One of the largest cost drivers in running a service like Flickr is storage. We’ve described multiple techniques to get this cost down over the years: use of COS, creating sizes dynamically on GPUs and perceptual compression. These projects have been very successful, but our storage cost is still significant.
At the beginning of 2016, we challenged ourselves to go further — to go a full year without needing new storage hardware. Using multiple techniques, we got there.

The Cost Story

A little back-of-the-envelope math shows storage costs are a real concern. On a very high-traffic day, Flickr users upload as many as twenty-five million photos. These photos require an average of 3.25 megabytes of storage each, totalling over 80 terabytes of data. Stored naively in a cloud service similar to S3, this day’s worth of data would cost over $30,000 per year, and continue to incur costs every year.

And a very large service will have over two hundred million active users. At a thousand images each, storage in a service similar to S3 would cost over $250 million per year (or $1.25 / user-year) plus network and other expenses. This compounds as new users sign up and existing users continue to take photos at an accelerating rate. Thankfully, our costs, and every large service’s costs, are different than storing naively at S3, but remain significant.

Cost per byte have decreased, but bytes per image from iPhone-type platforms have increased. Cost per image hasn’t changed significantly.

Storage costs do drop over time. For example, S3 costs dropped from $0.15 per gigabyte month in 2009 to $0.03 per gigabyte-month in 2014, and cloud storage vendors have added low-cost options for data that is infrequently accessed. NAS vendors have also delivered large price reductions.

Unfortunately, these lower costs per byte are counteracted by other forces. On iPhones, increasing camera resolution, burst mode and the addition of short animations (Live Photos) have increased bytes-per-image rapidly enough to keep storage cost per image roughly constant. And iPhone images are far from the largest.

In response to these costs, photo storage services have pursued a variety of product options. To name a few: storing lower quality images or re-compressing, charging users for their data usage, incorporating advertising, selling associated products such as prints, and tying storage to purchases of handsets.

There are also a number of engineering approaches to controlling storage costs. We sketched out a few and cover three that we implemented below: adjusting thresholds on our storage systems, rolling out existing savings approaches to more images, and deploying lossless JPG compression.

Adjusting Storage Thresholds

As we dug into the problem, we looked at our storage systems in detail. We discovered that our settings were based on assumptions about high write and delete loads that didn’t hold. Our storage is pretty static. Users only rarely delete or change images once uploaded. We also had two distinct areas of just-in-case space. 5% of our storage was reserved space for snapshots, useful for undoing accidental deletes or writes, and 8.5% was held free in reserve. This resulted in about 13% of our storage going unused. Trade lore states that disks should remain 10% free to avoid performance degradation, but we found 5% to be sufficient for our workload. So we combined our our two just-in-case areas into one and reduced our free space threshold to that level. This was our simplest approach to the problem (by far), but it resulted in a large gain. With a couple simple configuration changes, we freed up more than 8% of our storage.

Adjusting storage thresholds

Extending Existing Approaches

In our earlier posts, we have described dynamic generation of thumbnail sizes and perceptual compression. Combining the two approaches decreased thumbnail storage requirements by 65%, though we hadn’t applied these techniques to many of our images uploaded prior to 2014. One big reason for this: large-scale changes to older files are inherently risky, and require significant time and engineering work to do safely.

Because we were concerned that further rollout of dynamic thumbnail generation would place a heavy load on our resizing infrastructure, we targeted only thumbnails from less-popular images for deletes. Using this approach, we were able to handle our complete resize load with just four GPUs. The process put a heavy load on our storage systems; to minimize the impact we randomized our operations across volumes. The entire process took about four months, resulting in even more significant gains than our storage threshold adjustments.

Decreasing the number of thumbnail sizes

Lossless JPG Compression

Flickr has had a long-standing commitment to keeping uploaded images byte-for-byte intact. This has placed a floor on how much storage reduction we can do, but there are tools that can losslessly compress JPG images. Two well-known options are PackJPG and Lepton, from Dropbox. These tools work by decoding the JPG, then very carefully compressing it using a more efficient approach. This typically shrinks a JPG by about 22%. At Flickr’s scale, this is significant. The downside is that these re-compressors use a lot of CPU. PackJPG compresses at about 2MB/s on a single core, or about fifteen core-years for a single petabyte worth of JPGs. Lepton uses multiple cores and, at 15MB/s, is much faster than packJPG, but uses roughly the same amount of CPU time.

This CPU requirement also complicated on-demand serving. If we recompressed all the images on Flickr, we would need potentially thousands of cores to handle our decompress load. We considered putting some restrictions on access to compressed images, such as requiring users to login to access original images, but ultimately found that if we targeted only rarely accessed private images, decompressions would occur only infrequently. Additionally, restricting the maximum size of images we compressed limited our CPU time per decompress. We rolled this out as a component of our existing serving stack without requiring any additional CPUs, and with only minor impact to user experience.

Running our users’ original photos through lossless compression was probably our highest-risk approach. We can recreate thumbnails easily, but a corrupted source image cannot be recovered. Key to our approach was a re-compress-decompress-verify strategy: every recompressed image was decompressed and compared to its source before removing the uncompressed source image.

