I've prototyped an iPhone app that uses (internally) SQLite as its data base. The intent was to ultimately have it communicate with a server via PHP, which would use MySQL as the back-end database.
I just discovered Google App Engine, however, but know very little about it. I think it'd be nice to use the Python interface to write to the data store - but I know very little about GQL's capability. I've basically written all the working database code using MySQL, testing internally on the iPhone with SQLite. Will GQL offer the same functionality that SQL can? I read on the site that it doesn't support join queries. Also is it truly relational?
Basically I guess my question is can an app that typically uses SQL backend work just as well with Google's App Engine, with GQL?
I hope that's clear... any guidance is great.
True, Google App Engine is a very cool product, but the datastore is a different beast than a regular mySQL database. That's not to say that what you need can't be done with the GAE datastore; however it may take some reworking on your end.
The most prominent different that you notice right off the start is that GAE uses an object-relational mapping for its data storage scheme. Essentially object graphs are persisted in the database, maintaining there attributes and relationships to other objects. In many cases ORM (object relational mappings) map fairly well on top of a relational database (this is how Hibernate works). The mapping is not perfect though and you will find that you need to make alterations to persist your data. Also, GAE has some unique contraints that complicate things a bit. One contraint that bothers me a lot is not being able to query for attribute paths: e.g. "select ... where dog.owner.name = 'bob' ". It is these rules that force you to read and understand how GAE data store works before you jump in.
I think GAE could work well in your situation. It just may take some time to understand ORM persistence in general, and GAE datastore in specifics.
GQL offers almost no functionality at all; it's only used for SELECT queries, and it only exists to make writing SELECT queries easier for SQL programmers. Behind the scenes, it converts your queries to db.Query objects.
The App Engine datastore isn't a relational database at all. You can do some stuff that looks relational, but my advice for anyone coming from an SQL background is to avoid GQL at all costs to avoid the trap of thinking the datastore is anything at all like an RDBMS, and to forget everything you know about database design. Specifically, if you're normalizing anything, you'll soon wish you hadn't.
I think this article should help you.
Summary: Cloud computing and software development for handheld devices are two very hot technologies that are increasingly being combined to create hybrid solutions. With this article, learn how to connect Google App Engine, Google's cloud computing offering, with the iPhone, Apple's mobile platform. You'll also see how to use the open source library, TouchEngine, to dynamically control application data on the iPhone by connecting to the App Engine cloud and caching that data for offline use.
That's a pretty generic question :)
Short answer: yes. It's going to involve some rethinking of your data model, but yes, changes are you can support it with the GAE Datastore API.
When you create your Python models (think of these as tables), you can certainly define references to other models (so now we have a foreign key). When you select this model, you'll get back the referencing models (pretty much like a join).
It'll most likely work, but it's not a drop in replacement for a mySQL server.
Related
I started working on a small POC and decided to give Sails.js a try :)
Part of the POC we wanted to use DynamoDB since the project will eventually involve high scalability and we're not looking to hire full-time MongoDB expert at this point.
We used the module: https://github.com/gadelkareem/sails-dynamodb
Problem is there is no documentation and the module does not even work...
It seems the sails ORM is not ideal for DynamoDB and requires writing custom DB services. Does anyone have experience with this?
I was very excited to come across Sails but if it won't let us play nice with DynamoDB then it might very well be out as an option to us....
Anyone have experience with this or maybe something I'm missing?
One of the important plus of vogels is excellent documentation.
Sails-dynamodb adapter based on the vogels, but not all features are implemented in sails-dynamodb adapter. For example, vogels has Expression Filters.
Vogels able to create tables. Adapter can't. An adapter needs duplication table schema in sails files and dynamodb shell.
Vogels has some own types, such as uuid type, StringSet, NumberSet, TimeUUID. (Adapter can use it too, if includes Vogels and Joi lib)
Vogels and adapter have the same query (create, update, delete, find) capabilities.
