Introduction

Telepact is an API ecosystem for bridging programs across inter-process communication boundaries.

What makes Telepact different? It takes the differentiating features of the industry's most popular API technologies, and combines them together through 3 key innovations:

• JSON as a Query Language - API calls and SELECT -style queries are all achieved with JSON abstractions, giving first-class status to clients wielding only a JSON library

• Binary without code generation - Binary protocols are established through runtime handshakes, rather than build-time code generation, offering binary efficiency to clients that want to avoid code generation toolchains

• Hypermedia without HTTP - API calls can return functions with pre-filled arguments, approximating a link that can be followed, all achieved with pure JSON abstractions

These innovations allow Telepact to design for the minimalist consumer while giving clients the option to enrich the consumer experience by:

• Selecting less fields to reduce response sizes

• Generating code to increase type safety

• Using binary serialization to reduce request/response sizes

It's just JSON

No query params. No binary field ids. No required client libraries.

It's just JSON in, and JSON out.

Schema:

[{"fn.helloWorld": {}, "->": [{"Ok_": {"msg": "string"}}]}]

Request:

[{}, {"fn.helloWorld":{}}]

Response:

[{}, {"Ok_": {"msg": "Hello world!"}}]

Check out the full-stack example.

Explore

To learn how to write Telepact APIs, see the API Schema Guide. A JSON Schema is available for validation.

To learn how to serve a Telepact API, see the specific library docs:

• Typescript

• Python

• Java

• Go

For development assistance, see the SDK tool docs:

• CLI

• Conveniently retreive API schemas from running live Telepact servers, and use schemas to create mock servers and generate code bindings, all from the command line

• Browser Console

• Develop against a live Telepact server by reading rendered docs, drafting requests, and submitting live requests with json-friendly editors

• Prettier Plugin

• Consistently format your Telepact api schemas, especially the doc-strings

Find the latest versions of all libraries and sdks here.

For further reading, see Motivation.

Telepact does have a few unorthodox design decisions. To be best informed, you should read the explanations in the FAQ.

Licensing

Telepact is licensed under the Apache License, Version 2.0. See LICENSE for the full license text. See NOTICE for additional information regarding copyright ownership.

Schema Writing Guide

This guide will explain how to understand and write *.telepact.json files.

Type Expression

Types are expressed with a string, which may be contained within conventional JSON collection types. When using JSON objects in type expressions, the only allowed key type is "string" .

Type Expression Example allowed JSON values Example disallowed JSON values

"boolean"

true , false

null , 0

"integer"

1 , 0 , -1

null , 0.1

"number"

0.1 , -0.1

null , "0"

"string"

"" , "text"

null , 0

["boolean"]

[] , [true, false]

null , 0 , [null] {}

{"string": "integer"}

{} , {"k1": 0, "k2": 1}

null , 0 , {"k": null} , []

[{"string": "boolean"}]

[{}] , [{"k1": true, "k2": false}]

[{"k1": null}] , [{"k1": 0}] , [null] [0]

"any"

false , 0 , 0.1 , "" , [] , {}

null

Nullability

The ? symbol can be appended to type strings to indicate nullability. Note that it is not possible to express nullable arrays or objects.

Type Expression Example allowed JSON values Example disallowed JSON values

"boolean?"

null , true , false

0

"integer?"

null , 1 , 0 , -1

0.1

"number?"

null , 0.1 , -0.1

"0"

"string?"

null , "" , "text"

0

["boolean?"]

[] , [true, false, null]

null , 0 , {}

{"string": "integer?"}

{} , {"k1": 0, "k2": 1, "k3": null}

null , 0 , []

[{"string": "boolean?"}]

[{}] , [{"k1": null, "k2": false}]

[{"k1": 0}] , [null] [0]

"any?"

null , false , 0 , 0.1 , "" , [] , {}

(none)

Definitions

A Telepact Schema is an array of the following definition patterns:

• struct

• union

• function

• errors

• headers

Struct Definition

Type expressions can be encased in a structured object (product type). Struct definitions may be used in any type expression.

The ! symbol can be appended to a field name to indicate that it is optional.

