Event Handlers

Event Handlers
You can do more processing and customization using event handlers. Event handlers are JavaScript entry points to perform user-defined actions.
API Server calls the following types of event handlers for every request:
  • Requests.
     API Server invokes these event handlers 
     it receives the request but 
     it processes request/row mapping and reactive logic. 
  • Responses.
     API Server invokes these event handlers 
     it sends the (successful) final result to the client.
  • HTTP Options.
     API Server recognizes Cross-Origin Resource Sharing (CORS) OPTIONS requests and invokes these event handlers only for these requests.
  • Errors. 
    API Server invokes these event handlers when the request encounters an error or exception condition and 
     it sends the response to the client.
In this article:
Create an Event Handler
There is no limit to the number of event handlers you can create.
You can create only one HTTP Options event per API.
Follow these steps:
  1. With your API open, in the Create section, click 
    Request Events
  2. If your API does not have existing event handlers, the Welcome to Request and Response Events page appears. If your API has existing event handlers, they are listed on the Events page.
  3. Complete 
     of the following steps:
    • If you have not defined any event handlers, on the Welcome to Request and Response Events page, click 
      Create Event
    • If you have at least one event handler defined, click 
  4. Complete the following fields on the page, and then save your changes:
    Event type
    The type of event handler.
     request, response, options, error
    Event Handler name
    The name for your event handler. 
    Layer7 Live API Creator
     invokes event handlers in alphabetical order based on their name. To have 
    Layer7 Live API Creator
     invoke the event handler in a specific order, append the event handler name with a number sequence.
    The operations for this event handler. The following is an example of a response event:
    if (“SalesResports” == req.resourceName) {
    //log.debug(‘***SalesReports Req Event - json: ' + json);
    var data = JSON.parse(json); // json from the request
    var theOrder = data[0];
    var keptItemNum = 0;
    var keptItems = [];
    log.debug(“theOrder: " + JSON.stringify(theOrder));
    for ( var i = 0; i < theOrder.Order_DetailsList.length; i ++) {
    var eachItem = theOrder.Order_DetailsList(i);
    delete eachItem[“@metadata”];
    if (eachItem.SupplierName == “Pavlova, Ltd.”) {
    delete eachItem.OrderID;
    delete eachItem.ProductID;
    Insert a code example or use code completion. For more information about the code examples that are available from the code editor, see Code Completion.
    Select to indicate that this event handler is active.
The event handler is created.
How API Server Invokes Event Handlers
The following section describes how API Server invokes each event handler.
Request Events
API Server invokes request events using the following process:
  1. API Server performs initial authentication.
  2. API Server invokes the request event for every transaction, with the following variables pre-defined:
    Variable Name
    The raw JSON string that the client sends, if any. Set only for POST and PUT requests. You can modify this string.
     object. For more information about this object, see The req Object.
    The logger for the current request.
    The headers sent as part of the HTTP request. Excludes authorization headers.
  3. API Server establishes a connection to the database.
  4. API Server parses the JSON payload.
For more information about how API Server processes update requests, including how rules integrate with request eventd, see Logic Execution.
Request Event Examples
The following code snippets show examples of the code for a request event.
In this code example, API Server rejects all requests on 
 if the request does not use HTTPS:
// Reject all requests on SecretData if they do not use HTTPS
if ("SecretData" == req.resourceName && !req.clientUsesHttps) {
log.error('SecretData was accessed over HTTP by ' + req.clientAddress);
throw "Use HTTPS to access this resource";
In this code example, API Server deletes data from requests except the data that is included in the JSON payload:
// Remove extraneous data that the client sends but that is not needed.
if ( ! json) {
// Only requests with a JSON payload are needed.
var data = JSON.parse(json);
if (Array.isArray(data)) {
for (var i = 0; i < data.length; i++) {
delete data[i].uselessData;
else {
delete data.uselessData;
// Important: Serialize the data back to a string if you want to change it
json = JSON.stringify(data);
Response Events
You can add the following variables to the JavaScript for response event:
Variable name
The complete response object, which is sent back to the client after 
Layer7 Live API Creator
 has executed the response event (if any). You can (carefully) change or replace this object.
 object is different from request events. This variable is not a string, but rather an object or an array of objects. For a GET, this variable is a single object or an array of objects depending on the details of the request. For PUT/POST/DELETE, this variable is a single object with 
 elements. This variable can also be an error result. The 
 method must be able to stringify the JSON object.
For more information about this object, see The req Object.
The logger for the request.
For more information about this object, see The SysUtility Object.
Response Event Examples
The following code snippets show examples of the code for a response event.
The Sample database illustrates preparing a partner-specific response by materializing a resource and returning it as the response message. In this code example, API Server defines resource attribute aliases to match partner expectations:
// Remove all @metadata sections from the response for a specific table/resource
// Note: This would make it difficult for the client to update the data, but this is only an example.
// This only deals with top-level resources. To remove all
// @metadata sections from a complex resource, use recursion.
// An attribute 'TheVerb' is added to each array object or the single object
// with the name of request verb - GET, PUT, POST, PATCH, DELETE
// get the name used for metadata (configurable by project).
