Callouts and Callbacks: Limits and Usage

• External service callouts aren’t allowed when there are pending, uncommitted transactions.
  • In Flow, close your transaction, and resume Flow using the flow element Pause.
  • In Apex, perform the callout before any DML statement such as inserting, updating, or deleting objects. See Callout Limits and Limitations in Apex Developer Guide.
• Callouts to External Services time out after 120 seconds.

External Services Asynchronous Callbacks

Note External Services' asynchronous callback operations are supported in Apex but not in Flow.

The timeout duration of a callback operation is configured in the Apex client, and can be set for a maximum of twenty-four hours.

Accepted callback HTTP methods are PUT and POST. All other methods are ignored.

One callback parameter must contain only one callback operation, as a one-to-many relationship isn’t supported. If one-to-many is detected, only the first callback operation under the callback parameter is processed.

In Example API Specification With Callback Operation, the first callback parameter is applicationOutcomeApproved. It contains one callback operation #ref/…/ApplicationApproved. The third callback parameter is applicationError. It contains one callback operation "{$request.query.callbackUrlForErrorCases}": {"post": {.....}}} 3.3. We ignore the callback operation response declaration and always send 200, 404, 408 (timeout), or 500 HTTP response to the callback sender.

A callback URL can be defined as a query parameter or added to the request body parameter. A callback URL can be:

• a string type
• a property of an object type
• an element of an array

Some valid callback URL expression formats are:

• '{$request.query.callbackUrl}'
• '{$request.body#/callbackUrl}'
• '{$request.body#/callbackUrl/1}'

Other complex expressions aren't currently supported. For example, expressions with static text combined with variable references, or expressions that have multiple variable references. In these unsupported cases, the callback is considered invalid and the operation is attempted as a synchronous callout with a 120-second time out.

A callback operation can define an operationId. For example, in the Example API Specification With Callback Operation, see:

"operationId": "approvedCallback”

This operationId can’t be duplicated with any other operationId – with a regular top-level operation

"operationId": "SubmitApplication”

or any other callback operationId.

If a callback operation doesn’t declare an operationId, Salesforce uses its callback component reference name as the operationId - in this case - ApplicationRejected.

If the callback operation is declared inline, then its operationId is constructed as

parentAsyncOperationId_CB_callbackParameterName

For example, SubmitApplication_CB_applicationError.