[ { "struct.ExampleStruct1": { "field": "boolean", "anotherField": ["string"] } }, { "struct.ExampleStruct2": { "optionalField!": "boolean", "anotherOptionalField!": "integer" } } ]

Type Expression Example allowed JSON values Example disallowed JSON values

"struct.ExampleStruct1"

{"field": true, "anotherField": ["text1", "text2"]}

null , {}

"struct.ExampleStruct2"

{"optionalField!": true} , {}

null , {"wrongField": true}

["struct.ExampleStruct2"]

[{"optionalField!": true}]

[null] , [{"wrongField": true}]

Union

Type expressions can be encased in a tagged structured object (sum type). Unions may be used in any type expression.

At least one tag is required.

[ { "union.ExampleUnion1": [ { "Tag": { "field": "integer" } }, { "EmptyTag": {} } ] }, { "union.ExampleUnion2": [ { "Tag": { "optionalField!": "string" } } ] } ]

Type Expression Example allowed JSON values Example disallowed JSON values

"union.ExampleUnion1"

{"Tag": {"field": 0}} , {"EmptyTag": {}}

null , {} , {"Tag": {"wrongField": true}}

"union.ExampleUnion2"

{"Tag": {"optionalField!": "text"}} , {"Tag": {}}

null , {}

Function

Request-Response semantics can be defined with functions. A function is a combination of an argument struct and a result union. The result union requires at least the Ok_ tag. By convention, all non-Ok_ tags are considered errors.

Clients interact with servers through functions. The client submits JSON data valid against the function argument struct definition, and the server responds with JSON data valid against the function result union.

When referenced as a type in type expressions, the result union is unused. Functions cannot be used in type expressions that extend down from a top-level function argument.

[ { "fn.exampleFunction1": { "field": "integer", "optionalField!": "string" }, "->": [ { "Ok_": { "field": "boolean" } } ] }, { "fn.exampleFunction2": {}, "->": [ { "Ok_": {} }, { "Error": { "field": "string" } } ] } ]

Example Request Example Response

[{}, {"fn.exampleFunction1": {"field": 1}}]

[{}, {"Ok_": {"field": true}}]

[{}, {"fn.exampleFunction2": {}}]

[{}, {"Error": {"field": "text"}}]

Type Expression Example allowed JSON values Example disallowed JSON values

"fn.exampleFunction1"

{"field": 0} , {"field": 1, "optionalField!": "text"}

null , {}

"fn.exampleFunction2"

{}

null , {"wrongField": 0}

Errors

Errors definitions are similar to unions, except that the tags are automatically added to the result union of all user-defined functions. Errors definitions cannot be used in type expressions.

[ { "errors.ExampleErrors1": [ { "Error1": { "field": "integer" } }, { "Error2": {} } ] } ]

For example, if placed in the same schema, the above error definition would apply the errors Error1 and Error2 to both the fn.exampleFunction1 and fn.exampleFunction2 functions from the previous section, as indicated below (Note, the following example only illustrates the effect of the errors definition at schema load time; the original schema is not re-written.)

[ { "fn.exampleFunction1": { "field": "integer", "optionalField!": "string" }, "->": [ { "Ok_": { "field": "boolean" } }, { "Error1": { "field": "integer" } }, { "Error2": {} } ] }, { "fn.exampleFunction2": {}, "->": [ { "Ok_": {} }, { "Error": { "field": "string" } }, { "Error1": { "field": "integer" } }, { "Error2": {} } ] } ]

Don't over-error

NOTE: API designers should be careful to avoid using errors definitions to abstract "reusable" errors. Errors definitions are only intended for systemic server errors that could be encountered by any function.

For instance, in Telepact, there is no standard "Not found" error, because Telepact favors expressive data, such as using an empty optional field to replace conventional "Not found" patterns.

❌ Bad:

[ { "errors.GeneralErrors": [ { "NotFound": {} } ] }, { "fn.exampleFunction": {}, "->": [ { "Ok_": { "result": "string" } } ] } ]

✅ Good:

[ { "fn.exampleFunction": {}, "->": [ { "Ok_": { "result!": "string" } } ] } ]

Headers

Headers definitions are similar to function definitions in that they correlate to the request/response semantics, but only with respect to the headers object on the Telepact message. Both the request and response definitions resemble struct definitions, with a few exceptions:

• all header fields must be prepended with @

• all header fields are implicitly optional

• additional header fields not specified in the definition will be allowed during validation

Headers definitions cannot be used in type expressions.