// You CAN convert from the Java string value to a JavaScript string object here
var metadataName = "@metadata";
if (Array.isArray(json)) {
// If the response is an array of objects
for (var i = 0; i < json.length; i++) {
delete json[i][metadataName];
json[i].txsummary && delete json[i].txsummary[0][metadataName];
// You MUST convert to native JavaScript string
json[i].TheVerb = req.verb;
else {
// The response is a single object
delete json[metadataName];
json.txsummary && delete json.txsummary[0][metadataName];
json.TheVerb = req.verb;
 For more information:
In this code example, API Server adds the top 30 (by 
 where the employee is the sales rep by augmenting each employee in a GET of multiple employees:
// for a GET to an employee, add any PURCHASEORDERS where the employee is a sales rep
if ("GET" == req.verb && "demo:employee" == req.resourceName) {
var ordersAsSalesRep = [];
for (var i = 0; i < json.length; i++ ) {
// want purchase orders for this employee as sales rep
// top thirty purchase orders by amount_total
var detail = {
filter: '"salesrep_id" = ' + json[i].employee_id ,
order: '"amount_total" desc',
pagesize: 10,
offset: 0
if(json[i].employee_id) {
ordersAsSalesRep = SysUtility.getResource("PurchaseOrders", detail);
json[i].ordersAsSalesRep = ordersAsSalesRep;
In this code example, API Server includes the results of a resource GET by amending the transaction summary:
// for a PUT (perhaps to update the name) of one or more employees
// add the picture into the result
if ("PUT" == req.verb && "demo:employee" == req.resourceName) {
var txsummary = json.txsummary;
for (var i = 0, len = txsummary.length; i < len; i += 1) {
var meta = txsummary[i]["@metadata"];
if ("UPDATE" == meta.verb && "demo:employee" == meta.resource) {
var detail = { filter: '"employee_id" = ' + txsummary[i].employee_id };
var picInfo = SysUtility.getResource("the_employee_picture", detail);
if (picInfo && picInfo.length > 0) {
txsummary[i].the_picture = picInfo[0].picture;
Combine these examples and delete the typical transaction summary, redirecting it to be the result of the 
For more information about transaction summaries, see Transaction Summary.
HTTP Options Events
Protect your data from unauthorized access by creating an HTTP Options event for your API. You can get complete control over the response to CORS OPTIONS preflight requests with HTTP Options event.
For more information:
HTTP Options Event Example
The following code snippet shows an example of the code for an HTTP Options event. This code instructs the client web browser that this API expects calls only from the specified URLs. 
Layer7 Live API Creator
 invokes the event handler only if the request includes an 
 header or an 
return {
"Access-Control-Allow-Methods" : "GET,PUT,POST,DELETE",
"Access-Control-Allow-Origin" : "https://internal.acme.com,https://safe.acme.com",
"Access-Control-Allow-Headers" : "authorization,content-type,accept,x-requested-with,X-CALiveAPICreator-ResponseFormat",
"Access-Control-Max-Age" : 2400 ,
"HTTPStatusCode" : 200
Error Events
You can have API Server intercept error conditions using error events. You can change the error response that API Server sends to a response that the client of your API expects. In the error event, you define the code that handles the error by modifying the error response.
For more information about how to have API Server catch and process them using standard JavaScript try/catch blocks, see JavaScript Code Examples.
For example, API Server sends the following error:
"statusCode": 500,
"errorCode": 50029,
"errorMessage": "Logic Execution failure: entity mssql:Deal, rule Deal, line 59/13, reason Only one Primary Broker allowed in RU3115 at line number 59 at column number 13"
You can create an error event so that API Server modifies the error and sends the following error response instead:
"statusCode": 500,
"errorCode": 999,
"errorMessage": "Only one Primary Broker allowed"
Error Event Example
The following code snippet shows an example of the code for an error event. This code example modifies the error response in API Server to a custom error response for your client:
var temp = json;//JSON response object
json = {
success : false,
error: {
code: temp.errorCode,
statusCode: temp.statusCode,
errorMessage: temp.errorMessage,
message: temp.message
Advanced Usage
The following sections illustrate advanced usage of event handlers.
Modify the JSON Request
You can look up the IDs of parent records when the JSON contains a key by inserting metadata action tags.
 request event in the Business to Business (B2B) API sample illustrates this familiar pattern.
For more information about the B2B API sample, see B2B API Sample.
Modify the JSON Response
You have full control over the result. The following is an example:
  1. Define a new 
     JavaScript resource, for example:
    return { Happy: ['New', 'Year']};
    For more information about how to define a JavaScript resource, see Define JavaScript Resource Types.
  2. In the REST Lab, view the result by performing a GET request, which is a different formatting of the previous JSON, by completing the following steps:
    1. Ensure that you are functioning by defining a simple response event that includes the following code:
      // MUST be an object or an array, if not, it is coerced into one (and probably NOT what you want)
      if ('GET' == req.verb && 'MyResource' == req.resourceName) {
      json = ["I got what I expected"];
      else {
      // this is returned on ALL other requests, make sure you delete this !!!
      json = ["Not what I wanted"];
    2. Change to a more specific event handler without the else condition:
      // Note double equals, as you are comparing JavaScript strings with java.lang.String objects
      if ('GET' == req.verb && 'MyResource' == req.resourceName) {
      json = SysUtility.getResource('demo:customer');
The event handler takes a value from the request and modifies the JSON response from the 
Access Request Headers
Request events have full access to the 
 in the incoming request. Request events have access to the 
 object, for example:
//header names are usually lowercase
var contentType = headers["content-type"];
Modify the Response
Response events can modify the outgoing response in multiple ways. The 
 variable is an instance of the 
 Java class. 
For more information about the 
 Java class, see the Oracle documentation.
In the response event, you can modify the response by adding headers, adding cookies, and setting the response code, for example:
var NewCookie = Java.type("javax.ws.rs.core.NewCookie");
var cookie = new NewCookie("MyCookie", "My cookie value");
response.header("X-my-special-header", "My header value");