In my we talked about how we can imple­ment sem­a­phores with Step Func­tions. Anoth­er com­mon sce­nario that many peo­ple have is to han­dle errors from a block of states like we’re used to with a block. last post try-catch try {step1()step2()step3()} catch (States.Timeout) {...} catch (States.ALL) {...} With Step Func­tions, you can use and claus­es to from states. There are a num­ber of , and you can also han­dle cus­tom errors that are thrown by your Lamb­da func­tions. Retry Catch han­dle errors Task pre­de­fined sys­tem errors You can do this by adding the same clause to each of the Task states. Catch "Catch": [{"ErrorEquals": [ "States.ALL" ],"Next": "NotifyError"}] How­ev­er, this approach requires you to add the same boil­er­plate to every state. As your error han­dling strat­e­gy, or the state machine itself becomes more com­plex, this becomes a main­te­nance headache. Task For­tu­nate­ly, both and can be used on states too! Retry Catch Parallel Even if you’re not look­ing to per­form tasks in par­al­lel, you can still use it to sim­pli­fy your error han­dling. In this case, if I wrap , and into a sin­gle branch inside a state, then I can with one clause. Step1 Step2 Step3 Parallel catch unhan­dled errors from any of the steps Catch {"StartAt": "Try","States": {"Try": {"Type": "Parallel","Branches": [{"StartAt": "Step1","States": {"Step1": {"Type": "Task","Resource": "...","Next": "Step2"},"Step2": {"Type": "Task","Resource": "...","Next": "Step3"},"Step3": {"Type": "Task","Resource": "...","End": true}}}],"Catch": [{"ErrorEquals": [ "States.ALL" ],"Next": "NotifyError"}],"Next": "NotifySuccess"},...} One final caveat with this approach is that, a state wraps the out­put from its branch­es into an array. So if sub­se­quent states?—?such as the state in the exam­ple above?—?wants to use the out­put from then it’ll have to take that into con­sid­er­a­tion. Parallel NotifySuccess Step3 What you can do instead, is to add a state to unwrap the array, like this: Pass "UnwrapOutput": {"Type": "Pass","InputPath": "$[0]","Next": "NotifySuccess"} This tech­nique is use­ful when you want to apply the same error han­dling to block of states with­out hav­ing to resort­ing to boil­er­plates. You can add clause to the state to retry the entire block (i.e. from , even if errored). You can also add and for indi­vid­ual states to mix things up too. Retry Parallel Step1 Step3 Retry Catch So that’s it, a nice and short post to share with you a sim­ple tech­nique that I have found use­ful with Step Func­tions. I have been spend­ing a fair bit of time with Step Func­tions and enjoy­ing the ser­vice. Let me know in the com­ments if you have use cas­es that you find dif­fi­cult to imple­ment with Step Func­tions, I would love to hear what oth­ers are doing with it. Hi, my name is . I’m an and the author of . I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless. Yan Cui AWS Serverless Hero Production-Ready Serverless You can contact me via , and . Email Twitter LinkedIn Check out my new course, . Complete Guide to AWS Step Functions In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. Including basic concepts, HTTP and event triggers, activities, design patterns and best practices. Get your copy . here Come learn about operational for AWS Lambda: CI/CD, testing & debugging functions locally, logging, monitoring, distributed tracing, canary deployments, config management, authentication & authorization, VPC, security, error handling, and more. BEST PRACTICES You can also get off the face price with the code . 40% ytcui Get your copy . here

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Step Functions: apply try-catch to a block of states

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