Adapter allows without changing the code switch to another data base. Adapter encapsulates establishment of connection to database.
Conclusion - for most purposes this adapter is suitable for the work and do not need to work directly with the Vogels
Sails comes loaded with an ORM called "Waterline". There are some official waterline plugins such as mongodb, postgresql, mysql and then there are some unofficial ones created by the community. I'd assume right now that Dynamo is in the latter category since I have not come across it before. However, with that being said I would not take this experience as a reason to ditch Sails.js.
Sails.js is built with the intention that all of its components can be swapped out, this means you are not tied to a specific template engine, authentication libraries etc. and including your ORM choice.
Waterline is still being actively developed but it is sat at v0.12.1 as of writing this response. It isn't fully there yet so there will be the odd issues still around!
My recommendation? Take a look at swapping out waterline for a different ORM. Keep the flexibility Sails gives you and change out the component that doesn't meet your criteria. There are still many benefits to Sails you can utilise.
Vogels might be worth checking out: https://github.com/ryanfitz/vogels
Turning off waterline: Is there a way to disable waterline and use a different ORM in sails.js?
I would like to give a web app with a PostgreSQL database 100% offline functionality. In an ideal case the database should be completely replicated in the browser per user, and synchronized when online. So that the same code can be used to talk to both the offline and online database. I know this is possible with PouchDB and CouchDB, but have not found a solution that works with PostgreSQL. Is this at all possible?
Short answer: I don't know of anything like this that currently exists.
However, in theory, this could be made to work...(long answer:)
Write a PostgreSQL backend for levelup (one exists for MySQL: https://github.com/kesla/mysqldown)
Wire up pouch-server to read/write from your PostgreSQL db using pouchdb's existing leveldb adapter (which in turn will have to be configured to use your postgres backend). Congrats, you can now sync data using PouchDB!
Whether an approach like this is practical in reality for your application is a different question you'll have to answer.
You may be wondering, for example, "will I be able to sync an existing complex schema with multiple tables to the client with this approach?" The answer is probably not - the mysqldown implementation of leveldown uses a single MySQL table with three fields: id, key, and value (source), and I imagine any general-purpose PostgreSQL adapter would be similar (nothing says you can't do a special-purpose adapter just for your app though!).
On the other hand, if you were to implement a couchdb-compatible API (or a subset- you may not need attachments, for example) over your existing database schema, there's nothing stopping you from using PouchDB on the client to talk directly to that as if it were an actual CouchDB - just pop in the URL and call replicate()! Implementing the replication protocol might be a fair bit of work, since you'd need to track revisions and so on somewhere - but again, technically not impossible!
There are also implementations of levelup's backend storage that are designed for browsers. See level.js, which could be another way to sync between a server-side Postgres levelup backend and the browser.
TL;DR: There's tons of work being done around Javascript databases right now. Is syncing with Postgres impossible? probably not. Would it be a lot of work? Definitely. Worth it? Who knows, but it would be cool.
Without installing PostgreSQL on the client? No. Obviously you can cache data for offline use, but an entire RDBMS+procedural languages in Javscript, no.
We have native applications that run on Android, iOS, and Windows mobile. For other devices (such as BlackBerry) a mobile web solution. These applications currently do an initial large pull from our CMS and then parse XML from our CMS as its datastore. This data is then available offline on the device. We're looking for something more elegant than XML as we scale.
Here are options that we are weighing:
Option 1. Export a sqlite DB to Android, iOS, and Windows 7 phone that they would all then use as its datastore.
Pros: CMS exports the same data format to all devices
Cons: iOS isn't using CoreData like everything I've been reading says I should be using.
Option 2. Export sqlite DB to all platforms but have iOS insert data into CoreData. We are toying with idea of having CMS export JSON format to iOS and have iOS insert into CoreData since our delta updates to the application will be in JSON.
Pros: iOS is using CoreData and all of its benefits.
Cons: iOS now strays from all our other platforms such that it needs an intermediate solution (converting data into CoreData storage.)
