Consuming REST APIs

In addition to reading from and writing data to JSON, CSV, XML, Excel and Parquet files, the Data Exchange library also supports sending HTTP client requests to an HTTP (REST) service, as well as exposing procedures in your model through a built-in HTTP service component. This makes it the one-stop solution for communicating JSON, CSV, XML and Excel data with external REST APIs.

Asynchronous HTTP client

The Data Exchange library contains a fully asynchronous HTTP client library, based on the well-known libCurl library (see the libCurl documentation). Each request will be executed in parallel through a fixed number of concurrent connections, and upon each response a user-specified callback will be executed when the AIMMS engine is idle, or whenever the modeler has explicitly requested the Data Exchange library to execute the callbacks for all handled requests. This approach allows massive amounts of requests to be handled in parallel, tremendously decreasing the total time it takes to perform all requests when the service called is set up in a scalable manner.


The functions in the dex::client namespace offer an alternative for the httpclient library, fully integrated within the Data Exchange library which will most likely be necessary for API calls anyway to map request bodies and responses to identifiers in the model. Both offer similar functionality, although there are some differences, most notably

  • the httpclient library does automatic proxy discovery, while for dex::client requests proxy discovery must be performed manually via the dex::client::ProxyResolve() function and subsequently set via the curl PROXY option. The function dex::client::DetermineProxyServer() will do perform steps for you.

  • dex::client HTTP requests can make use of all libCurl functionality that is available via libCurl options but not in the httpclient library (e.g. SPNEGO authentication)

  • dex::client HTTP requests only support a fully asynchronous execution model, optimized for massive amounts of HTTP/API requests to be executed in parallel

Although the dex::client HTTP requests more than the httpclient library forces you to e.g. adhere strictly to its asynchronous model, or invoke certain functionality by specifically enabling it through the available options, the tight integration also allows for more advanced features such as the automatic generation of API client code, based on an OpenAPI 3.1 specification of REST API.

To initiate a request, call the function dex::client::NewRequest(), where you specify the URL of the request, the callback to be called for the response, the HTTP method (GET, POST, …), and, if necessary, the location of a request and/or response file.

dex::client::NewRequest("request-1", "", 'SimpleRequestCallback', httpMethod: 'GET', responseFile: "out/request-1.html");

This as an instruction to create a new HTTP request with a unique identifier request-1, which will get the contents of After the Data Exchange library receives the response, it will be store in the file out/request-1.html, after which a callback called SimpleRequestCallback, will be called. The Data Exchange library will not come up with a unique identifier for the request itself, but will leave it up to you to come up with an appropriate scheme to identify your requests yourself.

The request will not be executed yet, however. Prior to this, you first have the opportunity to set additional headers for the request using the function dex::client::AddRequestHeaders(), or add additional Curl options using the function dex::client::AddRequestOptions(), for instance, to specify a proxy that is to be used to connect to the given server, or whether libCurl should follow any redirects it encounters.

To actually execute the request, you should call the function dex::client::PerformRequest()


This will queue the request for execution on one of the concurrent connections maintained by the Data Exchange library for making HTTP requests. You can specify how many concurrent connections you want to be used to execute HTTP requests through the function dex::client::SetParallelConnections(). By default, the Data Exchange library will use up to 16 parallel connections. By increasing this maximum number of connections you may substantially decrease the total amount of time taken to execute a large number of requests, but you should also make sure that the server infrastructure handling these requests is comfortable handling the number of parallel connections you set.

Upon completion of the request, the specified callback function will be called, either when the AIMMS engine is idle, or within the flow of the calling procedure, by calling the procedure dex::client::WaitForResponses(). Each callback function should have the following three arguments:

  • theRequest, the specific request identifier for which the callback is called.

  • statusCode, the HTTP status code of the response.

  • errorCode, the Curl error code for the response in case the request was not successful.

If there was a libCurl error, the HTTP status code will be 0, and you can use the function dex::client::GetErrorMessage(), to retrieve a description of the Curl error that occurred, based on the errorCode argument.

