[
    {
        "collection": "order",     // name of collection
        "cap": null,               // Number of bytes 
        "index": [                 // Array of index details
            {"cartId": 1},         // An ascending order index
            {"status": -1},        // Descending order index
            {"orderId": 1},
            {"groupId": 1},
            {"currency": 1},
            {"createdAt": -1},
            {"orderNumber": 1, "type": 1},  // Composite index on orderNumber and type
            {"type": -1, "paymentStatus": -1, "payment.paymentMethod": -1}
        ]
    },
    {
        "collection": "collection_name",
        "cap": null,
        "index": [
            {"userId": -1},
            {"username": 1},
            {"orderId": 1, "_unique": 1},                       // creates a `unique index`
            {"createdAt": -1, "_expireAfterSeconds": 500}       // creates a `expires index` that will delete document after given number of seconds 
        ]
    }
    ....
    ....
    ....
]