schibsted / Jslt
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JSLT
JSLT is a complete query and transformation language for JSON. The language design is inspired by jq, XPath, and XQuery.
JSLT can be used as:
- a query language to extract values from JSON (
.foo.bar[0]), - a filter/check language to test JSON objects (
starts-with(.foo.bar[0], "http://")) , - a transformation language to convert between JSON formats.
Here is an example transform:
{
"time": round(parse-time(.published, "yyyy-MM-dd'T'HH:mm:ssX") * 1000),
"device_manufacturer": .device.manufacturer,
"device_model": .device.model,
"language": .device.acceptLanguage,
"os_name": .device.osType,
"os_version": .device.osVersion,
"platform": .device.platformType,
"user_properties": {
"is_logged_in" : boolean(.actor."spt:userId")
}
}
Quick reference
| Operation | Explanation |
|---|---|
. |
The context node |
.<name> |
Get value of key "<name>" inside an object |
.[<index>] |
Get value <index> inside an array |
.[<from> : <to>] |
Array slicing |
if (<expr>) <expr> else <expr> |
If test to decide which value to return |
let <name> = <expr> |
Define a variable |
$<name> |
Refer to a variable |
[for (<expr>) <expr>] |
Transform an array |
{for (<expr>) <expr> : <expr>} |
Transform an object |
def <name>(<name>, <name>...) <expr> |
Declare a function |
// <anything up to end of line> |
Comment |
{ <key> : <expr> } |
Object constructor |
{ <key> : <expr>, * : . } |
Specify one key, copy rest of input |
5 * 7 + 23.2 |
Arithmetic operations |
7 < 5 |
Comparators |
7 < 5 and .foo == "yes" |
Boolean operators |
Using the library
To include JSLT in your project, depend on:
<dependency>
<groupId>com.schibsted.spt.data</groupId>
<artifactId>jslt</artifactId>
<version>0.1.11</version>
</dependency>
At runtime JSLT depends on Jackson, and nothing else.
To transform one JsonNode into another, do:
import com.schibsted.spt.data.jslt.Parser;
import com.schibsted.spt.data.jslt.Expression;
JsonNode input = ...;
Expression jslt = Parser.compileString(transform);
JsonNode output = jslt.apply(input);
For more alternatives, see the javadoc.
Command-line
To run transforms on the command-line, first build with ./gradlew clean shadowJar. Then you can run with:
java -cp build/libs/*.jar com.schibsted.spt.data.jslt.cli.JSLT transform.jslt input.json
The result is written to standard out.
Extension functions
You can implement your own functions and add them to the language. See the extension function tutorial.
Feedback
If you have questions about how to use JSLT, please ask the question
on StackOverflow, with the tag jslt.
If you have problems, feature requests, or think you found a bug, please open an issue.
Status
The language design is not finished, so features may be added. The language as it stands is not likely to change.
The entire language is implemented, and all of the function library. Functions may be added.
The language has been used in production at Schibsted since January 2018, performing about 9 billion transforms per day, and many times more queries.
Building JSLT
To build JSLT as a jar file, run ./gradlew jar.
To build a fat jar with all dependencies included, run ./gradlew shadowJar.
To run the tests: ./gradlew check.
There is a pom.xml file, but Maven is not used for building JSLT,
and the file is not intended to work. It's only there to make Github
dependency tracking work.
More information
Developing a language for JSON processing: video of talk, slides only.
Running the playground yourself
Anthony Sparks is working on a VM-based implementation in Java
A paper describing (among other things) some of the ways Schibsted uses JSLT.
Visual Studio syntax highlighter for JSLT.
Pincette event sourcing framework uses JSLT.
LICENSE
Copyright (c) 2018 Schibsted Marketplaces Products & Technology AS
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
What is missing
Things to be done:
- Move the tests out into YAML files.
- Write a proper spec with EBNF and everything.
- Fix the syntax ambiguity problem with
letanddef. - Implement toString() throughout the object tree, so that it's possible to turn expressions back to strings.
- Optimizer:
- Optimize
... and boolean( ... )by removingboolean(). - Implement parse tree traversal API.
- Make sure entire tree is traversed (inside function decls and variables, for example).
- Complete constant folding. Particularly constant folding for variables would be valuable.
- Inlining of functions.
- Eliminate unused variables.
- Optimize
- Use property-based testing and fuzz testing to harden the parser.
See also the list of ideas.