I want to combine the descriptors of 5 images of the same object into a single descriptor file, so that I can recognize the object in any of the views of the reference images. I need to eliminate repetition of similar descriptors from getting saved from the 5 images. All I need is a set of non-repeated, unique descriptors of the object in all the 5 views. How can I get it?
Related
I have a large blob (azure) file with 10k json objects in a single array. This does not perform because of its size. As I look to re-architect it, I can either create multiple files with a single array in each of 500-1000 objects or I could keep the one file, but burst the single array into an array of arrays-- maybe 10 arrays of 1000 objects each.
For simplicity, I'd rather break into multiple files. However, I thought this was worth asking the question and seeing if there was something to be learned in the answers.
I would think this depends strongly on your use-case. The multiple files or multiple arrays you create will partition your data somehow: will the partitions be used mostly together or mostly separate? I.e. will there be a lot of cases in which you only read one or a small number of the partitions?
If the answer is "yes, I will usually only care about a small number of partitions" then creating multiple files will save you having to deal with most of your data on most of your calls. If the answer is "no, I will usually need either 1.) all/most of my data or 2.) data from all/most of my partitions" then you probably want to keep one file just to avoid having to open many files every time.
I'll add: in this latter case, it may well turn out that the file structure (one array vs an array-of-arrays) doesn't change things very much, since a full scan is a full scan is a full scan etc. If that's the case, then you may need to start thinking about how to move to the prior case where you partition your data so that your calls fall neatly within few partitions, or how to move to a different data format.
I'm building an app the attach multiple pictures to a recording. Hence, there is a "Recordings" entity with attributes "Name" and "URL". I want to attach multiple images to one recording.
So do I add another attribute "images" and store array of images? If yes, how is that possible?
Or do I create another Entity that has attributes "Image" and "RecordingID" that has all images of all recordings and each image is connected to it's recording with with the recording ID? If yes, how do I create a unique ID for the recordings?
Please answer which one is better performance wise, and with your choice explain its associated question.
It depends on how you use the data, as always.
If you use an array of images, Core Data will load all of them as soon as you need one of them. You'll either have every image in memory at the same time, or none of them. If you'll always use all of the images at the same time, this might be OK.
If you use a separate entity, you can load individual images when you need them. It's slightly more complex but can reduce memory requirements. It also makes more sense if you decide you need more metadata for each image, for example a creation date.
In both cases you'll be better off saving the image to a file and putting just the filename in your persistent store. It's usually best to keep binary blobs like images and sounds out of Core Data. Binary is OK if you know the value will be very small, but images can potentially be quite large.
I'm not sure why you want a unique ID in the second case where you don't use one in the first case. If you need a unique ID for some reason, the UUID class is a convenient way to generate one.
The situation
I use Labview 2012 on Windows 7
my test result data is written in text files. First, information about the test is written in the file (product type, test type, test conditions etc) and after that the logged data is written each second.
All data files are stored in folders, sorted to date and the names of the files contain some info about the test
I have years worth of data files and my search function now only works on the file names (opening each file to look for search terms costs too much time)
The goal
To write metadata (additional properties like Word files can have) with the text files so that I can implement a search function to quickly find the file that I need
I found here the way to write/read metadata for images, but I need it for text files or something similar.
You would need to be writing to data files that supports meta data to begin with (such as LabVIEW TDMS or datalog file formats). In a similar situation, I would simply use a separate file with the same name, but a different extension for example. Then you can index those file names, and if you want the data you just swap the meta data filename extension and you are good to go.
I would not bother with files and use database for results logging. It may be not what you wiling to do, but this is the ultimate solution for the search problem and it open a lot of data analytics possibilities.
The metadata in Word files is from a feature called "Alternative Data Streams" which is actually a function of NTFS. You can learn more about it here.
I can't say I've ever used this feature. I don't think there is a nice API for LabVIEW, but one could certainly be made. With some research you should be able to play around with this feature and see if it really makes finding files any easier. My understanding is that the data can be lost if transferred over the network or onto a non-NTFS thumbdrive.
I am going to convert a text file in the SQLite db form; I am concerned about these points because giving any effort to write code for it:
Will both text file or its corresponding sqlite db be of same size?