This is still a work-in-progress. We have compressed many images but to do our entire corpus is a lengthy process, and we had reached our zero-new-storage-gear goal by mid-year.

On The Drawing Board

We have several other ideas which we’ve investigated but haven’t implemented yet.

In our current storage model, we have originals and thumbnails available for every image, each stored in two datacenters. This model assumes that the images need to be viewable relatively quickly at any point in time. But private images belonging to accounts that have been inactive for more than a few months are unlikely to be accessed. We could “freeze” these images, dropping their thumbnails and recreate them when the dormant user returns. This “thaw” process would take under thirty seconds for a typical account. Additionally, for photos that are private (but not dormant), we could go to a single uncompressed copy of each thumbnail, storing a compressed copy in a second datacenter that would be decompressed as needed.

We might not even need two copies of each dormant original image available on disk. We’ve pencilled out a model where we place one copy on a slower, but underutilized, tape-based system while leaving the other on disk. This would decrease availability during an outage, but as these images belong to dormant users, the effect would be minimal and users would still see their thumbnails. The delicate piece here is the placement of data, as seeks on tape systems are prohibitively slow. Depending on the details of what constitutes a “dormant” photo these techniques could comfortably reduce storage used by over 25%.

We’ve also looked into de-duplication, but we found our duplicate rate is in the 3% range. Users do have many duplicates of their own images on their devices, but these are excluded by our upload tools.  We’ve also looked into using alternate image formats for our thumbnail storage.    WebP can be much more compact than ordinary JPG but our use of perceptual compression gets us close to WebP byte size and permits much faster resize.  The BPG project proposes a dramatically smaller, H.265 based encoding but has IP and other issues.

There are several similar optimizations available for videos. Although Flickr is primarily image-focused, videos are typically much larger than images and consume considerably more storage.

Conclusion

Optimization over several releases

Since 2013 we’ve optimized our usage of storage by nearly 50%.  Our latest efforts helped us get through 2016 without purchasing any additional storage,  and we still have a few more options available.

Peter Norby, Teja Komma, Shijo Joy and Bei Wu formed the core team for our zero-storage-budget project. Many others assisted the effort.

Norbert Potocki, Software Engineer @ Yahoo Inc.

Warm up: Why configuration management?

When working with large-scale software systems, configuration management becomes crucial; supporting non-uniform environments gets greatly simplified if you decouple code from configuration. While building complex software/products such as Flickr, we had to come up with a simple, yet powerful, way to manage configuration. Popular approaches to solving this problem include using configuration files or having a dedicated configuration service. Our new solution combines the extremely popular GitHub and cfg4j library, giving you a very flexible approach that will work with applications of any size.

Why should I decouple configuration from the code?

• Faster configuration changes (e.g. flipping feature toggles): Configuration can simply be injected without requiring parts of your code to be reloaded and re-executed. Config-only updates tend to be faster than code deployment.
• Different configuration for different environments: Running your app on a laptop or in a test environment requires a different set of settings than production instance.
• Keeping credentials private: If you don’t have a dedicated credential store, it may be convenient to keep credentials as part of configuration. They usually aren’t supposed to be “public,” but the code still may be. Be a good sport and don’t keep credentials in a public GitHub repo. :)

Meet the Gang: Overview of configuration management players

Let’s see what configuration-specific components we’ll be working with today:

Configuration repository and editor: Where your configuration lives. We’re using Git for storing configuration files and GitHub as an ad hoc editor.

Push cache : Intermediary store that we use to improve fetch speed and to ease load on GitHub servers.

CD pipeline: Continuous deployment pipeline pushing changes from repository to push cache, and validating config correctness.

Configuration library: Fetches configs from push cache and exposing them to your business logic.

Bootstrap configuration : Initial configuration specifying where your push cache is (so that library knows where to get configuration from).

All these players work as a team to provide an end-to-end configuration management solution.

The Coach: Configuration repository and editor

The first thing you might expect from the configuration repository and editor is ease of use. Let’s enumerate what that means:

• Configuration should be easy to read and write.
• It should be straightforward to add a new configuration set.
• You most certainly want to be able to review changes if your team is bigger than one person.
• It’s nice to see a history of changes, especially when you’re trying to fix a bug in the middle of the night.
• Support from popular IDEs – freedom of choice is priceless.
• Multi-tenancy support (optional) is often pragmatic.

So what options are out there that may satisfy those requirements? The three very popular formats for storing configuration are YAML, Java Property files, and XML files. We use YAML – it is widely supported by multiple programming languages and IDEs, and it’s very readable and easy to understand, even by a non-engineer.

We could use a dedicated configuration store; however, the great thing about files is that they can be easily versioned by version control tools like Git, which we decided to use as it’s widely known and proven.

Git provides us with a history of changes and an easy way to branch off configuration. It also has great support in the form of GitHub which we use both as an editor (built-in support for YAML files) and collaboration tool (pull requests, forks, review tool). Both are nicely glued together by following the Git flow branching model. Here’s an example of a configuration file that we use:

One of the goals was to make managing multiple configuration sets (execution environments) a breeze. We need the ability to add and remove environments quickly. If you look at the screenshot below, you’ll notice a “prod-us-east” directory in the path. For every environment, we store a separate directory with config files in Git. All of them have the exact same structure and only differ in YAML file contents.

