Kibana Expressions Service

Expressions service

Expression service exposes a registry of reusable functions primary used for fetching and transposing data and a registry of renderer functions that can render data into a DOM element. Adding functions is easy and so is reusing them.

An expression is a chain of functions with provided arguments, which given a single input translates to a single output. Each expression is representable by a human friendly string which a user can type.

Creating expressions

Here is a very simple expression string:

essql 'select column1, column2 from myindex' | mapColumn name=column3 fn='{ column1 + 3 }' | table

It consists of 3 functions:

• essql which runs given sql query against elasticsearch and returns the results
• mapColumn, which computes a new column from existing ones;
• table, which prepares the data for rendering in a tabular format.

The same expression could also be constructed in the code:

import { buildExpression, buildExpressionFunction } from 'src/plugins/expressions';

const expression = buildExpression([
  buildExpressionFunction<ExpressionFunctionEssql>('essql', [ q: 'select column1, column2 from myindex' ]),
  buildExpressionFunction<ExpressionFunctionMapColumn>('mapColumn', [ name: 'column3', expression: 'column1 + 3' ]),
  buildExpressionFunction<ExpressionFunctionTable>('table'),
]

Note: Consumers need to be aware which plugin registers specific functions with expressions function registry and import correct type definitions from there.

Running expressions

Expression service exposes execute method which allows you to execute an expression:

const executionContract = expressions.execute(expression, input);
const result = await executionContract.getData();

In addition, on the browser side, there are two additional ways to run expressions and render the results.

React expression renderer component

This is the easiest way to get expressions rendered inside your application.

<ReactExpressionRenderer expression={expression} />

Expression loader

If you are not using React, you can use the loader expression service provides to achieve the same:

const handler = loader(domElement, expression, params);

Creating new expression functions

Creating a new expression function is easy, just call registerFunction method on expressions service setup contract with your function definition:

const functionDefinition = {
   name: 'clog',
   args: {},
   help: 'Outputs the context to the console',
   fn: (input: unknown) => {
     // eslint-disable-next-line no-console
     console.log(input);
     return input;
   },
};

expressions.registerFunction(functionDefinition);

Creating new expression renderers

Adding new renderers is just as easy as adding functions:

const rendererDefinition = {
   name: 'debug',
   help: 'Outputs the context to the dom element',
   render: (domElement, input, handlers) => {
     // eslint-disable-next-line no-console
     domElement.innerText = JSON.strinfigy(input);
     handlers.done();
   },
};

expressions.registerRenderer(rendererDefinition);