[ { "headers.Example": { "@requestHeader": "boolean", "@anotherRequestHeader": "integer" }, "->": { "@responseHeader": "string" } } ]

Example Request Example Response

[{"@requestHeader": true}, {"fn.ping_": {}}]

[{"@responseHeader": "text"}, {"Ok_": {}}]

[{"@anotherRequestHeader": true}, {"fn.ping_": {}}]

[{"@unspecifiedHeader": true}, {"Ok_": {}}]

Example Invalid Request Example Invalid Response

[{"@requestHeader": 1}, {"fn.ping_": {}}]

[{"@responseHeader": 1}, {"Ok_": {}}]

Docstrings

All top-level definitions and union tags (including errors and function results) can include a docstring. Docstrings support markdown when rendered in the Telepact console.

Single-line

[ { "///": " A struct that contains a field. ", "struct.ExampleStruct": { "field": "boolean" } }, { "struct.ExampleUnion": [ { "///": " The default Tag that contains a field. ", "Tag": { "field": "integer" } } ] } ]

Multi-line

Multi-line docstrings are supported. For readability and ease of writing, schema writers are encouraged to use the Telepact Prettier plugin. (The Telepact console uses the prettier plugin in draft mode.)

[ { "///": [ " A struct that contains a field. ", " ", " Fields: ", " - field (type: boolean) " ], "struct.ExampleStruct": { "field": "boolean" } } ]

Automatic Definitions

Some definitions are automatically appended to your schema at runtime.

Standard Definitions

Standard definitions include utility functions, like fn.ping_ , and common errors, like ErrorInvalidRequest and ErrorUnknown_ . These are always included and cannot be turned off.

You can find all standard definitions here.

Auth Definitions

Auth definitions include the @auth_ header and the ErrorUnauthenticated_ and ErrorUnauthorized_ errors. These are included conditionally if the API writer defines a struct.Auth_ definition in their schema, for the auth header definition data type references it, as in "@auth_": "struct.Auth_" .

API writiers are strongly encouraged to place all auth-related data into the standard struct.Auth_ struct, as the @auth_ header is treated with greater sensitivity throughout the Telepact ecosystem.

You can find details about auth definitions here.

Mock Definitions

Mock definitions include mocking functions, like fn.createStub_ and fn.verify_ for use in tests. These definitions are included if the API is served with a MockServer rather than a Server in the Telepact server-side library.

You can find all mock definitions here.

Full Example

Schema

[ { "///": " A calculator app that provides basic math computation capabilities. ", "info.Calculator": {} }, { "///": " A function that adds two numbers. ", "fn.add": { "x": "number", "y": "number" }, "->": [ { "Ok_": { "result": "number" } } ] }, { "///": " A value for computation that can take either a constant or variable form. ", "union.Value": [ { "Constant": { "value": "number" } }, { "Variable": { "name": "string" } } ] }, { "///": " A basic mathematical operation. ", "union.Operation": [ { "Add": {} }, { "Sub": {} }, { "Mul": {} }, { "Div": {} } ] }, { "///": " A mathematical variable represented by a name that holds a certain value. ", "struct.Variable": { "name": "string", "value": "number" } }, { "///": " Save a set of variables as a dynamic map of variable names to their value. ", "fn.saveVariables": { "variables": { "string": "number" } }, "->": [ { "Ok_": {} } ] }, { "///": " Compute the result of the given x and y values. ", "fn.compute": { "x": "union.Value", "y": "union.Value", "op": "union.Operation" }, "->": [ { "Ok_": { "result": "number" } }, { "ErrorCannotDivideByZero": {} } ] }, { "///": " Export all saved variables, up to an optional limit. ", "fn.exportVariables": { "limit!": "integer" }, "->": [ { "Ok_": { "variables": ["struct.Variable"] } } ] }, { "///": " A function template. ", "fn.getPaperTape": {}, "->": [ { "Ok_": { "tape": ["struct.Computation"] } } ] }, { "///": " A computation. ", "struct.Computation": { "user": "string?", "firstOperand": "union.Value", "secondOperand": "union.Value", "operation": "union.Operation", "result": "number?", "successful": "boolean" } }, { "fn.showExample": {}, "->": [ { "Ok_": { "link": "fn.compute" } } ] }, { "errors.RateLimit": [ { "ErrorTooManyRequests": {} } ] }, { "headers.Identity": { "@user": "string" }, "->": {} } ]

