ImageFetcher

Categories

Rasters

Workflows

Fetches an image by performing an HTTP GET operation on the specified URL, and then returning the resulting data as the geometry of a raster feature.

If an error occurs while accessing the URL, or while converting the downloaded data into a raster, the translation will continue, and the feature will be output through the <Rejected> port.

Configuration

Parameters

Image URL

Enter the URL (for example, http://www.url.com) or select an attribute that contains a URL.

Image Type

Choose the image type. If the <Auto-Detect from HTTP Headers> option is chosen, the transformer will examine the ‘Content-Type’ header in the HTTP Response for a MIME type. If a known MIME type is found, the corresponding reader will be used to create the raster geometry.

Maximum Number of Concurrent HTTP Requests

This parameter controls how features are processed through the transformer. The value of this parameter indicates the maximum number of transfers that will be in progress at the same time.

When the parameter is set to 1, a transfer will be fully processed before the next transfer can begin.

When the parameter is set to a value greater than 1, the transformer will not wait for each transfer to finish before starting the next one. There may be multiple transfers occurring at the same time. This can provide significant performance benefits. This value should be adjusted based on the ability of the HTTP server to handle multiple requests at the same time, taking into consideration that multiple HTTPCaller transformers may be accessing the server at the same time.

This parameter must be set to 1 if the transformer is being used inside a custom transformer which has a loop output.

Maximum Requests per Interval

This is the maximum number of HTTP requests the transformer will start in a particular time interval. For example, if you wanted to limit the transformer to 50 requests per minute, you would set this parameter to 50.

Interval Length (seconds)

This is the time period over which requests will be limited. For example, if you wanted to limit the transformer to 50 requests per minute, you would set this parameter to 60. Some preset values are provided in the parameter list, or any number may be manually entered.

Request Timing

This controls the way that transfers are started within the time interval. There are two options:

  • Start requests as soon as possible: When this value is selected, requests will be started as soon as possible, as long as the maximum number of requests started in the preceding time interval does not exceed the maximum. This may lead to many requests being started at the beginning of the time interval, and then nothing happening until the interval has passed.

  • Start requests evenly through the interval: When this value is selected, the transformer will attempt to start requests evenly through the interval. That is, it will divide the interval length by the maximum number of requests, and then ensure that at least this much time elapses between starting each request. This is equivalent to inserting a Decelerator transformer before the HTTPCaller, and can ease the load on an HTTP server.

Save Cookies

If this parameter is set to Yes, any saved cookies that correspond to the URL being accessed will be sent with the request. In addition, any cookies contained in the response will be saved for use with subsequent HTTP requests.

Saved cookies will persist for the duration of the translation – they cannot be used in subsequent translations.

Saving cookies is useful when a web service keeps a "session". Then the following sequence of operations might be used:

  1. An HTTPCaller transformer is used to log in to the web service and a session cookie is saved.
  2. An ImageFetcher transformer is used to download an image from the web service, utilizing the saved session cookie.
  3. A second HTTPCaller is used to log out of the web service, which invalidates the saved cookie.

Follow Redirects

If this parameter is set to Yes, the transformer will follow any URL redirects it encounters, making a second request to a new location.

Verify HTTPS Certificates

If this parameter is set to Yes, the transformer will verify HTTPS certificates. This is done in two stages:

  1. Ensure that the certificate is for the host we are trying to connect to.
  2. Ensure that the certificate is valid.

Connection Timeout Length

This parameter can be used to set a timeout on HTTP connection requests. If a connection to the server cannot be created within the given number of seconds, the request will fail, and the feature will be output through the <Rejected> port. The default connection timeout is 60 seconds. If 0 is entered, there will be no timeout, and the transformer will wait indefinitely for a successful connection.

Transfer Timeout Length

This parameter can be used to set a timeout on HTTP transfers. If data stops being transferred for more than the given number of seconds, the request will fail, and the feature will be output through the <Rejected> port. The default transfer timeout is 90 seconds. If 0 is entered, there will be no timeout, and the transformer will wait indefinitely for data to resume being transferred.

Enable if the image is being retrieved from a server that requires user authentication.

