MongoDB does not trigger scaling

[dmitrizzle]

I've set up KEDA 2.16.1 on my Digital Ocean managed k8s 1.32.1-do.0. This is the status that I get when I check scaledobject:

There appears to be no obvious issues in logs (happy to share more) but scaling isn't working.

Here's the YAML I used for scaledobject:

apiVersion: keda.sh/v1alpha1
kind: ScaledObject
metadata:
  name: mongodb-job
spec:
  scaleTargetRef:
    name: film-q-deployment
  cooldownPeriod: 300
  minReplicaCount: 1
  maxReplicaCount: 3
  triggers:
    - type: mongodb
      metadata:
        connectionStringFromEnv: DATABASE_URI
        dbName: "app-db"
        collection: "job-queue"
        query: '{ "status": "waiting" }'
        queryValue: "1"
      authenticationRef:
        name: mongodb-trigger
---
apiVersion: keda.sh/v1alpha1
kind: TriggerAuthentication
metadata:
  name: mongodb-trigger
spec:
  secretTargetRef:
    - parameter: connectionString
      name: env-secrets
      key: DATABASE_URI

Note that I'm using sealed-secrets to store my MongoDB (Atlas) connection string.

Not sure where I should look next to fix this. Can anyone advise?

[dmitrizzle]

I think I figured this out and I'd like to share what I learned.

My confusion was with the queueValue. My objective was to start scaling when the { "status": "waiting" } query in MongoDB returned one or more documents. However, it appears that KEDA looks at this value and starts scaling after the number of returned documents is greater than queueValue (which, in this case, means 2 or greater). Also, unfortunately, setting queryValue to 0 does not work either, presumably because of how this value is used in calculations.

I'm new to k8s and KEDA so missing the above context made the job confusing. Eventually, I came across a solution that involves using scalingModifiers to achieve what I wanted:

apiVersion: keda.sh/v1alpha1
kind: ScaledObject
metadata:
  ...
spec:
  ...
+  advanced:
+    scalingModifiers:
+      formula: "(waitroom + 1)" # Modifies computed scaled value by adding +1
+      target: "1" # The final computed value matches 1 per replica
  triggers:
    - type: mongodb
+      name: waitroom
      metadata:
        connectionStringFromEnv: DATABASE_URI
        ...
        query: '{ "status": "waiting" }'
        queryValue: "1"
      authenticationRef:
        name: mongodb-trigger