coin-cache-api

This is a simple Rest API for caching fiat-conversion rates from a third-party API, and may potentially serve other useful data in the future. This API is built on Python, Flask, and Redis. Data from a third-party API are stored as a hashmap in Redis with a designated TTL. The client can either use the timestamp of the original third-party request for each token and currency, or the timestamp of the request to this API to determine a sane time to refetch results. Each token and currency combination is requested and cached individually.

Table of Contents

• Installation
• Running the development server
• Environment Configuration
• Endpoints
• Notes
• TODO

Installation

# Create a virtual environment
python3 -m venv venv

# Activate the virtual environment
. venv/bin/activate

# Install dependencies
pip install -r requirements.txt

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Running the development server

export FLASK_APP=src/api

flask run
# Your server will be available at http://127.0.0.1:5000/
# Make sure to send requests with the x-api-key header value which
# matches the key in your .env file

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Environment Configuration

Create a .env file with the following keys defined:

# Key used by x-api-key header to authorize protected routes - REQUIRED
API_KEY="THIS_IS_MY_API_KEY"

# Redis authentication - REQUIRED (if your Redis host requires AUTH)
REDIS_PASSWORD="MY_REDIS_SECRET"

# Redis host (if not localhost) - OPTIONAL
REDIS_HOST="domain.of.redishost"

# Redis port (if not default) - OPTIONAL
REDIS_PORT=6379

# Redis db, defaults to 0 - OPTIONAL
REDIS_DB=0

# API KEY for rest.coinapi.io - REQUIRED
THIRD_PARTY_KEY="xxxxxx-xxxx-xxxxxx-xxxxxxxxx-xxxxxxxx"

# Exchange Rate API URL - OPTIONAL
EXCHANGE_RATE_API="https://rest.coinapi.io/v1/exchangerate"

# TTL for cached API queries (third-party), defaults to 7200 (2 hours) - OPTIONAL
TTL = 3600

Additionally, see DEFAULTS in config.py for setting default tokens, currencies, etc.

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Endpoints

• /api/v1 - Version info
• /api/v1/rates - Get default conversion rates (GET|POST)
• /api/v1/rates?coins=BTC,ETH,DOT - Specify coins for which to query. Default currencies will be used.
• /api/v1/rates?currencies=USD,EUR - Specify currencies. Default coins will be used.
• /api/v1/rates?coins=BTC,ETH,DOT,ATOM&currencies=USD,YEN,EUR - Specify both coins and currencies, no defaults.
• /api/v1/health - Health check
• /api/v1/env - Environment variables

POST Example with cURL

NOTE You can specify coin or coins, currency or currencies as the GET request parameters with similar effect.

NOTE POST requests can also be sent as JSON data, passing coins and/or currencies as JSON parameters. Passing no arguments in a POST or GET will return the API default values.

curl --request POST http://127.0.0.1:5000/api/v1/rates --header "X-Api-Key:MY_SECRET_API_KEY" --data "coins=BTC,EUR&currencies=USD,EUR,YEN"

# Optionally, pipe it through jq for formatting JSON
curl --request POST http://127.0.0.1:5000/api/v1/rates --header "X-Api-Key:MY_SECRET_API_KEY" --data "coins=BTC,EUR&currencies=USD,EUR,YEN" | jq

SAMPLE OUTPUT

{
  "data": {
    "BTC": {
      "EUR": {
        "coin": "BTC",
        "currency": "EUR",
        "rate": 23268.18602491288,
        "timestamp": "2022-07-20T13:05:06.0000000Z"
      },
      "USD": {
        "coin": "BTC",
        "currency": "USD",
        "rate": 23838.256582523245,
        "timestamp": "2022-07-20T13:05:06.0000000Z"
      },
      "YEN": {}
    },
    "EUR": {
      "EUR": {
        "coin": "EUR",
        "currency": "EUR",
        "rate": 1.0,
        "timestamp": "2022-07-20T13:18:07.6269666Z"
      },
      "USD": {
        "coin": "EUR",
        "currency": "USD",
        "rate": 1.0245,
        "timestamp": "2022-07-20T13:17:57.0000000Z"
      },
      "YEN": {}
    }
  },
  "timestamp": 1658323087.77571
}

NOTE In this example, no conversion rates were found for YEN, which results in an empty JSON object. Empty results are not cached, and will be refetched on the next request.

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Notes

Using redis-cli, we can inspect the cached values from our third-party requests.

redis-cli

If authentication is required, enter the proper password (as defined in .env):

AUTH <my-secret-redis-password>

Inspecting the stored keys following a request, you may see something like the following:

127.0.0.1:6379> keys *
 1) "DOT/USD/expires"
 2) "DOT/USD"
 3) "BTC/USD/expires"
 4) "BTC/USD"
...

As the API results are cached as hashmaps (which, in Redis, do not accept a TTL), an /expires key/value is set, expiring when the configured TTL has been reached. These keys are checked to determine when a refetch should occur, which will overwrite the cached hashmaps. We can inspect one of these hashmaps as such:

127.0.0.1:6379> hgetall "BTC/USD"

The value for the /expires key, e.g., BTC/USD/expires, is set to TTL it was given:

127.0.0.1:6379> get "BTC/USD/expires"
"7200"

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TODO

• Use the same format and type for timestamps.
• Any error responses from the third-party API should be passed on to the client for handling.
• Add SimpleCache or Memcached for any non-Redis-cached routes

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