Async Workflow Queues

Async Workflow Queues enable asynchronous workflow starts via Kafka, providing rate limiting, buffering, and decoupling of workflow initiation from execution.

Overview

Async Workflow Queues provide:

Rate Limiting: Control workflow start rate per domain
Buffering: Queue workflow starts during high load
Decoupling: Separate request acceptance from execution
Backpressure: Prevent overload from workflow start storms
Multi-Tenancy: Per-domain queue configuration

Architecture

Client → Frontend → Kafka Topic → Async Worker → History Service
                     (Queue)      (Consumer)

Client submits workflow start request
Frontend validates and publishes to Kafka
Request acknowledged immediately
Async worker consumes from Kafka
Worker starts workflow via normal path
Rate limiting and retries handled by worker

Setup

Prerequisites

Kafka cluster (version 2.0+)
Cadence server with async workflow feature enabled
Worker service for consuming queue

Kafka Configuration

Create Kafka topics for async workflow queue:

# Create topic
kafka-topics.sh --create \
  --bootstrap-server kafka:9092 \
  --topic cadence-async-workflow-queue \
  --partitions 10 \
  --replication-factor 3 \
  --config retention.ms=86400000 \
  --config max.message.bytes=4194304

# Verify topic
kafka-topics.sh --describe \
  --bootstrap-server kafka:9092 \
  --topic cadence-async-workflow-queue

Partitions: More partitions = higher throughput, but more consumers needed Retention: How long requests stay in queue (24h = 86400000ms)

Server Configuration

Enable async workflow queues in config.yaml:

publicclient:
  hostPort: "127.0.0.1:7833"

kafka:
  clusters:
    test:
      brokers:
        - kafka1.example.com:9092
        - kafka2.example.com:9092
        - kafka3.example.com:9092
  topics:
    cadence-async-workflow-queue:
      cluster: test
  applications:
    async-wf-consumer:
      topic: cadence-async-workflow-queue
      cluster: test
      dlq-topic: cadence-async-workflow-dlq

services:
  worker:
    asyncWorkflowQueueProvider:
      enabled: true
      kafkaConfig:
        cluster: test
        topic: cadence-async-workflow-queue
        consumerGroup: cadence-async-wf-consumer
        concurrency: 100

Domain Configuration

Enable async workflow queue per domain:

# Get current configuration
cadence admin domain get-async-wf-config --domain my-domain

# Update configuration
cadence admin domain update-async-wf-config \
  --domain my-domain \
  --json '{
    "enabled": true,
    "predefinedQueueName": "cadence-async-workflow-queue",
    "queueType": "kafka",
    "queueConfig": {
      "encodingType": "JSON",
      "version": 0
    }
  }'

Configuration Fields:

enabled: Enable/disable async queue for domain
predefinedQueueName: Kafka topic name
queueType: Queue backend (currently only “kafka”)
queueConfig: Queue-specific configuration

Worker Service Setup

Async workers are part of the Cadence worker service:

# Start worker service with async queue consumer
./cadence-server --zone async_wf_kafka_queue start

Or via Docker:

services:
  cadence-worker:
    image: ubercadence/server:master-auto-setup
    command:
      - /bin/sh
      - -c
      - |
        /cadence-server --zone async_wf_kafka_queue start
    environment:
      - KAFKA_SEEDS=kafka:9092
    depends_on:
      - kafka

Usage

Starting Async Workflows

Go SDK:

import (
    "go.uber.org/cadence/client"
)

func startAsyncWorkflow(c client.Client) error {
    options := client.StartWorkflowOptions{
        ID:                           "workflow-1",
        TaskList:                     "my-task-list",
        ExecutionStartToCloseTimeout: time.Hour,
    }
    
    // Start asynchronously
    // Returns immediately after queueing to Kafka
    we, err := c.StartWorkflow(context.Background(), options, MyWorkflow)
    if err != nil {
        return err
    }
    
    // WorkflowExecution returned immediately
    // Actual workflow start happens asynchronously
    fmt.Printf("Queued workflow: %s\n", we.ID)
    
    return nil
}

Java SDK:

WorkflowOptions options = WorkflowOptions.newBuilder()
    .setWorkflowId("workflow-1")
    .setTaskList("my-task-list")
    .setExecutionStartToCloseTimeout(Duration.ofHours(1))
    .build();

MyWorkflow workflow = client.newWorkflowStub(MyWorkflow.class, options);

// Start asynchronously
WorkflowExecution execution = WorkflowClient.start(workflow::processOrder, order);

// Returns immediately after queueing
System.out.println("Queued workflow: " + execution.getWorkflowId());

CLI:

# Start workflow (async if domain configured)
cadence --do my-domain workflow start \
  --tl my-task-list \
  --wt MyWorkflow \
  --et 3600 \
  --input '{"orderId": "12345"}'

No special flags needed - async behavior automatic if domain is configured.

