Build Your Own SDK

How to build a new Language SDK with Looker's SDK codegen project

Overview

The high-level steps for building a new Language SDK include:

Create the language/platform run-time library (RTL)
Create the Language SDK code generator by descending from codeGen.ts or perhaps the TypeScript generator or Python generator
Create unit and functional tests
Complete the configuration of items for CI/CD (where applicable)

When implementing the syntax and features for the new language SDK, keep in mind the principles described in the Codegen rationale documentation:

consistency,
discoverability, and
correctness.

Most examples in this document will use the TypeScript SDK, which is typically our lead language SDK the other SDKs follow. The other language SDK implementations found in this repository will have similarly named or designed classes.

Run-time Library

The Run-Time Library (RTL) is most of the work when building a new SDK. When an SDK has a general-purpose runtime that's designed to handle authenticated REST API requests automatically, the methods and models that implement a particular API become very concise, which simplifies the code generator.

An important feature of these SDKs is the support for multiple API versions in the same SDK. This feature greatly aids iterative client-side code migration from an older SDK version to a new SDK version. Only the files generated from a versioned OpenApi specification will indicate which version of the API they consume, and all generated files must use exactly the same RTL code.

Configuration support

Any method of secure retrieval of API credentials and server locations can be supported by adopting the configuration provider pattern used by the reference SDKs. To support quickly getting started with an SDK, the existing SDKs support .ini file style configuration both for configuring the code generator and as a default configuration file.

All SDKs implement something like a readConfig() -> [String: String] method that can be overridden to customize how to determine configuration values. See securing your SDK credentials for more information on this topic, including links to readConfig() examples.

The TypeScript SDK configuration support is in apiSettings.ts.

Once the RTL can read the configuration values for base_url and API credentials, the HTTP request processor and AuthSession implementation work can begin.

If the Language SDK supports environment variable, configuration values can also be set via environment variables. Precedence rules should also be implemented.

Note: By design, SDKs never retain API credentials in runtime memory, but only use them when required to authenticate.

Request processing

All SDKs implement a transport interface or protocol that has most, if not all, of the following characteristics:

/**
 * Recognized HTTP methods
 */
export type HttpMethod =
  | 'GET'
  | 'POST'
  | 'PUT'
  | 'DELETE'
  | 'PATCH'
  | 'TRACE'
  | 'HEAD';

/** Interface for API transport values */
export interface ITransportSettings {
  [key: string]: any;
  /** base URL of API REST web service */
  base_url: string;
  /** standard headers to provide in all transport requests */
  headers?: Headers;
  /** whether to verify ssl certs or not. Defaults to true */
  verify_ssl: boolean;
  /** request timeout in seconds. Default to 30 */
  timeout: number;
  /** encoding override */
  encoding?: string | null;
  /** agent tag to use for the SDK requests */
  agentTag: string;
}

/**
 * Untyped basic HTTP response type for "raw" HTTP requests
 */
export interface IRawResponse {
  /** ok is `true` if the response is successful, `false` otherwise */
  ok: boolean;
  /** HTTP response code */
  statusCode: number;
  /** HTTP response status message text */
  statusMessage: string;
  /** MIME type of the response from the HTTP response header */
  contentType: string;
  /** The body of the HTTP response, without any additional processing */
  body: any;
}

/**
 * Transport plug-in interface
 */
export interface ITransport {
  /**
   * HTTP request function for atomic, fully downloaded raw HTTP responses
   *
   * Note: This method returns the result of the HTTP request without any error handling
   *
   * @param method of HTTP request
   * @param path request path, either relative or fully specified
   * @param queryParams name/value pairs to pass as part of the URL
   * @param body data for the body of the request
   * @param authenticator authenticator callback, typically from `IAuthSession` implementation
   * @param options overrides of default transport settings
   * @returns typed response of `TSuccess`, or `TError` result
   */
  rawRequest(
    method: HttpMethod,
    path: string,
    queryParams?: Values,
    body?: any,
    authenticator?: Authenticator,
    options?: Partial<ITransportSettings>
  ): Promise<IRawResponse>;

  /**
   * HTTP request function for atomic, fully downloaded responses
   * @param method of HTTP request
   * @param path request path, either relative or fully specified
   * @param queryParams name/value pairs to pass as part of the URL
   * @param body data for the body of the request
   * @param authenticator authenticator callback, typically from `IAuthSession` implementation
   * @param options overrides of default transport settings
   * @returns typed response of `TSuccess`, or `TError` result
   */
  request<TSuccess, TError>(
    method: HttpMethod,
    path: string,
    queryParams?: Values,
    body?: any,
    authenticator?: Authenticator,
    options?: Partial<ITransportSettings>
  ): Promise<SDKResponse<TSuccess, TError>>;