Authentication Method

Specify the method for authenticating with the server:

  • Basic (HTTP Basic authentication): This is the default choice, and the only method in widespread use and supported virtually everywhere. This sends a username and password over the network in plain text, easily captured by others.
  • Digest (HTTP Digest authentication): Digest authentication is more secure over public networks than the Basic method.
  • NTLM (HTTP NTLM authentication): NTLM is a proprietary protocol invented and used by Microsoft. It uses a challenge-response and hash concept similar to Digest to prevent the password from being eavesdropped.
  • Named Web Connection: Allows you to authenticate using a stored named web service connection. For more information, see Using Web Connections in the FME Workbench help.

Named Web Connection

If Authentication Method is Named Web Connection, specify the named connection to use for authentication. To add a new named connection, select Add Named Web Connection from the dropdown menu.

HTTP Authentication Username, HTTP Authentication Password

If Authentication Method is Basic, Digest, or NTLM, specify the username and password for authenticating.

HTTP Status Code

(Optional) Name the attribute to contain the HTTP response code returned by the server.

HTTP Error

(Optional) Name the attribute to contain a relevant error message if an error occurs. The feature will be output through the <Rejected> output port.

List Attribute for Response Headers

This parameter is optional. If it is set to a list attribute name, then HTTP headers returned by the server will be stored in the list attribute. The headers will be split into names and values. For example:

`_headers{0}.name' has value `X-XSS-Protection'

`_headers{0}.value' has value `1; mode=block'

`_headers{1}.name' has value `Date'

`_headers{1}.value' has value `Wed, 25 May 2011 17:20:20 GMT'

`_headers{2}.name' has value `Transfer-Encoding'

`_headers{2}.value' has value `chunked'

`_headers{3}.name' has value `Content-Type'

`_headers{3}.value' has value `text/html; charset=ISO-8859-1'

`_headers{4}.name' has value `Cache-Control'

`_headers{4}.value' has value `private, max-age=0'

Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.

Using the Text Editor

The Text Editor provides a convenient way to construct text strings (including regular expressions) from various data sources, such as attributes, parameters, and constants, where the result is used directly inside a parameter.

Text Editor

Using the Arithmetic Editor

The Arithmetic Editor provides a convenient way to construct math expressions from various data sources, such as attributes, parameters, and feature functions, where the result is used directly inside a parameter.

Arithmetic Editor

Conditional Values

Set values depending on one or more test conditions that either pass or fail.

Parameter Condition Definition Dialog

Content

Expressions and strings can include a number of functions, characters, parameters, and more.

When setting values - whether entered directly in a parameter or constructed using one of the editors - strings and expressions containing String, Math, Date/Time or FME Feature Functions will have those functions evaluated. Therefore, the names of these functions (in the form @<function_name>) should not be used as literal string values.

String Functions

These functions manipulate and format strings.

Special Characters

 A set of control characters is available in the Text Editor.

Math Functions

Math functions are available in both editors.
Date/Time Functions Date and time functions are available in the Text Editor.

Math Operators

These operators are available in the Arithmetic Editor.

FME Feature Functions

These return primarily feature-specific values.

FME Parameters

FME and workspace-specific parameters may be used.
Creating and Modifying User Parameters Create your own editable parameters.

Dialog Options - Tables

Transformers with table-style parameters have additional tools for populating and manipulating values.

Row Reordering

Enabled once you have clicked on a row item. Choices include:

  • Add a row
  • Remove a row
  • Move current row up one
  • Move current row down one
  • Move current row to top
  • Move current row to bottom

Cut, Copy, and Paste

Enabled once you have clicked on a row item. Choices include:

  • Cut a row - delete and copy to clipboard
  • Copy a row to the clipboard
  • Paste a row from the clipboard

Cut, copy, and paste may be used within a transformer, or between transformers.

Filter

Start typing a string, and the matrix will only display rows matching those characters. Searches all columns. This only affects the display of attributes within the transformer - it does not alter which attributes are output.

Import

Import populates the table with a set of new attributes read from a dataset. Specific application varies between transformers.

Reset/Refresh

Generally resets the table to its initial state, and may provide additional options to remove invalid entries. Behavior varies between transformers.

Note: Not all tools are available in all transformers.

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