SQLite would take less space than text file?
Or text file db is the one with lowest space?
"Hardware is cheap" - I'd strongly recommend not worrying about size differences, which will likely be insignificant anyway, and instead pick whichever solution best meets the rest of your needs. A text file can work just fine for simple projects, but a database has many more features that can help you organize, backup, and query your data much more efficiently and robustly.
For a more in-depth look at the pros and cons of both options, check out: database vs. flat files
Some things to keep in mind:
(NOTE about this answer: Files here references to internal/external storage, not SharedPrefs)
SQL:
Databases have overheads, which does take up size
If the database or a table goes corrupt, all data is lost(how bad this is depends on your app. Losing several thousand pictures: bad. Losing deletion log: not very bad)
Databases can be compressed(see this)
You can split up data into different tables, if you have issues with ID(or whatever way you identify row X), meaning one database can have several tables for each object where object X have identification conflicts with object Y. That basically means you can keep everything in one file, and still avoid conflicts with names. (Read more at the bottom of the answer)
Files:
Every single file has to be defined as its own separate file, which takes up space(name of the file)
You cannot store all attributes in one file without having to set up an advanced reader that determines the different types of data. If you don't do that, and have one file for each attribute, you will use a lot of space.
Reading thousands of lines can go slow, especially if you have several(say 100+) very big files
The OS uses space for each file, excluding the content. The name of the file for an instance, that takes up space. But something to keep in mind is you can keep all the data of an app in a single file. If you have an app where objects of two different types may have naming issues, you create a new database.
Naming conflicts
Say you have two objects, object X and Y.
Scenario 1'
Object X stores two variables. The file names are(x and y are in this case coordinates):
x.txt
y.txt
But in a later version, object Y comes in with the same two files.
So you have to assign an ID to object X and Y:
0-x.txt
0-y.txt
Every file uses 3 chars(7 total, including extension) on the name alone. This grows bigger the more complex the setup is. See scenario 2
But saving in the database, you get the row with ID 0 and find column X or Y.
You do not have to worry about the file name.
Further, if every object saves a lot of files, the reference to load or save each file will take up a lot of space. And that affects your APK file, and slowly pushes you up towards the 50 MB limit(google play limit).
You can create universal methods, but you can do the same with SQL and save space in the APK file. But compared to text files, SQL does save some space in terms of name.
Note though, that if you save 2-3 files(just to take a number) those few bytes going to names aren't going to matter
It is when you start saving hundreds of files, long names to avoid naming conflicts, that is when SQL saves you space. And if the table gets too big, you can compress it. You can zip text files to maybe save some space, but with one-liner files, there is not much to save.
Scenario 2
Object X and Y has three children each.
Every child has 3 variables it saves to the file system. If there was only one object with 3 children, it could have saved it like this
[id][variable name].txt
But because there is another parent with 3 children(of the same type, and they save the same files) the object's children who get saved last are the ones that stay saved. The first 3 get overwritten.
So you have to add the parent ID:
[parent ID][child ID][variable name].txt
And keep in mind, these examples are focused on a few objects. The amount of space saved is low, but when you save hundreds, if not thousands of files, that is when you start to save space.
Now, if you create a table, you can store your main objects(X and Y in this case). Then, you can either create the first table in a way that makes it recognisable whether the object is the parent or child, or you can create a second table. The second table have two ID values; One to identify the parent and one to identify the child. So if you want to find all the children of object 436, you simply write this query:
SELECT * FROM childrentable WHERE `parent_id`='436'
And that will give you all the attributes for all the children with object 436 as its parent.
And everything is stored in the Cursor when returned.
If you were to do the same with a file, this line(where Saver is the file saving and loading class):
Saver.load("0-436-file_name", context);
It is, of course, possible to use a for-loop to cycle the children ID(the 0 at the start), but you would also have to save how many children there are: You cannot get the files as easily, so you have to store values about thee amount of objects and the objects children.
