Working with the REST API

Interactions with MLDB occurs via a REST API (fully documented here), where REST stands for Representational State Transfer.


The structure of a REST API call is <verb> <resource> <arguments>, e.g. PUT /v1/datasets {"type": "sparse.mutable"}.

Calling the API over HTTP

The quickest way to get started with making HTTP calls to the MLDB API is with your web browser using our REST API Interactive Documentation.

Beyond that, if MLDB is running on a server named <host> on port <port> then you can use any of a widely-available variety of tools to make HTTP calls to http://<host>:<port>/<resource>, for example:

Calling the API over HTTP from Python with pymldb

If you are using the built-in Notebook interface or want to work with MLDB from Python, you can install pymldb, which gives you access to an MLDB-specific library to interact with the API over HTTP, while hiding the details of HTTP from you. The Using pymldb Tutorial will show you how to use pymldb.

Exposing functions as a REST API

For most predictive applications, you will want to expose a function over REST that implements the specific functionality required. There are two ways of doing this: by calling the GET /v1/functions/<function>/application route, or via a plugin exposing a custom route.

Making a function available via GET /v1/functions/<function>/application

As soon as a function is created, that route is automatically available, and so this version requires no extra work. However, as MLDB does not know the data types that the function will be called with, it is required to re-bind the function on every call, which can be very expensive. This can reduce performance by an order of magnitude.

In order to avoid this penalty, it is possible to pre-bind function calls using the sql.expression function type as follows:

PUT /v1/functions/wrapper {
    "type": "sql.expression",
    "params": {
        "expression": "original_function({args, arg2, ...})[output] AS *",
        "prepared": true

This will create a new function, /v1/functions/wrapper, which has an application route that will forward arg1 and arg2 (these should be replaced with the real function arguments) to the original_function function, and return the output field to the caller. The key element here is the "prepared": true, which causes the call to original_function to be pre-bound and so the call is very fast; rates in the hundreds of thousands of calls per second can be expected on a modern server for a simple function.

Making multiple predictions per REST call (high-level solution)

The /v1/functions/<function>/batch REST route allows for multiple predictions to be made in one call. This call will take a JSON object or array as input, apply the function to each element, and recreate the same structure with the result of the predictions. For example, if we set up the following function:

PUT /v1/functions/score_one {
    "type": "sql.expression",
    "params": {
        "expression": "horizontal_sum(input)",
        "prepared": true,
        "raw": true,
        "autoInput": true

(which effectively makes the horizontal_sum builtin available as a REST function, and uses autoInput and raw parameters to automatically associate the input parameter with the passed input value and the output of the horizontal_sum function to the result), then we can call it like so:

GET /v1/functions/score_one/batch { "input": [[1,2,3],[4,5],[6],[]] }

which will return us


with one output per input entry. Alternatively, we could call it with

GET /v1/functions/score_one/batch {
     "input": {
         "one":   [1,2,3],
         "two":   [4,5],
         "three": [6],
         "four":  [] } }

which will return us

{ "one": 6, "two": 9, "three": 6, "four": 0 } 

with the function being applied to each member of the object.

Allowing multiple predictions per REST call (low-level solution)

In some cases, it may make sense to batch several predictions together into the same REST call (especially to reduce network overhead). This can also be achieved using the sql.query function type and the row_dataset() table function:

PUT /v1/functions/batcher {
    "type": "sql.query",
    "params": {
        "query": "SELECT original_function(value) as value, column FROM row_dataset($input)",
        "output": "NAMED_COLUMNS"

The batcher function, when called, will take an argument input that should be either an array or an object. It will apply original_function to each element of the array or object, and return the same but with the elements replaced with the value of their functions. For example, if we use horizontal_sum for the original function, then applying the batched version to [[1,2,3],[4],[5,6,7],[]] will return [6,4,18,NULL].

For maximum performance, it can be combined with the wrapper trick above.

Making a function available via a plugin route

By defining a custom plugin, it is also possible to define completely custom routes for MLDB to operate on. The following example shows how to make a given function available via REST to respond to the GET /routes/predict request.

First, we create a Javascript plugin that will set itself up to respond to the given route:

var fnconfig = {
    type: "sql.expression",
    params: {
        expression: "horizontal_sum({*})",
        prepared: true
var predictfn = mldb.createFunction(fnconfig);

function handleRequest(relpath, verb, resource, params, payload, contentType, contentLength,
    if (verb == "GET" && relpath == "/predict") {
        return predictfn.callJson(JSON.parse(params[0][1]));
    throw "Unknown route " + verb + " " + relpath;


Next, we PUT that function to create the API:

PUT /v1/plugins/myapi {
    "type": "javascript",
    "params": {
        "source": {
            "main": "<above javascript code>"

Finally, we can GET the predict route in order to make a prediction:

GET /v1/plugins/myapi/routes/predict { "input": [ 1, 2, 3 ] }

which returns 6.

Working with APIs that are unable to send a body with a GET request

If you need to send a body with a GET request to MLDB and your interface doesn't support it, you can POST to the special route /v1/redirect/get, which was created exactly for that case. It basically acts as a proxy that redirects POST requests as GET to internal MLDB routes.

Here is an example

POST /v1/redirect/get {
    "target": "/v1/query",
    "body": {
        "q": "SELECT 'foo' AS bar"

Which returns