I'm trying to write a iOS note taking app that is blazingly fast for a large number of notes and that syncs without ever blocking the UI. (Don't worry, it's just a learning project, I know there are a billion note apps for iOS). I have decided to use Core Data (mostly because of the excellent posts by Brent Simmons about Vesper). I know UIManagedDocument can do async reads and writes and has a lot of functionality built in, so I'm wondering if there is any information on which would be faster for a fairly simple notes app. I can't really find a lot of information about people using UIManagedDocuments for anything other than a centralized, basically singleton, persistent store. Is it suitable for 1000s of documents? Would it be faster or slower than just a database of NSManagedObjects? It seems like most information I can find about Core Data is oriented towards people using NSManagedObject, so any information about UIManagedDocuments being used in production apps would be really helpful. At this point, the only thing I can think of is to just write the whole app both ways, load 10,000 notes into it, and see what happens.
Update
To clarify, I'm not learning iOS development and Objective-C, the "learning project" mostly means that I've never used Core Data and would like to learn how to write a really performant Core Data application.
UIManagedDocument is designed/intended for document based applications. One UIManagedDocument instance per document. If you are not building a document based application then you should not be using UIManagedDocument.
Everything that people like about UIManagedDocument can be accomplished with very little effort using the Core Data stack directly. UIManagedDocument abstracts you away from what your persistence layer is doing. Something you really do not want.
If you want a high performance Core Data application you do not want to be using UIManagedDocument. You will run into issues with it. It will do things at random times and cause performance issues.
You are far, far, better off learning the framework properly.
In the case of Vesper, those are not documents; they are too small. Think of documents as Word files, or Excel files. Large complicated data structures that are 100% isolated from each other.
Also, whether you use a UIManagedDocument or not, you will be using NSManagedObject instances. NSManagedObject, NSManagedObjectContext, NSPersistentStoreCoordinator are all foundational objects in Core Data. UIManagedDocument is just an abstraction layer on top.
Finally, Core Data is not a database. Thinking of it that way will get you into a jam. Core Data is an object model that can persist to disk and one of the persistence formats happens to be SQLite.
Update (Running Into Problems)
UIManagedDocument is an abstraction on top of Core Data. To use UIManagedDocument you actually need to learn more about Core Data than if you just used the primary Core Data stack.
UIManagedDocument uses a parent/child context internally. Don't understand that yet? See the point above. What it also means is that your requests for it to save are "taken under advisement" as opposed to being saved right then and there. This can lead to unexpected results if you don't understand the point of it or don't want it to save when it feels like it.
UIManagedDocument uses asynchronous saves and at most you can request that it save. Doesn't mean it is going to save now, nor does it mean you can easily stop and wait for the save to complete. You need to trust that it will complete it. In addition, it may decide to save at an inopportune moment.
When you start looking for performance gains with Core Data you tend to want to build the stack in a very specific way to maximize the benefit to your application. That is application dependent and with the abstracts in UIManagedDocument you get limited very quickly.
Even in a situation where I was building a document based application I would still not use UIManagedDocument. Just to much behind the curtain.
Performance is likely to come down to how the rest of your code is implemented, and not necessarily the difference between a UIManagedDocument or a NSManagedObject.
Think of UIManagedDocument as a specific niche implementation of CoreData, that already has the parts of CoreData you want baked into it's structure to save you (the developer) a little bit of time writing code. It's been purpose built for handling UIDocuments, and multi-threading.
Under the hood, it's likely that UIManagedDocument is using CoreData as well as you would (assuming you know what you're doing), but it's theoretically possible you could cut some corners by knowing the exact and inane details of your implementation.
If you're new to CoreData, or GCD and NSOperationQueue, then you'll likely save a ton of developer time by leveraging UIManagedDocument instead of rolling your own.
A very apt analogy would be to using a NSFetchedResultsController to run your UITableView, rather then rolling your own CoreData and UITableView implementation.
If you're new to objective-C, I'd recommend you bang out something functional with UIManagedDocument at first. Later you get lost in the weeds of dispatch_asynch() and NSFetchRequest, and pick up a few milliseconds of performance here and there.
Cheers!
Just store a plain text file on the disk for each note.
Keep a separate database (using sqlite or perhaps just -[NSDictionary writeToFile:atomically]) to store metadata, such as the last user modification date and the last server sync date for each note.
Periodically talk to the server, asking it for a list of stuff that has changed since the last time you did a sync, and sending any data on your end that has changed in the same time period.
This should be perfectly fast as long as you have less than a million note files. Do all network and filesystem operations on an NSOperationQueue.
Related
i am new to this Core data. When i search for tutorials, I'm seeing this sentence Core Data is not a database everywhere on the internet.
