Twitter Network Layer (a.k.a TNL)

The Twitter Network Layer (TNL) is a framework for interfacing with the Apple provided NSURLSession stack that provides additional levels of control and insight over networking requests, provides simple configurability and minimizes the cognitive load necessary to maintain a robust and wide-reaching networking system.

OSI Layering with TNL

The Twitter Network Layer sits on top of the connection/session layer provided by the Apple NSURL framework. Those frameworks are build on top of the HTTP/1.1 and HTTP/2. The layer chart appears like this:

/--------------------------------------\
|                                      |
|              User Layer              |
|       The actual user (Layer 8)      |
|                                      |
|--------------------------------------|
|                                      |
|          Application Layer           |
|       MVC, MVVM, etc (Layer 7e)      |
|                                      |
|--------------------------------------|
|                                      |
|     Concrete Operation/App Layer     |  <------ Operations, Requests &
|             TNL (Layer 7d)           |          Responses built on TNL
|                                      |
|--------------------------------------|
|                                      |
|     Abstract Operation/App Layer     |
|             TNL (Layer 7c)           |  <------ TNL
|                                      |
|--------------------------------------|
|                                      |
|         Connection/App Layer         |
|        NSURL Stack (Layer 7b)        |
|                                      |
|--------------------------------------|
|                                      |
|          Protocol/App Layer          |
|     HTTP/1.1 & HTTP/2 (Layer 7a)     |
|                                      |
|--------------------------------------|
|                                      |
|            Presentation Layer        |
| Encryption & Serialization (Layer 6) |
|                                      |
|--------------------------------------|
|                                      |
|            Session Layer             |
|      A Feature of TCP (Layer 5)      |
|                                      |
|--------------------------------------|
|                                      |
|            Transport Layer           |
|             TCP (Layer 4)            |
|                                      |
|--------------------------------------|
|                                      |
|         Routing/Network Layer        |
|              IP (Layer 3)            |
|                                      |
|--------------------------------------|
|                                      |
|            Data Link Layer           |
|          IEEE 802.X (Layer 2)        |
|                                      |
|--------------------------------------|
|                                      |
|            Physical Layer            |
|          Ethernet (Layer 1)          |
|                                      |
\--------------------------------------/

Brief Overview

Features

Twitter Network Layer provides a framework on top of Apple's NSURLSession framework with numerous benefits. Here are some of the features provided by TNL:

• All the features of NSURLSession, simplified where appropriate
• NSOperation based request operations (for NSOperation features)
• Strong separation of roles in the framework's objects
• Immutable/mutable pairings of requests (TNLHTTPRequest) and configurations (TNLRequestConfiguration)
• Encapsulated immutable responses (TNLResponse)
• Prioritization of requests
• Selectable response body consumption modes (NSData storage, callback chunking or saving to file)
• Request hydration (enables polymorphic requests and dynamically populated requests)
• Dynamic retrying with retry policies
• More events (request operation state transitions, progress, network state/condition updates, etc)
• Modular delegates for separation of roles and increased reusability and easy overriding

Usage

The high level concept of how to use TNL is rather straightforward:

1. Set up any reuseable settings (by doing any combination of the following):
   • Build shared accessors to resuable TNLRequestConfiguration instances
   • Implement a TNLRequestDelegate (if added functionality is desired beyond just handling the result)
   • Configure a TNLRequestConfiguration for reuse
   • Configure the TNLGlobalConfiguration
2. Set up any reusable TNLRequestOperationQueue objects once (ex: one for API requests, one for image requests, etc.)
   • [TNLRequestOperationQueue initWithIdentifier:]
3. Generate and enqueue any desired TNLRequestOperation with the following objects:
   • TNLRequest conforming object (including TNLHTTPRequest concrete class and NSURLRequest)
   • TNLRequestConfiguration (optional)
   • TNLRequestDelegate (optional)
4. Handle the events appropriately via the callbacks, particularly the completion callback that provides the TNLResponse
   • Delegate callbacks will go to the appropriate sub-protocol in the TNLRequestOperation's TNLRequestDelegate

HOWTO

Where To Start

Twitter Network Layer documentation starts with this README.md and progresses through the APIs via their documentation.

Overview of a Network Operation

Core Objects

The core objects of a service based architecture are request, response and operation/task/action (referred to as an operation from here on). The request encapsulates data to send and is not actionable; the response encapsulates the data received and is not actionable; and the operation is the object that delivers the request and retrieves the response and is the only actionable object in the set of core objects.