This solution makes working with environments simple and comes in very handy during local development or new production fleet rollout (see use cases at the end of this article). Here’s a sample config repo for a project that has only one “feature”:

Some of the products that we work with at Yahoo have a very granular architecture with hundreds of micro-services working together. For scenarios like this, it’s convenient to store configurations for all services in a single repository. It greatly reduces the overhead of maintaining multiple repositories. We support this use case by having multiple top-level directories, each holding configurations for one service only.

The sprinter: Push cache

The main role of push cache is to decrease the load put on the GitHub server and improve configuration fetch time. Since speed is the only concern here, we decided to keep the push cache simple: it’s just a key-value store. Consul was our choice, in part because it’s fully distributed.

You can install Consul clients on the edge nodes and they will keep being synchronized across the fleet. This greatly improves both the reliability and the performance of the system. If performance is not a concern, any key-value store will do. You can skip using push cache altogether and connect directly to Github, which comes in handy during development (see use cases to learn more about this).

The Manager: CD Pipeline

When the configuration repository is updated, a CD pipeline kicks in. This fetches configuration, converts it into a more optimized format, and pushes it to cache. Additionally, the CD pipeline validates the configuration (once at pull-request stage and again after being merged to master) and controls multi-phase deployment by deploying config change to only 20% of production hosts at one time.

The Mascot: Bootstrap configuration

Before we can connect to the push cache to fetch configuration, we need to know where it is. That’s where bootstrap configuration comes into play. It’s very simple. The config contains the hostname, port to connect to, and the name of the environment to use. You need to put this config with your code or as part of the CD pipeline. This simple yaml file binding Spring profiles to different Consul hosts suffices for our needs:

The Cool Guy: Configuration library

The configuration library takes care of fetching the configuration from push cache and exposing it to your business logic. We use the library called cfg4j (“configuration for java”). This library re-loads configurations from the push cache every few seconds and injects them into configuration objects that our code uses. It also takes care of local caching, merging properties from different repositories, and falling back to user-provided defaults when necessary (read more at http://www.cfg4j.org/).

Briefly summarizing how we use cfg4j’s features:

• Configuration auto-reloading: Each service reloads configuration every ~30 seconds and auto re-configures itself.
• Multi-environment support: for our multiple environments (beta, performance, canary, production-us-west, production-us-east, etc.).
• Local caching: Remedies service interruption when the push cache or configuration repository is down and also improves the performance for obtaining configs.
• Fallback and merge strategies: Simplifies local development and provides support for multiple configuration repositories.
• Integration with Dependency Injection containers – because we love DI!

If you want to play with this library yourself, there’s plenty of examples both in its documentation and cfg4j-sample-apps Github repository.

The Heavy Lifter: Configurable code

The most important piece is business logic. To best make use of a configuration service, the business logic has to be able to re-configure itself in runtime. Here are a few rules of thumb and code samples:

• Use dependency injection for injecting configuration. This is how we do it using Spring Framework (see the bootstrap configuration above for host/port values):

• Use configuration objects to inject configuration instead of providing configuration directly – here’s where the difference is:

Direct configuration injection (won’t reload as config changes)

Configuration injection via “interface binding” (will reload as config changes):

The exercise: Common use-cases (applying our simple solution)

Configuration during development (local overrides)

When you develop a feature, a main concern is the ability to evolve your code quickly.  A full configuration-management pipeline is not conducive to this. We use the following approaches when doing local development:

• Add a temporary configuration file to the project and use cfg4j’s MergeConfigurationSource for reading config both from the configuration store and your file. By making your local file a primary configuration source, you provide an override mechanism. If the property is found in your file, it will be used. If not, cfg4j will fall back to using values from configuration store. Here’s an example (reference examples above to get a complete code):

• Fork the configuration repository, make changes to the fork and use cfg4j’s GitConfigurationSource to access it directly (no push
  cache required):

• Set up your private push cache, point your service to the cache, and edit values in it directly.

Configuration defaults

When you work with multiple environments, some of them may share a configuration. That’s when using configuration defaults may be convenient. You can do this by creating a “default” environment and using cfg4j’s MergeConfigurationSource for reading config first from the original environment and then (as a fallback) from “default” environment.

Dealing with outages

Configuration repository, push cache, and configuration CD pipeline can experience outages. To minimize the impact of such events, it’s good practice to cache configuration locally (in-memory) after each fetch. cfg4j does that automatically.

Responding to incidents – ultra fast configuration updates (skipping configuration CD pipeline)

Tests can’t always detect all problems. Bugs leak to the production environment and at times it’s important to make a config change as fast as possible to stop the fire. If you’re using push cache, the fastest way to modify config values is to make changes directly within the cache. Consul offers a rich REST API and web ui for updating configuration in the key-value store.

Keeping code and configuration in sync

Verifying that code and configuration are kept in sync happens at the configuration CD pipeline level. One part of the continuous deployment process deploys the code into a temporary execution environment, and points it to the branch that contains the configuration changes. Once the service is up, we execute a batch of functional tests to verify configuration correctness.

