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Metronome provides programmatic notifications in the form of webhooks. If you configure a webhook URL, Metronome sends an HTTP POST request to that URL when certain events occur, such as an invoice being finalized. Your service can then react to that notification by updating customer state, sending an email, and so on.

Metronome's alerts are the most common source of webhooks. See our alerts documentation for more on how to set these up.

Metronome notifications have this structure:

"id": string, // a unique identifier for this specific notification
"type": string, // the notification type
"properties": {
// additional type-specific properties to describe the notification

The full list of IP addresses that webhook notifications may come from is below. This list may change, but if it does we'll give at least 30 days notice.
Notification configuration

Talk to your Metronome representative to understand and configure which type of notifications you should expect to receive.

To receive Metronome webhooks, you need a webhook handler listening at a publicly accessible HTTPS URL that performs the below tasks.

Acknowledge the notification

Upon receiving a notification, your endpoint must return a successful status code, such as 200 OK. If Metronome receives a status code >299, notification retries are attempted until a successful status code is returned.

If your webhook endpoint does not properly acknowledge the notification, Metronome continuously retries it with exponential backoff until it hits a 15-minute retry cadence. Once the retry process hits 15 minutes, Metronome repeats the notification until it is either accepted or two days have passed since the initial notification attempt (~200 retries).

It is recommended that you process webhooks asynchronously: store the webhook payload in a queue, return a 200 response code, and only then validate or process the payload. Removing webhook processing from the receiving path reduces the likelihood of your systems getting blocked.

Prepare for duplicate notifications

Under normal circumstances, Metronome sends each notification exactly once. However, there are a few situations that could cause you to receive the same notification multiple times:

  • RetriesAs mentioned above, if Metronome receives an error when attempting to deliver the notification to your webhook handler, we retry the notification. Depending on the nature of the error, it's possible that your endpoint receives a notification without acknowledging it, in which case your endpoint receives the same notification again.
  • Multiple webhook URLsIf you configure Metronome to notify multiple webhook URLs, or even the same URL multiple times, notifications are sent multiple times. If you want to ensure duplicate notifications are ignored, you can use the notification's id field to deduplicate.

Verify notifications

Because your webhook endpoint is a public URL, anyone could send a request to it. Before you take actions based on the notification, you should verify the information it contains. You can do this in two ways: by fetching data from the Metronome API yourself or by verifying the webhook request's signature.

Call the Metronome API

Webhook notifications contain only minimal information about the event that occurred. This means it's often useful to call the appropriate Metronome API endpoint to get the full details. For example, if you receive a webhook notification informing you that an invoice has been finalized, you can call the /customers/{customer_id}/invoices/{invoice_id} endpoint to fetch the details of the invoice mentioned in the notification. In this way, the notification serves as a hint that something has changed, but your code relies only on data obtained directly from the Metronome API.

Verify signatures

If the above strategy doesn't work for your use case, Metronome also provides a method to verify the authenticity of notifications as you receive them by using the Metronome-Webhook-Signature HTTP header. The value of this header is a cryptographic signature of the HTTP request, using a secret key set up when you configure your webhook.

Secret keys are unique per webhook

If you have multiple webhooks configured on your Metronome account, each webhook has its own secret key.

To validate the signature, first concatenate the value of the request's Date header and the exact bytes of the request body, separated by a newline character (\n). Then compute the HMAC-SHA256 of the resulting string, keyed by the webhook's secret key. Finally, compare the hexadecimal representation of the HMAC you computed with the one found in the Metronome-Webhook-Signature header. If they don't match, the webhook notification did not come from Metronome.

HMAC_SHA256(secret_key, DATE_HEADER + "\n" + BODY)

The Date header is included to aid in deduplication. You should ignore webhook requests that are older than five minutes, which means your webhook handler only needs to store recent notification IDs to prevent duplicates.

The body must be treated as bytes

When computing the signature, Metronome uses the exact bytes sent in the request body. Be careful to do the same in your code. If you try to use the parsed JSON body for verification purposes, you'll likely fail signature verification because serializing the data again is not guaranteed to produce the same JSON.

The following code example shows how to perform signature validation:

echo -n "$DATE_HEADER\n$BODY" | openssl dgst -sha256 -hmac $KEY

To test this, consider the following example webhook notification. The secret key for verification is correct-horse-battery-staple:

POST /webhook HTTP/1.1
User-Agent: Metronome
Content-Type: application/json
Date: Mon, 02 Jan 2006 22:04:05 GMT
Metronome-Webhook-Signature: b82652fa2246cf1d8a27e591f155c865f68b46c19b9213fd9c052f2419b4742b

"id": "b2c9e307-624e-4e7d-a5a4-1b74107d78c4",
"type": "widget_created",
"properties": {
"customer_id": "5f794d50-085a-4db6-8d15-286e518b7225",
"widget_id": "0891458d-b6f0-4fdd-a41e-380aae1a1e38"
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