This high level concept translates directly into a network architecture as we will have requests that encapsulate the data of an HTTP request which are Headers and a Body, responses that encapsulate the data of an HTTP response which are Headers and a Body, and last the operation that executes delivering the request and retrieving the response.

Core Object Breakdown

• request
  • encapsulates data to send
  • immutability provides stability
  • not actionable, just data
  • TNLRequest is the protocol for requests in TNL
  • TNLHTTPRequest and NSURLRequest are concrete classes (both are immutable/mutable pairs)
• response
  • encapsulates data received
  • immutability provides stability
  • not actionable, just data
  • TNLResponse is the object for responses in TNL (composite object that includes an NSHTTPURLResponse)
• operation
  • the executing object
  • delivers the request
  • retrieves the response
  • actionable (e.g. starting, canceling, priotiziation, modifying dependencies)
  • TNLRequestOperation is the operation in TNL (subclasses NSOperation) and is backed by NSURLSessionTask

Support Objects

In addition to a service architecture having requests, operations and responses; support objects are often present that aid in the management of the executing operations, configuration of their behavior and delegation of decisions or events.

The configuration object encapsulates how an operation behaves. It will have no impact on what is sent in the operation (that's the request), but can modify how it is sent. For instance, the configuration can indicate a maximum duration that the operation can take before it should fail.

The delegate object provides the extensibility of on demand decision making when prudent as well as the delivery of events as the operation executes.

The manager object coordinates the execution of multiple operations within a logical grouping.

Support Object Breakdown

• configuration
  • encapsulation of behavior settings
  • TNLRequestConfiguration is the config in TNL (applied per operation)
  • NSURLSessionConfiguration is the config in NSURL stack (applied per manager)
• delegate
  • provides extensibility
  • has callbacks for on demand decisions
  • has callbacks for events as they occur
  • TNLRequestDelegate is the delegate in TNL (provided per operation)
  • NSURLSessionDelegate is the delegate in NSURL stack (provided per manager)
• manager
  • coordinator of multiple operations
  • permits grouping of operations
  • TNLRequestOperationQueue is the manager object in TNL
  • NSURLSession is the manager object in NSURL stack

Note: You can already see there is a fundamental architecture difference between NSURLSession networking and Twitter Network Layer. The configuration and delegate per operation approach in TNL is much more scalable when dealing with dozens or hundreds of unique configurations and/or delegates given a plethora of requests and their needs. Coupling the configuration and/or delegate to the reusable manager object(s) is unwieldy and can lead to mistakes w.r.t. correct configuration and event on a per request basis.

Building a Request

TNL uses the TNLRequest as the interface for all network requests. In practice, the protocol is used in one of 3 ways:

1. Concrete TNLHTTPRequest

• Configuring a concrete TNLHTTPRequest object (or TNLMutableHTTPRequest)

2. NSURLRequest

• NSURLRequest explicitely conforms to TNLRequest protocol via a category in TNL making it supported as request object.
• However, since TNL rigidly segregates configuration from request, only the request properties on NSURLRequest are observed and the configuration properties of NSURLRequest are ignored.

3. Implementing a custom TNLRequest

• TNL supports having anything that conforms to TNLRequest as an original request for an operation.
• Makes it simple for an object that encapsulates the minimal amount of information necessary to take the place as the original request.
  • You could have a APPRetrieveBlobRequest that has 1 property, the identifier for the "Blob" call blobIdentifier.
  • That object doesn't need to have any methods that actually represent anything related to an HTTP request and that's ok. However, in order for the operation to send the original request, it needs to be able to be treated as an HTTP request, which is to say it must conform to TNLRequest. This can be done in 2 ways:
    1. have the object implement TNLRequest and have its methods that populate the values by the relevant properties (in our example, the blob identifier)
    2. have the delegate implement the request hydration callback to convert the opaque request into a well formed TNLRequest ready for HTTP transport.
  • See Custom TNLRequest examples

When it comes to making the choice, it can boil down to convenience vs simplicity of reuse. If you have a one shot request that has no reuse, options 1 and 2 will suffice. If you have a request that can be reused throughout the code base, option 3 clearly offers the cleanest interface. By having the caller only need to know the class of the request and the relevant values for populating the request, any concern over the HTTP structure is completely eliminated.

