.discourse.group/t/optional-outputs/650/8 "2023-05-16T08:05:50Z")
> [@MetRonnie](#): > > failure is not an option Ha - that’s my take away from this thread! 😀