I am developing a web service that will be invoked (using JSON) from client side each time the selection of a drop down changes.
The goal is to register each "intermediate" change (on client side) using the "OnSelectedIndexChanged" event and before submitting the form to the Server.
Each new selected value will be written to a shared txt file calling a relative web method via Ajax/JSON.
Would it be better to write these changes to a txt file (having to implement a lock/unlock policy to assure exclusive access) or rather define a DB table and save the changes there?
Everyday the web app will have around 10 to 20 active users that might potentially changes the DropDownLists and usually the right value will be selected at first, hence generally no more than one "intermediate" entry would be registered.
Thanks.
Don't use the filesystem. It's slow. Use mongodb via a node.js webserver.
http://howtonode.org/express-mongodb
Good Luck!
This sounds exactly like what you would want to use a database for, since ACID is already implemented there.
If you want a real headache (and a programming challenge!) trying to debug overlapping writes, resource starvation and deadlocks, by all means, go with a shared text file!
Related
I am new to sitecore and just trying to understand its architecture/design. Just curious to know how Intranet and Internet server communicates and how does the data flow happens between these two layers in on-prem and on AWS EC2 environment? I have surfed enough in the web and couldn't find the appropriate explanation.
Really appreciate if anyone can help me understand.
When u do a publish from CM, it puts a record in eventqueue table in Web Db.
all CD servers will hit the eventqueue table table for update and proceed.
default is 2 seconds once this hit happens.
In short, they communicate via events in the database(s). Note: This is very simplified but seeing it this way helped me understand how the events work and troubleshoot issues.
For example, when publishing an item, the publisher (running on CM or on a dedicated role) reads its data from the master database and writes it to the web database. When done, it raises an event by writing a row in the EventQueue table in web database. The CD server(s) picks up this event and clears its corresponding caches etc. causing a reload of that data from the web database.
All Sitecore databases have the EventQueue table and events goes to the table in different databases, depending on the type of event. An events is basically just a class name and a set of serialized data. Events can be raised "locally" and "globally" indicating if several instances should pick up the event. Think of a scenario where you have two CD servers sharing one web database, both CD's would have to pick up the event.
To keep track on what events has been processed, a "EQSTAMP" value is stored in the Properties table. It's named [database]_EQSTAMP_[InstanceName]. It's therefore essential that not two Sitecore instances share the same instance name. If not set, Sitecore will make an instance name by combining the hostname and IIS site name. The decimal Value of this timestamp corresponds to the hexadecimal Stamp column in the EventQueue table.
Normally, you should never have to play with these tables yourself, but I find it good to have some insights in how they work and keep an eye on them. They can grow in size and cause some issues. The CleanupEventQueue scheduled task is responsible for removing old processed events from the EventQueue tables. You may want to play with the scheduling of this agent if your EventQueue grows too large between cleanups.
Note: This is the most common way of communication between the servers. Later versions of Sitecore have other techniques as well, such as Rebus.
Event Queues. Why? How? When? article that explains it in detail, it also describes the pitfalls of using this mechanism in real life as well.
Please also be aware that Sitecore.Link project is a good place to get more knowledge regarding Sitecore functionality.
It accumulates Sitecore knowledge all around the web.
Thanks.
I am working on a CLI in Go that scrapes a webpage to collect the href attributes of all the links on the page into a slice. I want to store this slice in memory for some time so that the scraper is not being called on every execution of the CLI command. Ideally, the scraper would only be called after the cache expires or the user provides some sort of --update flag.
I came across the library go-cache and other similar libraries, but from what I could tell they only work for something that is continuously running, like a server.
I thought about writing the links to a file, but then how would I expire the results after a specific duration? Would it make sense to create a small server in the background that shuts down after a while in order to use a library like go-cache? Any help is appreciated.
There are two main approaches in these scenarios:
Create a daemon, service or background application that acts as your data repository. You can run it as an HTTP server / RPC server depending on your requirements. Your CLI application then interacts with this daemon as required;
Implement a persistence mechanism that will allow data to be written and read across multiple CLI application executions. You may use normal text files, databases or even an implementation of golang's encoding/gob to write and read your slice (a map would probably be better) to and from a binary file.
