[DRILL-8204] Allow Provided Schema for HTTP Plugin in JSON Mode

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.20.0
• Fix Version/s: 1.21.0
• Component/s: Storage - Other
• Assignee: Charles Givre
• Reporter: Charles Givre
• Created: 01/May/22 13:28
• Updated: 27/May/22 06:11
• Resolved: 05/May/22 12:06

Description

One of the challenges of querying APIs is inconsistent data. Drill allows you to provide a schema for individual endpoints. You can do this in one of two ways: either by
providing a serialized TupleMetadata of the desired schema. This is an advanced functionality and should only be used by advanced Drill users.

The schema provisioning currently supports complex types of Arrays and Maps at any nesting level.

1. 1. 1. Example Schema Provisioning:
         ```json
         "jsonOptions": {
         "providedSchema": [ { "fieldName": "int_field", "fieldType": "bigint" }

         , {
         "fieldName": "jsonField",
         "fieldType": "varchar",
         "properties":

         { "drill.json-mode":"json" }

         },

         { // Array field "fieldName": "stringField", "fieldType": "varchar", "isArray": true }

         , {
         // Map field
         "fieldName": "mapField",
         "fieldType": "map",
         "fields": [

         { "fieldName": "nestedField", "fieldType": "int" }

         ,

         { "fieldName": "nestedField2", "fieldType": "varchar" }

         ]
         }
         ]
         }
         ```

1. 1. 1. Example Provisioning the Schema with a JSON String
         ```json
         "jsonOptions":
         Unknown macro: {"jsonSchema"}

         "
         }
         ```

You can print out a JSON string of a schema with the Java code below.

```java
TupleMetadata schema = new SchemaBuilder()
.addNullable("a", MinorType.BIGINT)
.addNullable("m", MinorType.VARCHAR)
.build();
ColumnMetadata m = schema.metadata("m");
m.setProperty(JsonLoader.JSON_MODE, JsonLoader.JSON_LITERAL_MODE);

System.out.println(schema.jsonString());
```

This will generate something like the JSON string below:

```json
{
"type":"tuple_schema",
"columns":[

{"name":"a","type":"BIGINT","mode":"OPTIONAL"}

,

{"name":"m","type":"VARCHAR","mode":"OPTIONAL","properties":\{"drill.json-mode":"json"}

}
]
}
```

1. 1. Dealing With Inconsistent Schemas
      One of the major challenges of interacting with JSON data is when the schema is inconsistent. Drill has a `UNION` data type which is marked as experimental. At the time of
      writing, the HTTP plugin does not support the `UNION`, however supplying a schema can solve a lot of those issues.

1. 1. 1. Json Mode
         Drill offers the option of reading all JSON values as a string. While this can complicate downstream analytics, it can also be a more memory-efficient way of reading data with
         inconsistent schema. Unfortunately, at the time of writing, JSON-mode is only available with a provided schema. However, future work will allow this mode to be enabled for
         any JSON data.

1. 1. 1. 1. Enabling JSON Mode:
            You can enable JSON mode simply by adding the `drill.json-mode` property with a value of `json` to a field, as shown below:

```json
{
"fieldName": "jsonField",
"fieldType": "varchar",
"properties":

{ "drill.json-mode": "json" }

}
```