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Create Your First Dynamic Content

Harness AI SRE supports two expression languages for creating dynamic content: CEL (Common Expression Language) for logic and conditions, and Mustache for simple variable substitution.

CEL versus Mustache​

Choose the right expression language based on your needs:

FeatureCEL (${{expression}})Mustache ({{variable}})
PurposeLogic and computationSimple variable substitution
Use casesConditions, transformations, calculationsDisplay values, basic templating
CapabilitiesBoolean logic, regex, math, functionsVariable interpolation only
Syntax${{incident.severity == "0"}}{{incident.severity}}
Return typesAny type (string, number, boolean, array, object)String only
MixingCannot mix with Mustache in same fieldCannot mix with CEL in same field

When to Use Each​

Use CEL When You Need:​

  • Conditional logic: Filter or route based on conditions
  • Regex matching: Pattern matching on service names or messages
  • Calculations: Math operations, thresholds, percentages
  • Transformations: String manipulation, datetime formatting
  • Complex logic: Multi-field boolean expressions

Use Mustache When You Need:​

  • Simple display: Show field values in messages
  • Basic templating: Insert variables into text
  • Field mapping: Map source fields to destination fields
  • Static substitution: Replace placeholders with values

Where to Use CEL​

CEL is available in these contexts:

1. Alert Rule Conditions​

Filter incoming alerts before creating incidents.

Example:

alert.severity == "critical" && alert.source.matches("prod-.*")

Go to Use CEL in Alert Rules to learn more.

2. Runbook Trigger Conditions​

Control when runbooks automatically execute.

Example:

incident.severity == "0" && incident.environment == "production"

Go to Use CEL in Runbook Triggers to learn more.

3. Webhook Advanced Mapping Conditions​

Filter webhook payloads before creating alerts.

Example:

webhook.priority == "P1" && webhook.region.matches("us-.*")

Go to Use CEL in Webhooks to learn more.

4. Runbook Action Fields (Inline)​

Embed CEL expressions in text fields for dynamic content.

Example:

Incident ${{incident.title}} has severity ${{incident.severity == "0" ? "CRITICAL" : "Normal"}}

Go to Use CEL in Runbook Actions to learn more.

Where to Use Mustache​

Mustache is available in these contexts:

1. Runbook Action Fields​

Insert field values into messages, tickets, and notifications.

Example:

Incident {{incident.title}} detected in {{incident.environment}}

Go to Use Mustache in Runbook Actions to learn more.

2. Webhook Field Mapping​

Map webhook payload fields to alert properties.

Example:

{{webhook.alert.name}}

Go to Use Mustache in Webhooks to learn more.

Harness-Specific Information​

Feature Flag​

CEL expressions require the feature flag IR_CEL_CONDITIONS. Contact your Harness account team to enable this feature.

Expression Limits​

  • Max CEL expression length: 4,096 characters
  • No mixing: Cannot use both CEL and Mustache in the same field
  • No preview: Expressions cannot be tested before execution

Harness-Specific Data​

Severity values are strings, not numbers:

// ✅ Correct
incident.severity == "0"

// ❌ Wrong - comparing string to number
incident.severity == 0

Timestamps are milliseconds since Unix epoch:

incident.created_at > 1704067200000

Common Patterns​

Severity Checks​

Single severity:

incident.severity == "0"    // SEV0 (Critical)

Multiple severities:

incident.severity in ["0", "1", "2"]

String Operations​

Contains check:

incident.title.contains("database")

Regex matching:

incident.service.matches("^payment-.*")

Regex extraction:

regex.extract(Webhook.parsed_body, r"\"AlarmName\"\:\"(.*)\"")

Case conversion:

incident.environment.upperAscii()

String trimming:

incident.title.trim()

Replace text:

regex.replace(incident.description, r"\n", ", ")

Null Safety and Default Values​

Always check for null:

// ✅ Safe
incident.owner != null && incident.owner.contains("@example.com")

// ❌ May fail if owner is null
incident.owner.contains("@example.com")

Provide default values with orValue():

// Returns "Unknown" if field is null or empty
regex.extract(Webhook.parsed_body, r"\"AlarmName\"\:\"(.*)\"").orValue("")

// Chain multiple operations with safe defaults
incident.owner.orValue("Unassigned").trim()

Mustache Nested Fields​

Access nested data:

{{webhook.metadata.environment}}
{{alert.resource.name}}

Collection Operations​

CEL provides powerful collection operations for working with arrays and lists.

Get collection size:

size(incident.affected_services) > 3

Check if any item matches:

incident.tags.exists(t, t == "customer-impact")

Extract Harness service IDs from impacted services:

Activity.impacted_services.map(s, s.id)
// Returns: ["9280f15c-8c59-4c32-834b-3b36c06d269b", "a1b2c3d4-..."]

Get first service ID:

Activity.impacted_services[0].id

Filter services by name pattern:

Activity.impacted_services.filter(s, s.name.contains("api"))

Count services matching a condition:

size(Activity.impacted_services.filter(s, s.name.contains("api")))

Combine operations:

// Get IDs of all API services
Activity.impacted_services
  .filter(s, s.name.contains("api"))
  .map(s, s.id)
Activity Namespace

Activity.* provides access to incident and activity data in runbook action fields. Use Activity.impacted_services to get the list of Harness services affected by an incident, not just service names.

Examples​

CEL: Dynamic Slack Message​

${{incident.severity == "0" ? "[CRITICAL]" : "[ALERT]"}}
**Service**: ${{incident.service}}
**Environment**: ${{incident.environment}}
**Status**: ${{incident.status}}

${{incident.severity in ["0", "1"] ?
  "**IMMEDIATE ACTION REQUIRED**" :
  "Monitor and triage as needed"}}

View: ${{incident.url}}

Mustache: Simple Jira Ticket​

**Incident**: {{incident.short_id}}
**Title**: {{incident.title}}
**Service**: {{incident.service}}
**Environment**: {{incident.environment}}
**Severity**: SEV{{incident.severity}}

Link: {{incident.url}}

Learn More​

CEL Language Reference​

Harness AI SRE Guides​