Work with JSON strings

In this tutorial, we will see how to use Lenses SQL to process JSON strings using JsonPath, a sort of XPath for JSON objects.

Setting up our example 

We have a topic http_logs that collects the details of HTTP calls to a microservice. Basic details of the request are stored, like the URL and the HTTP method used. The payload of the requests is stored as well as a string.

We can create the topic and insert some example data through SQL Studio:

CREATE TABLE http_logs(_key string, method string, url string, content_type string, body string)
FORMAT (STRING, AVRO);

INSERT INTO http_logs(_key, method, url, content_type, body) VALUES
('event_1', 'POST', '/users', 'application/json', '{ "id": 1, "username": "juno", "contacts": [ { "type": "email", "value": "juno@example.org" }, { "type": "phone", "value": "12345" }] }'),
('event_2', 'POST', '/users', 'application/json', '{ "id": 2, "username": "max", "contacts": [ { "type": "email", "value": "max@example.org" }, { "type": "twitter", "value": "@max" }] }'),
('event_3', 'GET', '/users/1', '', ''),
('event_4', 'GET', '/users/2', '', ''),
('event_5', 'POST', '/users', 'application/json', '{ "id": 3, "username": "nic", "contacts": [ { "type": "email", "value": "nic@example.org" }, { "type": "phone", "value": "78910" }] }'),
('event_6', 'PUT', '/users/1', 'application/json', '{ "username": "juno", "contacts": [ { "type": "email", "value": "juno@domain.org" }] }'),
('event_7', 'POST', '/users', 'application/json', '{ "id": 4, "username": "john", "contacts": [ { "type": "email", "value": "john@example.org" }] }');

The HTTP method and the URL used for the request are stored in the method and url fields respectively, while the optional payload, and its content-type, are stored in the body and content_type fields.

As you can imagine the logs contained in this topic are quite generic, and different endpoints may have different content-types for their body. For this reason the best the system can do is storing the payload as a simple string, whenever that is possible.

This comes with some drawbacks: since the data is a simple string, and it is not structured, it is not possible to inspect it as we would do with a normal AVRO/JSON object.

Fortunately Lenses SQL offers a couple of handful functions that make our life easier in these kind of scenarios.

Extracting fields from a JSON string 

Our first task is to find the username of users created with a call to POST /users.

To do that we can use JSON_EXTRACT_FIRST(json_string, pattern), one of the string functions available in Lenses SQL. The first argument of the function is the string representing the JSON we want to manipulate. The second is a string representing a JsonPath.

JsonPath is a powerful way to traverse and extract elements from a JSON object. Explaining the full details of goes beyond the scope of this article, but in general it can be thought as a JSON version of XPath, the standard used to select elements from an XML document.

A nice way to try and test if your JsonPaths are doing what you intended, is using the JsonPath online evaluator.

In our case, we would like to extract the name of the user just created. The simple path $.username will do it!

Let’s try to use it in a SELECT that we can run in SQL Studio:

SELECT
    JSON_EXTRACT_FIRST(body, '$.username') as username
FROM
    http_logs

That query will produce the results

{ "username": "\"juno\"" }
{ "username": "\"max\"" }
{ "username": null }
{ "username": null }
{ "username": "\"nic\"" }
{ "username": "\"juno\"" }
{ "username": "\"john\"" }

As you can see we have two entries for juno. That’s because the user was first created, and then modified later, with a PUT call.

Also, there are some null values. This is because JSON_EXTRACT_FIRST was not able to extract the username, either because the payload was not valid JSON, or because the field was not found.

We can fix this restricting our query to user creation calls:

SELECT
   JSON_EXTRACT_FIRST(body, '$.username') as username
FROM
   http_logs
WHERE
   method = "POST" AND
   url = "/users"

We have now only valid results:

{ "username": "\"juno\"" }
{ "username": "\"max\"" }
{ "username": "\"nic\"" }
{ "username": "\"john\"" }

Filtering the results 

All Lenses SQL functions can be used in any part of the query. Thus JSON_EXTRACT_FIRST can be used in the projections, where, and group bys.

For example, you can run the query

SELECT
   JSON_EXTRACT_FIRST(body, '$.contacts[?(@.type=="email")].value') as email
FROM
   http_logs
WHERE
   JSON_EXTRACT_FIRST(body, '$.username') = '"max"'

to retrieve max’s e-mail:

{ "email" : "max@example.org" }

Extract multiple values from a JSON string 

So far we had fun using JSON_EXTRACT_FIRST, but we have not talked yet about its bigger brother, JSON_EXTRACT_ALL.

JSON_EXTRACT_ALL(json_string, pattern) works like JSON_EXTRACT_FIRST, except that it will return all the values that match the pattern. The results will be returned in an array, and when no results are found the empty array will be returned.

Let’s make use of it, extracting all the contact types used at the moment of the creation of the user:

SELECT
   JSON_EXTRACT_FIRST(body, '$.username') as username,
   JSON_EXTRACT_ALL(body, '$.contacts[*].type') as contact_types
FROM
   http_logs
WHERE
   method = "POST" AND
   url = "/users"

Running the query above we get what we desired:

{ "username": "\"juno\"", "contact_types": [ "email", "\"phone\"" ] }'),
{ "username": "\"max\"", "contact_types": [ "email", "\"twitter\"" ] }'),
{ "username": "\"nic\"", "contact_types": [ "email", "\"phone\"" ] }'),
{ "username": "\"john\"", "contact_types": [ "\"email\""] }')

Sql Processors 

JSON_EXTRACT_FIRST() and JSON_EXTRACT_ALL() are available also in the Streaming Engine, like most Lenses SQL functions.

Let’s say we want another topic continuously filled with the contact types used for user creations. We also want each record containing a single username-contact type pair. To achieve that we can take the query of the last example and adapt it a bit, using a Lateral Join:

SET defaults.topic.autocreate=true;

INSERT INTO contact_types
SELECT STREAM
   JSON_EXTRACT_FIRST(body, '$.username') as username,
   contact_type
FROM
   http_logs LATERAL
   JSON_EXTRACT_ALL(body, '$.contacts[*].type') as contact_type
WHERE
   method = "POST" AND
   url = "/users"

Conclusions 

JSON_EXTRACT_FIRST() and JSON_EXTRACT_ALL() simplifies your life every time you have to deal with JSON that is represented as a string value of a field in your topic.

The use of JsonPath make them very powerful and even complex operations are easily representable with it.

To know more about JsonPath you can check the author's original blog post, while to know more about Lenses SQL String functions you can check our documentation.