Snap! Websites Processes

Snap! Websites stackThe system process including all the steps I can think of at the moment.


Assuming you give the Snap! server permissions, it blocks IP addresses that misbehave with your firewall. Misbehavior is detected as spam posts, fast hits, or unwanted accesses.

Apache Server

Start the Apache server, check whether it is better (faster) to run Apache and have all special features (redirects and such) in the snap.cgi (i.e. dynamic) or in Apache (i.e. but that last option requires a reload of Apache every now and then!)

See whether we gain anything in having a static cache. The problem with a static cache is that it will not change pages depending on things such as a mobile phone or IE accessing your website.


This one can handle a fast cache. Slower than the static cache if we have many rules to access the cache but still certainly a lot faster than asking the main snap server for advice.

Snap! Server

Start the Snap! server which can then connect to the Cassandra proxy server to access all the data.

Because of the way the design works, objects we create in the server have an instance() function. This means all the objects are unique. We may want to look in a way to create a server instance, and all the plugins get loaded once on server start up, but they are included in the running instance (thread) by creating a clone instance and the server keeps a list of those. This means each time plugin A wants to connect to plugin B, A first needs to find the pointer to B, then call the connect function on B. At this time, this makes use of the instance() pointer.

The Snap! server listen for connections on a TCP/IP socket port 4004 by default. This allows for each snapserver to run on a separate computer from the Apache server.

The Ready Cache is a cache created by a backend (a permanently running process on a server that is not answering requests on the Internet) to prepare pages so they load much faster than by having to recreate them on the fly (at least it should be, especially if the page layout is complex.)

Cassandra Proxy Servers

The Cassandra Proxy Servers allows our Snap! Servers to quickly and always connect to a Cassandra node. It knows of several Cassandra nodes to connect to in order to maintain high connectivity. (i.e. if a node goes down, then the system know how to continue to run by switching and sending requests to a different node.)

The proxy also removes the need to do retries and other similar issues we can have using a direct connection.

Backend Processors

The Backend Processors are similar to the Snap! Servers, only they may run on computers that are not accepting front end connections. This is particularly useful to avoid taking processing time and large amount of memory on computers that are used to directly communicate with clients.


  • Apache receives a hit
    • Apache checks hit validity (modsecurity and alike)
    • Apache checks the static cache, return that file if present here
    • Apache transforms the request in order to run the snap.cgi application (using ?q=<path>)
  • snap.cgi takes over
    • connect to snapserver via TCP/IP
    • forwards the Apache request
    • wait for the reply
    • return the reply to Apache
  • snapserver accepts connections from all snap.cgi requests
    • create a snap_child object
    • parse the URL
    • connect to corresponding Cassandra database via a Cassandra Proxy Server
    • load the plugins that are enabled for that website
    • send bootstrap() signal
      • plug-ins attach themselves to all the necessary signals
      • plugin_exists() can already be used here
      • this is nearly the only thing happening here (connecting to signals) since you cannot be sure that all the plugins are connected until this signal is applied to all of them
    • send init() signal
      • plugins register their dynamic callbacks (i.e. name to function mapping)
      • note that we cannot call these callbacks yet... pretty much all the dynamic callbacks are used for are path to function mapping
    • send execute(path) signal
      • check whether the path is a redirect, if so, process as such
        • TBD: look into putting redirects directly in the Apache settings (probably with a redirect.conf file; but be sure it happens only for the given website so we need the correct server info and we may not be able to duplicate server entries...)
      • verify that the user has the necessary permissions to access this page
      • convert the path in a page
      • the path plugin capture this signal and transform it into a call to the dynamic callback as defined by the init() signal
      • at this point, the process depends on what was just signaled
    • ... (see below, each plugin has its own process, although they may use the default content/layout mechanism by calling those functions directly)
    • get the output and send it to snap.cgi via the TCP/IP connection; the output is one large array of UTF-8 text; everything is 100% buffered and sent at the end only if it worked; this way we can be sure to have the ability to change the headers if an error occurs and we change direction 180°
    • exit (no complete "de-initialization" necessary in Unix)

Note that the server and snap_child processes (i.e. the server forks and then runs the connection in the snap_child process) do not make use of threads except in one single case at this point: if it runs a process that require a two way FIFO, the input is handled by the main process and the output is handled by a thread. The is the best way to ensure that the FIFO doesn't block (i.e. input is missing, too much output not read).

Current Implementation. There is still one thread used in the server to handle the STOP and NLOG signals the server may receive via UDP. This thread will be removed once the snapcommunicator is available.

The following figure represents an installation. Each box represents a "physical" computer (these could of course be virtual computers, but if you have your own computers, really, physical computers is probably best since with such an installation and a proper load balancing, you can turn off/disconnect any one computer for maintenance and reconnect it later.)


The plugins should probably each have their own page with their own expected processes.

Right now, we consider that two main plugins can be listed here because they are the major processes happening pretty much all the time (i.e. mandatory plugins).


The init() and execute() signals are already part of the path plugin process. The init() requests that all the existing plugins register their name to function entries (using boost signals as the underlaying implementation.)

The execute() matches the path to the name which is then matched to a virtual function. A plugin has to implement that virtual function if it decided to execute a specific path.

However, before calling a function, it will:

(1) see whether the path should generate a redirection1;

We still have a snap_child::page_redirect() function, for module that have to dynamically redirect users (i.e. log in page, once the user is logged in, redirects you to the page you came from.)

(2) check that the current user has proper permissions to access the current page;

See talk about the Permissions feature [core] for details.

There is a verify_permissions() signal which tanslate in two signals:

  • get_user_rights() to retrieve the set of all the rights the user has; and
  • get_plugin_permissions() that retrieves sets of rights, in general we have one set per plugin, and each plugin may offer one or more right

The verification process terminates with the computation of the intersection of the user set with each one of the plugin sets; if any result is the empty set, then the user does not have permission.


The content (pages) is the main component of a website. It is therefore described here.


  • 1. Note that we could have "soft" redirections; that is, a redirection not sent back to the browser.
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