neumino
How to set consistency read_mode for query
Hello, we have serious problems with concurrent data updates in table for one row and sub array of data. In rethink documentation there is a https://www.rethinkdb.com/docs/consistency/ where they say that we must change read_mode for each query, per example
r.table('users', {readMode: 'majority'}).filter({role: 'admin'}).run(conn, callback);
How can be set with Thinky.io ? There is an option in method for creating models
thinky.createModel(
"table_name",
tableObjectWithFields,
{pk: "id"}
);
where the primary key is. Can we put read_mode: "majority" there also? Nothing in documentation can be found. Best regards
Oh, I didn't know that you could pass the read mode in the table command.
At the moment the only way to pass the read mode is to set it in run/execute.
It shouldn't too hard to add an option on a model.
So please... Now we tested with r.table() and concurrent records are gone... it work as expected.
You said: At the moment the only way to pass the read mode is to set it in run/execute.
Can we run like this:
Posts.filter({...}).run({readMode: "majority"}).then().error()
Why do you think it's possible with only callback? As far as I know, it works with or without callbacks.
I tested today, because all my code is based on run().then().error() promises and that doesn't work.
After setting like above, we loosing our data in row. Imagine situation like this:
Table: items
{
id: 21
name: xxxxx
comments: [
{ id: 1, name: ssss }, {id: 2, name: zzz}, ....
]
}
now, with 8 devices we sending update of the same item, but with different comments. At the end, instead of 8 comments total, we have only 1 (because of last device)... and this happening because devices sending data at the same time.
When we used r.table(), as I explained we have positive results, after sending item has 8 comments.
After adding readMode in run() this morning, all code works correctly, there is no errors, but we loosing comments again. Of course, adding comments one by one at the time is ok.
First if you just append, you can never lose a comment as long as the write is successful since RethinkDB provide atomicity per document.
r.table('posts').get(21).update(function(post) {
post.merge({
comments: post('comments').append({id: xxx: name: yyy})
})
})
Then readMode has to do with read, not write, so I'm not sure what your expectations are, but I'm pretty sure that readMode is passed to run. You can test it by bringing 3 servers, create one table with 3 replicas, kill 2, run a query with readMode outdated'/majority`.
Please provide your script that reproduces your issue.
We using save() on model
Items.get(1).run()
.then(function(item){
item.comments.push(newComment)
item.save()
.then(function(savedItem){
// ....
})
})
This is expected. What can happen is
server 1 get the item
server 2 get the same item
serve 1 write the document
server 2 overwrite the document
You can set any readMode you want, you can't prevent this case except if you have a global lock around your code (get/save). The query I wrote in https://github.com/neumino/thinky/issues/442#issuecomment-186276280 is what you want.
Yes, exactly... when I changed code to r.table() with update() and append() problem is gone.
Problem with readMode: single is when 3 clients reads and writes data with small time interval (ms). Client 2 reads data actually but replica is not finished yet. As they say, readMode: majority read always success written data.