### Object Download Request Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/24. S3-like Object Storage/README.md

Demonstrates an HTTP GET request for downloading an object from an S3-like storage. Specifies the host, date, and authorization headers.

```http
GET /bucket-to-share/script.txt HTTP/1.1
Host: foo.s3example.org
Date: Sun, 12 Sept 2021 18:30:01 GMT
Authorization: authorization string
```

--------------------------------

### Navigation Service Request Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/18. Google Maps/README.md

Example of a GET request to the navigation service to find a route between two locations. Origin and destination can be specified as addresses or coordinates.

```http
GET /v1/nav?origin=1355+market+street,SF&destination=Disneyland
```

--------------------------------

### Navigation Service Response Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/18. Google Maps/README.md

Example JSON response from the navigation service, detailing route information such as distance, duration, and instructions.

```json
{
  "distance": {"text":"0.2 mi", "value": 259},
  "duration": {"text": "1 min", "value": 83},
  "end_location": {"lat": 37.4038943, "Ing": -121.9410454},
  "html_instructions": "Head <b>northeast</b> on <b>Brandon St</b> toward <b>Lumin Way</b><div style=\"font-size:0.9em\">Restricted usage road</div>",
  "polyline": {"points": "_fhcFjbhgVuAwDsCal"},
  "start_location": {"lat": 37.4027165, "lng": -121.9435809},
  "geocoded_waypoints": [
    {
       "geocoder_status" : "OK",
       "partial_match" : true,
       "place_id" : "ChIJwZNMti1fawwRO2aVVVX2yKg",
       "types" : [ "locality", "political" ]
    },
    {
       "geocoder_status" : "OK",
       "partial_match" : true,
       "place_id" : "ChIJ3aPgQGtXawwRLYeiBMUi7bM",
       "types" : [ "locality", "political" ]
    }
  ],
  "travel_mode": "DRIVING"
}
```

--------------------------------

### Adaptive ETA Data Storage Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/18. Google Maps/README.md

Example data structure for storing user navigation paths using routing tiles for adaptive ETA and rerouting. This format helps in efficiently identifying users affected by traffic incidents.

```Plain Text
user_1: r_1, r_2, r_3, …, r_k
user_2: r_4, r_6, r_9, …, r_n
user_3: r_2, r_8, r_9, …, r_m
... 
user_n: r_2, r_10, r21, ..., r_l
```

--------------------------------

### Example Response for Specific User Score

Source: https://github.com/liquidslr/system-design-notes/blob/main/25. Real-time Gaming Leaderboard/README.md

This is an example JSON response when fetching the score and rank for a particular user.

```json
{
    "user_info": {
        "user_id": "user5",
        "score": 1000,
        "rank": 6,
    }
}
```

--------------------------------

### Example Response for Top 10 Scores

Source: https://github.com/liquidslr/system-design-notes/blob/main/25. Real-time Gaming Leaderboard/README.md

This is an example JSON response when fetching the top 10 players from the leaderboard.

```json
{
  "data": [
    {
      "user_id": "user_id1",
      "user_name": "alice",
      "rank": 1,
      "score": 12543
    },
    {
      "user_id": "user_id2",
      "user_name": "bob",
      "rank": 2,
      "score": 11500
    }
  ],
  ...
  "total": 10
}
```

--------------------------------

### Time-Series Data Example (Line Protocol)

Source: https://github.com/liquidslr/system-design-notes/blob/main/20. Metrics Monitoring and Alerting System/README.md

This is an example of time-series data in line protocol format, commonly used by monitoring software like Prometheus and OpenTSDB. It includes metric name, labels, timestamp, and value.

```text
CPU.load host=webserver01,region=us-west 1613707265 50

CPU.load host=webserver01,region=us-west 1613707265 62

CPU.load host=webserver02,region=us-west 1613707265 43

CPU.load host=webserver02,region=us-west 1613707265 53

...

CPU.load host=webserver01,region=us-west 1613707265 76

CPU.load host=webserver01,region=us-west 1613707265 83
```

