in a Meteor app, having real-time reactive updates between all connected clients is achieved with writing in collections, publishing and subscribing the right data. In normal case this means also database writes.
But what if I would like to sync particular data which does not need to be persistent and I would like to save the overhead of writing in the database ? Is it possible to use mini-mongo or other in-memory caching on the server by still preserving DDP synchronisation to all clients ?
Example
In my app I have a multiple collapsed threads and I want to show, which users currently expanded particular thread
Viewed by: Mike, Johny, Steven ...
I can store the information in the threads collection or make make a separate viewers collection and publish the information to the clients. But there is actually no meaning in making this information persistent an having the overhead of database writes.
I am confused by the collections documentation. which states:
OPTIONS
connection Object
The server connection that will manage this collection. Uses the default connection if not specified. Pass the return value of calling DDP.connect to specify a different server. Pass null to specify no connection.
and
... when you pass a name, here’s what happens:
...
On the client (and on the server if you specify a connection), a Minimongo instance is created.
But If I create a new collection and pass the option object with conneciton: null
// Creates a new Mongo collections and exports it
export const Presentations = new Mongo.Collection('presentations', {connection: null});
/**
* Publications
*/
if (Meteor.isServer) {
// This code only runs on the server
Meteor.publish(PRESENTATION_BY_MAP_ID, (mapId) => {
check(mapId, nonEmptyString);
return Presentations.find({ matchingMapId: mapId });
});
}
no data is being published to the clients.
TLDR: it's not possible.
There is no magic in Meteor that allow data being synced between clients while the data doesn't transit by the MongoDB database. The whole sync process through publications and subscriptions is triggered by MongoDB writes. Hence, if you don't write to database, you cannot sync data between clients (using the native pub/sub system available in Meteor).
After countless hours of trying everything possible I found a way to what I wanted:
export const Presentations = new Mongo.Collection('presentations', Meteor.isServer ? {connection: null} : {});
I checked the MongoDb and no presentations collection is being created. Also, n every server-restart the collection is empty. There is a small downside on the client, even the collectionHanlde.ready() is truthy the findOne() first returns undefined and is being synced afterwards.
I don't know if this is the right/preferable way, but it was the only one working for me so far. I tried to leave {connection: null} in the client code, but wasn't able to achieve any sync even though I implemented the added/changed/removed methods.
Sadly, I wasn't able to get any further help even in the meteor forum here and here
Related
I am using Spring-Boot, Spring-Data/JPA with Hazelcast client/server topology. In parts of my test application, I am calculating time when performing CRUD operations on the client side (the server is the one interacting with a relational db). I configured the map(Store) to be write-behind by setting write-delay-seconds to 10.
Spring-Data's save() returns the persisted entity. In the client app, therefore, the application flow will be blocked until the (server) returns the persisted entity.
Would like to know is there is an alternative in which case the client does NOT have to wait for the entity to persist. Was under the impression that once new data is stored in the Map, persisting to the backed happens asynchronously -> the client app would NOT have to wait.
Map config in hazelast.xml:
<map name="com.foo.MyMap">
<map-store enabled="true" initial-mode="EAGER">
<class-name>com.foo.MyMapStore</class-name>
<write-delay-seconds>10</write-delay-seconds>
</map-store>
</map>
#NeilStevenson I don't find your response particularly helpful. I asked on an earlier post about where and how to generate the Map keys. You pointed me to the documentation which fails to shed any light on this topic. Same goes for the hazelcast (and other) examples.
The point of having the cache in the 1st place, is to avoid hitting the database. When we add data (via save()), we need to also generate an unique key for the Map. This key also becomes the Entity.Id in the database table. Since, again, its the hazelcast client that generates these Ids, there is no need to wait for the record to be persisted in the backend.
The only reason to wait for save() to return the persisted object would be to catch any exceptions NOT because of the ID.
That unfortunately is how it is meant to work, see https://docs.spring.io/spring-data/commons/docs/current/api/org/springframework/data/repository/CrudRepository.html#save-S-.
Potentially the external store mutates the saved entry in some way.
Although you know it won't do this, there isn't a variant on the save defined.
So the answer seems to be this is not currently available in the general purpose Spring repository definition. Why not raise a feature request for the Spring Data team ?
Due to the Meteor Docs there are 'server-side', 'client-side' and 'local' Collections. Is there a way to change the 'status' (e.g. if it's server-side, client-side or local) on a running app?
Use Case: A Web-Application where users can register and login. They can store sensible data. Depending on the Users personal preferences he should be able to choose if that data is stored local or on the server (General decision - not from case to case).