Curious if anyone has mobile applications on various platforms that require persistent stores and how your team tackled it.
3/22/2013 for minor clarification and grammatical changes.
Stumbled upon this solution while doing research:
https://github.com/AlexDenisov/iActiveRecord
It behaves similarly to CoreData, such that it utilizes object graphs. It constructs the SQL statements behind the scenes for you so that you do not need to concern yourself with writing queries.
What I like about this solution is that iActiveRecord points to our exported SQLite DB from our CMS (which utilizes django) and we simply define our Obj-C class to match the schemas of the table (similar to defining data models in CoreData.) After the class schemas are defined to reflect our table schemas, we can start using objects without worrying about sql queries.
CoreData could not do this since it required us to write a "converter" to get exported data from our CMS into CoreData's datastore.
Of course all the extra bells and whistles of CoreData are missing, but for our use cases, utilizing iActiveRecord out weighed other options.
The real question is: can you really reuse any code on these platforms? Not much. Probably, too little to worry about it. In fact, if you don't have a common C/C++ library (in a broad sense, as in a set of functions) to process data in an SQLite database, there's not much code to reuse.
There may be cases where you heavily depend on certain SQL queries for good performance. Those queries may turn out to be very inefficient in Core Data. In this scenario I would go with SQLite and FMDB.
If the queries are simple enough and you mainly need to display/edit the data, then going with Core Data will be easier.
I want to write a high scalable web application for selling event tickets. I want to use NoSQL database, like Big Table or MongoDB and Cloud Service like Google App Engine (GAE) or Amazon Elastic Compute Cloud (Amazon EC2)
Is it posible using this type of database to be sure that two client will not be able to buy a ticket for the same place simultaneously? Or may be I will have to use RDBMS database and forget about Google App Engine?
Things like GAE's datastore can still support transactional semantics, for example:
http://code.google.com/appengine/docs/python/datastore/transactions.html
So yes, it is possible to do what you're seeking to do. (Note - GAE's Datastore is not exactly NoSQL, since it uses SQL-like queries.)
I have a problem with this question. Not all NoSQL databases are created equally, and different NoSQL databases have different ways they store data. Generally the thing you should be worried about are: data is actually written to disk and not just into memory. Most NoSQL databases can do this but not by default. Let's just say this is not a problem, you can usually tell the database like MOngo or Cassandra to write data to disk, can even tell how many servers at minimum the data should be written to.
The problem is that you may not get a true transactional support. When you deal with ecommerce it's important to have all or nothing type of transation where several operations either succeed completely or rolled back. There must be absolutely no chance that only part of your data is saved. For example, if you need to write data to more than one table (collection or document in NoSQL lingo), if server goes down in the middle of the process and your data is only written to one table, that's usually unacceptable in ecommerce.
I am not familiar with all NoSQL databases, but the ones I know don't have this option yet.
MySQL, on the other hand, does.
If transactional support or lack of it does not bother you, then I think its OK to use NoSQL as long as you tell it to save data to disk and not just into memory.
The answer is 'maybe.'
Depending on what you're trying to build, you many be able to use some of the techniques in this post:
http://kylebanker.com/blog/2010/06/07/mongodb-inventory-transactions/
Using something like get_or_insert you can easily ensure that two clients are not receiving the same resource simultaneously on Google App Engine. However, there are big differences between GAE and a RDBMS, so make sure you study them further before you make a decision.
We're beginning development of an in-house app in the iPhone Enterprise developer program. Since it's close to OS 3.0, we're reconsidering our original design of using SQLite and using Core Data instead. Here's some more info:
There is a legacy desktop application that this is replacing. We will reuse the existing back end.
We currently have a SQLite database generated as a proof of concept. This is basically a cut down version of the existing back end database.
We will be loading data from a remote site and storing it locally, where it will persist and need to be . We only update it if it has changed, which will be every month or two. We will most likely use XML or JSON to transfer the data.
