Accessing Data and Metadata
Some filters (e.g : AppendFilter can be configured using Simple Connect Expression Language.
Simple Connect Expression Language (ScEL for short) is an expression language based on regex that allows quick access and manipulating record fields and metadata.
The syntaxes to define an expression are of the form : <expression string> or "{{ <expression string> }}".
ScEL supports the following capabilities :
- Literal expressions
- Field Selector
- Nested Navigation
- String substitution
- Functions
Literal expressions
- String :
'Hello World' - Number :
42 - Boolean:
True - Nullable:
null
Field Selector
The expression language can be used to easily select one field from the input record :
$.username
Nested Navigation
To navigate down a struct value, just use a period to indicate a nested field value :
$.address.city
String substitution
The expression language can be used to easily build a new string field that concatenate multiple ones :
The user {{ $.username }} is living in city {{ $.address.city }}
Function
The expression language support function call :
The user {{ $.username }} is living in city {{ uppercase($.address.city) }}
Dynamic Field Selector
String substitution can be used to dynamically select a field :
The bellow example shows how to dynamically build a field selector by concatenating $. and
the first element present in the array field $.values.
{{ '$.'extract_array($.values, 0) }}
Note the used of double-quotes to define a substitution expressions
Built-in Functions
ScEL supports a number of predefined functions that can be used to apply a single transformation on a field.
| Function | Description | Syntax |
|---|---|---|
concat | Concatenate two or more string expressions. | {{ concat(expr1, expr2, ...) }} |
concat_ws | Concatenate two or more string expressions, using the specified separator between each. | {{ concat_ws(separator, prefix, suffix, expr1, expr2, ...) }} |
contains | Returns true if an array field’s value contains the specified value | {{ contains(array, 'value') }} |
converts | Converts a field’s value into the specified type | {{ converts(field, INTEGER) }} |
ends_with | Returns true if an a string field’s value end with the specified string suffix | {{ ends_with(field, 'suffix') }} |
equals | Returns true if an a string or number fields’s value equals the specified value | {{ equals(field, value) }} |
exists | Returns true if an the specified field exists | {{ ends_with(field, value) }} |
extract_array | Returns the element at the specified position of the specified array | {{ extract_array(array, 0) }} |
hash | Hash a given string expression, using murmur2 algorithm | {{ hash(field_expr) }} |
is_null | Returns true if a field’s value is null | {{ is_null(field) }} |
length | Returns the number of elements into an array of the length of an string field | {{ length(array) }} |
lowercase | Converts all of the characters in a string field’s value to lower case | {{ lowercase(field) }} |
matches | Returns true if a field’s value match the specified regex | {{ matches(field, 'regex') }} |
md5 | Compute the MD5 hash of string expression | {{ md5(field_expr) }} |
nlv | Sets a default value if a field’s value is null | {{ length(array) }} |
replace_all | Replaces every subsequence of the field’s value that matches the given pattern with the given replacement string. | {{ replace_all(field, 'regex', 'replacement') }} |
split | Split a string field’s value into an array using the specified regex or character | {{ split(field_expr, regex) }} or {{ split(field_expr, regex, limit) }} |
starts_with | Returns true if an a string field’s value start with the specified string prefix | {{ starts_with(field, 'prefix') }} |
trim | Trims the spaces from the beginning and end of a string. | {{ trim(field) }} |
uppercase | Converts all of the characters in a string field’s value to upper case | {{ uppercase(field) }} |
uuid | Create a Universally Unique Identifier (UUID) | {{ uuid() }} |
In addition, ScEL supports nested functions.
For example, the following expression can be used to replace all whitespace characters after transforming our field’s value into lowercase.
replace_all(lowercase($.field), '\\s', '-')
Limitation
Currently, FilePulse does not support user-defined functions (UDFs). So you cannot register your own functions to enrich the expression language.Scopes
In the previous section, we have shown how to use the expression language to select a specific field. The selected field was part of our the current record being processed.
Actually, ScEL allows you to get access to additional fields through the used of scopes. Basically, a scope defined the root object on which a selector expression must evaluated.
The syntax to define an expression with a scope is of the form : “$<scope>.<selector expression string>”.
By default, if no scope is defined in the expression, the scope $value is implicitly used.
ScEL supports a number of predefined scopes that can be used for example :
- To define the topic for the record.
- To define the key for the record.
- To get access to metadata about the source file.
- Etc.
| Scope | Description | Type |
|---|---|---|
$headers | The record headers | - |
$key | The record key | string |
$metadata | The file metadata | struct |
$offset | The offset information of this record into the source file | struct |
$system | The system environment variables and runtime properties | struct |
$timestamp | The record timestamp | long |
$topic | The output topic | string |
$value | The record value | struct |
$variables | The contextual filter-chain variables | map[string, object] |
Note, that in case of failures more fields are added to the current filter context (see : Handling Failures)
Record Headers
The scope headers allows defining the headers of the output record.
Record key
The scope key allows defining the key of the output record. Only string key is currently supported.
Source Metadata
The scope metadata allows read access to information about the file being processing.
| Predefined Fields (ScEL) | Description | Type |
|---|---|---|
$metadata.name | The file name | string |
$metadata.path | The file directory path | string |
$metadata.absolutePath | The file absolute path | string |
$metadata.hash | The file CRC32 hash | int |
$metadata.lastModified | The file last modified time. | long |
$metadata.size | The file size | long |
$metadata.inode | The file Unix inode | long |
Record Offset
The scope offset allows read access to information about the original position of the record into the source file.
The available fields depend on the configured FileInputRecord.
| Predefined Fields (ScEL) | Description | Type |
|---|---|---|
$offset.timestamp | The creation time of the record (millisecond) | long |
Information only available if RowFilterReader is configured.
| Predefined Fields (ScEL) | Description | Type |
|---|---|---|
$offset.startPosition | The start position of the record into the source file | long |
$offset.endPosition | The end position of the record into the source file | long |
$offset.size | The size in bytes | long |
$offset.row | The row number of the record into the source | long |
Information only available if BytesArrayInputReader is configured.
| Predefined Fields (ScEL) | Description | Type |
|---|---|---|
$offset.startPosition | The start position of the record into the source file (always equals to 0) | long |
$offset.endPosition | The end position of the record into the source file (equals to the file size) | long |
Information only available if AvroFilterInputReader is configured.
| Predefined Fields (ScEL) | Description | Type |
|---|---|---|
$offset.blockStart | The start position of the current block | long |
$offset.position | The position into the current block. | long |
$offset.records | The number of record read into the current block. | long |
System
The scope system allows accessing to the system environment variables and runtime properties.
| Predefined Fields (ScEL) | Description | Type |
|---|---|---|
$system.env | The system environment variables. | map[string, string] |
$system.props | The system environment properties. | map[string, string] |
Timestamp
The scope $timestamp allows defining the timestamp of the output record.
Topic
The scope $topic allows defining the target topic of the output record.
Value
The scope $value allows defining the fields of the output record
Variables
The scope $variables allows read/write access to a simple key-value map structure.
This scope can be used to share user-defined variables between Processing Filters.
Note : variables are not cached between records.