Checking Workflow Status

Workflow may not start immediately:

# Check if workflow has started
cadence --do my-domain workflow describe -w workflow-1

# List workflows
cadence --do my-domain workflow list

# Observe workflow (waits for start)
cadence --do my-domain workflow observe -w workflow-1

Rate Limiting

Consumer Rate Limiting

Control workflow start rate:

# Dynamic config
worker.asyncWorkflowConsumerStartRPS:
  - value: 1000
    constraints:
      domainName: "my-domain"

worker.asyncWorkflowConsumerConcurrency:
  - value: 100
    constraints:
      domainName: "my-domain"

Backpressure Handling

Kafka provides natural backpressure:

Queue Full: Kafka rejects new messages if retention exceeded
Slow Consumption: Messages accumulate in Kafka
Consumer Lag: Monitor consumer_lag metric

Monitoring

Key Metrics

Queue Depth:

# Messages waiting in queue
sum(kafka_consumer_lag{topic="cadence-async-workflow-queue"})

Consumption Rate:

# Workflows started per second
sum(rate(cadence_async_workflow_consumed_total[5m]))

Failure Rate:

# Failed workflow starts
sum(rate(cadence_async_workflow_failed_total[5m]))

End-to-End Latency:

# Time from queue to workflow start
histogram_quantile(0.95,
  rate(cadence_async_workflow_e2e_latency_bucket[5m])
)

CLI Monitoring

# Check domain configuration
cadence admin domain get-async-wf-config --domain my-domain

# Monitor Kafka consumer lag
kafka-consumer-groups.sh \
  --bootstrap-server kafka:9092 \
  --describe \
  --group cadence-async-wf-consumer

Alerting

High Consumer Lag:

alert: AsyncWorkflowHighLag
expr: kafka_consumer_lag{topic="cadence-async-workflow-queue"} > 10000
for: 5m
annotations:
  summary: "Async workflow queue backed up"

High Failure Rate:

alert: AsyncWorkflowHighFailures
expr: rate(cadence_async_workflow_failed_total[5m]) > 10
for: 5m
annotations:
  summary: "High async workflow failure rate"

Best Practices

Queue Configuration

Partition Count: Start with num_workers * concurrency / 1000 partitions
Retention: Set based on acceptable delay (24h typical)
Replication: Use RF=3 for durability
Max Message Size: Match Cadence payload limits

Consumer Tuning

Concurrency: Balance throughput vs. resource usage
Rate Limiting: Prevent overload of downstream services
Batch Size: Tune Kafka consumer fetch.min.bytes
Commit Interval: Balance consistency vs. throughput

Operational

Monitor Lag: Alert if lag exceeds threshold
DLQ: Configure dead-letter queue for failed starts
Capacity Planning: Size workers for peak load + headroom
Testing: Test queue behavior under load

Use Cases

Rate-Limited API

Protect backend from workflow start storms:

// API endpoint
func createOrder(w http.ResponseWriter, r *http.Request) {
    // Parse request
    var order Order
    json.NewDecoder(r.Body).Decode(&order)
    
    // Start workflow asynchronously
    // Returns immediately, rate limited by queue consumer
    we, err := cadenceClient.StartWorkflow(
        context.Background(),
        client.StartWorkflowOptions{
            ID:       fmt.Sprintf("order-%s", order.ID),
            TaskList: "order-processing",
        },
        ProcessOrderWorkflow,
        order,
    )
    
    if err != nil {
        http.Error(w, err.Error(), 500)
        return
    }
    
    // Respond immediately
    json.NewEncoder(w).Encode(map[string]string{
        "workflowId": we.ID,
        "status":     "queued",
    })
}

Batch Processing

Queue large batches of workflow starts:

func processBatch(items []Item) error {
    for _, item := range items {
        // Each workflow start goes to queue
        // No blocking on workflow execution
        _, err := cadenceClient.StartWorkflow(
            context.Background(),
            client.StartWorkflowOptions{
                ID: fmt.Sprintf("batch-item-%s", item.ID),
            },
            ProcessItemWorkflow,
            item,
        )
        if err != nil {
            log.Printf("Failed to queue: %v", err)
            // Continue with other items
        }
    }
    return nil
}

Decoupled Services

Decouple request service from workflow execution:

Request Service → Kafka → Async Worker → Workflow
    (Fast)         (Buffer)  (Rate Limited)  (Execution)

Request service can scale independently of workflow capacity.