The cool down: Summary

The presented solution is the result of work that we put into building huge-scale photo-serving services. We needed a simple, yet flexible, configuration management system. Combining Git, Github, Consul and cfg4j provided a very satisfactory solution that we encourage you to try.

I want to thank the following people for reviewing this article: Bhautik Joshi, Elanna Belanger, Archie Russell.

PS. You can also follow me on Twitter, GitHub, LinkedIn or my private blog.

In the last couple of months, Apple has released new features as part of iOS 9 that allow a deeper integration between apps and the operating system. Among those features are Spotlight Search integration, Universal Links, and 3D Touch for iPhone 6S and iPhone 6S Plus.

Here at Flickr, we have added support for these new features and we have learned a few lessons that we would love to share.

Spotlight Search

There are two different kinds of content that can be searched through Spotlight: the kind that you explicitly index, and the kind that gets indexed based on the state your app is in. To explicitly index content, you use Core Spotlight, which lets you index multiple items at once. To index content related to your app’s current state, you use NSUserActivity: when a piece of content becomes visible, you start an activity to make iOS aware of this fact. iOS can then determine which pieces of content are more frequently visited, and thus more relevant to the user. NSUserActivity also allows us to mark certain items as public, which means that they might be shown to other iOS users as well.

For a better user experience, we index as much useful information as we can right off the bat. We prefetch all the user’s albums, groups, and people they follow, and add them to the search index using Core Spotlight. Indexing an item looks like this:

// Create the attribute set, which encapsulates the metadata of the item we're indexing
CSSearchableItemAttributeSet *attributeSet = [[CSSearchableItemAttributeSet alloc] initWithItemContentType:(NSString *)kUTTypeImage];
attributeSet.title = photo.title;
attributeSet.contentDescription = photo.searchableDescription;
attributeSet.keywords = photo.keywords;
attributeSet.thumbnailData = UIImageJPEGRepresentation(photo.thumbnail, 0.98);

// Create the searchable item and index it.
CSSearchableItem *searchableItem = [[CSSearchableItem alloc] initWithUniqueIdentifier:[NSString stringWithFormat:@"%@/%@", photo.identifier, photo.searchContentType] domainIdentifier:@"FLKCurrentUserSearchDomain" attributeSet:attributeSet];
[[CSSearchableIndex defaultSearchableIndex] indexSearchableItems:@[ searchableItem ] completionHandler:^(NSError * _Nullable error) {
                       if (error) {
                           // Handle failures.
                       }
              }];

Since we have multiple kinds of data – photos, albums, and groups – we had to create an identifier that is a combination of its type and its actual model ID.

Many users will have a large amount of data to be fetched, so it’s important that we take measures to make sure that the app still performs well. Since searching is unlikely to happen right after the user opens the app (that’s when we start prefetching this data, if needed), all this work is performed by a low-priority NSOperationQueue. If we ever need to fetch images to be used as thumbnails, we request it with low-priority NSURLSessionDownloadTask. These kinds of measures ensure that we don’t affect the performance of any operation or network request triggered by user actions, such as fetching new images and pages when scrolling through content.

Flickr provides a huge amount of public content, including many amazing photos. If anybody searches for “Northern Lights” in Spotlight, shouldn’t we show them our best Aurora Borealis photos? For this public content – photos, public groups, tags and so on – we leverage NSUserActivity, with its new search APIs, to make it all searchable when viewed. Here’s an example:

CSSearchableItemAttributeSet *attributeSet = [[CSSearchableItemAttributeSet alloc] initWithItemContentType:(NSString *) kUTTypeImage];
// Setup attributeSet the same way we did before...
// Set the related unique identifier, so it matches to any existing item indexed with Core Spotlight.     
attributeSet.relatedUniqueIdentifier = [NSString stringWithFormat:@"%@/%@", photo.identifier, photo.searchContentType];
        
self.userActivity = [[NSUserActivity alloc] initWithActivityType:@"FLKSearchableUserActivityType"];
self.userActivity.title = photo.title;
self.userActivity.keywords = [NSSet setWithArray:photo.keywords];
self.userActivity.webpageURL = photo.photoPageURL;
self.userActivity.contentAttributeSet = attributeSet;
self.userActivity.eligibleForSearch = YES;
self.userActivity.eligibleForPublicIndexing = photo.isPublic;
self.userActivity.requiredUserInfoKeys = [NSSet setWithArray:self.userActivity.userInfo.allKeys];
        
[self.userActivity becomeCurrent];

Every time a user opens a photo, public group, location page, etc., we create a new NSUserActivity and make it current. The more often a specific activity is made current, the more relevant iOS considers it. In fact, the more often an activity is made current by any number of different users, the more relevant Apple considers it globally, and the more likely it will show up for other iOS users as well (provided it’s public).

Until now we’ve only seen half the picture. We’ve seen how to index things for Spotlight search; when a user finally does search and taps on a result, how do we take them to the right place in our app? We’ll get to this a bit later, but for now suffice it to say that you’ll get a call to the method application:continueUserActivity:restorationHandler: to our application delegate.

