File Poller
In this article
Overview
You can use this Snap to poll the target directory and find file names matching the specified pattern.
The Snap continues polling at the intervals specified in the Polling interval property until the timeout (specified in the Polling timeout property) is reached. After polling is done, the Snap lists all files whose names match the specified pattern.
This Snap can be used in situations where an operation must be triggered when a specific file is found in the target directory. The pipeline can be configured with additional Snaps to process the Snap's output and delete the matched file before the Polling interval value is reached.
The File Poller Snap uses the case-sensitive filter pattern, regardless of the operating system.
This Snap polls the target directory only. Subdirectories, if any, are ignored. Use the Directory Browser Snap if you want to poll files in the directory and all subdirectories, and to poll a directory only once.
Snap Type
The File Poller Snap is a Read-type Snap.
Prerequisites
Support for Ultra Pipelines
Works in Ultra Pipelines.
Limitations
For S3 folders, the Snap currently supports polling the target directory for a maximum of 10,000 files. If there are more than that, the Snap does not provide any output.
Known Issues
The Snap is expected to fail if there is no account selected. However, the Snap may execute successfully without any account if all the following conditions exist:
The Snap is executed in an EC2-instance Snaplex where your pipeline runs with an IAM role.
The S3 bucket accessed by the Snap includes the necessary permissions for use with the specific IAM role.
The following global property is set as a node property in the plex:
jcc.jvm_options = -DIAM_CREDENTIAL_FOR_S3=TRUE
Behavior Change
The File Poller Snap now honors the value specified in the Polling timeout field instead of polling indefinitely in case of poor file polling operations. To handle indefinite polling operations the polling is done in a separate thread. However, when the execution time exceeds the value specified in the Polling timeout, a timeout exception is written to the log to prevent the polling from getting stuck and the Snap continues polling depending on the Polling timeout.
If the Polling timeout value is greater than 0, the Snap polls until the end of polling window.
If it is less than 0, the Snap stops polling.
If it is -1, the Snap continues polling.
Supported Protocols
Account types supported by each protocol are as follows:
Protocol | Account types |
---|---|
sldb | no account |
s3 | AWS S3 |
ftp | Basic Auth |
sftp | Basic Auth, SSH Auth |
ftps | Basic Auth |
hdfs | no account |
smb | SMB |
wasb | Azure Storage |
wasbs | Azure Storage |
gs | Google Storage |
file | Local file system |
The FTPS file protocol works only in explicit mode. The implicit mode is not supported.
Required settings for account types are as follows:
Account Type | Settings |
---|---|
Basic Auth | Username, Password |
AWS S3 | Access-key ID, Secret key |
SSH Auth | Username, Private key, Key Passphrase |
SMB | Domain, Username, Password |
Azure Storage | Account name, Primary access key |
Google Storage | Approval prompt, Application scope, Auto-refresh token |
Snap Views
Type | Format | Number of Views | Examples of Upstream and Downstream Snaps | Description |
---|---|---|---|---|
Input | Document
|
|
| An optional document to evaluate expressions in the Directory and/or File filter properties. Note that each input document will trigger the execution of the Snap. |
Output | Document
|
|
| A full path in each document as a value for a key "path". If multiple files match the filter, the same number of documents will be provided in the output view after each interval. [
{
"path" : "sftp://sftp.smart.com/home/voo/test1.csv"
},
{
"path" : "sftp://sftp.smart.com/home/voo/test2.csv"
}
] |
Error | Error handling is a generic way to handle errors without losing data or failing the Snap execution. You can handle the errors that the Snap might encounter when running the pipeline by choosing one of the following options from the When errors occur list under the Views tab:
Learn more about Error handling in Pipelines. |
Snap Settings
Field Name | Field Type | Description | |
---|---|---|---|
Label*
| String | Specify the name for the Snap. You can modify this to be more specific, especially if you have more than one of the same Snap in your pipeline. | |
Directory
Default Value: N/A
| String/Expression | Specify the URL path to the directory where files will be searched in the following format: The supported file protocols are:
| |
File filter*
| String/Expression | Specify a GLOB pattern to be applied to select one or more files in the directory. The File filter property can be a JavaScript expression which will be evaluated with values from the input view document. [None] | |
Polling interval in seconds*
| Integer | Specify the time gap between each poll request (in seconds).
| |
Polling timeout*
Default value: 30 | Integer | Specify a period of time after which file polling must end. If the Polling timeout is set to:
| |
Polling-timeout unit
| Dropdown list | Specify a value for polling timeout.
| |
Only Output on Change
Default value: Selected | Checkbox | Select this check box to instruct the Snap to provide an output only when there is a change in the contents of the polled directory. When selected, the Snap provides an output during its initial run if it finds matching documents. However, it provides polling results in the next run only if the polled directory has newer files that match the pattern specified. | |
Number of retries
Minimum value: 0 | Integer | Specify the maximum number of retry attempts that the Snap must make in case there is a network failure, and the Snap is unable to read the target file. | |
Retry interval (seconds)
Minimum value: 1 Default value: 1 | Integer | Specify the minimum number of seconds for which the Snap must wait before attempting recovery from a network failure.
| |
Advanced properties | Use this field set to define specific settings for polling files. | ||
Properties | Dropdown list | Choose either of the following options:
| |
Values | String/Expression |
| |
Snap execution Default Value: Validate & Execute | Dropdown list | Select one of the following three modes in which the Snap executes:
|
Troubleshooting
Error | Reason | Resolution |
---|---|---|
| The library that we use for SFTP connections no longer supports deprecated signature protocols by default. This changed with the 4.33 GA release. | Add the algorithm to the
Learn more: Configuration Options |
| If you have set the Polling Timeout value to a few seconds, it results in the S3 request getting canceled.
| Increase the value of Polling Timeout (in seconds) for the Snap to work successfully. We recommend that you set the Polling Timeout value to the default value of 30 minutes or more to fetch all the data from S3. |
Examples
Write a List of Files in a Specific Directory
This example pipeline demonstrates how to list out files from a specific directory. After the Poller Snap lists the files, then write the output to a file and run the File Poller Snap again to check whether the new file was created as expected. To ensure that the File Poller Snap doesn't pick up any existing files, you use an unusual extension for this example.
Configure the File Poller Snap to poll a directory for all files with the extension ".JSON2"
.
Connect JSON Formatter and File Writer Snaps to process the output and write it to a file. Use the JSON Formatter Snap with the default settings. In the File Writer Snap, use the date.now() function to give the file a name, so a new file is created every time you run the pipeline.
When used in production, the output from the File Poller Snap can be used to trigger specific tasks as needed. In this example, you write it to a file. As expected, the file contains no output, as there is no file in the target directory with the extension ".JSON2"
.
Next, add the second File Poller Snap and configure it exactly as the first one. Once again, you add a JSON Formatter and File Writer Snap with the same settings as for the previous pair. But this time, the file created is not blank: It lists out the file that you created using the first three Snaps in the pipeline:
Download this pipeline.
Poll a Directory Using a Trigger Task from ServiceNow
In this example, you call a Trigger Task from ServiceNow to poll a directory for files of a specific type.
Download this pipeline
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