What Goes Where: Making Sense of Meteor's Client:Server Split - Discover Meteor

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JUL 7 2015 by Sacha Greif inARTICLES

What Goes Where: Making Sense of Meteor's

Client/Server Split
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One of Meteors biggest selling point is the way it blurs the line between client and server.

With Meteor, these two environments are no longer separate worlds: they share a common language, and

can even share common code!

But that blurring of lines comes at a cost: it can sometimes be tricky to figure out what should run where.

Client vs Server

Lets start from the beginning. Meteor runs using the JavaScriptlanguage, which is unique among

(nearly) every other programming languagesin that it can be executed by browsers. In other words,

Chrome, Safari, Firefox and all their friends can look at a bit of JavaScript code and make sense of it.

Try that with Ruby, Python, or C, and your browser will just look at you with a puzzled look on its face.

But wait! Browsers are not the only ones that can execute JavaScript code. So can servers, thanks to

Node.js.

This is where Meteor comes in: by providing layers on top of boththe server and the client, Meteor can

bridge the gap and make the two sides talk to each other more smoothly.

Meteors Control Methods

Meteor gives you three basic ways of controlling where a fileis executed:

In your app, you can use the /client and /server directoriesto restrict a file to only run in a

specific environment.

In packages, you can pass "client" or "server" as argumentsto the api.addFile() function to

only load a file in a specific environment.

You can also control where a code blockinsidea file is executed (supposing the file itself runs in both

environment) thanks to the Meteor.isClient and Meteor.isServer booleans.

For this article, well only focus on the first scenario: controlling where a file is loaded using the /client

and /server directories.

Meteors File Architecture

Its also important to point out that Meteor doesnt really care whereyou put your code past the
https://nodejs.org/


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client/server/both distinction. In other words, there is no practical difference between putting

your code in foo/bar.js and putting it in bar/foo.js .

That being said, if you need a little more structure we recommend checking out Microscopeto

see what a typical Meteor app looks like.

[Client] Browser-Specific JavaScript

One of the first JavaScript function youll encounter when learning the language is the humble alert() :

alert("Hello world!");

You can test this right now if youd like; all it does is pop open a system dialog containing the string you

passed as argument.

Now heres a question for you: if I log on to my server, open a Node.js console, and feed it this line of

JavaScript code, where will the alert show up?

If your answer was something along the line of What!? That doesnt make any sense! then

congratulations, you got it right. This was a trick question: alert() is clearly something that only makes

sense in the context of the browser, since it needs a system to open the dialog, a screen to render it on,

and a user to close it.

Now consider the following, used to set the scroll position of the body of a page to 300px from the top:

$("body").scrollTop(300);

A perfectly valid bit of JavaScript code (jQuerycode, to be exact), yet one that makes no sense in the

context of the server. The server has no concept of window, body, or scrolling. Its just a dumb box sitting

in the middle of some data center!

Hopefully youre starting to see the pattern: any JavaScript code that specifically targets the browser

should only run on the client (note that subscriptionsalso fall in this category).
http://docs.meteor.com/#/full/meteor_publishhttp://jquery.com/http://alert%28%27hello%20world%21%27%29/https://github.com/DiscoverMeteor/Microscope


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The server has no clue what this thing is

[Client] User-Specific JavaScript

Additionally, theres another category of code that should only run on the client: code that targets the

current user.

Again, it makes sense if you think about it: the server is in charge of serving your app to hundreds, if not

thousands of users. It doesnt havea current user.

The Method Exception

The only exception to the server doesnt know about the user rule is Meteor Methods.

A Method is a function that runs on the server, but is called from the client. This means the

server knows which specific usermade the call, and this is whyMeteor.user()

andMeteor.userId() (as well as the equivalent this.userId ) can be used inside Methods code,

even on the server.
http://docs.meteor.com/#/full/meteor_methods


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Theres another Meteor feature thats specific to the current user: Sessionvariables.

In fact, the same can be said about client-side variables in general: even if client-side variables were

somehow sent back to the server (and they arent), it wouldnt really make sense. If Alice defines x

equal to foo but Bob sets it equal to bar , how is the server supposed to know which one to use?

