I have looked around quite extensivly and could not find any example on how to use query results from the marklogic module inside node.js...
Most examples do a console.log() of results and thats it, but what if I need the query results (say in a JSON array and use these results later on?
Seems I am missing some node.js ascynch stuff here...
Example :
var marklogic = require('marklogic');
var my = require('./my-connection.js');
var db = marklogic.createDatabaseClient(my.connInfo);
var qb = marklogic.queryBuilder;
db.documents.query(
qb.where(qb.parsedFrom('oslo'))
).result( function(results) {
console.log(JSON.stringify(results, null, 2));
});
// I would like to use the results here
// console.log(JSON.stringify(results, null, 2))
Now question is I would like to use the results object later on in this script. I have tried using .then(), or passing it to a variable and returning that variable but no luck.
Regards,
hugo
Simple answer: you need to continue your business logic from the result() callback.
In more detail, your goal is to do something with the result of an asynchronous computation or request. Since JS has no native async capabilities (such as threads), callbacks are typically used to resume an operation asynchronously. The most important thing to realize is that you cannot return the result of an async computation or request, but must resume control flow after it completes. Defining lots of functions can help make this kind of code easier to read and understand.
This example illustrates what's happening:
process.nextTick(function() {
console.log('second')
})
console.log('first')
That program will log first, then second, because process.nextTick() asynchronously invokes the callback function provided to it (on the next turn of the event loop).
The answers at How do I get started with Node.js offer lots of resources to better understand async programming with node.js.
Related
The lambda's job is to see if a query returns any results and alert subscribers via an SNS topic. If no rows are return, all good, no action needed. This has to be done every 10 minutes.
For some reasons, I was told that we can't have any triggers added on the database, and no on prem environment is suitable to host a cron job
Here comes lambda.
This is what I have in the handler, inside a loop for each database.
sequelize.authenticate()
.then(() => {
for (let j = 0; j < database[i].rawQueries[j].length; j++) {
sequelize.query(database[i].rawQueries[j] => {
if (results[0].length > 0) {
let message = "Temporary message for testing purposes" // + query results
publishSns("Auto Query Alert", message)
}
}).catch(err => {
publishSns("Auto Query SQL Error", `The following query could not be executed: ${database[i].rawQueries[j])}\n${err}`)
})
}
})
.catch(err => {
publishSns("Auto Query DB Connection Error", `The following database could not be accessed: ${databases[i].database}\n${err}`)
})
.then(() => sequelize.close())
// sns publisher
function publishSns(subject, message) {
const params = {
Message: message,
Subject: subject,
TopicArn: process.env.SNStopic
}
SNS.publish(params).promise()
}
I have 3 separate database configurations, and for those few SELECT queries, I thought I could just loop through the connection instances inside a single lambda.
The process is asynchronous and it takes 9 to 12 seconds per invocation, which I assume is far far from optimal
The whole thing feels very very sub optimal but that's my current level :)
To make things worse, I now read that lambda and sequelize don't really play well together:
I am using sequelize because that's the only way I could get 3 connections to the database in the same invocation to work without issues. I tried mssql and tedious packages and wasn't able with either of them
It now feels like using an ORM is an overkill for this very simple task of a SELECT query, and I would really like to at least have the connections and their queries done asynchronously to save some execution time
I am looking into different ways to accomplish this and i went down the rabbit hole and I now have more questions than before! Generators? are they still useful? Observables with RxJs? Could this apply here? Async/Await or just Promises? Do I even need sequelize?
Any guidance/opinion/criticism would be very appreciated
I'm not familiar with sequelize.js but hope I can help. I don't know your level with RxJS and Observables but it's worth to try.
I think you could definitely use Observables and RxJS.
I would start with an interval() that will run the code every time you define.
You can then pipe the interval since it's an Observable, do the auth bit and do a map() to get an array of Observables (for each .query call, I am assuming all your calls, authenticate and query, are Promises so it's possible to transform them into Observables with from()). You can then use something like forkJoin() with the previous array to get a response after all calls are done.
In the .subscribe at the end, you would make the publishSns().
You can pipe a catchError() too and process errors.
The map() part might be not necessary and do it previously and have it stored in a variable since you don't depend on an authenticate value.