  /**
   * HTTP request function for a streamable response
   * @param callback that receives the stream response and pipes it somewhere
   * @param method of HTTP request
   * @param path request path, either relative or fully specified
   * @param queryParams name/value pairs to pass as part of the URL
   * @param body data for the body of the request
   * @param authenticator authenticator callback, typically from `IAuthSession` implementation
   * @param options overrides of default transport settings
   * @returns `T` upon success
   * @throws `ISDKErrorResponse` on failure
   */
  stream<T>(
    callback: (readable: Readable) => Promise<T>,
    method: HttpMethod,
    path: string,
    queryParams?: Values,
    body?: any,
    authenticator?: Authenticator,
    options?: Partial<ITransportSettings>
  ): Promise<T>;
}

Note: The latest version of these interfaces can always be found in transport.ts

The ITransportSettings interface shows the default properties for all HTTP requests and can also be used to override request settings for an individual HTTP request.

The IRawResponse interface shows the return value from lowest-level request processor rawRequest(). It does not handle any errors or type conversion, but returns the necessary information for error handling and automatic type conversion of the response.

The ITransport interface describes the two primary HTTP request methods. request<TSuccess, TError> is for an atomically completed HTTP request and response, and stream<T> is for streaming an HTTP response result from the request.

Request parameters

In the ITransport function parameters shown above:

method is the HTTP method for the request. Typically, this will be GET, PUT, POST, PATCH or DELETE.
path is either a fully specified url like https://my.server.com/app/path?q=foo or a relative path like /users.
- If the path is a full url, the request path does not change.
- If the path is relative, the path may change internal to the request processor internally to the request method based on other parameters (such as the authenticator) being passed in.
queryParams: Values is a dictionary typically of the form [name:value]. Nullable types are allowed. The run-time will automatically strip any optionally null values from the HTTP query string. The value is converted to a string and URI encoded, so callers to the request methods should not encode the values in the dictionary.
body is a data structure typically converted to JSON for the HTTP body param, unless the provided value is a string type. For string values, the body will be considered application/x-www-form-urlencoded.
authenticator is a callback or lambda function that accepts the request properties for the constructed URL as an argument and returns a modified set of request properties that includes authorization information for the request. If an authenticator is not provided for a request, no special treatment of the HTTP url is performed. If an authenticator is specified for a request, the HTTP url for that particular request will be:
- base_url (from ITransportSettings) +
- api_version (from the SDK method class) +
- path from the request parameter
options is a set of override values of ITransportSettings (typically, a longer timeout value or setting verify_ssl to false) for a specific request.
- agentTag is a string that identifies the SDK language and SDK version. Pass the tag with the x-looker-appid header.

The value for AgentTag is assigned in TypeScript with the following:

export const agentPrefix = 'TS-SDK'
export const LookerAppId = 'x-looker-appid'
...
this.authSession.settings.agentTag = `${agentPrefix} ${lookerVersion}.${this.apiVersion}`

Where lookerVersion is the version of Looker (like 23.18) and apiVersion is (currently) 4.0 (Note: API 3.x has been removed).

This results in the agentTag appearing like TS-SDK 22.6.4.0.

Additional attributes can be added to the agent tag by separating them with semicolons. (;)

Processing requests

The rawRequest() implementation:

constructs an HTTP request based on the properties passed into it, and returns the result of the response without any type conversion or error handling
sets the ok property to true if successful or false if the request fails.

The request() implementation:

sets the error status and data of the SDKResponse structure if an error occurs
sets the success status and data of an SDKResponse structure if the request succeeds
for successful requests, the response body is deserialized into the type indicated for the success value

The stream() implementation:

throws an error if a request error occurs
passes the streamable HTTP response to the stream parameter of the method
for successful requests, the response body is deserialized into the type indicated for the success value