Troubleshooting

High Consumer Lag

Problem: Kafka consumer lag increasing Solution:

Scale up worker instances
Increase consumer concurrency
Check for slow workflow starts
Review rate limiting settings
Verify workers are healthy

Workflows Not Starting

Problem: Workflows queued but not executing Solution:

# Check async worker status
cadence admin cluster health

# Verify Kafka connectivity
kafka-console-consumer.sh \
  --bootstrap-server kafka:9092 \
  --topic cadence-async-workflow-queue \
  --from-beginning

# Check worker logs
grep "async.*workflow" /var/log/cadence/worker.log

# Verify domain configuration
cadence admin domain get-async-wf-config --domain my-domain

Failed Workflow Starts

Problem: High failure rate for async workflow starts Solution:

Check DLQ for failed messages
Review error logs in worker
Verify workflow registration
Check task list worker availability
Validate workflow input payloads

Advanced Topics

Dead Letter Queue

Handle failed workflow starts:

kafka:
  topics:
    cadence-async-workflow-dlq:
      cluster: test
  applications:
    async-wf-consumer:
      topic: cadence-async-workflow-queue
      dlq-topic: cadence-async-workflow-dlq

Failed starts go to DLQ after max retries. Process DLQ separately:

# Monitor DLQ
kafka-console-consumer.sh \
  --bootstrap-server kafka:9092 \
  --topic cadence-async-workflow-dlq

# Replay from DLQ (custom tool)
./replay-dlq --topic cadence-async-workflow-dlq

Custom Queue Implementation

Implement custom queue backend:

type Queue interface {
    // Enqueue workflow start request
    Enqueue(ctx context.Context, req *StartRequest) error
    
    // Consume workflow start requests
    Consume(ctx context.Context) (<-chan *StartRequest, error)
}

Next Steps

Configure Isolation Groups for zone awareness
Set up Dynamic Config for queue tuning
Monitor with Web UI
Test with Bench for load validation

Development Tools

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Async Workflow Queues

Overview

Architecture

Setup

Prerequisites

Kafka Configuration

Server Configuration

Domain Configuration

Worker Service Setup

Usage

Starting Async Workflows

Checking Workflow Status

Rate Limiting

Consumer Rate Limiting

Backpressure Handling

Monitoring

Key Metrics

CLI Monitoring

Alerting

Best Practices

Queue Configuration

Consumer Tuning

Operational

Use Cases

Rate-Limited API

Batch Processing

Decoupled Services

Troubleshooting

High Consumer Lag

Workflows Not Starting

Failed Workflow Starts

Advanced Topics

Dead Letter Queue

Custom Queue Implementation

Next Steps

Build docs developers (and LLMs) love

Development Tools

Advanced

Documentation Index

​Overview

​Architecture

​Setup

​Prerequisites

​Kafka Configuration

​Server Configuration

​Domain Configuration

​Worker Service Setup

​Usage

​Starting Async Workflows

​Checking Workflow Status

​Rate Limiting

​Consumer Rate Limiting

​Backpressure Handling

​Monitoring

​Key Metrics

​CLI Monitoring

​Alerting

​Best Practices

​Queue Configuration

​Consumer Tuning

​Operational

​Use Cases

​Rate-Limited API

​Batch Processing

​Decoupled Services

​Troubleshooting

​High Consumer Lag

​Workflows Not Starting

​Failed Workflow Starts

​Advanced Topics

​Dead Letter Queue

​Custom Queue Implementation

​Next Steps

Build docs developers (and LLMs) love

Overview

Architecture

Setup

Prerequisites

Kafka Configuration

Server Configuration

Domain Configuration

Worker Service Setup

Usage

Starting Async Workflows

Checking Workflow Status

Rate Limiting

Consumer Rate Limiting

Backpressure Handling

Monitoring

Key Metrics

CLI Monitoring

Alerting

Best Practices

Queue Configuration

Consumer Tuning

Operational

Use Cases

Rate-Limited API

Batch Processing

Decoupled Services

Troubleshooting

High Consumer Lag

Workflows Not Starting

Failed Workflow Starts

Advanced Topics

Dead Letter Queue

Custom Queue Implementation

Next Steps