It’s important to note that if we wanted to make use of the userInfo in the NSUserActivity, iOS won’t give it back to you for free in this method. To get it, we have to make sure that we assigned an NSSet to the requiredUserInfoKeys property of our NSUserActivity when we created it. In their documentation, Apple also tells us that if you set the webpageURL property when eligibleForSearch is YES, you need to make sure that you’re pointing to the right web URL corresponding to your content, otherwise you might end up with duplicate results in Spotlight (Apple crawls your site for content to surface in Spotlight, and if it finds the same content at a different URL it’ll think it’s a different piece of content).

Universal Links

In order to support Universal Links, Apple requires that every domain supported by the app host an “apple-app-site-association” file at its root. This is a JSON file that describes which relative paths in your domains can be handled by the app. When a user taps a link from another app in iOS, if your app is able to handle that domain for a specific path, it will open your app and call application:continueUserActivity:restorationHandler:. Otherwise your application won’t be opened – Safari will handle the URL instead.

{
    "applinks": {
        "apps": [],
        "details": {
            "XXXXXXXXXX.com.some.flickr.domain": {
                "paths": [
                    "/",
                    "/photos/*",
                    "/people/*",
                    "/groups/*"
                ]
            }
        }
    }
}

This file has to be hosted on HTTPS with a valid certificate. Its MIME type needs to be “application/pkcs7-mime.” No redirects are allowed when requesting the file. If the only intent is to support Universal Links, no further steps are required. But if you’re also using this file to support Handoffs (introduced in iOS 8), then your file has to be CMS signed by a valid TLS certificate.

In Flickr, we have a few different domains. That means that each one of flickr.com, http://www.flickr.com, m.flickr.com and flic.kr must provide its own JSON association file, whether or not they differ. In our case, the flic.kr domain actually does support different paths, since it’s only used for short URLs; hence, its “apple-app-site-association” is different than the others.

On the client side, only a few steps are required to support Universal Links. First, “Associated Domains” must be enabled under the Capabilities tab of the app’s target settings. For each supported domain, an entry “applinks:” entry must be added. Here is how it looks for Flickr:

That is it. Now if someone receives a text message with a Flickr link, she will jump right to the Flickr app when she taps on it.

Deep linking into the app

Great! We have Flickr photos showing up as search results and Flickr URLs opening directly in our app. Now we just have to get the user to the proper place within the app. There are different entry points into our app, and we need to make the implementation consistent and avoid code duplication.

iOS has been supporting deep linking for a while already and so has Flickr. To support deep linking, apps could register to handle custom URLs (meaning a custom scheme, such as myscheme://mydata/123). The website corresponding to the app could then publish links directly to the app. For every custom URL published on the Flickr website, our app translates it into a representation of the data to be shown. This representation looks like this:

@interface FLKRoute : NSObject

@property (nonatomic) FLKRouteType type;
@property (nonatomic, copy) NSString *identifier;

@end

It describes the type of data to present, and a unique identifier for that type of data.

- (void)navigateToRoute:(FLKRoute *)route
{
    switch (route.type) {
        case FLKRouteTypePhoto:
            // Navigate to photo screen
            break;
        case FLKRouteTypeAlbum:
           // Navigate to album screen
            break;
        case FLKRouteTypeGroup:
            // Navigate to group screen
            break;
        // ...
        default:
            break;
    }
}

Now, all we have to do is to make sure we are able to translate both NSURLs and NSUserActivity objects into FLKRoute instances. For NSURLs, this translation is straightforward. Our custom URLs follow the same pattern as the corresponding website URLs; their paths correspond exactly. So translating both website URLs and custom URLs is a matter of using NSURLComponents to extract the necessary information to create the FLKRoute object.

As for NSUserActivity objects passed into application:continueUserActivity:restorationHandler:, there are two cases. One arises when the NSUserActivity instance was used to index a public item in the app. Remember that when we created the NSUserActivity object we also assigned its webpageURL? This is really handy because it not only uniquely identifies the data we want to present, but also gives us a NSURL object, which we can handle the same way we handle deep links or Universal Links.

The other case is when the NSUserActivity originated from a CSSearchableItem; we have some more work to do in this case. We need to parse the identifier we created for the item and translate it into a FLKRoute. Remember that our item’s identifier is a combination of its type and the model ID. We can decompose it and then create our route object. Its simplified implementation looks like this:

FLKRoute * FLKRouteFromSearchableItemIdentifier(NSString *searchableItemIdentifier)
{
    NSArray *routeComponents = [searchableItemIdentifier componentsSeparatedByString:@"/"];
    if ([routeComponents count] != 2) { // type + id
        return nil;
    }
    
    // Handle the route type
    NSString *searchableItemContentType = [routeComponents firstObject];
    FLKRouteType type = FLKRouteTypeFromSearchableItemContentType(searchableItemContentType);
    
    // Get the item identifier
    NSString *itemIdentifier = [routeComponents lastObject];
    