Concrete TNLRequest with TNLHTTPRequest

TNLHTTPRequest:

NSString *URLString = [NSString stringWithFormat:@"http://api.myapp.com/blob/%tu", blobIdentifier];
NSURL *URL = [NSURL URLWithString:URLString];
TNLHTTPRequest *request = [TNLHTTPRequest GETRequestWithURL:URL
                                           HTTPHeaderFields:@{@"User-Agent": [MYAPPDELEGATE userAgentString]}];

TNLMutableHTTPRequest:

NSString *URLString = [NSString stringWithFormat:@"http://api.myapp.com/blob/%tu", blobIdentifier];
NSURL *URL = [NSURL URLWithString:URLString];
TNLMutableHTTPRequest *mRequest = [[TNLMutableHTTPRequest alloc] init];
mRequest.HTTPMethodValue = TNLHTTPMethodValueGET;
mRequest.URL = URL;
[mRequest setValue:[MYAPPDELEGATE userAgentString] forHTTPHeaderField:@"User-Agent"];

NSURLRequest

NSString *URLString = [NSString stringWithFormat:@"http://api.myapp.com/blob/%tu", blobIdentifier];
NSURL *URL = [NSURL URLWithString:URLString];
NSMutableURLRequest *mRequest = [[NSMutableURLRequest alloc] init];
mRequest.HTTPMethod = @"GET";
mRequest.URL = URL;
[mRequest setValue:[MYAPPDELEGATE userAgentString] forHTTPHeaderField:@"User-Agent"];

Custom TNLRequest

1) Request Hydration

APPRetrieveBlobRequest *request = [[APPRetrieveBlobRequest alloc] initWithBlobIdentifier:blobIdentifier];

// ... elsewhere ...

- (void)tnl_requestOperation:(TNLRequestOperation *)op
              hydrateRequest:(APPRetrieveBlobRequest *)request // we know the type
                  completion:(TNLRequestHydrateCompletionBlock)complete
{
     NSString *URLString = [NSString stringWithFormat:@"http://api.myapp.com/blob/%tu", blobRequest.blobIdentifier];
     NSURL *URL = [NSURL URLWithString:URLString];
     TNLHTTPRequest *newReq = [TNLHTTPRequest GETRequestWithURL:URL
                                               HTTPHeaderFields:@{@"User-Agent": [MYAPPDELEGATE userAgentString]}];
     complete(newReq);
}

2) Request with HTTP support

APPRetrieveBlobRequest *request = [[APPRetrieveBlobRequest alloc] initWithBlobIdentifier:blobIdentifier];

// ... elsewhere ...

@implementation APPRetrieveBlobRequest

- (NSURL *)URL
{
    NSString *URLString = [NSString stringWithFormat:@"http://api.myapp.com/blob/%tu", self.blobIdentifier];
    return [NSURL URLWithString:URLString];
}

- (NSDictionary *)allHTTPHeaderFields
{
    return @{@"User-Agent":[MYAPPDELEGATE userAgentString]};
}

// POINT OF IMPROVEMENT:
// utilize polymorphism and have an APPBaseRequest class that implements
// the "allHTTPHeaderFields" so that all subclasses (including APPRetrieveBlobRequest)
// will inherit the desired defaults.
// This can apply to a wide range of HTTP related TNLHTTPRequest properties
// or even composition of subcomponents that are aggregated to a single property.
// For example: the host of the URL (api.myapp.com) could be provided
// as a property on APPBaseRequest that permits subclasses to override the host, and then
// the construction of the `URL` uses composition of variation properites that the subclasses
// can provide.

@end

Inspecting a Response

When an operation completes, a TNLResponse is populated and provided to the completion block or completion callback (depending on if you use a delegate or not). The TNLResponse has all the information necessary to understand how the operation completed, as well as what information was retrieve. Additionally, with response polymorphism, the response can be extended to provide better contextual information regarding the result, such as parsing the response body as JSON or converting the response body into a UIImage.

The way you deal with a TNLResponse should be systematic and straighforward:

1. deal with any errors on the response
   • TNLResponse has an operationError property but custom subclasses could expose other errors too.
   • Subclass response objects that have extra errors should consider having an anyError property for quick access to any error in the response.
2. deal with the status code of the response
   • It is important to know that a 404 is not an operation error so it won't be set as an error.
   • It is actually the status of the successful operation and needs to be handled accordingly.
   • For designs that want to treat HTTP Status codes that are not success as errors, they should expose an HTTP error on their response subclass(es).
3. deal with the response payload
   • This could be the response HTTP headers, the response body (as NSData or a file on disk), etc
   • response subclasses should consider deserializing their response's body into a model object and exposing it as a property for concrete interactions.

One benefit to using response polymorphism is the ability to handle the response and populate the hydrated response with the information that's pertinent to the caller. For example: if your network operation yields JSON, and all you care about is if that JSON came through or not, at hydration time you could check for any error conditions then parse out the JSON and if everything is good have a property on the custom TNLResponse subclasss that holds the NSDictionary result property (or nil if anything along the way prevented success).

