+ * `messageBindingTemplate` (*type:* `String.t`, *default:* `nil`) - Optional. The CEL expression used to modify how the destination-bound HTTP request is constructed. If a binding expression is not specified here, the message is treated as a CloudEvent and is mapped to the HTTP request according to the CloudEvent HTTP Protocol Binding Binary Content Mode. In this representation, all fields except the `data` and `datacontenttype` field on the message are mapped to HTTP request headers with a prefix of `ce-`. To construct the HTTP request payload and the value of the content-type HTTP header, the payload format is defined as follows: 1) Use the output_payload_format_type on the Pipeline.Destination if it is set, else: 2) Use the input_payload_format_type on the Pipeline if it is set, else: 3) Treat the payload as opaque binary data. The `data` field of the message is converted to the payload format or left as-is for case 3) and then attached as the payload of the HTTP request. The `content-type` header on the HTTP request is set to the payload format type or left empty for case 3). However, if a mediation has updated the `datacontenttype` field on the message so that it is not the same as the payload format type but it is still a prefix of the payload format type, then the `content-type` header on the HTTP request is set to this `datacontenttype` value. For example, if the `datacontenttype` is "application/json" and the payload format type is "application/json; charset=utf-8", then the `content-type` header on the HTTP request is set to "application/json; charset=utf-8". If a non-empty binding expression is specified then this expression is used to modify the default CloudEvent HTTP Protocol Binding Binary Content representation. The result of the CEL expression must be a map of key/value pairs which is used as follows: - If a map named `headers` exists on the result of the expression, then its key/value pairs are directly mapped to the HTTP request headers. The headers values are constructed from the corresponding value type's canonical representation. If the `headers` field doesn't exist then the resulting HTTP request will be the headers of the CloudEvent HTTP Binding Binary Content Mode representation of the final message. Note: If the specified binding expression, has updated the `datacontenttype` field on the message so that it is not the same as the payload format type but it is still a prefix of the payload format type, then the `content-type` header in the `headers` map is set to this `datacontenttype` value. - If a field named `body` exists on the result of the expression then its value is directly mapped to the body of the request. If the value of the `body` field is of type bytes or string then it is used for the HTTP request body as-is, with no conversion. If the body field is of any other type then it is converted to a JSON string. If the body field does not exist then the resulting payload of the HTTP request will be data value of the CloudEvent HTTP Binding Binary Content Mode representation of the final message as described earlier. - Any other fields in the resulting expression will be ignored. The CEL expression may access the incoming CloudEvent message in its definition, as follows: - The `data` field of the incoming CloudEvent message can be accessed using the `message.data` value. Subfields of `message.data` may also be accessed if an input_payload_format has been specified on the Pipeline. - Each attribute of the incoming CloudEvent message can be accessed using the `message.` value, where is replaced with the name of the attribute. - Existing headers can be accessed in the CEL expression using the `headers` variable. The `headers` variable defines a map of key/value pairs corresponding to the HTTP headers of the CloudEvent HTTP Binding Binary Content Mode representation of the final message as described earlier. For example, the following CEL expression can be used to construct an HTTP request by adding an additional header to the HTTP headers of the CloudEvent HTTP Binding Binary Content Mode representation of the final message and by overwriting the body of the request: ``` { "headers": headers.merge({"new-header-key": "new-header-value"}), "body": "new-body" } ``` Additionally, the following CEL extension functions are provided for use in this CEL expression: - toBase64Url: map.toBase64Url() -> string - Converts a CelValue to a base64url encoded string - toJsonString: map.toJsonString() -> string - Converts a CelValue to a JSON string - merge: map1.merge(map2) -> map3 - Merges the passed CEL map with the existing CEL map the function is applied to. - If the same key exists in both maps, if the key's value is type map both maps are merged else the value from the passed map is used. - denormalize: map.denormalize() -> map - Denormalizes a CEL map such that every value of type map or key in the map is expanded to return a single level map. - The resulting keys are "." separated indices of the map keys. - For example: { "a": 1, "b": { "c": 2, "d": 3 } "e": [4, 5] } .denormalize() -> { "a": 1, "b.c": 2, "b.d": 3, "e.0": 4, "e.1": 5 } - setField: map.setField(key, value) -> message - Sets the field of the message with the given key to the given value. - If the field is not present it will be added. - If the field is present it will be overwritten. - The key can be a dot separated path to set a field in a nested message. - Key must be of type string. - Value may be any valid type. - removeFields: map.removeFields([key1, key2, ...]) -> message - Removes the fields of the map with the given keys. - The keys can be a dot separated path to remove a field in a nested message. - If a key is not found it will be ignored. - Keys must be of type string. - toMap: [map1, map2, ...].toMap() -> map - Converts a CEL list of CEL maps to a single CEL map - toDestinationPayloadFormat(): message.data.toDestinationPayloadFormat() -> string or bytes - Converts the message data to the destination payload format specified in Pipeline.Destination.output_payload_format - This function is meant to be applied to the message.data field. - If the destination payload format is not set, the function will return the message data unchanged. - toCloudEventJsonWithPayloadFormat: message.toCloudEventJsonWithPayloadFormat() -> map - Converts a message to the corresponding structure of JSON format for CloudEvents - This function applies toDestinationPayloadFormat() to the message data. It also sets the corresponding datacontenttype of the CloudEvent, as indicated by Pipeline.Destination.output_payload_format. If no output_payload_format is set it will use the existing datacontenttype on the CloudEvent if present, else leave datacontenttype absent. - This function expects that the content of the message will adhere to the standard CloudEvent format. If it doesn't then this function will fail. - The result is a CEL map that corresponds to the JSON representation of the CloudEvent. To convert that data to a JSON string it can be chained with the toJsonString function. The Pipeline expects that the message it receives adheres to the standard CloudEvent format. If it doesn't then the outgoing message request may fail with a persistent error.
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