Alice and Bob arguing about what the value of x should be

[Client] Templates & Stylesheets

Although this might change in the future, both templates and stylesheets are currently only executed on

the client.

By extension, this also means any template-related JavaScript code (such as template helpers, template

callbacks, and event handlers) will only be executed on the client as well.

The one exception to this is emailtemplate: since emails are sent out by the server, you can use a

package such as handlebars-serverto define specific, server-only templates and styles.
https://github.com/cmather/meteor-handlebars-serverhttp://docs.meteor.com/#/full/session


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[Both] Collections

So if templates, stylesheets, template-related code, user-specific code, and browser-related code all run

on the client, what does that leave for the server?

A couple important things, as youll soon see. But before we can get there, we need to talk about code

that needs to run on boththe client andserver.

The main example of this is collectionscode. Another one of Meteors innovations is the ability to mirror

some of your data on the client, and this is done through the afore-mentioned collections.

For that reason, it makes sense to define collections in both environments (in other words, in any

directory besides /client or /server ). Of course, collection-related code (such as helpers or validation

code) will also come along for the ride.

[Both] Methods

Meteor also features something called optimistic UI (a.k.a. latency compensation). This is a fancy way

of saying that if an operation needs to involve the server (which would necessarily slow it down), the

client will try to complete that operation by itself first, and then amend the result if needed once it gets

the servers actual reply.

Yet this can only happen if the client knows what the operation consists of, which is why method code is

often shared among both environments.

[Server] Secure Data

Finally, we get to the server, and just the server.

Unlike the client, the server has direct access to your whole database, which gives it more control over

your data.

For example, one of the main tasks of the server is publishingdata to the client. This is done through

publications, which are server-only. Since publications control which subset of your data should be made

public (and which subset should remain secure), it stands to reason that this is not something you want

your users to be able to control.

In fact, generally speaking any operation that involves private data like email addresses or API keys will

be best handled by the server. After all, you wouldnt want to expose that data to any random user!


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[Server] Time-Sensitive Operations

Finally, another category of server-only operations is anything involving time: its trivial for a user to fake

their browsers time (not to mention the mess involved with timezones), so you cant rely on it for any

serious timekeeping.

So cronjob-type operations that needs to happen at regular intervals, rate-limiting, and timestamping

should all be left to the server.

A Real-World Example

Lets bring all of this together by looking at the code of Microscope, the app that you build in Discover

Meteor.

Client

First, the client directory contains the following items:

helpers contains global template helpers and code for other client-only features such as error

messages.

stylesheets contains the apps styles.

templates contains the apps templates as well as their companion JavaScript code, split into

subgroups.

main.html and main.js hold the apps main template and its code.

Both

The lib folder contains files shared across client and server:

collections contains the code for the Posts, Comments, and Notifications collections.

permissions.js contains code used to define who can do what, which applies both to client and

server.

router.js deals with client-side and server-side routes.

Server

And finally, server contains server-only code:
https://github.com/DiscoverMeteor/Microscope


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fixtures.js contains code that will seed our database with content the first time we run the app.

pubications.js contains our publication code.

Conclusion

This is by no means meant to be an exhaustive list of every single use case youll encounter while

building Meteor apps.

So I strongly recommend checking out the Meteor documentationwhenever youre not sure if

something should run on the client, server, or both.

But hopefully, it will be enough to give you a few tools to help you figure things out the next time youre

stuck at that Save As dialog wondering where to put your code.

Share this article: Tweet !"# %&' ())*+, ,-.',/+,0 23JaimeJaime

Sacha Greifis a product and web designer who has worked with companies such as

Hipmunkand Le Monde. He now works on open-source Meteor app Telescope, and

Sidebar, a design newsletter.

You can follow him on Twitter.

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Jan 2 2014 inARTICLE

Understanding Meteor Publications & Subscriptions

The way Meteor handles an apps data is one of the frameworks greatest assets, but also one

of the hardest things to wrap your head around...Read more

Allow & Deny: A Security PrimerPREVIOUS ARTICLE

Building Reusable Components With The

Template Controller Pattern NEXT ARTICLE
https://www.discovermeteor.com/blog/meteor-components-template-controller-pattern/https://www.discovermeteor.com/blog/allow-deny-a-security-primer/https://www.discovermeteor.com/blog/understanding-meteor-publications-and-subscriptions/https://www.discovermeteor.com/blog/understanding-meteor-publications-and-subscriptions/https://www.discovermeteor.com/blog/understanding-meteor-publications-and-subscriptions/https://www.discovermeteor.com/category/article/


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15 Comments !1

! !

richylai !