Things you can inspect on a response by default:

• the operation error (if one occurred)
• the original request
• the response info
  • this object encapsulates the information of the HTTP response including:
    • the source of the response (local cache or network load)
    • the response body (as data or temporarySavedFile if the operation was configured to maintain the data)
    • the final NSURLRequest that loaded the response
    • the final NSURLResponse object
  • it also provides convenience accessors
    • the response's HTTP status code
    • the final NSURL
    • all the HTTP header fields
• the response metrics
  • detailed metric information such as execution timings, durations, bytes sent/received, attempt counts, etc.
  • this is the detail that TNL exposes for every request/operation/response that really empowers programmers to maximize impact with their networking.

Simple Network Operations

Twitter Network Layer provides a highly robust API for building network operations with a great deal of flexibility and extensibility. However, there are often occasions when you just need to execute an operation and need things to be as simple as possible. Twitter Network Layer provides all the convenience necessary for getting what needs to be done as simply as possible.

 NSString *URLString = [NSURL URLWithString:@"http://api.myapp.com/settings"];
 NSURLRequest *request = [NSURLRequest requestWithURL:URLString];
 [[TNLRequestOperationQueue defaultOperationQueue] enqueueRequest:request
                                                       completion:^(TNLRequestOperation *op, TNLResponse *response) {
     NSDictionary *json = nil;
     if (!response.operationError && response.info.statusCode == 200) {
         json = [NSJSONSerialization JSONObjectWithData:response.info.data options:0 error:NULL];
     }
     if (json) {
        [self _myapp_didCompleteSettingsRequestWithJSON:json];
     } else {
        [self _myapp_didFailSettingsRequest];
     }
 }];

Configuring Behavior

Often the vanila configuration for an operation will suffice, however it is common to need particular behaviors in order to get specific use cases to work. Let's take, as an example, firing network operation when a specific metric is hit. In this case, we don't care about storing the response body and we also want to avoid having a cache that could get in the way.

 NSURL *URL = [NSURL URLWithString:@"http://api.myapp.com/recordMetric?hit=true"];
 TNLHTTPRequest *request = [TNLHTTPRequest GETRequestWithURL:URL HTTPHeaderFields:nil];
 TNLMutableRequestConfiguration *config = [TNLMutableRequestConfiguration defaultConfiguration];
 config.responseDataConsumptionMode = TNLResponseDataConsumptionModeNone;
 config.URLCache = nil; // Note: 'URLCache' is now 'nil' by default in TNL, but the illustration still works
 TNLRequestOperation *op = [TNLRequestOperation operationWithRequest:request
                                                       configuration:config
                                                          completion:^(TNLRequestOperation *o, TNLResponse *response) {
     assert(response.info.source != TNLResponseSourceLocalCache);
     const BOOL success = response.info.statusCode == 202;
     [self didSendMetric:success];
 }];
 [[TNLRequestOperationQueue defaultOperationQueue] enqueueOperation:op];

Now, sometimes, you may want to have the same defaults for certain kinds of operations. That can easily be accomplished with a category or some other shared accessor.

 @interface TNLRequestConfiguration (APPAdditions)
 + (instancetype)configurationForMetricsFiring;
 @end

 @implementation TNLRequestConfiguration (APPAdditions)

 + (instancetype)configurationForMetricsFiring
 {
     static TNLRequestConfiguration* sConfig;
     static dispatch_once_t onceToken;
     dispatch_once(&onceToken, ^{
         TNLMutableRequestConfiguration *mConfig = [TNLMutableRequestConfiguration defaultConfiguration];
         mConfig.URLCache = nil; // Note: 'URLCache' is now 'nil' by default in TNL, but the illustration still works
         mConfig.responseDataConsumptionMode = TNLResponseDataConsumptionModeNone;
         sConfig = [mConfig copy];
     });
     return sConfig;
 }

 @end

 @implementation TNLMutableRequestConfiguration (APPAdditions)

 + (instancetype)configurationForMetricsFiring
 {
     return [[TNLRequestConfiguration configurationForMetricsFiring] mutableCopy];
 }

 @end

Building an Advanced API Layer

Twitter Network Layer was designed from the ground up with REST APIs in mind. From simple APIs to complex API layers that require a complicated system for managing all operations, TNL provides the foundation needed.

