Ohkay, so here is what I am trying to do.
I have a c program that connects to Google Chromes "Web Data" Sqlite db and it can read and write to it when chrome is not launched. But the minute Chrome is launched , i only have read access to the db.
Is there any way i can make my program perform writing operations onto the db while chrome is open?
like temporarily shut down chrome's access to the db for a few miliseconds to insert 1 row into the db and then let chrome take charge again?
Willing to put a bounty on this .. please help.
I'm not sure if google WebData has same implementation as sqlite even if it is same sql engine, but there is possibility to open same SQLite file and perform lock when you are trying to write and release lock when you finished writing, below is link to doc for SQLite.
http://www.sqlite.org/lockingv3.html - concurrency
http://www.sqlite.org/c3ref/busy_handler.html - locking handler
I presume that the only problem you will have is, does google chrome ever release this lock :)
Related
I'm referring to SQLite-PCL tutorial here: https://code.tutsplus.com/tutorials/an-introduction-to-xamarinforms-and-sqlite--cms-23020
I'm very new to SQLite, so I'm lacking in knowledge in lots of basic things - I have tried Googling, can't understand most of it.
Is the call to new SQLiteConnection actually opens up the database or just saying that "the road to the database has been established, whether you access it or not is up to you"?
How do I check if there's already existing database in the devices? And if there is, how do I access it? I have Googled this, but it all seems to be a bit extreme - can't I just call simple OPEN the database?
Is it okay to have multiple SQLiteConnection instances to the same database, if I can be sure that I'm not going to do multiple transaction at the same time?
After I have INSERT into the database, close the app, open up the app back - how do I make sure that there is database created in previous session? Any way to debug this? Because I have no idea if the database is there or not, and I don't know how to access it either..
SQLiteConnection returns a connection object that is used to make subsequent queries
use File.Exists to see if the db file is already present
Yes
Again, use File exists to see if the db file is physically present
Xamarin's ToDo sample provides a good overview of using SQLite with Forms.
I have a QML application which user intract with. There is a timer that listen to server for work order then insert all info to SQLite db in application.Also user make change on data (update,delete etc...) in SQLite.
My question is , How to prevent multi operation on SQLite table. Only one operation must take effect on SQLite(select,delete,insert,update....) I don't know but , Can Mutex.lock structure use for this. Or Is there a something wrong with multiple operation on SQLite
First thing you should do is read up on SQLite locking, they have a section in the docs about it: https://www.sqlite.org/lockingv3.html
The summarisation is that SQLite does locking on modifications such as an insert or update, but won't create a lock when reading. However if a lock exists whilst a modification is in progress the read won't be able to access the database.
I wouldn't worry too much about locking on reading, the state should be fine to be shared at that stage.
I have a C application that inserts data directly to the database of my Meteor application. The app works fine (withoud delays) when I run it in development mode (with "meteor"). However, if I run the app as a node app (bundled) and with external MongoDB, there's an annoying delay in screen updates (5-10s).
I have noticed some previous discussions about this:
Meteor: server-side db insert delays
Using node ddp-client to insert into a meteor collection from Node
Questions:
Is there any other way than building a server-side API for doing the db inserts through Meteor?
Why the delay is only when using external MongoDB?
Is there a way in Meteor to shorten this database polling interval?
You need to enable oplog tailing. Without oplog tailing, when your C program makes a database write, the Meteor server doesn't realise anything has changed until it polls MongoDB again. With oplog tailing, it can pick up the changes much more quickly and efficiently. In development mode, oplog tailing is enabled automatically, but for production it needs some additional setup.
Your MongoDB must be set up as a replica set (a replica set of one node does work).
You have to pass in a mongo URL for the replica set's local database with the environment variable MONGO_OPLOG_URL.
For more information, see this article.
I have a SQLite database that is used by two processes. I am wondering, with the most recent version of SQLite, while one process (connection) starts a transaction to write to the database will the other process be able to read from the database simultaneously?
I collected information from various sources, mostly from sqlite.org, and put them together:
First, by default, multiple processes can have the same SQLite database open at the same time, and several read accesses can be satisfied in parallel.
In case of writing, a single write to the database locks the database for a short time, nothing, even reading, can access the database file at all.
Beginning with version 3.7.0, a new “Write Ahead Logging” (WAL) option is available, in which reading and writing can proceed concurrently.
By default, WAL is not enabled. To turn WAL on, refer to the SQLite documentation.
SQLite3 explicitly allows multiple connections:
(5) Can multiple applications or multiple instances of the same
application access a single database file at the same time?
Multiple processes can have the same database open at the same time.
Multiple processes can be doing a SELECT at the same time. But only
one process can be making changes to the database at any moment in
time, however.
For sharing connections, use SQLite3 shared cache:
Starting with version 3.3.0, SQLite includes a special "shared-cache"
mode (disabled by default)
In version 3.5.0, shared-cache mode was modified so that the same
cache can be shared across an entire process rather than just within a
single thread.
5.0 Enabling Shared-Cache Mode
Shared-cache mode is enabled on a per-process basis. Using the C
interface, the following API can be used to globally enable or disable
shared-cache mode:
int sqlite3_enable_shared_cache(int);
Each call sqlite3_enable_shared_cache() effects subsequent database
connections created using sqlite3_open(), sqlite3_open16(), or
sqlite3_open_v2(). Database connections that already exist are
unaffected. Each call to sqlite3_enable_shared_cache() overrides all
previous calls within the same process.
I had a similar code architecture as you. I used a single SQLite database which process A read from, while process B wrote to it concurrently based on events. (In python 3.10.2 using the most up to date sqlite3 version). Process B was continually updating the database, while process A was reading from it to check data. My issue was that it was working in debug mode, but not in "release" mode.
In order to solve my particular problem I used Write Ahead Logging, which is referenced in previous answers. After creating my database in Process B (write mode) I added the line:
cur.execute('PRAGMA journal_mode=wal') where cur is the cursor object created from establishing connection.
This set the journal to wal mode which allows for concurrent access for multiple reads (but only one write). In Process A, where I was reading the data, before connecting to the same database I included:
time.sleep(0.5)
Setting a sleep timer before a connection was made to the same database fixed my issue with it not working in "release" mode.
In my case: I did not have to manually set any checkpoints, locks, or transactions. Your use case might be different than mine however, so research is most likely required. Nevertheless, I hope this post helps and saves everyone some time!
Try to be more clear, I'm in lack of ideas in this problem, even it sounds like a classic.
My application is running on weblogic 10.3.3 application server, and for database I am using Oracle database 11g. My problem is that there is table in db, let's say "user.", there is column, let's say "columnA", in this table. This table is updating by some module of application.
What I want if when value of column is "abc.", then I have to show alert to console(IP). {IP can be retrieved from DB as it is configured in DB. this ip will be other linux system other than linux machine where oracle database is installed.} Updating is continuously done on my table from module of application. Please tell me from where should I start?, what should I read. I am not able to understand what should be approach. Any help is much appreciated.
Can u provide me any begginner.s link of oracle db listener?
You probably want to look at setting up a Trigger in the database
http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28370/triggers.htm
An alternative to a trigger would be to log update queries against the table (to a log file) and have a process monitor the log, sending out alerts when something happens.