Valid Client/Server Interactions

-> [{}, {"fn.ping_": {}}] <- [{}, {"Ok_": {}}]

-> [{}, {"fn.add": {"x": 1, "z": 2}}] <- [{}, {"ErrorInvalidRequestBody_": {"cases": [{"path": ["fn.add", "z"], "reason": {"ObjectKeyDisallowed": {}}}, {"path": ["fn.add"], "reason": {"RequiredObjectKeyMissing": {"key": "y"}}}]}}]

-> [{}, {"fn.add": {"x": 1, "y": 2}}] <- [{}, {"Ok_": {"result": 3}}]

-> [{}, {"fn.saveVariables": {"a": 1, "b": 2}}] <- [{}, {"Ok_": {}}]

-> [{}, {"fn.showExample": {}}] <- [{}, {"Ok_": {"link": {"fn.compute": {"x": {"Constant": {"value": 5}}, "y": {"Variable": {"name": "b"}}, "op": {"Mul": {}}}}}}]

-> [{"@user": "bob"}, {"fn.compute": {"x": {"Constant": {"value": 5}}, "y": {"Variable": {"name": "b"}}, "op": {"Mul": {}}}}] <- [{}, {"Ok_": {"result": 10}}]

-> [{"@user": "bob"}, {"fn.compute": {"x": {"Variable": {"name": "a"}}, "y": {"Constant": {"value": 0}}, "op": {"Div": {}}}}] <- [{}, {"ErrorCannotDivideByZero": {}}]

-> [{}, {"fn.getPaperTape": {}}] <- [{}, {"Ok_": {"tape": [{"user": null, "firstOperand": {"Constant": {"value": 1}}, "secondOperand": {"Constant": {"value": 2}}, "operation": {"Add": {}}, "result": 3, "successful": true}, {"user": "bob", "firstOperand": {"Constant": {"value": 5}}, "secondOperand": {"Variable": {"name": "b"}}, "operation": {"Mul": {}}, "result": 10, "successful": true}, {"user": "bob", "firstOperand": {"Variable": {"name": "a"}}, "secondOperand": {"Constant": {"value": 0}}, "operation": {"Div": {}}, "result": null, "successful": false}]}}]

-> [{}, {"fn.exportVariables": {}}] <- [{}, {"Ok_": {"variables": [{"name": "a", "value": 1}, {"name": "b", "value": 2}]}}]

-> [{}, {"fn.exportVariables": {"limit!": 1}}] <- [{}, {"Ok_": {"variables": [{"name": "a", "value": 1}]}}]

-> [{}, {"fn.add": {"x": 1, "y": 2}}] <- [{}, {"ErrorTooManyRequests": {}}]

-> [{}, {"fn.showExample": {}}] <- [{}, {"ErrorTooManyRequests": {}}]

Appendix

auth-telepact-json

[ { "///": [ " The @auth_ header is the conventional location for sending credentials to ", " the server for the purpose of authentication and authorization. " ], "headers.Auth_": { "@auth_": "struct.Auth_" }, "->": {} }, { "///": " A standard error. ", "errors.Auth_": [ { "///": " The credentials in the _auth header were missing or invalid. ", "ErrorUnauthenticated_": { "message!": "string" } }, { "///": " The credentials in the _auth header were insufficient to run the function. ", "ErrorUnauthorized_": { "message!": "string" } } ] } ]