Here's a TypeScript code sample for streaming the download of a query's CSV result:

const request: IRequestRunInlineQuery = {
  body: {
    client_id: q.client_id || undefined,
    column_limit: q.column_limit || undefined,
    dynamic_fields: q.dynamic_fields || undefined,
    fields: q.fields || undefined,
    fill_fields: q.fill_fields || [],
    filter_config: q.filter_config || undefined,
    filter_expression: q.filter_expression || undefined,
    filters: q.filters,
    limit: limit.toString(10),
    model: q.model!,
    pivots: q.pivots || undefined,
    query_timezone: q.query_timezone || undefined,
    row_total: q.row_total || undefined,
    sorts: q.sorts || [],
    subtotals: q.subtotals || undefined,
    total: typeof q.total !== 'undefined' ? q.total : false,
    view: q.view!,
    vis_config: q.vis_config || undefined,
    visible_ui_sections: q.visible_ui_sections || undefined,
  },
  result_format: 'csv',
};
const csvFile = './query.csv';
const writer = fs.createWriteStream(csvFile);
await sdk.stream.run_inline_query(async (readable: Readable) => {
  return new Promise<any>((resolve, reject) => {
    readable.pipe(writer).on('error', reject).on('finish', resolve);
  });
}, request);

Request parameter encoding

This section discusses how parameters should be processed by the run-time before submitting the HTTP request to the endpoint.

Path parameters

include all path parameters
url-encode the values for a path containing variables e.g., /users/{user_id}

Query parameters

url-encode all values
date values should be formatted in UTC time format like "2020-03-15T13:16:34.692-07:00"
skip null or undefined values

Body object

include all required properties (this is typically enforced by the declared method's interface)
skip properties that are optional and null or undefined

AuthSession implementations

When the codegen project began, there was one method of authenticating for Looker API requests, which was providing the API credentials client_id and client_secret to the API. AuthSession is commonly the name of this class. Due to the complexities of Node vs. Browser implementations and authentication methods, the reference implementation for the TypeScript SDK is called NodeSession but for this document we'll pretend it's called AuthSession.

Recent (Looker 7.6 and above) API implementations additionally offer an OAuth authentication flow for API calls. Both API3 credentials and OAuth2 authentication methods should be supported by AuthSession implementations. There will be additional authentication methods in future Looker releases, but they should follow very similar patterns to these methods.

What all AuthSession implementations have in common is:

the use of an authorization token (typically added into a request's properties by the authentication callback) to authenticate the API user's request
the session tracks authorization token expiration. If it is expired, the session automatically re-authenticates the API user by reading the necessary credential information from readConfig() then logs the user back in, discards those retrieved credentials, and saves the new authorization token for subsequent API requests.

NOTE: The token expiration tracking should be set for 10 - 15 seconds before the indicated token expiration period to ensure the token for any API request is always active.

Automatic authentication

The first time an SDK method makes an API request, the SDK's AuthSession automatically logs in for the API user and adds the authorization token will be added to the HTTP request, then the SDK completes the method request.

Similarly, the SDK also automatically refreshes tokens when they are about to expire if an SDK request is near the expiration period.

Important AuthSession methods

The authenticate() method must be implemented, because that adds the authorization information (token) into an API request.

The getToken() method must be implemented, because that is the crux of authorizing API usage. There are many ways to create or retrieve an API authorization token, but typically the Authorization header of the request should use this value as a Bearer token.

The login([sudoUserId]) and logout() methods may not be required for a given SDK run-time scenario. e.g., for several Browser-based implementations of the TypeScript SDK such as same origin requests or as a Looker extension, the run-time environment can provide API authentication support without requiring login() and logout().

When an SDK supports significantly different runtime behavior, use a different AuthSession implementation. For example, the TypeScript SDK has:

CSRFSession for browser-based same-origin usage
BrowserSession for browser-based CORS usage
NodeSession for node-based usage
OAuthSession for OAuth2-based usage
ProxySession for proxy-based usage

Code generator

The code generator "template" is written in TypeScript. The ICodegen interface (currently found at the bottom of sdkModels.ts) describes the properties and methods used to generate an API language binding for an SDK.

codeGen.ts is the base implementation of all code generators, and is partially abstract. Specific generators must override these abstract methods.

Prologues and Epilogues

Source code files typically have imports and source code comments at the start of a file, and some type of closure or clean-up code at the end.

The various Prologue properties are for the standard SDK methods, streaming, and model (type/structure/class) files.

Strong typing

We strongly prefer strong typing for language SDKs, as mentioned in correctness.

Some languages may not support strong typing or generics. In that case, we recommend using name/value pairs and verifying the names against the endpoint's specification at run-time before submitting the HTTP request to the endpoint.

Methods

The IMethod interface in sdkModels.ts describes the properties and methods to use for generating a method declaration.

REST API endpoints return responses that vary in type. There is always at least one success and one error response type. Many endpoints can return multiple success and error response types.

For languages that have union types, method declarations should use union types. The IMethod.responses is available for iteration.