You can timestamp entries and simply remove them after their ttl expires by explicitly deleting them, or by simply not rewriting them during subsequent executions, according to the strategy / approach selected above.
The scope and number of examples for such an open ended question is too myriad to post in a single answer and will most likely require multiple specific questions.
Use a database and store as much detail as you can (fetched_at, host, path, title, meta_desc, anchors etc). You'll be able to query over the data later and it will be useful to have it in a structured format. If you don't want to deal with a db dependency you could embed something like boltdb (pure go) or sqlite (cgo).
I'm developing an app extension that needs to share data with the containing app. I created an app group and moved the core data store of the main app to that folder. From the extension I can create the managed object context and save data to the store and I can also access it from the containing app. Now I have two independent applications accessing the same core data store. This sounds like a recipe for disaster to me. Is what I have set up sufficient for sending data from the extension to the containing app or should I look for another way?
In this situation you'll have two entirely independent Core Data stacks accessing the same persistent store file.
Assuming that you're using SQLite, you're fine, at least as far as data integrity. Core Data uses SQLite transactions to save changes and SQLite is fine with multiple processes using the same file. Neither process will corrupt data for the other or mess up the file.
You will have to deal with keeping data current in the app. For example if someone uses the share extension to create new data while the app is running. You won't get anything like NSManagedObjectContextDidSaveNotification in this case. You'll need to find your own way to ensure you get any new updates.
In many cases you can make this almost trivial-- listen for UIApplicationDidBecomeActiveNotification, which will be posted any time your app comes to the foreground. When you get it, check the persistent store for new data and load it.
If you want to get a little more elegant, you could use something like MMWormhole for a sort-of file based IPC between the app and the extension. Then the extension can explicitly inform the app that there's new data, and the app can respond.
Very interesting answer from Tom Harrington. However I need to mention about MMWormhole. I've found that MMWormhole uses NSFileCoordinator and apple tells that:
Using file coordination in an app extension to access a container
shared with its containing app may result in a deadlock in iOS
versions 8.1.x and earlier.
What Apple suggests for safe save operations you can read here:
You can use CFPreferences, atomic safe save operations on flat
files, or SQLite or Core Data to share data in a group container
between multiple processes even if one is suspended mid transaction.
Here is the link to the Apple Technical Note TN2408.
I'm in the middle of brainstorming a cloud sync solution for a Core Data app that I am currently developing. I'm planning to open source the code for this once its done, for anyone to use with their Core Data apps, so input from the community on how this system should work is much appreciated :-) Here's what I'm thinking:
Server Side
Storage Provider
As with all cloud sync systems, storage is a major piece of the puzzle. There are many ways to handle this. I could set up my own server for storage, or use a service like Amazon S3, but because I'm starting out with $0 capital, at this moment, a paid storage solution isn't a viable option. After some thought, I decided to settle with Dropbox (an already well established cloud sync application and storage provider). The pros of using Dropbox are:
It's free (for a limited amount of space)
In addition to being a storage service, it also handles cloud sync
They recently released an Objective-C SDK which makes it much easier to interface with it in Mac and iPhone apps
In case I decide to switch to a different storage provider in the future, I intend to add "services" to this cloud sync framework, basically allowing anyone to create a service class to interface with their choice of storage provider, which can then simply be plugged into the framework.
Storage Structure
This is a really difficult part to figure out, so I need as much input as I can here. I've been thinking about a structure like this:
CloudSyncFramework
======> [app name]
==========> devices
=============> (device id)
================> deviceinfo
================> changeset
==========> entities
=============> (entity name)
================> (object id)
A quick explanation of this structure:
The master "CloudSyncFramework" (name undecided) folder will contain separate folders for each app that uses the framework
Each app folder contains a devices folder and an entities folder
The devices folder will contain a folder for each device that is registered with the account. The device folder will be named according to the device ID, obtained using something like [[UIDevice currentDevice] uniqueIdentifier] (on iOS) or a serial number (on Mac OS).
