Resumability

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Intro

In this article, we’ll cover how Trigger.dev handles resumability of long-running workflows. We’ll also cover how to use keys to ensure that your workflows are resumable.

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What is resumability?

Resumability is the ability of a workflow run to continue from where it left off after a failure or interruption. For example, if a workflow run is interrupted due to a network failure, it should be able to resume from where it left off when the network is back up.

We accomplish this by storing the state of the workflow run in a database, and when a workflow run is resumed we will call the Trigger.run function again.

We use step “keys” to determine which steps have already been executed. If a step has already been executed, we will skip it and move on to the next step.

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How to use keys

Like we mentioned above, we use step keys to determine which steps have already been executed. They are defined by you inside your Trigger.run function, for example when you call slack.postMessage:

await slack.postMessage("⭐️ New Star", {
  channelName: "github-stars",
  text: `@${starredBy} just starred ${repoName}!`,
});

In this example, the key is the string "⭐️ New Star". This means that if the workflow is interrupted and then resumed, the slack.postMessage step will be skipped because it has already been executed.

If you make multiple calls to slack.postMessage, you should use different keys for each call. For example:

await slack.postMessage("⭐️ New Star", {
  channelName: "github-stars",
  text: `@${starredBy} just starred ${repoName}!`,
});

await slack.postMessage("🚨 Critical Issue", {
  channelName: "critical-issues",
  text: `@${assignee} just opened a critical issue in ${repoName}!`,
});

If you are calling a step multiple times with the same key, it will only be executed once. For example, if you call slack.postMessage with the key "⭐️ New Star" twice, it will only be executed once.

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How to use keys with loops

If you are using a loop, you should use the loop index as the key. For example:

for (let i = 0; i < 10; i++) {
  await ctx.waitFor(`Wait ${i}`, { seconds: 30 });

  await slack.postMessage(`⭐️ New Star ${i}`, {
    channelName: "github-stars",
    text: `@${starredBy} just starred ${repoName}!`,
  });
}

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When to use keys

The following functions in Trigger.dev require a key parameter:

• Integration actions (e.g. slack.postMessage)
• delays
• sendEvent
• fetch

Logging does not require a key, as we are currently using the log message as the key.