If it is not a database, why are we using it as a database?
For what purpose Core Data is initially created?
Is there any other way Core Data was used before/will be used in
future (Other than as a DB)?
Sorry for my English.
Thanks for the time.. (:
Actually Core Data is a bridge between the code and the underlying database (like SQLite or XML...). Core Data is not a database, but uses SQLite (or XML...) for persistence. The main purpose of Core Data is to manage memory objects and object graphs easily without having to do it manually through a SQLite library for instance. It is possible to use Core Data without persistence is you want (using In-Memory stores).
Here is the documentation : https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/index.html#//apple_ref/doc/uid/TP40001075-CH2-SW1
Bye!
If it is not a database, why are we using it as a database?
"We" are not necessarily doing so, depending on who you mean by "we". Core Data can be used in a database-like manner, keeping in mind the Core Data vs. SQL differences others have noted. But that's not the only possible use.
Statements that Core Data isn't a database are mostly intended to prevent people from thinking of Core Data in the same sense as SQL. That sort of thinking leads to badly designed data models, since the approach is different. It can be used as a database in the generic sense of the storing structured data but it's important to not assume that it works like other databases you may have used.
For what purpose Core Data is initially created?
Is there any other way Core Data was used before/will be used in future (Other than as a DB)?
Core Data was created to fill what might have been perceived as a missing piece in Apple's frameworks. They generally take the MVC approach. There were classes to help in designing and implementing views (in these pre-iOS days that meant AppKit) and with controller classes (for example NSArrayController, also an OS X class). But model support was limited to NSCoding, which required a lot of repetitive code. Custom solutions also had trouble scaling to large amounts of data-- for example NSCoding doesn't help you load only part of a large document graph, because it recursively works its way through the entire object hierarchy.
Core Data was added with the purpose of making it easier to design and implement the model layer of an app. It's no accident that the document you edit to design the data is called a data model and not a schema.
The idea was (and is) that you could
Design your data model visually in Xcode
Create and use instances of your model objects
Read and save those objects in a file
...all without ever needing to write your own code to figure out how to convert your model objects to and from something that could be written into a file, or with the mechanics of opening a file, reading/writing it, and saving it. You'd just create objects as needed and then call save on NSManagedObjectContext. The small bit of code that was concerned with locating and opening the file was all pretty much the same in any app, so while it was still required it was mostly no longer the app developer's concern (and in iOS 10, NSPersistentContainer eliminates even this). You'd also get the benefit of only needing to load the parts of your object graph that you currently needed instead of loading everything every time.
As you've noticed, in practice Core Data is commonly used more or less like a database, and it works for that. But it's not designed to be limited to such uses.
Yes it is true , Core data is not a Database, though internally it saves data using sqlite. We use Coredata for persistent data which means we should be able to save the data and use it even after closing and reopening the app. There are various ways to store data like Sqlite,Plist,UserDefaults and CoreData. Coredata does not maintain any relations like SQlite. It means there are no keys like primary and foreign etc. Coredata allows you to deal with data in Object Oriented Approach. You can easily work with data operations even you don't have knowledge about DB queries.
A few weeks ago, I decided to learn Core Data for my new project and apply it to my entire model. There was a steep learning curve, but eventually I got familiar with the stack and I'm now rather comfortable with at least the basic concepts and the few common pitfalls such as thread concurrency.
I have to say, the first few weeks after getting comfortable where pretty amazing. NSFetchedResultsController give you a good way to communicate between my model and my controllers. However the more I use Core Data, the more annoying it gets.
As a concrete example, my app fetches a few pieces of data from my server (the posts) which appear in a feed. Each post has an owner, of class User, which I also fetch from the server. Now, Core Data has been great for managing the realtionship between a post and a user. The relationship is updated automatically and getting the post's origin is as simple as calling post.owner. However, there are also inconveniences:
1.Core Data forces objects to the disk that I do not want forced to the disk. This is probably the main issue. With the posts, I do not want them to be forced to disk, and would rather make calls to the server again. Why? Because the more posts I store persistently, the more housekeeping there is to do. A post can be edited, deleted, flagged, etc... and keeping those posts locally means having to plan updates.
2.Having to constantly worry about concurrency of contexts, objects and the likes. I wrote an object factory that always returns objects on the right thread and the right context, but even then bugs occur here and there, which quickly becomes frustrating.
3.Decreased performance. Perhaps the least important one at this point, going from cached objects to Core Data has taken a (barely noticeable) toll on the performance of my application (most notably the feed).
So what are your recommendations regarding Core Data? Would you suggest a different approach to Core Data?