As a pattern for creating concrete API operations, one of the first places to extend TNL for your API layer is by concretely building API requests and responses. For requests, you implement a TNLRequest for every request your API provides with properties that configure each request appropriately. Those requests should be subclassing a base request that does the busy work of pulling together the generic properties that the subclasses can override to construct the HTTP properties of the request. Each subclassed request then overrides only what is necessary to form the valid HTTP request. For things that are context or time sensitive, such as request signing, request hydration should be used to fully saturate the custom API request at the time the request is going to sent (vs at the time it was enqueued).

Following from custom API requests are custom API responses. At a minimum, it makes sense to have an API response that subclasses TNLResponse. To provide an even simpler interface to callers, you can implement a response per request. For response hydration, you merely extract whatever contextually relevant information is valuable for an API response and set those properties on you custom subclass of TNLResponse (such as API error, JSON result, etc).

If the API layer is advanced enough, it may warrant further encapsulation with a managing object which is often referred to as an API client. The API client would manage the queuing of requests, the delegate implementation for operations (including hydration for requests and subclassing responses so they hydrate too), the vending of operations, authentication/signing of requests, high level retry plus timeout behavior, custom configurations and oberving responses for custom handling.

Client Architecture

With an API client architecture, the entire HTTP structure is encapsulated and callers can deal with things just the objects they care about. No converting or validating. No configuring. The power of TNL is completely utilized by API client freeing the caller of any burden.

 APISendMessageRequest *request = [[APISendMessageRequest alloc] init];
 request.sender = self.user;
 request.receiver = otherUser;
 request.message = message;
 self.sendOp = [[APIClient sharedInstance] enqueueRequest:request
                                           completion:^(TNLRequestOperation *op, APIResponse *response) {
    [weakSelf messageSendDidComplete:op withResponse:(id)response];
 }];

 // ... elsewhere ...

 - (void)messageSendDidComplete:(TNLRequestOperation *)op withResponse:(APISendMessageResponse *)response
 {
     assert(self.sendOp == op);
     self.sendOp = nil;
     if (!sendMessageResponse.wasCancelled) {
        if (sendMessageResponse.succeeded) {
            [self updateUIForCompletedMessageSendWithId:sendMessageResponse.messageId];
        } else {
            [self updateUIForFailedMessageSendWithUserErrorTitle:sendMessageResponse.errorTitle
                                                    errorMessage:sendMessageResponse.errorMessage];
        }
     }
 }

 // Insight:
 // Presumably, APISendMessageResponse would subclass a base response like APIBaseResponse.
 // Following that presumption, it would make sense that APIBaseResponse would expose
 // wasCancelled, succeeded, errorTitle and errorMessage while APISendMessageResponse would
 // expose messageId (since that is part of the response payload that is specific to the request).
 // It would likely make sense that if the API used JSON response bodies,
 // the base response would also expose a "result" property (NSDictionary) and
 // APISendMessageResponse's implementation for messageId is just:
 //    return self.result[@"newMessageId"];

Using the Command-Line-Interface

Twitter Network Layer includes a target for building a macOS tool called tnlcli. You can build this tool run it from Terminal from you Mac, similar to cURL or other networking command line utilities.

Usage

Usage: tnlcli [options] url

    Example: tnlcli --request-method HEAD --response-header-mode file,print --response-header-file response_headers.json https://google.com

Argument Options:
-----------------

    --request-config-file <filepath>     TNLRequestConfiguration as a json file
    --request-headers-file <filepath>    json file of key-value-pairs for using as headers
    --request-body-file <filepath>       file for the HTTP body

    --request-header "Field: Value"      A header to provide with the request (will override the header if also in the request header file). Can provide multiple headers.
    --request-config "config: value"     A config setting for the TNLRequestConfiguration of the request (will override the config if also in the request config file). Can provide multiple configs.
    --request-method <method>            HTTP Method from Section 9 in HTTP/1.1 spec (RFC 2616), such as GET, POST, HEAD, etc

    --response-body-mode <mode>          "file" or "print" or a combo using commas
    --response-body-file <filepath>      file for the response body to save to (requires "file" for --response-body-mode
    --response-headers-mode <mode>       "file" or "print" or a combo using commas
    --response-headers-file <filepath>   file for the response headers to save to (as json)

    --dump-cert-chain-directory <dir>    directory for the certification chain to be dumped to (as DER files)

    --verbose                            Will print verbose information and force the --response-body-mode and --responde-headers-mode to have "print".
    --version                            Will print ther version information.

License

Copyright 2014-2020 Twitter, Inc.

Licensed under the Apache License, Version 2.0: https://www.apache.org/licenses/LICENSE-2.0

Security Issues?

Please report sensitive security issues via Twitter's bug-bounty program (https://hackerone.com/twitter) rather than GitHub.