I am currently working on a desktop app with AdobeAir, JS, and a SQlite DB. I have an issue where sometimes the DB is opening too slowly and the first query is then not successful.
I am using an Async connection, and I made a few functions that handle each type of query, like:
function CoreExecuteSelect(sql, selectStmt, callbackFunction) {
openConnection();
air.trace(sql);
selectStmt.sqlConnection = sqlConnection;
selectStmt.text = sql;
selectStmt.addEventListener(air.SQLEvent.RESULT, callbackFunction);
selectStmt.addEventListener(air.SQLErrorEvent.ERROR, onDatabaseError1);
selectStmt.execute();
}
So I believe, sometimes the openConnection() is not yet finished when the selectStmt.execute() runs, this is then causing trouble because I don't get data back. What would be the best way for me to wait for the DB to be open before sending the first queries ?
Thanks for your help.
Related
I am new to nodejs/Express.js development.
I have built my backend service with Express.js / Typescript and I have multiple routes / api endpoints defined. One is like this:
app.post('/api/issues/new', createNewIssue);
where browser will send a post request when a user submits a new photo (also called an issue in my app).
The user can send an issue to another user, and the backend will first query the database to find the number of issues that matches the conditions of "source user" and "destination user", and then give the new issue an identifying ID in the form srcUser-dstUser-[number], where number is the auto-incremented count.
The createNewIssue function is like this:
export const createNewIssue = catchErrors(async (req, res) => {
const srcUser = req.header('src_username');
const dstUser = req.header('dst_username');
// query database for number of issues matching "srcUser" and "dstUser"
...
const lastIssues = await Issue.find( {where: {"srcUser": srcUser, "dstUser": dstUser}, order: { id: 'DESC'}});
const count = lastIssues.length;
// create a new issue Entity with the ID `srcUser-dstUser-[count+1]`
const newIssue = await createEntity(Issue, {
...
id: `srcUser-dstUser-${count+1}`,
...
});
res.respond({ newIssue: newIssue});
})
Say the backend receives multiple requests with the same srcUser and dstUser attributes at the same time, will there be collisions where multiple new issues are created with the same id?
I have read some documentation about nodejs being single-threaded, but I'm not sure what that means definitely for this specific scenario.
Besides business logic in this scenario, I have some confusions in general about Express JS / Node JS:
When there is only one cpu core, Express JS process multiple concurrent requests asynchronously: it starts processing one and does not wait for it to finish, instead continues to process the next one. Is this understanding accurate?
When there are multiple cpu cores, does Express JS / Node Js utilize them all in the same manner?
Node.js will not solve this problem for you automatically.
While it will only deal with one thing at a time, it is entirely possible that Request 2 will request the latest ID in the database while Request 1 has hit the await statement at the same point and gone to sleep. This would mean they get the same answer and would each try to create a new entry with the same ID.
You need to write your JavaScript to make sure that this doesn't happen.
The usual ways to handle this would be to either:
Let the database (and not your JavaScript) handle the ID generation (usually by using a sequence.
Use transactions so that the request for the latest ID and the insertion of the new row are treated as one operation by the database (so it won't start the same operation for Request 2 until the select and insert for Request 1 are both done).
Test to make sure createEntity is successful (and doesn't throw a 'duplicate id' error) and try again if it fails (with a limit in case it keeps failing in which case it should return an error message to the client).
The specifics depend on which database you use. I linked to the Postgresql documentation for the sake of example.
I am having some issues trying to connect to a matrix server using the matrix-js-sdk in a react app.
I have provided a simple code example below, and made sure that credentials are valid (login works) and that the environment variable containing the URL for the matrix client is set. I have signed into element in a browser and created two rooms for testing purposes, and was expecting these two rooms would be returned from matrixClient.getRooms(). However, this simply returns an empty array. With some further testing it seems like the asynchronous functions provided for fetching room, member and group ID's only, works as expected.
According to https://matrix.org/docs/guides/usage-of-the-matrix-js-sd these should be valid steps for setting up the matrix-js-sdk, however the sync is never executed either.
const matrixClient = sdk.createClient(
process.env.REACT_APP_MATRIX_CLIENT_URL!
);
await matrixClient.long("m.login.password", credentials);
matrixClient.once('sync', () => {
debugger; // Never hit
}
for (const room of matrixClient.getRooms()) {
debugger; // Never hit
}
I did manage to use the roomId's returned from await matrixClient.roomInitialSync(roomId, limit, callback), however this lead me to another issue where I can't figure out how to decrypt messages, as the events containing the messages sent in the room seems to be of type 'm.room.encrypted' instead of 'm.room.message'.
Does anyone have any good examples of working implementations for the matrix-js-sdk, or any other good resources for properly understanding how to put this all together? I need to be able to load rooms, persons, messages etc. and display these respectively in a ReactJS application.
It turns out I simply forgot to run startClient on the matrix client, resulting in it not fetching any data.
I have an app where I spawn several BrowserWindows, with html forms, and I'd like to collect all the data (in order to save it, to be able to spawn them in the same state at a restart) at a press of a button.
At the moment, the only solution I found to do so, is to have each BrowserWindow do ipcRenderer.send every single time any variable changes (not too hard to do with Vuejs 'watchers'), but this seems demanding and inefficient.
I also thought of doing 'executeJavascript' to each window but that does not allow to capture the return value afaik.
I'd just like to be able to send a message from main when a request for saving is made, and wait for the windows to respond before saving all.
EDIT
I found a slightly better way, it looks like this
app.js
// wait for update reponses
ipc.on('update-response', (evt,args) => {
updates[evt.sender.id] = args;
if(Object.keys(updates).length == BrowserWindow.getAllWindows().length) {
// here I do what I need to save my settings, using what is stored in 'updates'
// ...