--------------------------------

### Geocoding API Response Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/18. Google Maps/README.md

Example JSON response from the Google Geocoding API, showing formatted address and location details.

```JSON
{
   "results" : [
      {
         "formatted_address" : "1600 Amphitheatre Parkway, Mountain View, CA 94043, USA",
         "geometry" : {
            "location" : {
               "lat" : 37.4224764,
               "lng" : -122.0842499
            },
            "location_type" : "ROOFTOP",
            "viewport" : {
               "northeast" : {
                  "lat" : 37.4238253802915,
                  "lng" : -122.0829009197085
               },
               "southwest" : {
                  "lat" : 37.4211274197085,
                  "lng" : -122.0855988802915
               }
            }
         },
         "place_id" : "ChIJ2eUgeAK6j4ARbn5u_wAGqWA",
         "plus_code": {
            "compound_code": "CWC8+W5 Mountain View, California, United States",
            "global_code": "849VCWC8+W5"
         },
         "types" : [ "street_address" ]
      }
   ],
   "status" : "OK"
}
```

--------------------------------

### Example Folder Response

Source: https://github.com/liquidslr/system-design-notes/blob/main/23. Distributed Email Service/README.md

Illustrates the structure of a response when retrieving a list of email folders. Includes folder identifier, name, and owner reference.

```json
[
  {
    "id": "string",
    "name": "string",
    "user_id": "string"
  }
]
```

--------------------------------

### Geocoding API Request Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/18. Google Maps/README.md

Example of a request to the Google Geocoding API to convert an address to latitude and longitude coordinates.

```HTTP
https://maps.googleapis.com/maps/api/geocode/json?address=1600+Amphitheatre+Parkway,+Mountain+View,+CA
```

--------------------------------

### Object Upload Request Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/24. S3-like Object Storage/README.md

Illustrates an HTTP PUT request for uploading an object to an S3-like storage. Includes headers for host, date, authorization, content type, length, and metadata.

```http
PUT /bucket-to-share/script.txt HTTP/1.1
Host: foo.s3example.org
Date: Sun, 12 Sept 2021 17:51:00 GMT
Authorization: authorization string
Content-Type: text/plain
Content-Length: 4567
x-amz-meta-author: Alex

[4567 bytes of object data]
```

--------------------------------

### Example Message Response

Source: https://github.com/liquidslr/system-design-notes/blob/main/23. Distributed Email Service/README.md

Shows the format of a response for a single email message. Details sender, recipients, subject, body, and read status.

```json
{
  "user_id": "string",
  "from": {"name": "string", "email": "string"},
  "to": [{"name": "string", "email": "string"}],
  "subject": "string",
  "body": "string",
  "is_read": "boolean"
}
```

--------------------------------

### In-Memory Sharding Algorithm

Source: https://github.com/liquidslr/system-design-notes/blob/main/27.  Digital Wallet/README.md

An example algorithm for partitioning user accounts across multiple Redis nodes based on account ID hash. This is crucial for scaling beyond a single node's capacity.

```java
String accountID = "A";
Int partitionNumber = 7;
Int myPartition = accountID.hashCode() % partitionNumber;
```

--------------------------------

### Calculate User Percentile with Cron Job

Source: https://github.com/liquidslr/system-design-notes/blob/main/25. Real-time Gaming Leaderboard/README.md

This example illustrates how a cron job can periodically analyze score distributions to determine a user's percentile rank. This is useful when direct rank calculation is difficult.

```shell
10th percentile = score < 100
20th percentile = score < 500
...
90th percentile = score < 6500
```

--------------------------------

### Raw Ad Click Event Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/21. Ad Click Event Aggregation/README.md

Represents a single ad click event with details like ad ID, timestamp, user, IP, and country.

```text
[AdClickEvent] ad001, 2021-01-01 00:00:01, user 1, 207.148.22.22, USA
```

--------------------------------

### Optimized Adaptive ETA Data Storage Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/18. Google Maps/README.md

An optimized data storage approach for adaptive ETA, storing only the origin tile and progressively coarser resolution tiles until the destination is covered. This reduces storage requirements.

```Plain Text
user_1, r_1, super(r_1), super(super(r_1)), ...
```

--------------------------------

### List objects in a bucket with prefix (SQL)

Source: https://github.com/liquidslr/system-design-notes/blob/main/24. S3-like Object Storage/README.md

