Add Ensure Lookup Can Be Retried as an Inngest TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1420 TILs and counting..._
+_1421 TILs and counting..._
 
 ---
 
@@ -410,6 +410,7 @@ _1420 TILs and counting..._
 
 ### Inngest
 
+- [Ensure Lookup Can Be Retried](inngest/ensure-lookup-can-be-retried.md)
 - [Exit Function Early Without Retries](inngest/exit-function-early-without-retries.md)
 
 ### Internet

inngest/ensure-lookup-can-be-retried.md

@@ -0,0 +1,57 @@
+# Ensure Lookup Can Be Retried
+
+A common thing to do in a workflow step is to look up a record. This might be a
+record that was created or updated around the time that the workflow was
+triggered.
+
+You need to be sure the record was found before proceeding. That might end up
+looking like this:
+
+```typescript
+export default inngest.createFunction(
+  { id: "record-user-purchase" },
+  { event: "app/record.purchase" },
+  async ({ event, step }) => {
+    const checkoutSession =
+      await step.run("find checkout session", async () => {
+        const cs = provider.lookupSession(event.checkoutSessionId)
+
+        return cs;
+      });
+
+    if(!checkoutSession) {
+      throw new Error('Checkout session not found')
+    }
+  }
+);
+```
+
+This approach has a subtle problem which is that the error for a missing
+checkout session is raised _outside_ the step that sets `checkoutSession`. As
+inngest does a series of retries, the memorized `checkoutSession` step won't be
+rerun and the error will continue to be thrown.
+
+It is better to raise the error _within_ the lookup step:
+
+```typescript
+export default inngest.createFunction(
+  { id: "record-user-purchase" },
+  { event: "app/record.purchase" },
+  async ({ event, step }) => {
+    const checkoutSession =
+      await step.run("find checkout session", async () => {
+        const cs = provider.lookupSession(event.checkoutSessionId)
+
+        if(!cs) {
+          throw new Error('Checkout session not found')
+        }
+
+        return cs;
+      });
+  }
+);
+```
+
+If the checkout session is missing on the first couple tries, the step will
+have a chance to retry the lookup and maybe eventually find what it is looking
+for.