If the status code is 200 (OK), then you can proceed to request the response headers using the function dex::client::GetResponseHeaders(), request additional info about the request from libCurl using the function dex::client::GetInfoItems() (e.g. the total request time, or the final destination of your request in case of redirects), or can use the function dex::ReadFromFile() to read the response data into identifiers in your model in case of REST call to some REST API.

The Data Exchange library will automatically close a request as soon as the specified callback function has been called, not to leave any resources in use unnecessary. It will, however, not remove any request and/or response files or memory streams you specified, unless the memory stream names start with ## (see Memory streams).

The library has been tested to be able to call a very simple HTTP service (i.e., with an empty response) for 100,000 times over 256 parallel connections within 20 or so seconds, so should able to deal with a more realistic number of calls to a non-trivial service as well. Note that in this case, the time taken to deal with the response in the callback (e.g. reading the data in AIMMS identifiers) may substantially add to the overall time to make and handle all requests.

Using OAuth2 for API authorization

The two most common ways to authorize the use of APIs is through the use of

While the use of API keys is fairly straightforward, and requires no additional support in AIMMS to use, the implementation of the OAuth2 protocol can be quite intricate, and some authorization flows require support in AIMMS to function at all. For this reason, the Data Exchange library provides all means necessary to effortlessly authorize the use of an API through the OAuth2 protocol.

With the OAuth2 protocol, a client application (i.e., your model) can be authorized to access an API. This authorization can take place at two levels:

  • at the application level, where the application itself will be authorized to access the API (called the Client Credentials flow), or

  • the application can get access to the API on behalf of the user operating the application through a UI (called the Authorization Code flow).

For both of these authorization flows, the result of a successful authorization through the OAuth2 protocol will be a Bearer token, which, when added to an API call, will authorize the application to access the API with a given level of access for a limited period of time.

Every client application with access to an API is identified through a client id and a client secret, which are handed out by the administrator of the API. The level of access to the API is set via one or more scopes, which the API administrator also needs to provide to client applications.

To get OAuth2 to work, you further need some end points of the identity platform that is used by the API for authentication and authorization. For both authorization at the application and user level, you will need to know the token endpoint, where the application can retrieve the Bearer token.

When requesting authorization on behalf of an end-user of an application, you further need to supply the authorization endpoint of the identity platform, where the end-user needs to authenticate herself with the identity platform, as well as a redirect url through which the identity platform can inform the application about the result of the end-user authentication.

Using the Client Credentials flow

To use the OAuth2 Client Credentials flow, you need to specify the following information

  • Identify your AIMMS model as a API client, by adding a client name to the set dex::oauth::APIClients,

  • Provide the client id, client secret and token endpoint for the API client via the string parameter dex::oauth::APIClientStringData, and

  • Provide the requested access level through the scope provided by the API administrator.

With this information, you can now add a Bearer authorization token to any dex::client request theRequest, by calling

dex::oauth::AddBearerToken(apiClient, theRequest);

prior to the actual call to dex::client::PerformRequest(). The function dex::oauth::AddBearerToken() will check whether there is still a Bearer token for the given apiClient valid up to an interval of dex::oauth::TokenValidityThreshold seconds of the token’s expiration time, and if not, request a new Bearer token.

Using the Authorization Code flow

To use the OAuth2 Authorization Code flow, you need to provide, on top of the information you also need for the Client Credentials flow

  • the authorization endpoint where the end-user needs to authenticate herself,

  • the path part of the redirect URL where the used identity platform will need to forward the result of the end-user authorization step to the application.

The following example shows the Authorization Code flow specification required for a client application authorized through the Azure Active Directory identity platform.

dex::oauth::APIClients := data { MS };

dex::oauth::APIClientStringData('MS',dex::oauth::apidata) :=$ data {
                authorizationEndpoint : "<tenant-id>/oauth2/v2.0/authorize",
                tokenEndpoint : "<tenant-id>/oauth2/v2.0/token",
                clientId : "<client-id>",
                clientSecret : "*******************************",
                scope: "offline_access"


To get a refresh token for Azure AD, you should add the offline_access scope.