internal-telepact-json

[ { "///": " Ping the server. ", "fn.ping_": {}, "->": [ { "Ok_": {} } ], "errors": "^errors\.Validation$" }, { "///": " Get the telepact schema of this server. ", "fn.api_": {}, "->": [ { "Ok_": { "api": [{"string": "any"}] } } ], "errors": "^errors\.Validation$" }, { "ext.Select": {} }, { "///": " The @time_ header indicates the request timeout honored by the client. ", "headers.Time_": { "@time_": "integer" }, "->": {} }, { "///": [ " If @unsafe_ is set to true, response validation by the server will be ", " disabled. The server will the client-provided the value of @unsafe_ header ", " in the response. " ], "headers.Unsafe_": { "@unsafe_": "boolean" }, "->": { "@unsafe_": "boolean" } }, { "///": " The @select_ header is used to select fields from structs. ", "headers.Select_": { "@select_": "ext.Select" }, "->": {} }, { "///": [ " The @bin_ header indicates the valid checksums of binary encodings ", " negotiated between the client and server. If the client sends a @bin_ header ", " with any value, the server will respond with a @bin_ header with an array ", " containing the currently supported binary encoding checksum. If te client's ", " provided checksum does not match the server's checksum, the server will also ", " send an @enc_ header containing the binary encoding, which is a map of field ", " names to field ids. The response body may also be encoded in binary. ", " ", " The @pac_ header can also be used to indicate usage of 'packed' binary ", " encoding strategy. If the client submits a @pac_ header with a true value, ", " the server will respond with a @pac_ header with a true value. " ], "headers.Binary_": { "@bin_": ["integer"], "@pac_": "boolean" }, "->": { "@bin_": ["integer"], "@enc_": {"string": "integer"}, "@pac_": "boolean" } }, { "///": " The @warn_ header is used to send warnings to the client. ", "headers.Warning_": {}, "->": { "@warn_": ["any"] } }, { "///": [ " The @id_ header is used to correlate requests and responses. The server will ", " reflect the client-provided @id_ header as-is. " ], "headers.Id_": { "@id_": "any" }, "->": { "@id_": "any" } }, { "///": " A type. ", "union.Type_": [ { "Null": {} }, { "Boolean": {} }, { "Integer": {} }, { "Number": {} }, { "String": {} }, { "Array": {} }, { "Object": {} }, { "Any": {} }, { "Base64String": {} }, { "Bytes": {} }, { "Unknown": {} } ] }, { "///": " A reason for the validation failure in the body. ", "union.ValidationFailureReason_": [ { "TypeUnexpected": { "expected": "union.Type_", "actual": "union.Type_" } }, { "NullDisallowed": {} }, { "ObjectKeyDisallowed": {} }, { "RequiredObjectKeyPrefixMissing": { "prefix": "string" } }, { "ArrayElementDisallowed": {} }, { "NumberOutOfRange": {} }, { "ObjectSizeUnexpected": { "expected": "integer", "actual": "integer" } }, { "ExtensionValidationFailed": { "reason": "string", "data!": {"string": "any"} } }, { "ObjectKeyRegexMatchCountUnexpected": { "regex": "string", "expected": "integer", "actual": "integer", "keys": ["string"] } }, { "RequiredObjectKeyMissing": { "key": "string" } }, { "FunctionUnknown": {} } ] }, { "///": " A parse failure. ", "union.ParseFailure_": [ { "IncompatibleBinaryEncoding": {} }, { "///": " The binary decoder encountered a field id that could not be mapped to a key. ", "BinaryDecodeFailure": {} }, { "JsonInvalid": {} }, { "ExpectedJsonArrayOfAnObjectAndAnObjectOfOneObject": {} }, { "ExpectedJsonArrayOfTwoObjects": {} } ] }, { "///": " A validation failure located at a path explained by a reason. ", "struct.ValidationFailure_": { "path": ["any"], "reason": "union.ValidationFailureReason_" } }, { "///": " A standard error. ", "errors.Validation_": [ { "///": " The server implementation raised an unknown error. ", "ErrorUnknown_": {} }, { "///": " The headers on the request are invalid. ", "ErrorInvalidRequestHeaders_": { "cases": ["struct.ValidationFailure_"] } }, { "///": " The body on the request is invalid. ", "ErrorInvalidRequestBody_": { "cases": ["struct.ValidationFailure_"] } }, { "///": " The headers on the response are invalid. ", "ErrorInvalidResponseHeaders_": { "cases": ["struct.ValidationFailure_"] } }, { "///": " The body that the server attempted to put on the response is invalid. ", "ErrorInvalidResponseBody_": { "cases": ["struct.ValidationFailure_"] } }, { "///": " The request could not be parsed as a telepact Message. ", "ErrorParseFailure_": { "reasons": ["union.ParseFailure_"] } } ] } ]