    // Build the route object
    FLKRoute *route = [FLKRoute new];
    route.type = type;
    route.parameter = itemIdentifier;
    
    return route;
}

Now we have all our bases covered and we’re sure that we’ll drop the user in the right place when she lands in our app. The final application delegate method looks like this:

- (BOOL)application:(nonnull UIApplication *)application continueUserActivity:(nonnull NSUserActivity *)userActivity restorationHandler:(nonnull void (^)(NSArray * __nullable))restorationHandler
{
    FLKRoute *route;
    NSString *activityType = [userActivity activityType];
    NSURL *url;
    
    if ([activityType isEqualToString:CSSearchableItemActionType]) {
        // Searchable item from Core Spotlight
        NSString *itemIdentifier = [userActivity.userInfo objectForKey:CSSearchableItemActivityIdentifier];
        route = FLKRouteFromSearchableItemIdentifier(itemIdentifier);
        
    } else if ([activityType isEqualToString:@"FLKSearchableUserActivityType"] ||
               [activityType isEqualToString:NSUserActivityTypeBrowsingWeb]) {
        // Searchable item from NSUserActivity or Universal Link
        url = userActivity.webpageURL;
        route = [url flk_route];
        
    }
    
    if (route) {
        [self.router navigateToRoute:route];
        return YES;
    } else if (url) {
        [[UIApplication sharedApplication] openURL:url]; // Fail gracefully
        return YES;
    } else {
        return NO;
    }
}

3D Touch

With the release of iPhone 6S and iPhone 6S Plus, Apple introduced a new gesture that can be used with your iOS app: 3D Touch. One of the coolest features it has brought is the ability to preview content before pushing it onto the navigation stack. This is also known as “peek and pop.”

You can easily see how this feature is implemented in the native Mail app. But you won’t always have a simple UIView hierarchy like Mail’s UITableView, where a tap anywhere on a cell opens a UIViewController. Take Flickr’s notifications screen, for example:

If the user taps on a photo in one of these cells, it will open the photo view. But if the user taps on another user’s name, it will open that user’s profile view. Previews of these UIViewControllers should be shown accordingly. But the “peek and pop” mechanism requires you to register a delegate on your UIViewController with registerForPreviewingWithDelegate:sourceView:, which means that you’re working in a much higher layer. Your UIViewController’s view might not even know about its subviews’ structures.

To solve this problem, we used UIView’s method hitTest:withEvent:. As the documentation describes, it will give us the “farthest descendant of the receiver in the view hierarchy.” But not every hitTest will necessarily return the UIView that we want. So we defined a protocol, FLKPeekAndPopTargetView, that must be implemented by any UIView subclass that wants to support peeking and popping from it. That view is then responsible for returning the model used to populate the UIViewController that the user is trying to preview. If the view doesn’t implement this protocol, we query its superview. We keep checking for it until a UIView is found or there aren’t any more superviews available. This is how this logic looks:

+ (id)modelAtLocation:(CGPoint)location inSourceView:(UIView*)sourceView
    
    // Walk up hit-test tree until we find a peek-pop target.
    UIView *testView = [sourceView hitTest:location withEvent:nil];
    id model = nil;
    while(testView && !model) {
      
        // Check if the current testView conforms to the protocol.
        if([testView conformsToProtocol:@protocol(FLKPeekAndPopTargetView)]) {
            
            // Translate location to view coordinates.
            CGPoint locationInView = [testView convertPoint:location fromView:sourceView];
            
            // Get model from peek and pop target.
            model = [((id<FLKPeekAndPopTargetView>)testView) flk_peekAndPopModelAtLocation:locationInView];
            
        } else {
            //Move up view tree to next view
            testView = testView.superview;
        }
    }
    
    return model;
}

With this code in place, all we have to do is to implement UIViewControllerPreviewingDelegate methods in our delegate, perform the hitTest and take the model out of the FLKPeekAndPopTargetView‘s implementor. Here’s is the final implementation:

- (UIViewController *)previewingContext:(id<UIViewControllerPreviewing>)previewingContext
              viewControllerForLocation:(CGPoint)location {
    
    id model = [[self class] modelAtLocation:location inSourceView:previewingContext.sourceView];
    UIViewController *viewController = nil;
    if ([model isKindOfClass:[FLKPhoto class]]) {
        viewController = // ... UIViewController that displays a photo.
    } else if ([model isKindOfClass:[FLKAlbum class]]) {
        viewController = // ... UIViewController that displays an album.
    } else if ([model isKindOfClass:[FLKGroup class]]) {
        viewController = // ... UIViewController that displays a group.
    } // ...
    return viewController;
    
}

- (void)previewingContext:(id<UIViewControllerPreviewing>)previewingContext
     commitViewController:(UIViewController *)viewControllerToCommit {
    
    [self.navigationController pushViewController:viewControllerToCommit animated:YES];
    
}

Last but not least, we added support for Quick Actions. Now the user has the ability to quickly jump into a specific section of the app just by pressing down on the app icon. Defining these Quick Actions statically in the Info.plist file is an easy way to implement this feature, but we decided to go one step further and define these options dynamically. One of the options we provide is “Upload Photo,” which takes the user to the asset picker screen. But if the user has Auto Uploadr turned on, this option isn’t that relevant, so instead we provide a different app icon menu option in its place.

