Cast Functions

What is a cast function and why I need it?

Different kind of table could store data in many different ways. You could have a numeric value stored as a string that was supposed to be an integer, or you could also have a date field stored in different formats in different tables but you would like to have date fields all in the same format (for example you receive a date with the format YYYYMMDD and you want to cast it to ISO format).

Cast functions meant to solve all these problems giving you full control over the format that data are going to have in your projections. Cast functions are simple JavaScript functions that receive the value coming from the Kafka topics and return the value in the format you need for you projection fields.

This enable you to define the output format and type of the imported fields.

note

Since JavaScript is untyped, a conversion function needs some care to be implemented correctly

Click on the Cast Functions voice of the left menu to see all the Cast Functions.

Cast Function Default

The Console provides your project with a set of default cast functions ready to use, so that you can start to create your projection without writing any line of code to cast your data in the correct format. Hover the zoom icon to see the implementation of the default cast functions.

If you need more control over the casting of your data, you can also create your custom cast functions.

caution

If value to cast is invalid (e.g. a character as input of a cast to defaultCastToInteger), all default cast functions returns undefined or null, based on the cases. These fallback values are saved as NULL in MongoDB.
Be aware of that if your fields are not nullable, in these cases you should define your own Custom Cast Functions with the fallback you need.

Create a Custom Cast Function

To define your own custom cast functions click on the Create button above the Custom cast functions table. This will open a drawer where you can define the property of your own cast function.

• Name: is the the name of your cast function. It cannot contains spaces or special characters.
• Returned Type: is the type of the value returned by the cast function.
• Expression: is the javascript implementation of the cast function. It needs to be an exported function as default.

Let's see an example

Name: castToIntBase10 Returned Type: Number Expression:

module.exports = function (valueToCast, fieldName, logger) {

return parseInt(valueToCast, 10)

}

As you can see in the example above, the cast function accepts three arguments:

• valueToCast: it is the value received as it's received from the data source
• fieldName: the name of the field associated with the value (e.g.: restaurantName)
• logger: the logger you can use to log in your function (an instance of the Pino logger).

To know the technical limitation you have in your cast function, read here

How and when are updated the Default Cast Functions?

As default, your project is provided with a set of default cast functions you can see in the Default cast functions table.

What happens when the Console changes the default cast function?

Let’s analyze each case:

The implementation of a default cast Function is changed, but the name castFunctionId is unchanged

In this case, nothing will change in your configuration. You will continue to use the same implementation you had before.

A new default cast function is added

In this case, the new cast function will be added to your configuration.

If a new default cast function is added on the Console, it will be available in the Cast Function section when you visit the Design area (even if it is not yet stored on your git provider in your fastdata-config.json). When you save the configuration it will be stored in the configuration file.

A default cast function is deleted

In this case, although a default cast function has been deleted from which Console generates, it will still be available in your configuration (e.g.: Console remove “defaultCastUnitTimestampToISOString”). Let's see an example:

• Default cast functions as seen in your project

• Default cast functions generated for new projects

As you can see, in your project you keep going to be able to use all the default cast functions, although defaultCastUnitTimestampToISOString, in our example, is no more supported. If you create a new project, this default cast function will not be provided instead.

Technical limitation

In your custom cast functions you can import only the node modules present in the following list:

• lodash.get
• mongodb
• ramda

caution

It is used the node version 12.