I'm certain my solution isn't the only one or the best but i think it would work.
Hope it helps and let me know if it works!
I am still quite new to Node.js and can't seem to find anything to help me around this.
I am having an issue of getting the query from my last record and adding it to my variable.
If I do it like below: -
let lastRecord = Application.find().sort({$natural:-1}).limit(1).then((result) => { result });
Then I get the value of the variable showing in console.log as : -
Promise { <pending> }
What would I need to do to output this correctly to my full data?
Here is it fixed:
Application.findOne().sort({$natural:-1}).exec().then((lastRecord) => {
console.log(lastRecord); // "lastRecord" is the result. You must use it here.
}, (err) => {
console.log(err); // This only runs if there was an error. "err" contains the data about the error.
});
Several things:
You are only getting one record, not many records, so you just use findOne instead of find. As a result you also don't need limit(1) anymore.
You need to call .exec() to actually run the query.
The result is returned to you inside the callback function, it must be used here.
exec() returns a Promise. A promise in JavaScript is basically just a container that holds a task that will be completed at some point in the future. It has the method then, which allows you to bind functions for it to call when it is complete.
Any time you go out to another server to get some data using JavaScript, the code does not stop and wait for the data. It actually continues executing onward without waiting. This is called "asynchronisity". Then it comes back to run the functions given by then when the data comes back.
Asynchronous is simply a word used to describe a function that will BEGIN executing when you call it, but the code will continue running onward without waiting for it to complete. This is why we need to provide some kind of function for it to come back and execute later when the data is back. This is called a "callback function".
This is a lot to explain from here, but please go do some research on JavaScript Promises and asynchronisity and this will make a lot more sense.
Edit:
If this is inside a function you can do this:
async function someFunc() {
let lastRecord = await Application.findOne().sort({$natural:-1}).exec();
}
Note the word async before the function. This must me there in order for await to work. However this method is a bit tricky to understand if you don't understand promises already. I'd recommend you start with my first suggestion and work your way up to the async/await syntax once you fully understand promises.
Instead of using .then(), you'll want to await the record. For example:
let lastRecord = await Application.find().sort({$natural:-1}).limit(1);
You can learn more about awaiting promises in the MDN entry for await, but the basics are that to use a response from a promise, you either use await or you put your logic into the .then statement.
I'm still learning the node.js ropes and am just trying to get my head around what I should be deferring, and what I should just be executing.
I know there are other questions relating to this subject generally, but I'm afraid without a more relatable example I'm struggling to 'get it'.
My general understanding is that if the code being executed is non-trivial, then it's probably a good idea to async it, as to avoid it holding up someone else's session. There's clearly more to it than that, and callbacks get mentioned a lot, and I'm not 100% on why you wouldn't just synch everything. I've got some ways to go.
So here's some basic code I've put together in an express.js app:
app.get('/directory', function(req, res) {
process.nextTick(function() {
Item.
find().
sort( 'date-modified' ).
exec( function ( err, items ){
if ( err ) {
return next( err );
}
res.render('directory.ejs', {
items : items
});
});
});
});
Am I right to be using process.nextTick() here? My reasoning is that as it's a database call then some actual work is having to be done, and it's the kind of thing that could slow down active sessions. Or is that wrong?
Secondly, I have a feeling that if I'm deferring the database query then it should be in a callback, and I should have the actual page rendering happening synchronously, on condition of receiving the callback response. I'm only assuming this because it seems like a more common format from some of the examples I've seen - if it's a correct assumption can anyone explain why that's the case?
Thanks!
You are using it wrong in this case, because .exec() is already asynchronous (You can tell by the fact that is accepts a callback as a parameter).
To be fair, most of what needs to be asynchronous in nodejs already is.
As for page rendering, if you require the results from the database to render the page, and those arrive asynchronously, you can't really render the page synchronously.
Generally speaking it's best practice to make everything you can asynchronous rather than relying on synchronous functions ... in most cases that would be something like readFile vs. readFileSync. In your example, you're not doing anything synchronously with i/o. The only synchronous code you have is the logic of your program (which requires CPU and thus has to be synchronous in node) but these are tiny little things by comparison.