SQL query to list objects within a specific bucket that match a given prefix. This is a common operation in object storage systems.

```sql
SELECT * FROM object WHERE bucket_id = "123" AND object_name LIKE `abc/%`
```

--------------------------------

### FIX Protocol Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/28. Stock Exchange/README.md

This is an example of a FIX protocol message for a securities transaction. It includes various fields like message type, sender/target IDs, timestamps, and order details.

```plaintext
8=FIX.4.2 | 9=176 | 35=8 | 49=PHLX | 56=PERS | 52=20071123-05:30:00.000 | 11=ATOMNOCCC9990900 | 20=3 | 150=E | 39=E | 55=MSFT | 167=CS | 54=1 | 38=15 | 40=2 | 44=15 | 58=PHLX EQUITY TESTING | 59=0 | 47=C | 32=0 | 31=0 | 151=15 | 14=0 | 6=0 | 10=128 |
```

--------------------------------

### Get file revisions

Source: https://github.com/liquidslr/system-design-notes/blob/main/15. Google Drive/Readme.md

Retrieves the revision history for a specific file.

```APIDOC
## GET /files/list_revisions

### Description
Retrieves a list of revisions for a given file.

### Method
GET

### Endpoint
https://api.example.com/files/list_revisions

### Parameters
#### Query Parameters
- **file_id** (string) - Required - The unique identifier of the file whose revisions are to be listed.

### Request Example
```json
{
  "example": "Request to list revisions for a file"
}
```

### Response
#### Success Response (200)
- **revisions** (array) - A list of file revisions, each containing details like version, timestamp, and author.

#### Response Example
```json
{
  "revisions": [
    {
      "version": "1.2",
      "timestamp": "2023-10-27T10:00:00Z",
      "author": "user@example.com"
    },
    {
      "version": "1.1",
      "timestamp": "2023-10-26T15:30:00Z",
      "author": "user@example.com"
    }
  ]
}
```
```

--------------------------------

### Get Message Details

Source: https://github.com/liquidslr/system-design-notes/blob/main/23. Distributed Email Service/README.md

Retrieves all information about a specific email message.

```APIDOC
## GET /v1/messages/{:message_id}

### Description
Get all information about a particular message.

### Method
GET

### Endpoint
/v1/messages/{:message_id}

### Parameters
#### Path Parameters
- **message_id** (string) - Required - The ID of the message to retrieve.

### Response
#### Success Response (200)
- **user_id** (string) - Reference to the account owner.
- **from** (object) - Sender information.
  - **name** (string) - Sender's name.
  - **email** (string) - Sender's email address.
- **to** (array) - List of recipients.
  - **name** (string) - Recipient's name.
  - **email** (string) - Recipient's email address.
- **subject** (string) - Subject of the email.
- **body** (string) - Body of the email.
- **is_read** (boolean) - Indicates if the message has been read.
```

--------------------------------

### Get Specific User's Rank

Source: https://github.com/liquidslr/system-design-notes/blob/main/25. Real-time Gaming Leaderboard/README.md

Retrieves the score and rank for a specific user.

```APIDOC
## GET /v1/scores/{:user_id}

### Description
Fetches the score and rank for a specific user identified by their user ID.

### Method
GET

### Endpoint
/v1/scores/{:user_id}

### Parameters
#### Path Parameters
- **user_id** (string) - Required - The unique identifier of the user whose information is being requested.

### Response
#### Success Response (200)
- **user_info** (object) - An object containing the user's details.
  - **user_id** (string) - The user's unique identifier.
  - **score** (integer) - The user's current score.
  - **rank** (integer) - The user's current rank on the leaderboard.

### Response Example
{
    "user_info": {
        "user_id": "user5",
        "score": 1000,
        "rank": 6
    }
}
```

--------------------------------

### Hotel Reservation API Request Example

Source: https://github.com/liquidslr/system-design-notes/blob/main/22. Hotel Reservation System/README.md

