A custom enrichment sidecar can add live evidence, runbook links, or dashboard context to Alertmanager notifications before forwarding them to another system. Alertmanager provides the generic webhook receiver and its notification payload; the sidecar itself is an integration you build and operate, not a built-in Alertmanager feature.
Where the sidecar fits
Prometheus sends alerts to Alertmanager, which groups and routes notifications according to labels. A receiver can be a built-in integration or a generic webhook. In the sidecar design, Alertmanager sends an HTTP POST to your service; that service enriches the notification and forwards it to the final destination.
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- Prometheus evaluates alerting rules and sends alerts to Alertmanager.
- Alertmanager groups and routes them, then POSTs a notification to the configured webhook endpoint.
- The sidecar reads the notification, gathers relevant context, and adds it to the downstream message or payload.
- The sidecar forwards the result to a destination such as a chat, incident-management, or alert-management service.
The webhook boundary and payload are documented by Alertmanager; the enrichment and forwarding behavior are architectural choices. Alertmanager’s notification template reference and configuration reference describe the relevant notification and receiver capabilities.
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What the webhook gives the sidecar
Alertmanager sends a grouped notification as JSON. The standard payload includes notification-level information such as its version, group key, receiver, overall status, group and common labels, common annotations, the external Alertmanager URL, and a count of alerts omitted from a truncated notification. Each alert object can include its status, labels, annotations, start and end timestamps, generator URL, and fingerprint. See the notification examples for payload examples.
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Preserve the fields that let people and downstream systems recognize what happened and where it came from. Add explanatory material as context rather than rewriting alert identity.
- Status: retain whether the notification is firing or resolved.
- Labels: preserve the values used to identify and group alerts. Alert labels participate in deduplicating identical instances, so avoid changing them merely to attach enrichment.
- Annotations: use these for explanatory information such as a summary, description, or runbook URL.
- Source references: retain the generator URL and fingerprint where useful for investigation and correlation.
- Timing: keep the start and end timestamps so recipients can distinguish a new event from a resolution.
Alertmanager’s Alerts API documentation explains the role of labels and annotations. If your downstream format differs from Alertmanager’s payload, map the fields explicitly and make sure resolved alerts remain distinguishable.
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Choose where context should come from
Use rule annotations for known context
If a summary, description, or runbook link is known when you define the alert, put it in the alert’s annotations. That keeps static, alert-specific guidance close to the rule and makes it available in the notification.
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A sidecar is useful when context must be looked up at notification time—for example, a relevant dashboard or evidence retrieved from another system. This is an architectural use of the webhook, not a feature that Alertmanager documents as automatic enrichment. Keep retrieved material in a downstream context field or message section, and leave identity labels intact.
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Choose the right webhook payload shape
The standard webhook provides a broad notification envelope. Alertmanager also supports payload templates for a receiving service that needs a tailored JSON structure. A custom template can reduce downstream mapping, but it also creates another format to maintain. Ensure the rendered payload is valid JSON and includes every field the receiver needs, including status and correlation information.
Consider a dedicated receiver
Alertmanager offers multiple integrations as well as the generic webhook. If a dedicated receiver already supports the destination and required context, it may avoid custom forwarding code. A generic webhook is the flexible option when you need a custom service in the path. Alerta, for example, documents an inbound integration for Prometheus Alertmanager notifications in its webhook integration guide.
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Design delivery and failure behavior explicitly
The webhook documentation describes the request format; it does not define the sidecar’s retry policy, timeout behavior, idempotency, dead-letter handling, or what happens when enrichment is unavailable. Those properties depend on your implementation and deployment. Do not treat Alertmanager’s alert-submission behavior as an end-to-end guarantee for delivery from Alertmanager through a sidecar to the final destination.
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- Set bounded timeouts for lookups to external evidence systems so enrichment cannot wait indefinitely.
- Define a fallback: decide whether to forward the original notification, forward a partial enrichment, or fail the delivery when a lookup is unavailable.
- Make forwarding behavior safe for repeated notifications; preserve a stable identifier such as the alert fingerprint so consumers can correlate them.
- Specify how the sidecar handles failed forwarding, retries, and messages it cannot deliver, then validate those choices in the deployed system.
These are engineering recommendations, not guarantees supplied by Alertmanager. Test both normal and failure paths, including resolved notifications and unavailable enrichment sources.
Protect the notification path
Alert payloads and enrichment links may expose operational details. Prometheus’s security model warns that anyone with access to the Alertmanager HTTP endpoint can access its data and can create or resolve alerts. Restrict endpoint access, and configure authentication and transport security appropriate to your deployment. Apply access controls to the sidecar and any systems from which it retrieves evidence as well.
Managed and self-hosted deployments
The webhook pattern is relevant to both self-managed Prometheus deployments and managed Prometheus services, but deployment-specific endpoint, access, and integration details vary. Amazon Managed Service for Prometheus documents Alertmanager API endpoints in its user guide. Compare operational responsibilities, integration requirements, regional availability, and current service terms using the provider’s current documentation.
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