Many languages do not have union types. In that circumstance, the IMethod.primaryResponse is the success return type.

IMethod.errorResponses is an array of all error responses for that method.

IMethod.responseModes is a set of response types ('binary', 'string', 'unknown'). Use this property to generate the comment indicating whether the method returns a binary response. When a method returns a binary response, the run-time request processor must not convert the response to UTF8.

IMethod.allParams returns the parameters for the function in declaration order, with required parameters followed by optional parameters.

If the language doesn't have "named parameter" support the generator supports generation of request structures. TypeScript is a language without named parameter support, so a request interface is created for methods with complex arguments. The code generator's needRequestTypes property should be set to true in this circumstance. In the TypeScript generator, the methodHeaderDeclaration function has this line:

const requestType = this.requestTypeName(method);

If the method requires a request type and needRequestTypes is true, the codeGen.ts implementation of requestTypeName() will get or create the existing request type and return its name as the result of the function. Refer to the TypeScript generator for more information if your language needs a request type.

See the TypeScript SDK methods.ts for method declaration examples.

Models

The IType interface in sdkModels.ts describes the properties and methods to use for generating a type declaration.

Type declarations are less complex than model declarations. Once the general template for type declarations is defined, the typeMap function that implements translations of OpenAPI types into the SDK language needs to be written.

Here's an example typeMap for Kotlin:

  typeMap(type: IType): IMappedType {
    super.typeMap(type)
    const mt = this.nullStr
    const ktTypes: Record<string, IMappedType> = {
      boolean: { default: mt, name: 'Boolean' },
      byte: { default: mt, name: 'binary' },
      date: { default: mt, name: 'Date' },
      datetime: { default: mt, name: 'Date' },
      double: { default: mt, name: 'Double' },
      float: { default: mt, name: 'Float' },
      int32: { default: mt, name: 'Int' },
      int64: { default: mt, name: 'Long' },
      integer: { default: mt, name: 'Int' },
      number: { default: mt, name: 'Double' },
      object: { default: mt, name: 'Any' },
      password: { default: mt, name: 'Password' },
      string: { default: mt, name: 'String' },
      uri: { default: mt, name: 'UriString' },
      url: { default: mt, name: 'UrlString' },
      void: { default: mt, name: 'Void' },
    }

    if (type.elementType) {
      // This is a structure with nested types
      const map = this.typeMap(type.elementType)
      switch (type.className) {
        case 'ArrayType':
          return { default: this.nullStr, name: `Array<${map.name}>` }
        case 'HashType':
          // TODO figure out this bizarre string template error either in IntelliJ or TypeScript
          // return {name: `Map<String,${map.name}>`, default: '{}'}
          if (map.name === 'String') map.name = 'Any' // TODO fix messy hash values
          return { default: this.nullStr, name: 'Map<String' + `,${map.name}>` }
        case 'DelimArrayType':
          return { default: this.nullStr, name: `DelimArray<${map.name}>` }
      }
      throw new Error(`Don't know how to handle: ${JSON.stringify(type)}`)
    }

    if (type.name) {
      return (
        ktTypes[type.name] || { default: this.nullStr, name: `${type.name}` }
      )
    } else {
      throw new Error('Cannot output a nameless type.')
    }
  }

See the TypeScript SDK models.ts for type/structure declaration examples.

Streams

Streaming declarations are basically the same as Methods, but with a parameter for a streaming callback.

See the TypeScript SDK streams.ts for streaming method examples.

API version tracking

The constants file (e.g. constants.ts) for an SDK has the Looker API version and Looker release version as variables. The update mechanism for these variables is in reformatter.ts.

Code reformatting

Some languages have command-line code reformatters readily available. If your SDK language has a code reformatter, define it in reformatter.ts and the generated source code will automatically be reformatted when the code generation is finishing up.

Tests

A minimum testing threshold must be met before a Language SDK can be accepted. This section describes the tests to implement for a Language SDK.

Files that support testing scenarios can be found in the test folder.

data.yml contains test data that can be used to create functional tests.
data.yml.json is automatically translated from data.yml.
openApiRef.json is a sample OpenAPI 3.x json-formatted Looker API specification
swaggerRef.json is a sample OpenAPI 2.x (Swagger) json-formatted Looker API specification

Unit tests

Unit tests are tests that do not require a running API server.

run-time library tests

SDK configuration values
- reading .ini sections by name
- parsing true and false values, e.g.

