Initializing help system before first use

# REST API Overview

You can develop a client application by using the FICO® Xpress Executor REST API, which is defined using the OpenAPI Specification.

For details about the REST API, see the Executor 4 REST API reference. You can also browse the API specification using the Swagger UI. To access the Swagger UI, go to the FICO Platform - Core Services Components page and then select Configure > Swagger UI from the Executor 4 component menu.

A client application can interact with any of the documented endpoints, although the most common use case is to execute the model using `POST /api/executions`. Other endpoints provide programmatic access to the model and the execution configuration.

The REST API is versioned. Breaking changes will not be introduced without a major version change. (A breaking change is the removal of any field, or the introduction of a flow that would prevent an existing properly written client from functioning.) Minor version changes might introduce new fields or headers that an existing client can safely ignore. The API is designed according to the "principle of least astonishment."

The API uses content negotiation and custom headers to convey additional information. (For details about response codes and associated media types, see the Executor 4 REST API reference or the API specification.) Because a client application cannot know in advance what response code it will receive, it should accept both the expected and exceptional content types to accommodate the responses from both successful and unsuccessful requests. For example, when your client submits an execution request to `/api/executions`, the request should include the following headers:

    ACCEPT: application/vnd.com.fico.executor.v2+octet-stream, application/vnd.com.fico.executor.v2+json

The response to any request includes a `FICO-EXECUTION-HOST` header, which specifies the hostname of the Xpress Executor node that received and processed the request. You can use this header to track the nodes. New nodes might be created if gears are scaled up or down, or if a node goes down and is recreated by the platform.

The `/api/executions` endpoint returns additional headers that contain information about the execution, such as the execution ID and the location of the run log.

An error response contains an outer error with a high-level code and a description. The outer error can contain one or more nested inner errors with more specific details about the problem that was encountered.

Authentication is handled by FICO Platform. For more information, see the FICO Platform - Core Services help.

Parent Topic Executor 4 FICO Platform Component Guide

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