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Overview

This endpoint demonstrates advanced data transformation capabilities by aggregating cryptocurrency market data and global statistics, while using flatmap filters to clean and reshape the response. HTTP Method: GET
Endpoint: /cryptos/{currency}

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Parameters

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currency
string
required
The fiat currency to display prices in (e.g., “usd”, “eur”, “gbp”)

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What It Demonstrates

  • Aggregation with Collections: Combines cryptocurrency data with global market statistics
  • Advanced Transformation: Uses flatmap filters to move and delete fields
  • Safe JSON Encoding: Handles potentially unsafe JSON responses
  • Field Filtering: Selects specific nested fields from global data
  • Data Grouping: Organizes global statistics under market key

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Request Example

curl http://localhost:8080/cryptos/usd

curl http://localhost:8080/cryptos/eur

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Expected Response

{
  "coins": [
    {
      "id": "bitcoin",
      "symbol": "btc",
      "name": "Bitcoin",
      "image": "https://assets.coingecko.com/coins/images/1/large/bitcoin.png",
      "current_price": 43250.50,
      "market_cap": 846000000000,
      "ath": 69045
    },
    {
      "id": "ethereum",
      "symbol": "eth",
      "name": "Ethereum",
      "image": "https://assets.coingecko.com/coins/images/279/large/ethereum.png",
      "current_price": 2890.75,
      "market_cap": 347000000000,
      "ath": 4878
    }
  ],
  "market": {
    "total_market_cap": {
      "btc": 52345678.90,
      "eth": 789012345.67
    },
    "total_volume": {
      "btc": 1234567.89,
      "eth": 9876543.21
    },
    "market_cap_percentage": {
      "btc": 52.5,
      "eth": 18.3
    },
    "updated_at": 1709481600
  }
}

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Backend Services Called

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1. CoinGecko Market Data

  • Host: https://api.coingecko.com
  • URL Pattern: /api/v3/coins/markets?vs_currency={currency}&ids=bitcoin%2Cethereum&order=market_cap_desc&per_page=100&page=1&sparkline=false
  • Purpose: Fetches current market data for Bitcoin and Ethereum
  • Encoding: safejson

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2. CoinGecko Global Data

  • Host: https://api.coingecko.com
  • URL Pattern: /api/v3/global
  • Purpose: Fetches global cryptocurrency market statistics
  • Target: data (extracts data from the nested response)
  • Group: market

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KrakenD Configuration

{
  "@comment": "Feature: Aggregation & Transformation with collections",
  "endpoint": "/cryptos/{currency}",
  "backend": [
    {
      "host": ["https://api.coingecko.com"],
      "url_pattern": "/api/v3/coins/markets?vs_currency={currency}&ids=bitcoin%2Cethereum&order=market_cap_desc&per_page=100&page=1&sparkline=false",
      "encoding": "safejson",
      "extra_config": {
        "proxy": {
          "flatmap_filter": [
            {
              "type": "move",
              "args": ["collection", "coins"]
            },
            {
              "type": "del",
              "args": [
                "coins.*.ath_change_percentage",
                "coins.*.ath_date",
                "coins.*.atl",
                "coins.*.atl_change_percentage",
                "coins.*.atl_date",
                "coins.*.circulating_supply",
                "coins.*.fully_diluted_valuation",
                "coins.*.high_24h",
                "coins.*.low_24h",
                "coins.*.market_cap_change_24h",
                "coins.*.market_cap_change_percentage_24h",
                "coins.*.market_cap_rank",
                "coins.*.max_supply",
                "coins.*.price_change_24h",
                "coins.*.price_change_percentage_24h",
                "coins.*.roi",
                "coins.*.total_supply",
                "coins.*.total_volume"
              ]
            }
          ]
        }
      }
    },
    {
      "host": ["https://api.coingecko.com"],
      "url_pattern": "/api/v3/global",
      "allow": [
        "total_market_cap.btc",
        "total_market_cap.eth",
        "total_volume.btc",
        "total_volume.eth",
        "market_cap_percentage.btc",
        "market_cap_percentage.eth",
        "updated_at"
      ],
      "target": "data",
      "group": "market"
    }
  ]
}

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Key Configuration Options

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encoding: "safejson"

Handles potentially unsafe JSON responses from the CoinGecko API, preventing issues with special characters or malformed data.

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flatmap_filter

Advanced transformation operations applied to the backend response: Move Operation: 
{
  "type": "move",
  "args": ["collection", "coins"]
}
Moves the collection array to a coins field in the response. Delete Operation: 
{
  "type": "del",
  "args": ["coins.*.ath_change_percentage", ...]
}
Removes unnecessary fields from each coin object in the collection, reducing payload size.

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target

Extracts a specific field from the backend response. The global endpoint returns {data: {...}}, so target: "data" extracts the inner object.

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allow with Nested Fields

Filters nested fields using dot notation (e.g., total_market_cap.btc), allowing precise control over complex response structures.

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Use Cases

  • Cryptocurrency price tracking applications
  • Portfolio management systems
  • Market analysis dashboards
  • Trading platform integrations
  • Financial data aggregation services

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