I'm not sure what Item is, but if I had to guess what .find().sort() does is build a query string internally to the system. It does not actually run the query (talk to the DB) until .exec is called. .exec takes a callback, so it will communicate with the DB asynchronously. When that communication is done, the callback is called.
Using process.nextTick does nothing in this case. That would just delay the calling of its code until the next event loop which there is no need to do. It has no effect on synchronicity or not.
I don't really understand your second question, but if the rendering of the page depends on the result of the query, you have to defer rendering of the page until the query completes -- you are doing this by rendering in the callback. The rendering itself res.render may not be entirely synchronous either. It depends on the internal mechanism of the library that defines the render function.
In your example, next is not defined. Instead your code should probably look like:
app.get('/directory', function(req, res) {
Item.
find().
sort( 'date-modified' ).
exec(function (err, items) {
if (err) {
console.error(err);
res.status(500).end("Database error");
}
else {
res.render('directory.ejs', {
items : items
});
}
});
});
});
I managed to create a module to handle all the database call. It uses this lib: https://github.com/developmentseed/node-sqlite3
My issues are the following.
Everytime I make a call, I need to make sure the database exist, and if not to create it.
Plus, as all the calls are asynchronous, I end up having loads of functions in functions in callbacks ... etc.
It pretty much looks like this:
getUsers : function (callback){
var _aUsers = [];
var that = this;
this._setupDb(function(){
var db = that.db;
db.all("SELECT * FROM t_client", function(err, rows) {
rows.forEach(function (row) {
_aUsers.push({"cli_id":row.id,"cli_name":row.cli_name,"cli_path":row.cli_path});
});
callback(_aUsers);
});
});
},
So, is there any way I can export my module only when the database is ready and fully created if it does not exist yet?
Does anyone see a way around the "asynchronous" issue?
You could also try using promises or fibers ...
I don't think so. If you make it synchronous, you are taking away the advantage. Javascript functions are meant to be that way. Such a situation is referred to as callback hell. If you are facing problems managing callbacks then you can use these libraries :
underscore
async
See these guides to understand basics of asynchronous programming
Node.js: Style and structure
Using underscore.js managing-callback-spaghetti-in-nodejs
Using async.js node-js-async-programming
I'm trying to design a system in Node.js (an attempt at solving one of my earlier problems, using Node's concurrency) but I'm running into trouble figuring out how to draw a plan of how the thing should operate.
I'm getting very tripped up thinking in terms of callbacks instead of returned values. The flow isn't linear, and it's really boggling my ability to draft it. How does one draw out an operational flow for an event-driven system?
I need something I can look at and say "Ok, yes, that's how it will work. I'll start it over here, and it will give me back these results over here."
Pictures would be very helpful for this one. Thanks.
Edit: I'm looking for something more granular than UML, specifically, something that will help me transition from a blocking and object-oriented programming structure, where I'm comfortable, to a non-blocking and event driven structure, where I'm unfamiliar.
Based on http://i.stack.imgur.com/9DDQP.png what you need is a good flow library that allows you to pipeline sync and async calls in node.
One such library is https://github.com/isaacs/slide-flow-control (look at the slide preso there too) and the code outline for what you need to do is below.
It is self documenting and as you see it is quite concise, pure nodejs, uml, img's etc. not required.
var chain = require("slide/chain")
, asyncMap = require("slide/async-map")
;
// start processing
main_loop(function() {
console.log("its done"); // when finished
});
function main_loop(cb) {
var res = [];
// each entry in chain below fires sequentially i.e. after
// the previous function completes
chain
( [ [start_update_q, "user-foo"]
, [get_followed_users, chain.last]
, [get_favorites, chain.last]
, [calc_new_q]
, [push_results, chain.last]
]
, res
, cb
)
}
function get_favorites(users, cb) {
function fn(user, cb_) {
get_one_users_favorites(user, cb_);
}
// this will run thru get_favorites in parallel
// and after all user favorites are gotten it will fire
// callback cb
asyncMap(users, fn, cb);
}
// code in the various functions in chain here,
// remember to either return the callback on completion.
// or pass it as an arg to the async call you make within the
// function as above i.e. asyncMap will fire cb on completion
UML might be appropriate. I'd look at the behavior diagrams.