Current Approach: It's working fine if I either instantiate the Collection local CollectionName = new Mongo.Collection(null); or server side CollectionName = new Mongo.Collection('collectionName');.
But I can't think of an approach to make it possible to the user that he can change the Collection status.
Is there a way to do this?
Or is a workaround needed (e.g. Create both, a local and server-side Collaction, and just decide which to use for insert/update/find - what would mean a lot of duplicate code?!).
Edit: To make thinks clear: I want the user to be able to choose if his data is stored in a collection which is synced with the server or a collection without any syncing.
No, you can't change the type of a collection on a running app.
I think you are confused about what these terms mean. "Client-side" collections aren't permanently stored in localstorage. It just means it's a collection that's in the browser's memory. Just as "server-side" collections are those that reside in the server's memory. The difference is not how it's defined, but where the code runs. Most collections have a client-side and a server-side counterpart, and they are kept synchronized via pub/sub. Server-side collections are also synchronized with MongoDB (using the oplog).
Local collections can live in both places, but "local" means they aren't synchronized with anything.
I probably don't fully understand what you are trying to do, but local collections do not persist data.
If you pass null as the name, then you’re creating a local collection. It’s not synchronized anywhere; it’s just a local scratchpad that supports Mongo-style find, insert, update, and remove operations. (On both the client and the server, this scratchpad is implemented using Minimongo.)
This means any data added to them on the client will be blown away when the user closes their browser (unless you are also using one of the local collection persist meteor packages) and any data added to them on the server will be blown away when the meteor app is restarted. So I don't think you really want to use local collections.
Instead, I would use a regular collection (where a name is passed to the constructor) and either the standard allow or deny options (not really recommended anymore...but still a valid approach) or Meteor methods (the preferred approach) to control who can change data and what data is allowed to change.
Or, another option could be to pass your publication function a list of fields that the user wishes to see on the client for that given session. To do this you defined a new publication that receives a displayFields argument that you then use as the field specifier options in your collection .find().
Meteor.publish("userData", function (userId, displayFields) {
// validate the structure and contents of displayFields
// retrieve the data but only use the fields that the user requested
return UserData.find({user_id: userId}, {fields: displayFields});
});
Then on the client side you would subscribe to this and pass in the fields the user wishes to make visible on the client.
var displayFields = {
firstname: 1,
lastname: 0,
//...
};
this.subscribe("userData", [displayFields]);
on meteor server side, this looks fine, which maps server side collection to publication
if (Meteor.isServer) { // This code only runs on the server
Meteor.publish('tasks', function tasksPublication() { return Tasks.find(); }); }
and the following on the client side is also understandable, which maps subscription to publication by name.
Meteor.subscribe('tasks');
But I couldn't find in any tutorial or docs explaining how subscription and client side collections are mapped to each other. There is no code mapping tasks (subscription) to Tasks (client side collection). Meteor might assume the client collection uses the same name as the server side by both including the same collection declaration (Tasks = new Mongo.collection('Tasks');). But what if I want to use a different collection name on the client side? or what if the info sent by the server is a mix of fields from multiple collections, how do clients know which collections to store this info when they get it from subscription?
This is part of the way Meteor works. It automatically synchronises data in collections between client and server. You don't need to worry about it, and you can't change it.
Your helper methods can pull data from different server collections, and put the data in arrays - you can read from different collections to do this, and you can do it reactively, so when the source collection changes it will run your helper again.
You can also define client only collections, which don't get saved to the server.
So you can probably do what you want, and if you then want to save something from your smooshed data, you would probably write a piece of code to extract the data to be updated into an object, and then save that.
Lets breakdown the code:
Meteor.publish('tasks', function() {
return Tasks.find();
});
Here we have defined a publication name 'tasks' which supplied data it has received from the function return Tasks.find().
Similarly, when subscribing, we refer to that particular publication- tasks in this case and we get those data.
Now coming to the part of linking it to the collection. In Meteor, when you define a collection, it should be defined on both client and server. So when you define a collection like Tasks = new Mongo.collection('tasklists');, On server, the Tasks object refers to the collection tasklists that the server will use to communicate with the mongoDB server. On the client, an object with the name Tasks is created to interact with the database minimongo created in client for tasklists. The Minimongo is a client side API in JS for MongoDB. (You can consider it as a client side replica of the mongoDB database).
So, on client side, you can define Tasks as anything as long as it is an object for the mongoDB collection tasklists- e.g AnyName=new Mongo.collection('tasklists');
Regarding how publication and subscription will know, which collection are we talking about: Publications send across something known as cursor which is related to a particular document(s) and collection in 'mongoDB'. As long as you get the collection name( tasklists) correct, you can have different object names(Tasks) on client and server.