Here’s how you can create Quick Actions:

NSMutableArray<UIApplicationShortcutItem *> *items = [NSMutableArray array];
    
[items addObject:[[UIApplicationShortcutItem alloc] initWithType:@"FLKShortcutItemFeed"
                                                  localizedTitle:NSLocalizedString(@"Feed", nil)]];
    
[items addObject:[[UIApplicationShortcutItem alloc] initWithType:@"FLKShortcutItemTakePhoto"
                                                  localizedTitle:NSLocalizedString(@"Upload Photo", nil)] ];

[items addObject:[[UIApplicationShortcutItem alloc] initWithType:@"FLKShortcutItemNotifications"
                                                  localizedTitle:NSLocalizedString(@"Notifications", nil)]];
    
[items addObject:[[UIApplicationShortcutItem alloc] initWithType:@"FLKShortcutItemSearch"
                                                  localizedTitle:NSLocalizedString(@"Search", nil)]];
    
[[UIApplication sharedApplication] setShortcutItems:items];

And this is how it looks like when the user presses down on the app icon:

Finally, we have to handle where to take the user after she selects one of these options. This is yet another place where we can make use of our FLKRoute object. To handle the app opening from a Quick Action, we need to implement application:performActionForShortcutItem:completionHandler: in the app delegate.

- (void)application:(UIApplication *)application performActionForShortcutItem:(UIApplicationShortcutItem *)shortcutItem completionHandler:(void (^)(BOOL))completionHandler {
    FLKRoute *route = [shortcutItem flk_route];
     [self.router navigateToRoute:route];
    completionHandler(YES);
}

Conclusion

There is a lot more to consider when shipping these features with an app. For example, with Flickr, there are various platforms the user could be using. It is important to make sure that the Spotlight index is up to date to reflect changes made anywhere. If the user has created a new album and/or left a group from his desktop browser, we need to make sure that those changes are reflected in the app, so the newly-created album can be found through Spotlight, but the newly-departed group cannot.

All of this work should be totally opaque to the user, without hogging the device’s resources and deteriorating the user experience overall. That requires some considerations around threading and network priorities. Network requests for UI-relevant data should not be blocked because we have other network requests happening at the same time. With some careful prioritizing, using NSOperationQueue and NSURLSession, we managed to accomplish this with no major problems.

Finally, we had to consider privacy, one of the pillars of Flickr. We had to be extremely careful not to violate any of the user’s settings. We’re careful to never publicly index private content, such as photos and albums. Also, photos marked “restricted” are not publicly indexed since they might expose content that some users might consider offensive.

In this blog post we went into the basics of integrating iOS 9 Search, Universal Links, and 3D Touch in Flickr for iOS. In order to focus on those features, we simplified some of our examples to demonstrate how you could get started with them in your own app, and to show what challenges we faced.

Archie Russell, Peter Norby, Saeideh Bakhshi

At Flickr our users really care about image quality.  They also care a lot about how responsive our apps are.  Addressing both of these concerns simultaneously is challenging;  higher quality images have larger file sizes and are slower to transfer.   Slow transfers are especially noticeable on mobile devices.   Flickr had historically aimed for high quality at the expense of larger files, but in late 2014 we implemented a method to both maintain image quality and decrease the byte-size of the images we serve to users.   As image appearance is very important to our users,  we performed an extensive user test before rolling this change out.   Here’s how we did it.

Background:  JPEG Quality Settings

Fig 1.    JPEG settings vs file size for a test image.

JPEG compression has several tuneable knobs.   The q-value is the best known of these; it adjusts the level of spatial detail stored for fine details;  a higher q-value typically keeps more detail.    However,  as q-value gets very close to 100,  file size increases dramatically,  usually without improving image appearance.

If file size and app performance isn’t an issue,  dialing up q-value is an easy way to get really nice-looking images; this is what Flickr has done in the past.    And if appearance isn’t very important,  dialing down q-value is a viable option.    But if you want both,  you’re kind of stuck.   Additionally,  q-value isn’t one-size-fits-all,  some images look great at q-value 80 while others don’t.

Another commonly adjusted setting is chroma-subsampling,  which alters the amount of color information stored in a JPEG file.    With a setting of 4:4:4,  the two chroma (color) channels in a JPG have as much information as the luminance channel.   In an image with a setting of 4:2:0, each chroma channel has only a quarter as much information as in an a 4:4:4 image.

q=96,  chroma=4:4:4 (125KB)	q=70, chroma=4:4:4 (67KB)
q=96, chroma=4:2:0 (62KB)	q=70, chroma=4:2:0 (62KB)

Table 1:   JPEG stored at different quality and chroma levels.   The upper left image is saved at high quality and chroma level; notice the color and detail in the folds of the red flag.   The lower right image has the lowest quality;  notice artifacts along the right edges of the red flag.