Each device folder contains two files: deviceinfo and changeset. deviceinfo contains information about the device (e.g. OS version, last sync date, model, etc.) and the changeset file contains information about objects that have changed since the device last synchronized. Both files will just be simple NSDictionaries archived into files using NSKeyedArchiver.
Each Core Data entity has a subfolder under the entities folder
Under each entity folder, every object that belongs to that entity will have a separate file. This file will contain a JSON dictionary with the key-value pairs.
Simultaneous Sync
This is one of the areas where I am almost completely clueless. How would I handle 2 devices connecting and syncing with the cloud at the same time? There seems to be a high risk of things getting out of sync here, or even data corruption.
Handling migrations
Once again, another clueless area here. How would I handle migrations of the Core Data managed object model? The easiest thing to do here seems to be just to wipe the cloud data store clean and upload a new copy of the data from a device which has undergone the migration process, but this seems somewhat risky, and there may be a better way.
Client Side
Converting NSManagedObjects into JSON
Converting attributes into JSON isn't a very hard task (theres lots of code for it floating around the web). Relationships are the key problem here. In this stackoverflow post, Marcus Zarra posts code in which the relationship objects themselves are added to the JSON dictionary. However, he mentions that this can cause an infinite loop depending on the structure of the model, and I'm not sure if this would work with my method, because I store each object as an individual file.
I've been trying to find a way to get an ID as a string for an NSManagedObject. Then I could save relationships in JSON as an array of IDs. The closest thing I found was [[managedObject objectID] URIRepresentation], but this isn't really an ID for an object, its more of a location for the object in the persistent store, and I don't know if its concrete enough to use as a reference for an object.
I suppose I could generate a UUID string for each object and save it as an attribute, but I'm open for suggestions.
Syncing changes to the cloud
The first (and still best) solution that popped into my head for this was to listen for the NSManagedObjectContextObjectsDidChangeNotification to get a list of changed objects, then update/delete/insert those objects in the cloud data store. After the changes have been saved, I would need to update the changeset file for every other registered device to reflect the newly changed objects.
One problem that comes up here is, how would I handle a failed or interrupted sync?. One idea I have is to first push changes to a temporary directory on the cloud, then once that has been confirmed as successful, to merge it with the master data on the cloud so that an interruption in the middle of the sync won't corrupt data. Then I would save records of the objects that need to be updated in the cloud into a plist file or something, to be pushed during the next time the app is connected to the internet.
Retrieving changed objects
This is fairly simple, the device downloads its changeset file, figures out which objects need to be updated/inserted/deleted, then acts accordingly.
And that sums up my thoughts for the logic that this system will use :-) Any insight, suggestions, answers to problems, etc. is greatly appreciated.
UPDATE
After lots of thinking, and reading TechZens suggestions, I have come up with some modifications to my concept.
The largest change I've thought up is to make each device have a separate data store in the cloud. Basically, every time the managed object context saves (thanks TechZen), it will upload the changes to that device's data store. After those changes are updated, it will create a "changeset" file with change details, and save it into the changeset folders of the OTHER devices that are using the application. When the other devices connect to sync, they will go through the changeset folder and apply each changeset to the local data store, then update their respective data stores in the cloud as well.
Now, if a new device is registered with the account, it will find the newest copy of the data out of all the devices and download that for use as its local storage. This solves the problem of simultaneous sync and reduces the chances for data corruption because there is no "central" data store, each devices touches only its data and just updates changes rather than every device accessing and modifying the same data at the same time.
There's some obvious conflict situations to deal with, mainly in relation to deleting objects. If a changeset is downloading instructing the app to delete an object that is currently being edited, etc. there needs to be ways to deal with this.
You want to look at this pessimistic take on cloud sync: Why Cloud Sync Will Never Work.
It covers a lot of the issues that you are wrestling with. Many of them are largely intractable.
It is very, very, very difficult to synchronize information period. Adding in different devices, different operating systems, different data structures, etc snowballs the complexity often fatally. People have been working on variants of this problem since the 70s and things really haven't improve much.
