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The DataBus

The DataBus is a key-value store that holds all data during test execution. Think of it as a shared memory where blocks can read and write data using named slots. 
┌─────────────┐
│   DataBus   │
├─────────────┤
│ response    │ ← HttpRequest writes here
│ parsed      │ ← JsonParser writes here
│ validation  │ ← Validate writes here
└─────────────┘

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Writing to the DataBus

Blocks write their output to named slots using the output field: 
{
  "block": "HttpRequest",
  "input": { "url": "https://api.example.com/users/1" },
  "output": "response"  // ← Writes to 'response' slot
}
After this block executes, the DataBus contains: 
{
  response: {
    status: 200,
    body: '{"id": 1, "name": "John"}',
    headers: { /* ... */ }
  }
}

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Reading from the DataBus

Use ${} syntax to read from slots: 
{
  "block": "JsonParser",
  "input": "${response.body}",  // ← Reads from 'response.body'
  "output": "parsed"
}
You can access nested properties using dot notation: 
"input": "${response.headers.content-type}"
"input": "${parsed.data.user.email}"
"input": "${validation.passed}"

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Variable Resolution

The DataBus resolves variables when blocks execute: 
{
  "pipeline": [
    {
      "id": "fetch",
      "block": "HttpRequest",
      "input": { "url": "${BASE_URL}/users" },  // ← Resolves ${BASE_URL}
      "output": "users"
    },
    {
      "id": "process",
      "block": "JsonParser",
      "input": "${users.body}",  // ← Resolves ${users.body}
      "output": "parsed"
    }
  ]
}

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Resolution Order

Variables are resolved from multiple sources in this priority:
  1. Environment - From .env file (accessed via ${env.VARIABLE})
  2. Context - Defined in the test file
  3. DataBus - From previous blocks
{
  "context": {
    "BASE_URL": "${env.API_URL}",  // From .env
    "API_KEY": "hardcoded-key"
  },
  "tests": [{
    "pipeline": [
      {
        "block": "HttpRequest",
        "input": {
          "url": "${BASE_URL}/data",        // From context
          "headers": {
            "Authorization": "${API_KEY}"    // From context
          }
        },
        "output": "response"
      },
      {
        "block": "JsonParser",
        "input": "${response.body}",         // From DataBus
        "output": "data"
      }
    ]
  }]
}

​
Data Transformation

Data can be transformed as it flows: 
{
  "pipeline": [
    {
      "block": "HttpRequest",
      "output": "raw"           // Raw HTTP response
    },
    {
      "block": "JsonParser",
      "input": "${raw.body}",
      "output": "parsed"        // Parsed JSON
    },
    {
      "block": "ValidateContent",
      "input": {
        "from": "parsed.message",
        "as": "text"
      },
      "output": "validation"    // Validation result
    }
  ]
}
DataBus state after each block:
1

After HttpRequest

{
  raw: {
    status: 200,
    body: '{"message": "Hello"}',
    headers: {}
  }
}
2

After JsonParser

{
  raw: { /* ... */ },
  parsed: {
    message: "Hello"
  }
}
3

After ValidateContent

{
  raw: { /* ... */ },
  parsed: { /* ... */ },
  validation: {
    passed: true,
    checks: { /* ... */ }
  }
}

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Context Variables

Define reusable variables in context: 
{
  "context": {
    "BASE_URL": "https://api.example.com",
    "API_VERSION": "v1",
    "ENDPOINT": "${BASE_URL}/${API_VERSION}/users"
  },
  "tests": [{
    "pipeline": [{
      "block": "HttpRequest",
      "input": { "url": "${ENDPOINT}" }
    }]
  }]
}

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Environment Variables

Reference environment variables:
.env
API_URL=https://api.example.com
API_KEY=secret123

{
  "context": {
    "BASE_URL": "${env.API_URL}",
    "API_KEY": "${env.API_KEY}"
  }
}

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Debugging Data Flow

Enable debug logging to see data flow: 
export LOG_LEVEL=DEBUG
npx semtest test.json
You’ll see: 
[DEBUG] DataBus.set('response', {...})
[DEBUG] DataBus.get('response.body') → '{"id": 1}'
[DEBUG] Resolving ${response.body} → {"id": 1}

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Best Practices

// Good
"output": "userProfile"
"output": "authToken"
"output": "validationResult"

// Bad
"output": "data"
"output": "result"
"output": "temp"

// Simpler to reference
"output": "userId"    // Use ${userId}

// vs
"output": "user"      // Use ${user.data.profile.id}

{
  "context": {
    "AUTH_HEADER": "Bearer ${env.API_KEY}"
  },
  "tests": [{
    "pipeline": [
      {
        "block": "HttpRequest",
        "input": {
          "headers": { "Authorization": "${AUTH_HEADER}" }
        }
      }
    ]
  }]
}

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Next Steps

Input Formats

Learn different ways to pass data to blocks

Output Formats

Learn how blocks can output data