This JSON object represents a sample request payload for making a hotel reservation. It includes dates, hotel and room identifiers, and an idempotency key to prevent double bookings.

```json
{
  "startDate":"2021-04-28",
  "endDate":"2021-04-30",
  "hotelID":"245",
  "roomID":"U12354673389",
  "reservationID":"13422445"
}
```

--------------------------------

### Get Top 10 Players

Source: https://github.com/liquidslr/system-design-notes/blob/main/25. Real-time Gaming Leaderboard/README.md

Retrieves the top 10 players from the leaderboard, including their rank and score.

```APIDOC
## GET /v1/scores

### Description
Fetches the top 10 players currently on the leaderboard.

### Method
GET

### Endpoint
/v1/scores

### Response
#### Success Response (200)
- **data** (array) - An array of player objects, each containing user_id, user_name, rank, and score.
- **total** (integer) - The total number of players returned in the data array (expected to be 10 for this endpoint).

### Response Example
{
  "data": [
    {
      "user_id": "user_id1",
      "user_name": "alice",
      "rank": 1,
      "score": 12543
    },
    {
      "user_id": "user_id2",
      "user_name": "bob",
      "rank": 2,
      "score": 11500
    }
  ],
  "total": 10
}
```

--------------------------------

### Get Payment Status

Source: https://github.com/liquidslr/system-design-notes/blob/main/26. Payment System/README.md

Retrieves the execution status of a single payment order using its unique identifier.

```APIDOC
## GET /v1/payments/{:id}

### Description
Retrieves the execution status of a single payment order based on the provided payment order ID.

### Method
GET

### Endpoint
/v1/payments/{:id}

### Parameters
#### Path Parameters
- **id** (string) - Required - The unique identifier of the payment order (`payment_order_id`) to query.
```

--------------------------------

### Get Candlesticks

Source: https://github.com/liquidslr/system-design-notes/blob/main/28. Stock Exchange/README.md

Retrieves historical candlestick data for a given stock symbol over a specified time range and resolution.

```APIDOC
## Get Candlesticks

### Description
Retrieves historical candlestick data for a given stock symbol over a specified time range and resolution.

### Method
GET

### Endpoint
/marketdata/candles

### Parameters
#### Query Parameters
- **symbol** (String) - Required - The stock symbol.
- **resolution** (Long) - Required - Window length of the candlestick chart in seconds.
- **startTime** (Long) - Required - Start time of the window in epoch.
- **endTime** (Long) - Required - End time of the window in epoch.

### Response
#### Success Response (200)
- **candles** (Array) - Array with each candlestick data.
  - **open** (Double) - Open price of each candlestick.
  - **close** (Double) - Close price of each candlestick.
  - **high** (Double) - High price of each candlestick.
  - **low** (Double) - Low price of each candlestick.
```

--------------------------------

### Payment Service API - Get Payment Status

Source: https://github.com/liquidslr/system-design-notes/blob/main/26. Payment System/README.md

This endpoint retrieves the execution status of a single payment using the `payment_order_id`.

```http
GET /v1/payments/{:id}
```

--------------------------------

### Order Book Data Structures (Pseudo Code)

Source: https://github.com/liquidslr/system-design-notes/blob/main/28. Stock Exchange/README.md

Defines the core classes for an order book implementation, including PriceLevel, Book, and OrderBook. These structures are foundational for managing buy and sell orders.

```pseudo
class PriceLevel{
    private Price limitPrice;
    private long totalVolume;
    private List<Order> orders;
}

class Book<Side> {
    private Side side;
    private Map<Price, PriceLevel> limitMap;
}

class OrderBook {
    private Book<Buy> buyBook;
    private Book<Sell> sellBook;
    private PriceLevel bestBid;
    private PriceLevel bestOffer;
    private Map<OrderID, Order> orderMap;
}
```

--------------------------------

### API Endpoint for Specific User Score

Source: https://github.com/liquidslr/system-design-notes/blob/main/25. Real-time Gaming Leaderboard/README.md

Use this GET endpoint to retrieve the score and rank for a specific user by their ID.

```http
GET /v1/scores/{:user_id}
```

--------------------------------

### Get Execution API Endpoint

Source: https://github.com/liquidslr/system-design-notes/blob/main/28. Stock Exchange/README.md