The fundamental problem is that if you leave the system flexible and customizable, then the complexity of synchronizing all the variations explodes exponentially as a function of the number of customization. If you make it rigid, you can sync but you are limited in what you can sync.
How would I handle 2 devices
connecting and syncing with the cloud
at the same time?
If you figure that out, you will be rich. It's a big issue for current cloud sync providers. They real problem here is that your not "syncing" your merging. Software sucks at merging because its very hard to establish a predefined rule set to describe all the possible merges.
The simplest system is to establish either a canonical device or a device hierarchy such that the system always knows which input to choose. This however, destroys flexibility.
How would I handle migrations of the
Core Data managed object model?
The migration of the Core Data model is largely irrelevant to the server. That's something that Core Data manages internally to itself. Model migration updates the model i.e. the entity graph, not the actual data.
Converting NSManagedObjects into JSON
Modeling relationships is hard especially with tools that don't support it as easily as Core Data does. However, the URI of a permanent managed object ID is supposed to serve as a UUID that nails the object down to a specific location in a specific store on a specific device. It's not technically guaranteed to be universally unique but its close enough for all practical purposes.
Syncing changes to the cloud
I think you're confusing implementation details of Core Data with the cloud itself. If you use NSManagedObjectContextObjectsDidChangeNotification you will evoke network traffic every time the observed context changes regardless of whether those changes are persisted or not. Depending on the app, this could drive connections thousands of times in a few minutes. Instead, you only want to sync when context is saved at the most.
One problem that comes up here is, how
would I handle a failed or interrupted
sync?
You don't commit changes until the sync completes. This is a big problem and leads to corrupt data. Again, you can have flexibility, complexity and fragility or inflexibility, simplicity and robustness.
Retrieving changed objects: This is
fairly simple, the device downloads
its changeset file, figures out which
objects need to be
updated/inserted/deleted, then acts
accordingly
It's only simple if you have an inflexible data structure. Describing changes to a flexible data structure is a nightmare.
Not sure if I have helped any. None of the problems have elegant solutions. Most designer end up with rigidity and/or slow, brute force iterative merging.
Take a serious look at RestKit.
It is an open source project that aims to help with integrating iOS apps with cloud data, including but not limited to the scenario where there is a core-data model for that data on the client.
I have recently started to use it in one of my projects, and found it to be quite useful. In the core-data scenario, you implement declarative mappings between your data model and the content you GET from and POST to the server, and it takes care of things like injecting objects from the cloud into your client model, posting new objects to the server and incorporating server-generated objects IDs into your client-side model, doing all of this in a background thread and taking care of all the core-data context threading issues and so on.
RestKit by no means is a mature product, but is has a fairly good foundation and quite a few things that can use help from other contributors. Especially, if your goal is to create an open source solution, it would be great to contribute and improve something like this rather than re-invent a new solution. Unless of course, your see serious differences between what you have in mind and other existing solutions :-)
Since this post was current, there are several new options available. It is possible to develop a solution, and there are apps shipping with these solutions.
Here is a short list of the main Core Data sync options:
Apple's native Core Data/iCloud sync. (Had a rocky start. Seems better now.)
TICDS
Wasabi Sync, a paid service.
Simperium (Seems abandoned.)
ParcelKit with Dropbox Datastore API
Ensembles, the most recent. (Disclosure: I am the founder of the project)
It's like Apple answered my question for me with the announcement of the iCloud SDKs, which come complete with Core Data integration. Win!
In what way is the Windows registry meant to be used? I know it's alright to store a small amount of user preferences, but is it considered bad practice to store all your users data there? I would think it would depend on the data set, so how about for small amounts of data, say, less than 2KB, in 100 or so different key/value pairs. Is this bad practice? Would a flat file or SQLite db be a better practice?
I'm going to take a contrarian view.
The registry is a fine place to put configuration data of all types. In general it is faster than most configuration files and more reliable (individual operations on the registry are transacted so if your app crashes during a write the registry isn't corrupted - in general that isn't the case with ini files).