By default, the Data Exchange library will encode the client id and secret as part of the form data that is being sent to the authorization endpoint. However, by setting the useBasicAuthorization member of the dex::oauth::APIClientStringData identifier to any non-default string value, you can instruct the Data Exchange library to use basic authentication instead, if your Identity Provider expects this.

When running the AIMMS application locally on your desktop, AIMMS will, by default, instantiate a fixed redirect URL http://localhost/oauth2. You can configure the path in the redirect URL by changing the string parameter dex::api::OAuth2CallbackPath, which has a default value of oauth2.

For an on-premise PRO server, or for the AIMMS Cloud Platform, the redirect URL will be the fixed URL https://<pro-server-dns-name>/extensions/<app-name>/oauth2/. You need to provide this redirect URL to the API administrator to allow the application to be authorized when run from the desktop. If the published name of your application on the on-premise PRO server or the AIMMS Cloud Platform contains spaces, you should URL-encode the application name before registering it with the identity platform.

With these settings, you can again call the function dex::oauth::AddBearerToken() to add a Bearer token to your API request. In this case, however, the end-user will need to authenticate herself with the identity platform through a browser session that will be initiated by AIMMS on the first call, and optionally on additional calls when a previous Bearer token has expired and cannot be refreshed.


The OAuth2 Authorization Code flow support will work for AIMMS sessions running locally on the desktop for all AIMMS versions. For WebUI session running from an on-premise PRO server or from the AIMMS Cloud Platform, AIMMS version 4.84+ and PRO version 2.41+ are required to support the redirect URLs to be routed back to the AIMMS session backing your WebUI session. Also, the use of OAuth requires the use of HTTPS on your on-premise PRO server, and the library will assume HTTPS traffic on port 443. If your PRO on-premise server uses a different port, then you can configure it through the parameter dex::oauth::WebUIOAuthCallbackPort.

Debugging client requests

When you experience trouble invoking a URL using dex::client requests, here are a number of guidelines that may help you tackle it:

  • libCurl doesn’t automatically follow redirects, and is pretty strict on checking revocation lists by default. This may cause HTTP requests to fail with sometimes hard to follow error messages. In addition, the HTTP client in the Data Exchange library does not perform automatic proxy discovery, which may cause HTTP requests to fail because the proper proxy is not used during the request. The following code will sensible defaults to prevent all of these issues:

    dex::client::ProxyResolve("", proxyURL);   ! determine proxy URL, assuming the same proxy result for any URL
    stringOptions(dex::client::stropt) := { 'PROXY' : proxyURL };   ! instruct libcurl to use the given proxy
    intOptions(dex::client::intopt) := { 'HTTPPROXYTUNNEL' : 1, 'SSL_OPTIONS' : 2, 'FOLLOWLOCATION' : 1, 'MAXREDIRS' : 10 };
    dex::client::SetDefaultOptions(intOptions, stringOptions);

The procedure dex::client::DetermineProxyServer will set these defaults options for you.

  • If your request contains a request body, the HTTP client will deduce the content type of the request body from the file extension containing the body, or if it cannot deduce it, set it to application/octetstream. You may need to set the Content-Type header to a proper value to make the request succeed, specifically when you do a POST request with URL-encoded parameters, as follows

    dex::client::AddRequestHeader(reqId, "Content-Type", "application/x-www-form-urlencoded");
  • A good way to debug HTTP requests is to enable request tracing by specifying a trace file in the dex::client::NewRequest() function. The resulting file will contain all available tracing information made available by libCurl, including all verbatim request and response headers and bodies.


    If you use trace files to debug the communication between libCurl and a website, be aware that the trace file will expose all headers, potentially including those that contain API keys or credentials necessary to access a web service. In such case, you are advised to carefully delete trace files directly after use. You should never create trace files in production.