Github user amberarrow commented on a diff in the pull request:
https://github.com/apache/apex-malhar/pull/368#discussion_r74608010
--- Diff: docs/operators/fsInputOperator.md ---
@@ -0,0 +1,101 @@
+File Input Operator
+=============
+
+## Operator Objective
+This operator scans a directory for files. Files are then read and split
into tuples, which are emitted. The default implementation scans a single
directory. The operator is fault tolerant. It tracks previously read files and
current offset as part of checkpoint state. In case of failure the operator
will skip files that were already processed and fast forward to the offset of
the current file. Supports partitioning and changes to number of partitions.
The directory scanner is responsible to only accept the files that belong to a
partition.
+
+File Input Operator is **idempotent**, **fault-tolerant** and
**partitionable**.
+
+## Operator Usecase
+1. Read all files of a directory and then keep scanning it for newly added
files.
+
+## Operator Information
+1. Operator location: ***malhar-library***
+2. Available since: ***1.0.2***
+3. Operator state: ***Stable***
+3. Java Packages:
+ * Operator:
***[com.datatorrent.lib.io.fs.AbstractFileInputOperator](https://www.datatorrent.com/docs/apidocs/com/datatorrent/lib/io/fs/AbstractFileInputOperator.html)***
+
+### AbstractFileInputOperator
+This is the abstract implementation that serves as base class for scanning
a directory for files and read the files one by one. This class doesnât have
any ports.
+
+
+
+## Properties, Attributes and Ports
+### <a name="props"></a>Properties of AbstractFileInputOperator
+| **Property** | **Description** | **Type** | **Mandatory** | **Default
Value** |
+| -------- | ----------- | ---- | ------------------ | ------------- |
+| *directory* | absolute path of directory to be scanned | String | Yes |
N/A |
+| *scanIntervalMillis* | Interval in milliseconds after which directory
should be scanned for new files | int | No | 5000 |
+| *emitBatchSize* | Number of tuples to emit in a batch | int | No | 1000 |
+| *partitionCount* | Desired number of partitions count | int | No | 1 |
+| *maxRetryCount* | Maximum number of times the operator will attempt to
process a file | No | 5 |
+| *scanner* | Scanner to scan new files in directory |
[DirectoryScanner](#DirectoryScanner) | No | DirectoryScanner |
+
+#### <a name="JDBCStore"></a>Properties of DirectoryScanner
+| **Property** | **Description** | **Type** | **Mandatory** | **Default
Value** |
+| -------- | ----------- | ---- | ------------------ | ------------- |
+| *filePatternRegexp* | regex to select files from input directory |
String | No | N/A |
+
+## Abstract Methods
+void emit(T tuple): Abstract method that emits tuple read from file.
+T readEntity(): Abstract method to read file entity (can be a line or
block).
+
+## Partitioning of File Input Operator
+Configure parameter "partitionCount" to define desired number of
partitions.
+
+```xml
+ <property>
+ <name>dt.operator.{OperatorName}.prop.partitionCount</name>
+ <value>4</value>
+ </property>
+```
+
+where {OperatorName} is the name of the input operator.
+Above lines will partition operator statically 4 times. Above value can be
changed accordingly to change the number of static partitions.
+
+#### Dynamic Partitioning of JDBC Poller
+Implement ```getNewPartitionCount()``` method to do dymanic partitioning.
+
+## Derived Classes
+### 1. AbstractFTPInputOperator
+The class is used to read files from FTP file system.
+#### Properties, Attributes and Ports
+#### <a name="props"></a>Properties of AbstractFTPInputOperator
+| **Property** | **Description** | **Type** | **Mandatory** | **Default
Value** |
+| -------- | ----------- | ---- | ------------------ | ------------- |
+| *host*| Hostname of ftp server.| String | Yes | N/A |
+| *port*| Port of ftp server.| int | No | 21 (default ftp port) |
+| *userName*| Username which is used for login to the server. | String |
No | anonymous |
+| *password*| Password which is used for login to the server. | String |
No | gues |
+
+### 2. AbstractParquetFileReader
+Reads Parquet files from input directory using GroupReadSupport. Derived
classes need to implement
[convertGroup(Group)](https://www.datatorrent.com/docs/apidocs/com/datatorrent/contrib/parquet/AbstractParquetFileReader.html#convertGroup(Group))
method to convert Group to other type.
--- End diff --
Since we are being explicit about what derived classes need to do here,
please verify that we do the same for all the other operators described here.
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