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Callback Mechanism

Revel API Gateway supports callbacks. This functionality allow to receive the response to a request in a asynchronous manner.

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How does the callback mechanism work?

A typical request/response lifecycle looks like this:

Request/Response model

Request/Response model

An HTTP connection is established and a response is sent when it is ready. This is fine for requests that don’t have a long execution time or have a large payload to be delivered. To avoid these drawback, we can use callback mechanism to postpone the response.

Request/Response model with Callback mechanism

Request/Response model with Callback mechanism

When using callback mechanism, we will get a confirmation response (status code 202 with an empty payload) that indicates, API gateway has accepted our request, but the actual response to the provided request, with the same Correlation-Id, will be delivered to the callback URL which is provided via HTTP header - Callback-Url.

When callback mechanism should be used?

Callback mechanism should be used when:

  • Response body might be larger than 10MB;
  • Request might take longer than 29 seconds (timeout);
  • Response should be sent to a different URL than the request;
  • Response should be handled in an asynchronous manner;

Preconditions for a client that would like to use callback mechanism

To use callback mechanism, client should implement these functionalities:

  • URL that is provided inside the Callback-Url should accept POST requests.
  • URL that is provided inside the Callback-Url should return 2xx status code. This will indicate to API Gateway that the response was successfully delivered.

Possible error messages

  • When a timeout occurs (request took longer than 29 seconds) API Gateway will return HTTP status code - 504
  • When response body exceeds 10MB, API Gateway will return HTTP status code - 413
  • When Callback-Url is sent for an endpoint that does not support callback mechanism, API Gateway will return HTTP status code - 412

Callback mechanism retry logic

If API Gateway fails to deliver the callback response to the client it will:

  • Do a maximum of 3 retries;
  • Retry if the client returned one of these HTTP status codes:
    • 429
    • 500
    • 502
    • 503
    • 504

Endpoints that support callback mechanism

Not every endpoint supports a callback mechanism. Here is a list of endpoints that do support it.

"enterprise/Establishment/{anyInt}/menu"
"reports/hourly_sales/json"
"inventory/purchase-orders/create.json"
"reports/customers/json"
"inventory/api/reorder/vendoritems"
"reports/product_mix/data"
"reports/labor/json"
"reports/product_mix/daily/data.xlsx"
"reports/sales_summary/sales_history"
"reports/orders/data.csv"
"resources/MenuProduct"
"reports/orders/json"
"inventoryx/api/inventory-log/Product"
"inventory/purchase-orders/data.json"
"enterprise/Establishment/"
"enterprise/Establishment/{anyInt}/new_list_all"
"reports"
"reports/all_establishment_sales/data.json"
"reports/house_account_summary/json"
"weborders/attributes"
"reports/sales_summary"
"reports/insights_multi_day_hourly_sales"
"reports/void_discount_summary/json"
"weborders/cart_totals"
"reports/operations"
"resources/Modifier/set/{anyInt}"
"reports/labor"
"resources/Customer"
"reports/operations/json"
"reports/voids_summary/json"
"inventoryx/reports/physical_inventory/{anyInt}/finalized-detail-data.json"
"reports/product_mix/daily/data.csv"
"inventory/vendors/items/export"
"inventory/purchase-orders/search-item.json"
"reports/cash_office/json"
"resources/Discount"
"reports/product_mix/pdf"
"reports/payment_summary/json"
"resources/SystemSetting"
"reports/hourly_sales"
"weborders/products"
"resources/Product"
"weborders/menu"
"resources/Permission"
"resources/Modifier"
"reports/product_mix"
"specialresources/cart/submit/"
"resources/ProductModifierClass"
"resources/DeclaredTips"
"resources/VendorOrderableItem"
"resources/BusinessActionLog"
"reports/productcomponent/data.json"
"weborders/v2/products"
"resources/GiftCard"
"resources/OrderItemDataDiff"
"reports/sales_summary/json"

Examples

This section will show examples, how to send a request to API Gateway with and without callback mechanism.

Request without callback mechanism

An example of a request without Callback-Url using cURL

curl -v --location \
--request GET 'https://api.revelsystems.com/resources/BankDrop' \
--header 'Client-Id: <client-id-goes-here>' \
--header 'Authorization: Bearer <token>'

After we will process your request, we will send a response back with a Correlation-Id in headers.

< HTTP/1.1 200 OK
< Date: Wed, 06 Oct 2021 11:11:15 GMT
< Connection: keep-alive
< Content-Type: application/json; charset=utf-8
< Content-Length: 120
< Correlation-Id: 9b9c4f01-90e9-4837-90b0-c858dff25f13
<

{
    "meta": {
        "limit": 20,
        "next": null,
        "offset": 0,
        "previous": null,
        "time_zone": null,
        "total_count": 0
    },
    "objects": []
}

Request with callback mechanism

An example of a request with Callback-Url using cURL

curl -v --location \
--request GET 'https://api.revelsystems.com/resources/BankDrop' \
--header 'Callback-Url: <callback-url-goes-here>' \
--header 'Client-Id: <client-id-goes-here>' \
--header 'Authorization Bearer <token>'

After sending a request to a whitelisted endpoint with Callback-Url you will receive 202 and a Correlation-Id in headers.

< HTTP/1.1 202 ACCEPTED
< Date: Wed, 06 Oct 2021 10:53:45 GMT
< Connection: keep-alive
< Content-Type: application/json
< Correlation-Id: a995d20e-8cd5-4ed9-a396-fcd889bf7aaa
< Content-Length: 0

After we will process your request, we will send a POST request to your provided Callback-Url where you will see JSON content.

{
  "body": {
    "meta": {
      "limit": 20,
      "next": null,
      "offset": 0,
      "previous": null,
      "time_zone": null,
      "total_count": 0
    },
    "objects": []
  },
  "method": "GET",
  "mimeType": "application/json; charset=utf-8",
  "statusCode": 200
}
  • body - will contain actual response content without any modifications
  • method - original method was called by you
  • mimeType - original type of data we have received during your request processing
  • statusCode - original status we have received during your request processing

Correlation-Id HTTP header inside the response can be mapped with the original request, because the original request response returns the same Correlation-Id.

Accept-Encoding: gzip, deflate
Accept: */*
Connection: keep-alive
Correlation-Id: a995d20e-8cd5-4ed9-a396-fcd889bf7aaa
Content-Length: 213
Content-Type: application/json

After you will receive a request to your Callback-Url we expect to receive 2xx as a response to our request.

Updated 6 months ago