Marcelo MD is totally right: Storing things like operation percentage complete in the registry (or any other non volitile storage) is a horrible idea. On the other hand storing data like the most recently used files is just fine - the registry was built for just that kind of problem.
A number of the other commenters on this post talking about the MRU list have discussed the problem of what happens when the MRU list gets out of sync due to application crashes. I'm wondering why storing the MRU list in a flat file in per-user storage is any better?
I'm also not sure what the "security implications" of storing your data in the registry are. The registry is just as secure as the filesystem - the registry and the filesystem use the same ACL mechanism to protect their data.
If you ARE going to store your user data in a file, you should absolutely put your data in %APPDATA%\CompanyName\ApplicationName at least - that way if two different developers create an application with the same name (how many "Media Manager" applications are there out there?) you won't have collisions.
For me, simple user configuration items and user data is better to be stored in either a simple XML configuration file, a SQLLite db, or a MS SQL Server Compact db. The exact storage medium depends on the specifics of the implementation.
I only use the registry for things that I need to set infrequently and that users don't need to be able to change/see. For example, I have stored encrypted license information in the registry before to avoid accidental user removal of the data.
Using the registry to store data has mainly one problem: It's not very user-friendly. Users have virtually no chance of backing up their settings, copying them to another computer, troubleshooting them (or resetting them) if they get corrupted, or generally just see what their software is doing.
My rule of thumb is to use the registry only to communicate with the OS. Filetype associations, uninstaller entries, processes to run at startup, those things obviously have to be in the registry.
But data that is for use in your application only belongs in a file in your App Data folder. (whiever one of the 3+ App Data folders Microsoft currently wants you to use, anyway)
As each user has directory space in Windows already dedicated to storing application user data, I use it to store the user-level data (preferences, for instance) there.
In C#, I would get it by doing something like this:
Environment.GetFolderPath( Environment.SpecialFolder.ApplicationData);
Typically, I'll store SQLite files there or whatever is appropriate for the application.
If your app is going to be deployed "in the enterprise", keep in mind that administrators can tweak the registry using group policy tools. For example, if firefox used the registry for things like the proxy server, it would make deployment a snap because an admin can use the standard tools in active directory to set it up. If you use anything else, I dont think such things can be done very easily.
So don't dismiss the registry all together. If there is a chance an admin might want to standardize parts of your configuration across a network, put the setting in the registry.
I think Microsoft is encouraging use of isolated storage instead of the Windows registry.
Here's an article that explains how to use it in .Net.
You can find those files in Windows XP under Documents & Settings\\Local Settings\ App Data\Isolated Storage. The data is in .dat files
I would differentiate:
On the one hand there is application specific configuration data that is needed for the app to run, e.g. IP addresses to connect to, which folders to use for what sort of files etc, and non trivial per user settings.
Those I put in a config file, ini format for simple stuff, xml if it gets more complex.
On the other hand there is trivial per user settings (best example: window positions and layout). To avoid cluttering the config files (which some users will want to edit themselves, so few and clearly arranged entries are a must), I like to put those in the registry (with conservative defaults being set in the app if no settings in the registry can be found).
I mainly do it like istmatt sais: I store config files inside the %APPDATA% folder. Usually in %APPDATA%\ApplicationName, I don't like the .NET default of APPDATA%\CompanyName\ApplicationName\Version, that level of detail and complexity is counterproductive for most small to medium sized applications.
I disagree with the example of Marcelo MD of not storing recently used files in the registry. IMO this is exactly the volatile sort of user specific information that can be stored there.
(His example of what not to do is very good, though!)
To me it seems easier to think of what you should NOT put there.
e.g: dynamic data, such as an editor's "last file opened" and per project options. It is really annoying when your app loses sync with the registry (file deletion, system crash, etc) and retrieves information that is not valid anymore, possibly deadlocking the user.
At an earlier job I saw a guy that stored a data transfer completness percentage there, Writing the new values at every 10k or so and having the GUI retrieve this value every second so it could show on the titlebar.