Perceptual JPEG Compression

Ideally we’d have an algorithm which automatically tuned all JPEG parameters to make a file smaller, but which would limit perceptible changes to the image.  Technology exists that attempts to do this and can decrease image file size by 30-50%. This compression ratio is highly dependent on image content and dimensions.

compressed: 112KB	non-compressed: 224KB

Fig 2. Compressed cropped JPEG is 50% smaller than not-compressed cropped JPEG, above, with no obvious defects.  Compression ratio is similar for a compressed 2048-pixel wide JPEG (475KB) of the entire scene and its corresponding not-compressed JPEG (897KB). 

We were pleased with perceptually compressed images in non-structured examinations.  The compressed images were smaller and nearly indistinguishable from their sources.   But we wanted to really quantify how well the technology worked before considering incorporating it into Flickr.  The standard computational tools for evaluating compression, such as SSIM, are fairly simplistic and don’t do a great job at modeling how a user sees things.  To really evaluate this technology had to use a better measure of perceptibility:  human minds.

To test whether our image compression would impact user perception of image quality, we put together a “taste test.”  The taste test is constructed as a game with multiple rounds where users look at both compressed and uncompressed images.  Users accumulate points the longer they play, and get more points for doing well at the game.  We maintained a leaderboard to encourage participation and used only internal testers.The game’s test images came from a diverse collection of 250 images contributed by Flickr staff.  The images came from a variety of cameras and included a number of subjects from photographers with varying skill levels.

Fig 3. A sampling of images used in our taste test.

In each round, our test code randomly select a test image, and present two variants of this image side by side.  50% of the time we present the user two identical images; the rest of the time we present one compressed image and one uncompressed image.  We ask the tester if the two images look the same or different and we’d expect a user choosing randomly OR a user unable to distinguish the two cases would answer correctly about half the time.  We randomly swap the location of the compressed images to compensate for user bias to the left or the right.  If testers choose correctly, they are presented with a second question: “Which image did you prefer, and why?”

Fig 4. Screenshot of taste test.

Our test displays images simultaneously to prevent testers noticing a longer load time for the larger, non-compressed image.  The images are presented with either 320, 640, or 1600 pixels on their longest side.  The 320 & 640px images are shown for 12 seconds before being dimmed out.  The intent behind this detail is to represent how real users interact with our images.  The 1600px images stay on screen for 20 seconds, as we expect larger images to be viewed for longer periods of time by real users.   We award 100 points per round, regardless of whether a tester chose correctly and also award a bonus of 400 points when a tester correctly identifies whether images were identical or different.  We update the tester’s score every five tests so that the user perceives an increasing score without being rewarded immediately for any particular behavior.

Taste Test Outcome and Deployment

We ran our taste test for two weeks and analyzed our results.    Although we let users play as long as they liked,  we skipped the first result per user as a “warm-up” and considered only the subsequent ten results,  this limited the potential for users training themselves to spot compression artifacts.   We disregarded users that had fewer than eleven results.

Table 2.   Taste test results.   Testers select “identical” at nearly the same rate, whether the input is identical or not.

When our testers were presented with two identical images, they thought the images were identical only 68.8% of the time(!), and when presented with a compressed image next to a non-compressed image,  our testers thought the images were identical slightly less often:  67.6% of the time.  This difference was small enough for us,  and our statisticians told us it was statistically insignificant.  Our image pairs were so similar that multiple testers thought all images were identical and reported that the test system was buggy. We inspected the images most often labeled different, and found no significant artifacts in the compressed versions.

So even in this side-by-side test,  perceptual image compression is just barely noticeable when images are presented side-by-side.  As the Flickr website wouldn’t ever show compressed and uncompressed images at the same time, and the use of compression had large benefits in storage footprint and site performance, we elected to go forward.

At the beginning of 2014 we silently rolled out perceptual-based compression on our image thumbnails (we don’t alter the “original” images uploaded by our users).  The slight changes to image appearance went unnoticed by users, but user interactions with Flickr became much faster,  especially for users with slow connections, while our storage footprint became much smaller.  This was a best-case scenario for us.

Evaluating perceptual compression was a considerable task,  but it gave the confidence we needed to apply this compression in production to our users.    This marked the first time Flickr had adjusted image settings in years, and, it was fun.

Fig 5.  Taste test high score list

Epilogue

After eighteen months of perceptual compression at Flickr,  we adjusted our settings slightly to shrink images an additional 15%.   For our users on mobile devices,  15% fewer bytes per image makes for a much more responsive experience.We had run a taste test on this newer setting and users were were able to spot our compression slightly more often than with our original settings.   When presented a pair of identical images, our testers declared these images identical 65.2% of the time,  when presented with different images,  of our testers declared the images identical 62% of the time.   It wasn’t as imperceptible as our original approach, but, we decided it was close enough to roll out.

Boy were we wrong!   A few very vocal users spotted the compression and didn’t like it at all.    The Flickr Help Forum had a very lively thread which Petapixel picked up.  We beat our heads against the wall considered our options and came up with a middle path between our initial and follow-on approaches,  giving us smaller, faster-to-load files while still maintaining the appearance our users expect.

Through our use of perceptual compression,  combined with our use of on-the-fly resize and COS,  we’ve been able to decrease our storage footprint dramatically, while simultaneously improving user experience. It’s a win all around but we’re not done yet — we still have a few tricks up our sleeves.