Publications also know the current user.

Valentin Zambelli !

Hey Sacha! I think it is worth mentioning that the server has access to all your data withoutneeding a publication. This means methods that run only on the servercan return data that the

client is not subscribed to. I made the mistake at first of putting all my methods into the [BOTH]

directory, which meant I needed to give the client way more access to data than they needed.

COMMENTS
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! !

If you only need one thing and it doesn't have to be real time (like the .count() on a whole

collection) itwouldbe better to write a server only method for it and return the result directly.

You can even persist it by writing directly to a minimongo collection.

! !

Brad !

I wasjust looking for some discussion on this. So in the event that you need to

reference unpublished data, do you use [SERVER] methods, and then when thepublished data is su"cient, run the method in [BOTH]. I've been contemplating if I'm

approaching my collection operations the "Meteor way" lately... I was debating moving

methods to the [BOTH] folder but this very subject made me hesitate. I wasn't sure if

the client stub would just trip up and wait for the server, throw an exception, etc.

! !

Daniel Arrizza !

When a method is called is run on the client side (the method simulation/stub) doesn't it

only work with the data that is available on the client? That was my assumption.

! !

Valentin Zambelli !

Correct, I don't see how this contradicts my statement? That is exactly why I

advocate having some methods in the [SERVER] scope only. You can access all

the data you want without having to send it to the client.

! !

cluxter !

I think I understand what you mean. Let's take the example of a

chatroom. Alice, Bob and Eve are in the public chatroom. Alice and Bob

are chatting in a private chatroom. In this case Alice has access to her

messages only, so does Bob with his messages. But when they are

chatting, they need to see the messages of each other, without giving Eve

any access to their messages. So if I understand well, I would put a

method in [BOTH] that would take care of their respective messages,

filtering the publication thanks to "userId". They would consequentlyenjoy the latency compensation (but only for their own messages). Then I

would put a method in [SERVER] which is able to see all of the messages,

and this method would send the Bob's messages to Alice according to

what Alice is allowed to see (= the messages they exchanged in their

private conversation) and vice versa. However Eve will never receive any

of the messages that Alice and Bob are exchanging, because each user

has access to its own messages only - the messages from other people

they are chatting with are like "bonuses" that the server send them

because they are chatting with someone in private. Is it what you meant?

Valentin Zambelli !
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Sense%20of%20Meteor%27s%20Client%2FServer%20Split&s_o=default#https://www.discovermeteor.com/blog/what-goes-where/#comment-2121005049https://disqus.com/by/disqus_vF8gxUB3V4/https://www.discovermeteor.com/blog/what-goes-where/#comment-2295909198https://disqus.com/embed/comments/?base=default&version=b7774234ec45ba8fdcfd0eef50361935&f=themeteorbook&t_i=what-goes-where&t_u=https%3A%2F%2Fwww.discovermeteor.com%2Fblog%2Fwhat-goes-where%2F&t_d=What%20Goes%20Where%3A%20Making%20Sense%20of%20Meteor%27s%20Client%2FServer%20Split&t_t=What%20Goes%20Where%3A%20Making%20Sense%20of%20Meteor%27s%20Client%2FServer%20Split&s_o=default#https://disqus.com/embed/comments/?base=default&version=b7774234ec45ba8fdcfd0eef50361935&f=themeteorbook&t_i=what-goes-where&t_u=https%3A%2F%2Fwww.discovermeteor.com%2Fblog%2Fwhat-goes-where%2F&t_d=What%20Goes%20Where%3A%20Making%20Sense%20of%20Meteor%27s%20Client%2FServer%20Split&t_t=What%20Goes%20Where%3A%20Making%20Sense%20of%20Meteor%27s%20Client%2FServer%20Split&s_o=default#https://disqus.com/embed/comments/?base=default&version=b7774234ec45ba8fdcfd0eef50361935&f=themeteorbook&t_i=what-goes-where&t_u=https%3A%2F%2Fwww.discovermeteor.com%2Fblog%2Fwhat-goes-where%2F&t_d=What%20Goes%20Where%3A%20Making%20Sense%20of%20Meteor%27s%20Client%2FServer%20Split&t_t=What%20Goes%20Where%3A%20Making%20Sense%20of%20Meteor%27s%20Client%2FServer%20Split&s_o=default#


12/14

! !