I was thinking of a hybrid caching + Core Data where I store the information I will actually use many times (such as users) persistently and then use the RAM for things like posts, or simply creating posts without an NSManagedContext. Input welcome!
Core Data forces objects to the disk that I do not want forced to the disk.
It does no such thing. If you don't want to save your Post objects to the persistent store, don't put them in Core Data and don't make them managed objects. Your User object can have a posts property even if the Post object is not managed by Core Data. Managed objects can have properties of any type, not only to other managed objects.
Having to constantly worry about concurrency of contexts, objects and the likes.
Concurrency is complex no matter how you model your data. It's a fundamentally complex problem. You're encountering it with Core Data because you're using Core Data. If you use something else, you'll deal with it there.
Decreased performance.
"Product" menu --> "Analyze" and run Instruments to find out why. There's no reason this should happen, and you have the tools to discover what's actually going on.
I am building a healthKit based app and am wondering how to best save healthKit data persistently.
My current approach is to get the data and save it as attributes of a custom class object and then save it in core data as NSData.
In terms of performance is Realm faster than CoreData?
According to http://qiita.com/moriyaman/items/1a2916f4c2b79e934370 CoreData is apparently slower than FMDB which is slower than Realm. Can someone confirm if this is true even after taking into account faults and indexes?
Disclaimer: I work at Realm
The question of which persistence product will perform best among the solutions you mentioned is highly dependent on the type/amount/frequency of your data. Since Core Data and FMDB are layers over SQLite, they can't be faster than SQLite by design, but they provide enough convenience to be worthwhile to many users. On the other hand, Realm isn't based on SQLite, but rather its own custom database engine that was designed specifically for modern smartphones. It was designed to strike a better balance between powerful features, simple API without adding a large performance hit.
You can see public benchmarks comparing Realm/SQLite/Core Data/FMDB in Realm's launch blog post here: http://realm.io/news/introducing-realm#fast
Finally, your approach of serializing HealthKit information into NSData using something like NSCoding is going to be terribly inefficient. No matter the persistence solution you choose, you'll be better served by using the serialization built in to those products rather than storing an already serialized data blob.
As I commented to #jpsim, it is difficult to simply compare the performance of Core Data to lower-level frameworks like FMDB, or differently-abstracted frameworks like Realm. Which approach you choose will dramatically impact how you build your program, which will tend to shuffle the performance problems around to different places.
Core Data and SQLite solve very different problems. SQLite is a relational database. Core Data is an object persistence engine. I'm not an expert on Realm, but it seems to be trying to strike a balance between those two approaches with more low-level control than Core Data affords, but a closer tie-in to the object model than SQLite. The fact that Realm (at least in my impressions of it) gives you more low-level control opens up opportunities for you to optimize things, or to mess things up IMO. That's neither good nor bad, it just makes it hard to apples-to-apples compare them, and particularly makes generic "performance benchmarks" problematic. The question isn't whether it's possible for someone to write faster code using engine A vs. engine B. The question is whether you will likely write acceptably performant code in each engine, while avoiding bugs, and minimizing development time.
In general, I believe HealthKit data is supposed to be stored in HealthKit in order to protect privacy. You should be careful about storing this data in your own storage anyway. Be particularly aware of the guidelines about iCloud:
Apps using the HealthKit framework that store users’ health information in iCloud will be rejected
I don't know how this will impact documents that you store and are then backed-up to iCloud. Just leaving the data in HealthKit is the best way to not have to worry about such problems.
In any case, though, performance is just one axis to consider. You didn't indicate anything to suggest that you have very special performance problems (for example, that you're handling tens of thousands of records, or real-time data, or something like that). So I would focus first on what tool meet your general needs best, and then do some basic experiments to make sure the performance is reasonable, and then optimize as you find issues.
I am trying to build an app for my website. I was under the impression that all I had to do was get and put json from the server to my phone. But then today I came across Core Data ( I am new to iOS).
So my question is, that to make a faster iOS app, is it a normal practice to fetch json data and save it as core data? Would apps like Facebook, Twitter follow this approach? or is just fetching and parsing json is a normal practice, and core data is not needed.
I am sorry if the question is dumb.
It is normal to retrieve data from a server (XML or JSON) and keep it in memory, if the memory foot print is reasonable. If you are talking about hundreds upon thousands of rows from a database, then persistent storage, with a dedicated data model(s) is probably the best choice; you can read it when needed.
If your needs are such that a complex data model is needed, one-to-many and/or many-to-many relationships, then consider Core Data (or SQLite directly).
You define your needs first, then try to define the data model that fits your needs (custom objects or maybe just a few instances of NSDictionary), then decide how that data needs to persist and how you plan on interacting with that data.