mock-internal-telepact-json

[ { "///": " A stubbed result for matching input. ", "ext.Stub": {} }, { "///": " A call of a function. ", "ext.Call": {} }, { "///": " The number of times a function is allowed to be called. ", "union.CallCountCriteria_": [ { "Exact": { "times": "integer" } }, { "AtMost": { "times": "integer" } }, { "AtLeast": { "times": "integer" } } ] }, { "///": " Possible causes for a mock verification to fail. ", "union.VerificationFailure_": [ { "TooFewMatchingCalls": { "wanted": "union.CallCountCriteria_", "found": "integer", "allCalls": ["ext.Call"] } }, { "TooManyMatchingCalls": { "wanted": "union.CallCountCriteria_", "found": "integer", "allCalls": ["ext.Call"] } } ] }, { "///": [ " Create a function stub that will cause the server to return the stub result ", " when the stub argument matches the function argument on a request. ", " ", " If ignoreMissingArgFields is true, then the server will skip field ", " omission validation on the stub argument, and the stub will match calls ", " where the given stub argument is Exactly a json sub-structure of the request ", " function argument. ", " ", " If generateMissingResultFields is true, then the server will skip field ", " omission validation on the stub result, and the server will generate the ", " necessary data required to make the result pass on response validation. " ], "fn.createStub_": { "stub": "ext.Stub", "strictMatch!": "boolean", "count!": "integer" }, "->": [ { "Ok_": {} } ], "errors": "^errors\.Validation$" }, { "///": [ " Verify a call was made with this mock that matches the given call and ", " multiplicity criteria. If allowPartialArgMatch is supplied as true, then ", " the server will skip field omission validation, and match calls where the ", " given call argument is Exactly a json sub-structure of the actual argument. " ], "fn.verify_": { "call": "ext.Call", "strictMatch!": "boolean", "count!": "union.CallCountCriteria_" }, "->": [ { "Ok_": {} }, { "ErrorVerificationFailure": { "reason": "union.VerificationFailure_" } } ], "errors": "^errors\.Validation$" }, { "///": [ " Verify that no interactions have occurred with this mock or that all ", " interactions have been verified. " ], "fn.verifyNoMoreInteractions_": {}, "->": [ { "Ok_": {} }, { "ErrorVerificationFailure": { "additionalUnverifiedCalls": ["ext.Call"] } } ], "errors": "^errors\.Validation$" }, { "///": " Clear all stub conditions. ", "fn.clearStubs_": {}, "->": [ { "Ok_": {} } ], "errors": "^errors\.Validation$" }, { "///": " Clear all call data. ", "fn.clearCalls_": {}, "->": [ { "Ok_": {} } ], "errors": "^errors\.Validation$" }, { "///": " Set the seed of the random generator. ", "fn.setRandomSeed_": { "seed": "integer" }, "->": [ { "Ok_": {} } ], "errors": "^errors\.Validation$" }, { "errors.Mock_": [ { "///": " The mock could not return a result due to no matching stub being available. ", "ErrorNoMatchingStub_": {} } ] } ]

FAQ

Who exactly needs to use Telepact libraries?

Telepact boasts a flexible development environment for clients, who are allowed to bring as much or as little Telepact tooling as they like, including no tooling at all in favor of industry standard JSON and network libraries.

The server, however, MUST use a Telepact library to serve its Telepact API. Doing so ensures a rich Telepact ecosystem for developers and clients, including features such as automatic API retrieval for mocking, documentation browsing using the console, request validation, opt-in binary, and response field selection.

Why have both optional and nullable fields?

Telepact allows API designers to mark a field as optional (the field might be omitted) as well as mark the field type as nullable (the field might appear with a null value).

These design options are both present to maximize the design expressiveness of the API. Telepact leverages optionality to accomplish the expressiveness of undefined in languages like TypeScript. While null is a value that can be passed around like a string or number, undefined or optionality can not be passed around but is rather an incidental property of the shape of the data itself. This is especially useful in update APIs where, for example, if you want to erase just one field of a model, where null can be used to indicate the erasure of data, and optionality can be used to omit all fields except the one field you want to erase.

Why do optional fields keep the ! symbol in the request/response payloads?

If a field is marked as optional in the schema, such as "field!": "integer" , it will keep that ! symbol on live payloads, such as "field!": 42 .

This pattern serves two purposes (1) to keep the schema and live payloads as similar as possible, and (2) to alert code writers of the optional edge case. If a client encounters something like response['field!'] in code, the !

immediately alerts the code reader that an undefined -like value might be returned from the code expression.

Why can I not define nullable arrays or objects?

Nullability is indicated on base types by appending type strings with ? , but since collection types are defined with native JSON array and object syntax, using [] and {} respectively, there is no way to append ? to these expressions since free ? characters are not legal JSON syntax.