There are two developers on this project and we both have strong SQL skills but neither one has used Core Data.
My questions are: what is the benefit of Core Data over SQLite, what would the benefit be in this specific instance and do the benefits justify learning a new framework instead of using existing strong SQL skills?
EDIT:
I just noticed this question: Core Data vs SQLite 3. I guess my questions therefore are:
If I have to check if a specific item either exists or has an update, which is easy using SQL, does Core Data still make sense? Can I load the first object in a graph and check the version number without loading the whole graph?
If we already know SQL, does the advantages of Core Data for this one project justify us learning it?
As you've read Core Data vs SQLite 3, you know that Core Data and the persistence mechanism (SQLite in this case) are largely orthogonal. Core Data is really about managing an object graph and it's main use case is for the model component of an MVC architecture. If your application fits nicely into this architecture, it's probably worth using Core Data as it will save you a lot of code in the model component. If you already have a working model component (e.g. from the existing desktop app), then Core Data won't buy you much. A hybrid approach is possible-- you can do your own persistence/querying and build a Core Data in memory store which you populate with the result of a query and use this in-memory store via Core Data as the model component for your app. This isn't common, but I've done it and there are no major roadblocks.
To answer your specific questions:
You can assign a version number to the entire persistent store and retrieve that information via +[NSPersistentStore metadataForPersistentStoreWithURL:error:], without even opening the store. An equivalent +setMetadata:forPersistentStoreWithURL:error also exists, of course. If you want to store the version info in an entity instance instead of in the persistent store metadata, you can load only a single object. With an SQLite persistent store, Core Data does a very good job of fetching only what you need.
The NSPredicate API, is very easy to learn and it seems to do a decent job of compilation to SQL. At least for databases of the size you could fit on an iPhone it's certainly been adequate (performance wise) in my experience. I think the SQL vs. Core Data question is slightly misguided, however. Once you get the result of a query what are you going to do with it? If you roll your own, you'll have to instantiate objects, handle faulting/uniqueing (if you don't want to load the entire result of a query into memory immediately) and all of the other object graph management facilities already provided by Core Data.
It sounds like you already have the project designed using SQLite, and you have experience in that area.
So the bottom line is, does it make sense to port this project, will Core Data give me anything that I didn't already have in my original design?
Assuming that the original design was done properly, based on the requirements ON THIS PROJECT, it's probably not worth it.
But that's not the end of the discussion. There are other things to think about: Will my next project have such light database requirements? Do I need to ship soon, due to timeline or budget constraints? Assuming I'm going to have to learn Core Data sooner or later, doesn't it make sense to do it now? Am I possibly interested in porting my code over to the Mac?
The answers to these questions may lead you to the decision that yes, it is indeed worth it to go back to the drawing board so to speak, and learn what Core Data is all about.
To get to your final question: What are the advantages? Well, Core Data is a higher level abstraction of your database, it is also data store agnostic (so if a future version of the iPhone were to ditch SQLite for an embedded version of MySQL... unlikely, but it's an example) then Core Data would require VERY few changes to the code to make it work with the new data store. Core Data will provide a great deal of quick portability to the Mac platform. Core Data will handle versioning of your data model, whereas unless you have a framework or a workflow to manage it, direct access to SQLite won't.
I'm sure other answerers can come up with other advantages, and maybe some good reasons why NOT to mess with Core Data. Incidentally, in a similar situation, my decision was to port to the higher level, newer framework. But in my case, it was for a side project, and ship date and budget were non-factors.
Not to detract from this forum, but you might find more respondents with contextually relevant experience at the Apple iPhone DevForum.
Speaking from a purely project management perspective, it sounds like you know how to build what you want to build using SQLite, and so it would make more sense to me for you to start along that route.
That being said, CoreData builds on top of SQLite and if you are trying to leverage other parts of the system in conjunction with your data, e.g. using KVC/KVO or bindings, then you may quickly find that this functionality is worth the learning curve.
= Mike