// and now reset updates for next time
updates = {}
}
});
// now send the requests for updates
BrowserWindow.getAllWindows().map(w => w.send('update'));
renderer.js
ipcRenderer.on('update', () => {
// collect the data
// var data = ...
ipcRenderer.send('update-response', data);
})
and obviously on the renderer side I am listening to these 'update' messages and sending data with 'udpate-response'.
But it seems a bit complicated and so I am sure there is a simpler way to achieve this using the framework.
EDIT 2
I realized that the above does not always work, because for some reason, the evt.sender.id do not match the ids obtained from BrowserWindows.getAllWindows(). I worked around that by sending ids in the request, and having the responder include it. But this is all so much fine for so very little...
I was thinking about how to make an instant messaging application, and wanted to not have to send an AJAX request so often (one every .2s), and I came across the following idea:
Send an AJAX request from the user side, to the server.
Only respond once there is a change in the MySQL database
And then send the next AJAX request once the response has been recorded and parsed
I'm aware of how to do the first and third steps, but the second one is going over my head.
I'm assuming that for step 2, I'll need to store the request somewhere, while the PHP script is continuously looping and looking for some changes, and once there is a change, the saved request would be responded to.
EDIT
Didn't know about WebSockets, should've used those.
You could use recursion and query the database every 2 seconds, until you find new data to be served to the user. So basically you could do something like
public function isDataUpdated($lastId) {
$query = "SELECT * FROM `messages` WHERE `messages`.`message_id` > $lastId";
return (bool)(count($this->executeSQL($query)) > 0);
}
public function fetchNewMessages () {
if ($this->isDataUpdated($_GET['last_id'])) {
/* We have new data! Send it to the user */
} else {
sleep(2); // wait for 2 seconds
$this->fetchNewMessages(); // we use recursion to query the database every 2 seconds to find new data
}
}
Although, it is not the best of solutions, it would hopefully work. I would recommend taking a look at Sockets in PHP to better achieve what you want
I'm trying to set up mongodb on Windows 8 using node.js, Does anyone know why im getting this error. C:\users\phill\node_modules\mongodb\lib\mongodb\mongo_client.js:359 it also says at collection = db collection,,, can't call method 'collection' of null. I'm having a hard time setting it up. My goal is to be able to add to mongo db, and see that I add or pull up what I added, but adding something is good enough for me for now. I'm trying every thing I can find, even straight from the website, I tried everything I see on here as well. Think it maybe it's the way I have things set up. My node.js is saved in my c: drive there is a file that says, program files(86x) in there I have node_modules, npm and such. The path ends up being, computer > windows (C:) > program files(86x) > nodejs. My Mongodb is saved right on my C: drive the path end up being windows (C:) > mongodb-win32-x86_64-2008plus-2.4.8. In my C: I also created a file data and in it created another db. I have been told i should just use mongoose, I'm just learning so i open to any advice, links or anything that will help. I have one last question as well, i learned php and then found out about sql injections and stuff like that, i am not seeing anything about security at all, should i expect the same as well. For this i get text not defined, but i have been getting errors with everthing i have done, best i did was get stuck on a right concern screen.
var MongoClient = require('mongodb').MongoClient;
MongoClient.connect("mongodb://localhost:27017/integration_test", function(err, db) {
test.equal(null, err);
test.ok(db != null);
db.collection("replicaset_mongo_client_collection").update({a:1},
{b:1}, {upsert:true}, function(err, result) {
test.equal(null, err);
test.equal(1, result);
db.close();
test.done();
});
});
Tried this as well and getting a error,C:\users\phill\node_modules\mongodb\lib\mongodb\mongo_client.js:359.... at collection = db collection,,, can't call method 'collection' of null. im calling it in command prompt node filename.js I'm saving it where my node.js file is, I have pulled up files before and created a server.
var Db = require('mongodb').Db,
MongoClient = require('mongodb').MongoClient,
Server = require('mongodb').Server,
ReplSetServers = require('mongodb').ReplSetServers,
ObjectID = require('mongodb').ObjectID,
Binary = require('mongodb').Binary,
GridStore = require('mongodb').GridStore,
Grid = require('mongodb').Grid,
Code = require('mongodb').Code,
BSON = require('mongodb').pure().BSON,
assert = require('assert');
var db = new Db('test', new Server('localhost', 27017));
// Fetch a collection to insert document into
db.open(function(err, db) {
var collection = db.collection("simple_document_insert_collection_no_safe");
// Insert a single document
collection.insert({hello:'world_no_safe'});
// Wait for a second before finishing up, to ensure we have written the item to disk
setTimeout(function() {
// Fetch the document
collection.findOne({hello:'world_no_safe'}, function(err, item) {
assert.equal(null, err);
assert.equal('world_no_safe', item.hello);
db.close();
})
}, 100);
});
In your first code example, you said:
For this i get text not defined
I assume you meant "test not defined?" Your script only requires the mongodb library, and I don't believe test is a core nodejs function, so that would explain the error.
To reference the driver documentation for db.collection(), an assert library is used, but also properly imported (as you did in your second example).
Within your callback to db.open(), you don't check if an error occurred. That might shed some light on why db is null in that function.
Regarding your question about the equivalent of SQL injection with MongoDB, the main areas of concern are places where you might pass untrusted input into evaluated JavaScript, or using such input to construct free-form query objects (not simply using a string, but more like dropping an object into your BSON query). Both of these links should provide more information on the subject:
What type of attacks can be used vs MongoDB?
How does MongoDB address SQL or Query injection?