This apparent design constraint, albeit coincidental, aligns with Telepact's design goals of expressibility without redundant design options. In Telepact, null represents "empty" (while optional represents "unknown"). Since array and object collection types can already express "emptiness," nullability is unnecessary.

Why is there nothing like a standard 404 Not Found error?

Telepact provides several standard errors to represent common integration issues, such as request and response incompatibilities and authentication/authorization errors, all reminisicent of the 400, 500, 401 and 403 error codes, respectively, but there is no standard error that approximates 404 Not Found.

Instead, API designers are encouraged to abstract concepts as data whenever possible, and the 404 Not Found use-case can be trivially represented with an empty optional value.

But the given 400-like Bad Request errors are too precise. Why is a more general-purpose "Bad Request" error not available?

Telepact has several errors to communicate request invalidity with respect to the API schema, but there is no out-of-the-box "Bad Request" error that a server can raise from some custom validation logic in the server handler.

Overly generalized abstractions, such as a catch-all "Bad Request", are unpreferred in Telepact in favor of precise data types. Where necessary, all "Bad Request" use-cases can be enumerated in function results alongside the Ok_ tag. API designers are encouraged to prefer data abstractions over errors wherever possible, such as preferring empty optionals over "Not Found" errors.

Why do functions in Telepact not support positional arguments?

Telepact functions are automatically associated with an argument struct and a result struct that API designers can use to define arguments and return values, respectively. The arguments being supplied via the argument struct will be inherently unordered due to the nature of JSON objects, and there is no way to approximate traditional positional arguments at the root of the Request Message Body.

This design decision is intentional. Positional arguments are a significant risk that provoke backwards incompatible changes through seemingly innocent API changes, such as changing the order of arguments or appending new arguments. This problem is especially pronounced in generated code for many programming languages. By making the design entry point a struct, API designers are predisposed for backwards-compatible changes like appending optional struct fields.

Why is there no Enum type as seen in C or Java?

Telepact achieves enumerated types with unions, which are very similar to enums as seen in C or Java, except that a struct is automatically attached to each value. The traditional enum can be approximated by simply leaving all union structs blank.

Why force servers to perform response validation?

Telepact automatically performs validation of responses against the Telepact schema, and there is no setting for servers to turn off this behavior.

This design decision is intentional. It helps maintain the high standard of type safety in the Telepact ecosystem by denying API providers the option of categorizing malformed data as an inconvenience and are instead forced to deal with hard failures through bug reports. Hard failures also help draw attention to type safety deficits early in the development phase.

Clients who are uniquely vulnerable to hard server failures and who find it advantageous to receive the malformed data anyway are able to turn off this response validation by submitting their requests with the {"@unsafe_":true}

header.

If all I want is compact binary serialization, why not just use gRPC?

Telepact and gRPC both have compact binary serialization for drastically improved efficiency over conventional serialization such as JSON. However, Telepact brings a critical new innovation to the space of RPC and binary serialization in that it does not leak the ABI into the API.

ABI, or Application Binary Interface, is the actual interface between RPC clients and servers using such compact serialization protocols. The data passing through this interface is unreadable due to conventional json keys being encoded as numbers. Because an encoding is in play, clients and servers need to agree on what numbers represent which fields all throughout the API. gRPC and other conventional RPC frameworks accomplish this by having the clients and servers both base their field ids on the same information during code generation by leaking these ABI field ids into the API schema itself. Unfortunately, this adds an unusual cognitive burden for developers designing such APIs, because they now need to guard against interface drift between the API and the ABI, typically by complying with a set of policies concerning how those field ids are defined and how they can change.

Telepact breaks free from the conventional practice of defining and maintaining field ids, and instead accomplishes client and server agreement over field ids through a client-server handshake at runtime. In consequence, Telepact boasts a far simpler developer experience during the API design phase as well as the unique privilege of allowing clients to leverage binary serialization without generated code.

Why can't I have other non-error result union values?

The only required tag for the function result union is Ok_ . All other tags in the result union that are not Ok_ are, by definition, "not okay", and will be interpreted as an error in all circumstances. API designers are encouraged to prefix additional result union tags with Error or equivalent to improve readability and recognition of errors.

Why can't I associate a union tag to something besides a struct?