/**
 * strip quotes from the value if the same "quote" character is the start and end of the string
 */
fun unQuote(value: String?): String {
    if (value === null) return ""
    if (value.isBlank()) return ""
    val quote = value.substring(0, 1)
    if ("\"`'".contains(quote)) {
        if (value.endsWith(quote)) {
            // Strip matching characters
            return value.substring(1, value.length - 1)
        }
    }
    return value
}

fun isTrue(value: String?): Boolean {
    val low = unQuote(value?.toLowerCase())
    return low == "true" || low == "1" || low == "t" || low == "y" || low == "yes"
}

fun isFalse(value: String?): Boolean {
    val low = unQuote(value?.toLowerCase())
    return low == "false" || low == "0" || low == "f" || low == "n" || low == "no"
}

fun asBoolean(value: String?): Boolean? {
    if (isTrue(value)) return true
    if (isFalse(value)) return false
    return null
}

readConfig() overrides
HTTP parameter encoding
HTTP request processing

generator tests

method declaration
stream declaration
type declaration
required parameters
optional parameters

See sdkModels.spec.ts and python.gen.spec.ts for examples.

Functional or integration tests

Functional tests should include representative methods from the API. For Looker SDK tests, a looker.ini must be in the root of the repository, configured for a running Looker server.

GET functions
PUT functions
POST functions
PATCH functions
DELETE functions
search functions
binary response types
streaming (if applicable)

See methods.spec.ts and test_methods.py examples.

CI/CD

Detailed plans for Continuous Integration/Continuous Deployment (CI/CD) are still being established but CI will likely use Travis for this repository.

Both CI and CD have prerequisites that are described below.

Release requirements

The following items must succeed for an SDK update to be ready for release:

yarn wipe && yarn gen
bin/smoke

The monthly release of Looker always has API changes. Updates for the supported SDKs must pass the two steps above before publishing to their respective package managers.

Smoke testing

When the Language SDK test suite is complete, add all or some parts of the tests to the smoke test script. This way, tests for the Language SDK will be part of the entire SDK Codegen test suite.

The smoke tests require the test/reference instance of Looker configured in looker.ini to be up and running.

Packaging

Most language platforms include a scriptable way to use their package manager. Many language platforms also have multiple package managers. Choose the package manager that is best supported, most broadly used, and supports the latest versions of the language.

If a "standard" package manager exists for the platform/language, deployment to that package manager must be supported before the Language SDK can be placed into open beta. For example, the TypeScript SDK is available via the Node Package Manager and the Python SDK is on PyPi.

A script or command to deploy to the relevant package manager must be provided for the Language SDK.

Package configuration

The package versioning information must contain the Looker Release version.

The package needs a README that:

introduces the SDK
describes how to get started
links to the SDK-Codegen repository
indicate an MIT or Apache license. Package managers often derive licensing information automatically by examining the origin of the package. (SDK-Codegen has an MIT license.)
Earn bonus points by linking to additional SDK examples.

Packages must use semantic versioning. Alpha and beta tags can be used in versioning.

Deployment scripts

Credential stores for deployment may vary so this document doesn't recommend a pattern, but each package manager's credentials must be secured and never checked in to this repository.

Deployment scripts should be able to retrieve the secured credentials and automate publication of a package. This script is a prerequisite for Continuous Deployment.

An SDK must pass the release requirements before it can be released.

Continuous Deployment

CD needs more planning.

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byosdk.md

byosdk.md

Build Your Own SDK

Overview

Run-time Library

Configuration support

Request processing

Request parameters

Processing requests

Request parameter encoding

Path parameters

Query parameters

Body object

AuthSession implementations

Automatic authentication

Important AuthSession methods

Code generator

Prologues and Epilogues

Strong typing

Methods

Models

Streams

API version tracking

Code reformatting

Tests

Unit tests

run-time library tests

generator tests

Functional or integration tests

CI/CD

Release requirements

Smoke testing

Packaging

Package configuration

Deployment scripts

Continuous Deployment

Files

byosdk.md

Latest commit

History

byosdk.md

File metadata and controls

Build Your Own SDK

Overview

Run-time Library

Configuration support

Request processing

Request parameters

Processing requests

Request parameter encoding

Path parameters

Query parameters

Body object

AuthSession implementations

Automatic authentication

Important AuthSession methods

Code generator

Prologues and Epilogues

Strong typing

Methods

Models

Streams

API version tracking

Code reformatting

Tests

Unit tests

run-time library tests

generator tests

Functional or integration tests

CI/CD

Release requirements

Smoke testing

Packaging

Package configuration

Deployment scripts

Continuous Deployment