I'm currently implementing a realtime search function in my app and I've come across some behaviour which I'm confused about.
The background is: I have two subscriptions from the same MongoDB database on my server, named posts.
The first subscription subscribes to the latest 50 posts, and sends the data to the MiniMongo collection Posts.
The second subscriptions subscribes to the post matching whatever search is entered by the user, and sends this data to MiniMongo collection PostsSearch as per below.
// client
Posts = new Mongo.Collection('posts');
PostsSearch = new Mongo.Collection('postsSearch');
// server
Meteor.publish('postsPub', function(options, search) {
return Posts.find(search, options);
});
Meteor.publish('postsSearchPub', function(options, search) {
var self = this;
var subHandle = Posts.find(search, options).observeChanges({
added: function (id, fields) {
self.added("postsSearch", id, fields);
}
});
self.ready();
});
My question is, we know from the docs:
If you pass a name when you create the collection, then you are
declaring a persistent collection — one that is stored on the server
and seen by all users. Client code and server code can both access the
same collection using the same API.
However this isn't the case with PostsSearch. When a user starts searching on the client, the functionality works perfectly as expected - the correct cursors are sent to the client.
However I do not see a postsSearch in my MongoDB database and likewise, PostsSearch isn't populated on any other client other than my own.
How is this happening? What is self.added("postsSearch", id, fields); appearing to do that's it's able to send cursors down the wire to the client but not to the MongoDB database.
According to this doc, self.added("postsSearch", id, fields); informs the client-side that a document has been added to the postsSeach collection.
And according to Meteor.publish:
Alternatively, a publish function can directly control its published record set by calling the functions added (to add a new document to the published record set), ...
So I'm guessing that self.added does both of these operations: Adds a document to the published record set, and informs the client (that has subscribed to the current publication) of this addition.
Now if you see Meteor.subscribe:
When you subscribe to a record set, it tells the server to send records to the client. The client stores these records in local Minimongo collections, with the same name as the collection argument used in the publish handler's added, changed, and removed callbacks. Meteor will queue incoming records until you declare the Mongo.Collection on the client with the matching collection name.
This suggests 2 things:
You have to subscribe in order to receive the data from the server-side database.
Some kind of client-side code must exist in order to create a client-only postsSearch collection. (this is because you said, this collection doesn't exist on server-side database).
The 2nd point can be achieved quite easily, for example:
if(Meteor.isClient) {
postsSearch = new Mongo.Collection(null);
}
In the above example, the postsSearch collection will exist only on the client and not on the server.
And regarding the 1st, being subscribed to postsSearchPub will automatically send data for the postsSearch collection to the client (even if said collection doesn't exist in the server-side database. This is because of the explicit call to self.added).
Something to check out: According to this doc, self.ready(); calls the onReady callback of the subscription. It would be useful to see what is there in this callback, perhaps the client-only postsSearch collection is defined there?
From the doc:
this.added(collection, id, fields)
Call inside the publish function.
Informs the subscriber that a document has been added to the record set.
This means that the line self.added("postsSearch", id, fields); emulates the fact that an insert has been done to the PostsSearch collection although it's obviously not the case.
Concerning the absence of MongoDB collection, it could be related to Meteor laziness which creates the MongoDB collection at first insert, not sure though.
Same situation like this question, but with current DerbyJS (version 0.6):
Using imported docs from MongoDB in DerbyJS
I have a MongoDB collection with data that was not saved through my
Derby app. I want to query against that and pull it into my Derby app.
Is this still possible?
The accepted answer there links to a dead link. The newest working link would be this: https://github.com/derbyjs/racer/blob/0.3/lib/descriptor/query/README.md
Which refers to the 0.3 branch for Racer (current master version is 0.6).
What I tried
Searching the internets
The naïve way:
var query = model.query('projects-legacy', { public: true });
model.fetch(query, function() {
query.ref('_page.projects');
})
(doesn't work)
A utility was written for this purpose: https://github.com/share/igor
You may need to modify it to only run against a single collection instead of the whole database, but it essentially goes through every document in the database and modifies it with the necessary livedb metadata and creates a default operation for it as well.
In livedb every collection has a corresponding operations collection, for example profiles will have a profiles_ops collection which holds all the operations for the profiles.
You will have to convert the collection to use it with Racer/livedb because of the metadata on the document itself.
An alternative if you dont want to convert is to use traditional AJAX/REST to get the data from your mongo database and then just put it in your local model. This will not be real-time or synced to the server but it will allow you to drive your templates from data that you dont want to convert for some reason.