This meaning you have to save more values in more files to be able to get the files you saved in the first place. And this is a really hard way to do things. A database would help you not have to write files to keep track of how many files you saved. The database would return [x] results on each query. So if object 436 has no children, SQL returns 0 rows. But in files, you would have to save 0 as the amount of children. Guessing file names lead to I/O exceptions.
I would expect the text file to be smaller as it has no overhead: all the things a Database gives you have a cost in terms of space.
It sounds like space is the only thing that matters to you, and that you expect to change the contents of the text file often (you call it a 'text file db'). Please note that there is no such thing as a 'text file db'. Reading and writing to it will be very slow compared to a proper db (such as SQLite). Adding different record types (Tables in a db) will complicate your like and I wouldn't want to try to maintain any sort of referential links between record types in a text file.
Hope that helps -
I'm working on a piece of software that stores files in a file system, as well as references to those files in a database. Querying the uploaded files can thus be done in the database without having to access the file system. From what I've read in other posts, most people say it's better to use a file system for file storage rather then storing binary data directly in a database as BLOB.
So now I'm trying to understand the best way to set this up so that both the database a file system stay in sync and I don't end up with references to files that don't exist, or files taking up space in the file system that aren't referenced. Here are a couple options that I'm considering.
Option 1: Add File Reference First
//Adds a reference to a file in the database
database.AddFileRef("newfile.txt");
//Stores the file in the file system
fileStorage.SaveFile("newfile.txt",dataStream);
This option would be problematic because the reference to the file is added before the actual file, so another user may end up trying to download a file before it is actually stored in the system. Although, since the reference to the the file is created before hand the primary key value could be used when storing the file.
Option 2: Store File First
//Stores the file
fileStorage.SaveFile("newfile.txt",dataStream);
//Adds a reference to the file in the database
//fails if reference file does not existing in file system
database.AddFileRef("newfile.txt");
This option is better, but would make it possible for someone to upload a file to the system that is never referenced. Although this could be remedied with a "Purge" or "CleanUpFileSystem" function that deletes any unreferenced files. This option also wouldn't allow the file to be stored using the primary key value from the database.
Option 3: Pending Status
//Adds a pending file reference to database
//pending files would be ignored by others
database.AddFileRef("newfile.txt");
//Stores the file, fails if there is no
//matching pending file reference in the database
fileStorage.SaveFile("newfile.txt",dataStream); database
//marks the file reference as committed after file is uploaded
database.CommitFileRef("newfile.txt");
This option allows the primary key to be created before the file is uploaded, but also prevents other users from obtaining a reference to a file before it is uploaded. Although, it would be possible for a file to never be uploaded, and a file reference to be stuck pending. Yet, it would also be fairly trivial to purge pending references from the database.
I'm leaning toward option 2, because it's simple, and I don't have to worry about users trying to request files before they are uploaded. Storage is cheap, so it's not the end of the world if I end up with some unreferenced files taking up space. But this also seems like a common problem, and I'd like to hear how others have solved it or other considerations I should be making.
I want to propose another option. Make the filename always equal to the hash of its contents. Then you can safely write any content at all times provided that you do it before you add a reference to it elsewhere.
As contents never change there is never a synchronization problem.
This gives you deduplication for free. Deletes become harder though. I recommend a nightly garbage collection process.
What is the real use of the database? If it's just a list of files, I don't think you need it at all, and not having it saves you the hassle of synchronising.
If you are convinced you need it, then options 1 and 2 are completely identical from a technical point of view - the 2 resources can be out of sync and you need a regular process to consolidate them again. So here you should choose the options that suits the application best.
Option 3 has no advantage whatsoever, but uses more resources.
Note that using hashes, as suggested by usr, bears a theoretical risk of collision. And you'd also need a periodical consolidation process, as for options 1 and 2.
Another questions is how you deal with partial uploads and uploads in progress. Here option 2 could be of use, but you could also use a second "flag" file that is created before the upload starts, and deleted when the upload is done. This would help you determine which uploads have been aborted.
To remedy the drawback you mentioned of option 1 I use something like fileStorage.FileExists("newfile.txt"); and filter out the result for which it returns a negative.
In Python lingo:
import os
op = os.path
filter(lambda ref: op.exists(ref.path()), database.AllRefs())