You are still mixing up the concept of pub/sub and methods. "Giving

access to data" I guess can have two meanings: Let me look at data or

let me do something with data. Publications/Subscriptions let you look at

data, so to speak. Methods let you do something with the data. In a

traditional web application the client would send a GET request to look at

data and then a POST or PUT request to prompt the server to do

something with the data.

The twist with Meteor is that you can cache data on the client and resolve

POST/PUT requests directly on the client (this is grossly generalized and

not what Meteor is doing, but in essence I guess it is correct). This is

done by subscribing to data and putting methods in the [BOTH] scope.

Your example: What you could do is have 3 collections: one for public

chatrooms, one for private chatrooms and one for messages. For each

chatroom you save the IDs of the users that are part of it. When a user

logs in they would subscribe to all public and private chatrooms they are

a part of, so that you can display their names in a menubar. When you

! !

cluxter !

Since client A never sees the notifications of client B this is completely

unnecessary.

Couldn't we filter the publication to restrict it to what user A is supposed

to see, and nothing else?

! !

Valentin Zambelli !

If we are saving the notifications with the user directly (in an array in the

profile field for example) you cannot. Lets spin the example further: If you

want to send a notification email each time an in-app notification isgenerated you will need to access the users email address. But you don't

want to let users see the email address of other users. To get around the

need to give users access to the email address of other users you move

the logic into the [SERVER] scope.

! !

cluxter !

Got it! Thank you very much for your time and your answers! :-)

cluxter !

I still don't et it. What I understood so far was: when a method is in
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13/14

! !

[BOTH] scope, you can call it from both server and client. But client will

only access to data it subscribed to (= only data server decides to

publish to this client). So there is no risk (as long as the method restricts

the range of the publication to what the client is allowed to see) that the

client will see more data than what it is supposed to, right? And

apparently you agree:

When a method is called is run on the client side (the methodsimulation/stub) doesn't it only work with the data that is available on

the client?

Correct, I don't see how this contradicts my statement?

So why do you say?:

You can access all the data you want without having to send it to the

client.

Why would I send it to the client? Isn't the method supposed to restrict

the range of the subscription to what the client is authorized to see? I'm

confused.

! !

Valentin Zambelli !

I think you are mixing up 2 concepts. Pub/sub and methods are notdirectly related. Methods do not restrict your access to data per se. (deny

and allow does that). One thing that I think you are missing is that

methods in the [SERVER] scope can also be accessed from the client.

The di#erence is, that you don't need to publish the data that the method

is working on, since it will only execute in the server environment (i.e.

directly accessing the database). When a method is in the [BOTH] scope

you NEED to give the client access to the data, otherwise the method will

fail.

Think of it this way: Minimongo is an intelligent cache. When you put a

method in the [BOTH] scope the client will try to access this cache and

afterwards tell the server:" Hey, this is what I did, take a look at it." When

you put it in the [SERVER] scope the client simply tells the server:" Hey,

please do this thing,thanks a lot" (much like a traditional http requests

works).

The upside of having the method in the [BOTH] directory is latency

compensation: The client sees the change almost instantly, withouthaving to wait for the server to come back with a response. The

downside is that the client will need to subscribe to the data .
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14/14

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Made in Osaka & Melbourne. 2015 Tom Colemanand Sacha Greif

! !

Sacha Greif ! Mod

Great explanation :)

! !

Daniel Arrizza !

You're right. I often use the 'isSimulation' property for cases when there's

server only portions of the method - usually to do with creating activityfeeds/notifications/sending emails.

! !

Sacha Greif ! Mod

Good point :)
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Documents

What Goes Where: Making Sense of Meteor's Client:Server Split - Discover Meteor