Retrieve details of order executions within a specified time range. The orderId parameter is optional.

```http
GET /execution?symbol={:symbol}&orderId={:orderId}&startTime={:startTime}&endTime={:endTime}
```

--------------------------------

### Insert New User into Leaderboard

Source: https://github.com/liquidslr/system-design-notes/blob/main/25. Real-time Gaming Leaderboard/README.md

Use this SQL statement to insert a new user into the leaderboard table if they do not already exist. This is typically the first interaction when a user scores.

```sql
INSERT INTO leaderboard (user_id, score) VALUES ('mary1934', 1);
```

--------------------------------

### Order Matching Algorithm Pseudocode

Source: https://github.com/liquidslr/system-design-notes/blob/main/28. Stock Exchange/README.md

This pseudocode details the process of handling new and canceled orders within an order book, including validation, order creation, and matching logic. It uses a FIFO approach for matching orders at the same price level.

```pseudocode
Context handleOrder(OrderBook orderBook, OrderEvent orderEvent) {
    if (orderEvent.getSequenceId() != nextSequence) {
        return Error(OUT_OF_ORDER, nextSequence);
    }

    if (!validateOrder(symbol, price, quantity)) {
        return ERROR(INVALID_ORDER, orderEvent);
    }

    Order order = createOrderFromEvent(orderEvent);
    switch (msgType):
        case NEW:
            return handleNew(orderBook, order);
        case CANCEL:
            return handleCancel(orderBook, order);
        default:
            return ERROR(INVALID_MSG_TYPE, msgType);

}

Context handleNew(OrderBook orderBook, Order order) {
    if (BUY.equals(order.side)) {
        return match(orderBook.sellBook, order);
    } else {
        return match(orderBook.buyBook, order);
    }
}

Context handleCancel(OrderBook orderBook, Order order) {
    if (!orderBook.orderMap.contains(order.orderId)) {
        return ERROR(CANNOT_CANCEL_ALREADY_MATCHED, order);
    }

    removeOrder(order);
    setOrderStatus(order, CANCELED);
    return SUCCESS(CANCEL_SUCCESS, order);
}

Context match(OrderBook book, Order order) {
    Quantity leavesQuantity = order.quantity - order.matchedQuantity;
    Iterator<Order> limitIter = book.limitMap.get(order.price).orders;
    while (limitIter.hasNext() && leavesQuantity > 0) {
        Quantity matched = min(limitIter.next.quantity, order.quantity);
        order.matchedQuantity += matched;
        leavesQuantity = order.quantity - order.matchedQuantity;
        remove(limitIter.next);
        generateMatchedFill();
    }
    return SUCCESS(MATCH_SUCCESS, order);
}
```

--------------------------------

### Initiate Payment

Source: https://github.com/liquidslr/system-design-notes/blob/main/26. Payment System/README.md

Initiates a payment transaction. This endpoint accepts payment details and orchestrates the payment process through various services.

```APIDOC
## POST /v1/payments

### Description
Initiates a payment transaction. This endpoint accepts payment details and orchestrates the payment process through various services.

### Method
POST

### Endpoint
/v1/payments

### Request Body
- **buyer_info** (object) - Required - Information about the buyer.
- **checkout_id** (string) - Required - Identifier for the checkout session.
- **credit_card_info** (object) - Required - Details of the credit card used for payment.
- **payment_orders** (array) - Required - A list of payment orders associated with this transaction.
  - **seller_account** (string) - Required - The seller's account identifier (e.g., IBAN).
  - **amount** (string) - Required - The monetary amount for the payment order. Represented as a string to avoid floating-point inaccuracies.
  - **currency** (string) - Required - The currency of the payment (e.g., USD).
  - **payment_order_id** (string) - Required - A unique identifier for the payment order, used for idempotency and deduplication with the PSP.

### Request Example
```json
{
  "buyer_info": {...},
  "checkout_id": "some_id",
  "credit_card_info": {...},
  "payment_orders": [
    {
      "seller_account": "SELLER_IBAN",
      "amount": "3.15",
      "currency": "USD",
      "payment_order_id": "globally_unique_payment_id"
    },
    {...}
  ]
}
```
```