A few starting points:
Core Data Overview - Shoud help you decide if you should use it
RestKit - Just a suggestion
Tutorial on Data Persistence
Good luck.
I remember facing this anomaly not so long ago.
As already pointed out in some of the comments, it depends on your needs.
Not all data are eligible to be saved in core data after retrieving it from the web side. You might have integrity issues with that. To do the checks for large chunks of data might have even severe overheads. But if you feel that certain data are not likely to change very often then you can employ this technique for some portions.
If you decide to stick with Request/Fetch data, be sure you process the requests using NSOperation, GCD or NSThread, in order to avoid UI freezes.
Although, they are used for same purpose, they all have advantages and disadvantage, plz check out this topic on NSOperation vs Grand Central Dispatch
I hope this helps.
I'm developing an iOS application using Core Data. I want to have the persistent store located in a shared location, such as a network drive, so that multiple users can work on the data (at different times i.e. concurrency is not part of the question).
But I also want to offer the ability to work on the data "offline", i.e. by keeping a local persistent store on the iPad. So far, I read that I could do this to some degree by using the persistent store coordinator's migration function, but this seems to imply the old store is then invalidated. Furthermore, I don't necessarily want to move the complete store "offline", but just a part of it: going with the simple "company department" example that Apple offers, I want users to be able to check out one department, along with all the employees associated with that department (and all the attributes associated with each employee). Then, the users can work on the department data locally on their iPad and, some time later, synchronize those changes back to the server's persistent store.
So, what I need is to copy a core data object from one store to another, along with all objects referenced through relationships. And this copy process needs to also ensure that if an object already exists in the target persistent store, that it's overwritten rather than a new object added to the store (I am already giving each object a UID for another reason, so I might be able to re-use the UID).
From all I've seen so far, it looks like there is no simple way to synchronize or copy Core Data persistent stores, is that a fair assessment?
So would I really need to write a piece of code that does the following:
retrieve object "A" through a MOC
retrieve all objects, across all entities, that have a relationship to object "A"
instantiate a new MOC for the target persistent store
for each object retrieved, check the target store if the object exists
if the object exists, overwrite it with the attributes from the object retrieved in steps 1 & 2
if the object doesn't exist, create it and set all attributes as per object retrieved in steps 1 & 2
While it's not the most complicated thing in the world to do, I would've still thought that this requirement for "online / offline editing" is common enough for some standard functionality be available for synchronizing parts of persistent stores?
Your point of views greatly appreciated,
thanks,
da_h-man
I was just half-kidding with the comment above. You really are describing a pretty hard problem - it's very difficult to nail this sort of synchronization, and there's seldom, in any development environment, going to be a turn-key solution that will "just work". I think your pseudo-code description above is a pretty accurate description of what you'll need to do. Although some of the work of traversing the relationships and checking for existing objects can be generalized, you're talking about some potentially complicated exception handling situations - for example, if updating an object, and only 1 out 5 related objects is somehow out of date, do you throw away the update or apply part of it? You say "concurrency" is not a part of the question, but if multiple users can "check out" objects at the same time, unless you plan to have a locking mechanism on those, you would start having conflicts when trying to make updates.
Something to check into are the new features in Core Data for leveraging iCloud - I doubt that's going to help with your problem, but it's generally related.
Since you want to be out on the network with your data, another thing to consider is whether Core Data is the right fit to your problem in general. Since Core Data is very much a technology designed to support the UI and MVC pattern in general, if your data needs are not especially bound to the UI, you might consider another type of DB solution.
If you are in fact leveraging Core Data in significant ways beyond just modeling, in terms of driving your UI, and you want to stick with it, I think you are correct in your analysis: you're going to have to roll your own solution. I think it will be a non-trivial thing to build and test.
An option to consider is CouchDB and an iOS implementation called TouchDB. It would mean adopting more of a document-oriented (JSON) approach to your problem, which may in fact be suitable, based on what you've described.
From what I've seen so far, I reckon the best approach is RestKit. It offers a Core Data wrapper that uses JSON to move data between remote and local stores. I haven't fully tried it yet, but from what the documentation reads, it sounds quite powerful and ideally suited for my needs.
You definetly should check these things:
Parse.com - cloud based data store
PFIncrementalStore https://github.com/sbonami/PFIncrementalStore - subclass of NSIncrementalStore which allows your Persistent Store Coordinator to store data both locally and remotely (on Parse Cloud) at the same time
All this stuff are well-documented. Also Parse.com is going to release iOS local datastore SDK http://blog.parse.com/2014/04/30/take-your-app-offline-with-parse-local-datastore/ wich is going to help keep your data synced.