A designer might want to treat a union tag like a struct field, and associate any data type with a tag. However, in Telepact, all tags in unions are associated with a struct, which means you can't associate union tags with simpler data types, like booleans or strings.

This restriction is in place to uphold Telepact's value of prioritizing effective software evolution. Unions, like functions, are entrypoints to unique execution paths in software, so if software evolves such that an execution path requires a new "argument" like a integer, that requirement will percolate up to the entrypoint. If the proverbial API designer chose to associate the union tag directly to a boolean, the API would require a breaking change in the form of creating another tag to make room for this new integer "argument" to sit next to the original boolean. In contrast, Telepact establishing the expectation that all union tags are associated with structs means the backwards compatible option of adding a new struct field is always available to software designers dealing with the needs of evolving software.

Why can I not omit fn.* fields using the "@select_" header?

The "@select_" header is used to omit fields from structs, which includes union structs, but not the argument struct included with function definitions.

The function type exists so that API providers may incorporate "links" into their API design, such that the appearance of a function type payload can simply be copied and pasted verbatim into the body a new message. Tooling like the Telepact console specifically utilizes this technique to allow end-users to "click through" graphs designed by the API provider.

Omitting fields in the argument struct disrupts the API provider's ability to established well-defined links, and consequently, the "@select_" header is disallowed from omitting fields in function argument structs.

Versions

Kind Package Registry Version

Library (Go) github.com/telepact/telepact/lib/go

Go module (proxy.golang.org) v1.0.0-alpha.181

Library (Java) io.github.telepact:telepact

Maven Central 1.0.0-alpha.185

Library (Python) telepact

PyPI 1.0.0-alpha.185

Library (TypeScript) telepact

npm 1.0.0-alpha.185

SDK (CLI) telepact-cli

PyPI 1.0.0-alpha.191

SDK (Console) telepact-console

npm 1.0.0-alpha.192

SDK (Prettier) prettier-plugin-telepact

npm 1.0.0-alpha.185

Telepact CLI

The CLI is a tool for various development jobs, such as fetching API schemas, generating code, and starting up mock servers for testing purposes.

Installation

pipx install telepact-cli

Usage

telepact --help

Usage: telepact [OPTIONS] COMMAND [ARGS]...

Options: --help Show this message and exit.

Commands: codegen Generate code bindings for a Telepact API schema. compare Compare two Telepact API schemas for backwards compatibility. demo-server Start a demo Telepact server. fetch Fetch a Telepact API schema to store locally. mock Start a mock server for a Telepact API schema.

telepact codegen --help

Usage: telepact codegen [OPTIONS]

Generate code bindings for a Telepact API schema.

Options: --schema-http-url TEXT telepact schema directory --schema-dir TEXT telepact schema directory --lang TEXT Language target (one of "java", "py", "ts", or "go") [required] --out TEXT Output directory [required] --package TEXT Java package (use if --lang is "java") --help Show this message and exit.

telepact compare --help

Usage: telepact compare [OPTIONS]

Compare two Telepact API schemas for backwards compatibility.

Options: --new-schema-dir TEXT New telepact schema directory [required] --old-schema-dir TEXT Old telepact schema directory [required] --help Show this message and exit.

telepact fetch --help

Usage: telepact fetch [OPTIONS]

Fetch a Telepact API schema to store locally.

Options: --http-url TEXT HTTP URL of a Telepact API [required] --output-dir TEXT Directory of Telepact schemas [required] --help Show this message and exit.

telepact mock --help

Usage: telepact mock [OPTIONS]

Start a mock server for a Telepact API schema.

Options: --http-url TEXT HTTP URL of a Telepact API --dir TEXT Directory of Telepact schemas --port INTEGER Port to run the mock server on --path TEXT Path to expose the mock API (default: /api) --generated-collection-length-min INTEGER Minimum length of generated collections --generated-collection-length-max INTEGER Maximum length of generated collections --disable-optional-field-generation Disable generation of optional fields (enabled by default) --disable-message-response-generation Disable generation of message responses (enabled by default) --disable-random-optional-field-generation Disable randomization of optional field generation (enabled by default) --help Show this message and exit.

NOTE: The mock command is an empowering development tool for clients. You do not need to develop against a live server; you can use the mock command to set up a "middle-man" server that will validate requests for schema compliance and return schema-compliant auto-generated responses (which can be overrideen with manual stubs if desired.)