What could cause a meteor app to sometimes create two documents? - mongodb

Question

I have a meteor app that takes the text of an article and splits it into paragraphs, sentences, words, and characters and then stores that in a json which I then save as a document in a collection. The document I am testing now ends up as 15133 bytes in mongodb.
When I insert the document it takes about 20 or 30 seconds to insert. Then sometimes it starts going through my article creation routine again and inserts another document. Sometimes it ends up inserting 3 or more documents. Sometimes it behaves as it should and only inserts 1 document into the collection.
What should I be looking for that could be causing this behavior?
Here is my code, as requested:
Meteor.methods({
'createArticle': function (text, title) {
var article = {}
article.title = title
article.userID = "sdfgsdfg"
article.text = text
article.paragraphs = []
var paragraphs = splitArticleIntoParagraphs(text)
console.log("paragraphs", paragraphs)
_.each(paragraphs, function (paragraph, p) {
if (paragraph !== "") {
console.log("paragraph", paragraph)
article.paragraphs[p] = {}
article.paragraphs[p].read = false
article.paragraphs[p].text = paragraph
console.log("paragraphs[p]", article.paragraphs[p])
var sentences = splitParagraphIntoSentences(paragraph)
article.paragraphs[p].sentences = []
}
_.each(sentences, function (sentence, s) {
if (sentence !== "") {
article.paragraphs[p].sentences[s] = {}
console.log("sentence", sentence)
article.paragraphs[p].sentences[s].text = sentence
article.paragraphs[p].sentences[s].read = false
console.log("paragraphs[p].sentences[s]", article.paragraphs[p].sentences[s])
var wordsForward = splitSentenceIntoWordsForward(sentence)
console.log("wordsForward", JSON.stringify(wordsForward))
article.paragraphs[p].sentences[s].forward = {}
article.paragraphs[p].sentences[s].forward.words = wordsForward
// var wordsReverse = splitSentenceIntoWordsReverse(sentence)
_.each(wordsForward, function (word, w) {
if (word) {
// console.log("word", JSON.stringify(word))
// article.paragraphs[p].sentences[s] = {}
// article.paragraphs[p].sentences[s].forward = {}
// article.paragraphs[p].sentences[s].forward.words = []
article.paragraphs[p].sentences[s].forward.words[w] = {}
article.paragraphs[p].sentences[s].forward.words[w].wordID = word._id
article.paragraphs[p].sentences[s].forward.words[w].simp = word.simp
article.paragraphs[p].sentences[s].forward.words[w].trad = word.trad
console.log("word.simp", word.simp)
var characters = word.simp.split('')
console.log("characters", characters)
article.paragraphs[p].sentences[s].forward.words[w].characters = []
_.each(characters, function (character, c) {
if (character) {
console.log("character", character, p, s, w, c)
article.paragraphs[p].sentences[s].forward.words[w].characters[c] = {}
article.paragraphs[p].sentences[s].forward.words[w].characters[c].text = character
article.paragraphs[p].sentences[s].forward.words[w].characters[c].wordID = Words.findOne({simp: character})._id
}
})
}
})
}
})
})
// console.log("article", JSON.stringify(article))
// console.log(JSON.stringify(article.paragraphs[10].sentences[1].forward))//.words[4].characters[0])
console.log("done")
var id = Articles.insert(article)
console.log("id", id)
return id
}
})
I call the method here:
Template.articleList.events({
"click #addArticle": function(event) {
event.preventDefault();
var title = $('#title').val();
var text = $('#text').val();
$('#title').value = '';
$('#text').value = '';
$('#text').attr('rows', '3');
Meteor.call('createArticle', text, title);
}
})

Answer 1

An important thing to keep in mind is that meteor methods do not work very well when it comes to perform CPU intensive tasks. Since your meteor server only works in a single thread, any kind of blocking computations - like yours - will affect all client connections, e.g. delaying DDP heartbeats. This - in turn - can result in clients thinking that the connection was dropped.
As #ghybs suggested in one of the comments your method is probably triggered several times by an impatient DDP client who thinks that server has disconnected. The easiest way to prevent this behavior is by adding noRetry flag to Meteor.apply as explained here:
https://docs.meteor.com/api/methods.html#Meteor-apply
I believe Meteor.call does not have this option.
Another strategy would be trying to make sure that your methods are idempotent, i.e. calling them more than once should not produce any additional effects. This is usually true - at least when you use method simulation - because retrying db insert will re-use the same document id which will fail on the second try. For some reason this is not happening in your case.
Finally, the problem you described clearly shows that probably a different pattern should be used for a computationally expensive task like yours. If I were you, I would start by splitting the job in several steps:
First I would make sure that the document is uploaded to the server with a POST request rather than through DDP.
Then I would implement a "process file" server side method that grabs the file which is already on the server or in the database (in case you used files collection). The first thing the method should do would be calling this.unblock(), but thats not all.
Ideally the computational task should be executed in a separated process. Only when that process is completed the method would return telling the actual caller that the job is done. But since we called this.unblock() that caller can perform different tasks, e.g. calling other methods/subscriptions while waiting for the result.
Sometimes, having a separated process will not be good enough. I've experienced situations where I had to delegate the task to another worker server(s).