uWSGI Subscription Server

Some components of the uWSGI stack require a key-value mapping system.

For example the The uWSGI FastRouter needs to know which server to contact for a specific request.

In big networks with a lot of nodes manually managing this configuration could be a real hell. uWSGI implements a subscription system where the node itself announces its presence to Subscription Servers, which will in turn populate their internal dictionaries.

uwsgi --fastrouter :1717 --fastrouter-subscription-server

This will run an uWSGI fastrouter on port 1717 and create an empty dictionary where the hostname is the key and the uwsgi address is the value.

To populate this dictionary you can contact, the address of the subscription server.

For every key multiple addresses can exist, enabling load balancing (various algorithms are available).

A node can announce its presence to a Subscription Server using the subscribe-to or subscribe2 options.

uwsgi --socket --wsgi myapp -M --subscribe-to

The FastRouter will map every request for uwsgi.it to

To now add a second node for uwsgi.it simply run it and subscribe:

uwsgi --socket --wsgi myapp --master --subscribe-to

Dead nodes are automatically removed from the pool.

The syntax for subscribe2 is similar but it allows far more control since it allows to specify additional options like the address to which all requests should be forwarded. Its value syntax is a string with “key=value” pairs, each separated by a comma.

uwsgi -s --wsgi myapp --master --subscribe2 server=,key=uwsgi.it,addr=

For a list of the available subscribe2 keys, see below.

The subscription system is currently available for cluster joining (when multicast/broadcast is not available), the Fastrouter, the HTTP/HTTPS/SPDY router, the rawrouter and the sslrouter.

That said, you can create an evented/fast_as_hell HTTP load balancer in no time.

uwsgi --http :80 --http-subscription-server --master

Now simply subscribe your nodes to the HTTP subscription server.

You can check the subscription server stats and/or the subscribed nodes through the http-stats-server option.

uwsgi --http :80 --http-subscription-server --http-stats-server --master

You can also forward subscription requests to another server with the option http-resubscribe.

uwsgi --http :80 --http-subscription-server --http-resubscribe --master

Securing the Subscription System

The subscription system is meant for “trusted” networks. All of the nodes in your network can potentially make a total mess with it.

If you are building an infrastructure for untrusted users or you simply need more control over who can subscribe to a Subscription Server you can use openssl rsa public/private key pairs for “signing” you subscription requests.

# First, create the private key for the subscriber. DO NOT SET A PASSPHRASE FOR THIS KEY.
openssl genrsa -out private.pem
# Generate the public key for the subscription server:
openssl rsa -pubout -out test.uwsgi.it_8000.pem -in private.pem

The keys must be named after the domain/key we are subscribing to serve, plus the .pem extension.


If you’re subscribing to a pool for an application listening on a specified port you need to use the domain_port.pem scheme for your key files. Generally all of the DNS-allowed chars are supported, all of the others are mapped to an underscore.

An example of an RSA protected server looks like this:

master = 1
http = :8000
http-subscription-server =
subscriptions-sign-check = SHA1:/etc/uwsgi/keys

The last line tells uWSGI that public key files will be stored in /etc/uwsgi/keys.

At each subscription request the server will check for the availability of the public key file and use it, if available, to verify the signature of the packet. Packets that do not correctly verify are rejected.

On the client side you need to pass your private key along with other subscribe-to options. Here’s an example:

socket =
subscribe-to =,5,SHA1:/home/foobar/private.pem
psgi = test.psgi

Let’s analyze the subscribe-to usage:

  • is the subscription server we want to subscribe to.

  • test.uwsgi.it:8000 is the subscription key.

  • 5 is the modifier1 value for our psgi app

  • SHA1:/home/private/test.uwsgi.it_8000.pem is the <digest>:<rsa> couple for authenticating to the server (the <rsa> field is the private key path).


Please make sure you’re using the same digest method (SHA1 in the examples above) both on the server and on the client.

To avoid replay attacks, each subscription packet has an increasing number (normally the unix time) avoiding the allowance of duplicated packets. Even if an attacker manages to sniff a subscription packet it will be unusable as it is already processed previously. Obviously if someone manages to steal your private key he will be able to build forged packets.

Using SSH keys

SSH-formatted keys are generally loved by developers (well, more than classic PEM files).

Both –subscribe-to and –subscribe2 (see below) support SSH private keys, while for the server part you have the encode the public key in pkcs8:

ssh-keygen -f chiavessh001.pub -e -m pkcs8


This is the keyval version of –subscribe-to. It supports more tricks and a (generally) more readable syntax:

uwsgi --socket 127.*:0 --subscribe2 server=,key=ubuntu64.local:9090,sign=SHA1:chiavessh001

Supported fields are:

  • server the address of the subscription server

  • key the key to subscribe (generally the domain name)

  • addr the address to subscribe (the value of the item)

  • socket the socket number (zero-based), this is like ‘addr’ by take the uWSGI internal socket number

  • weight the load balancing value

  • modifier1 and modifier2

  • sign <algo>:<file> the signature for the secured system

  • check it takes a file as argument. If it exists the packet is sent, otherwise it is skipped

  • sni_key set the keyfile to use for SNI proxy management

  • sni_crt set the crt file to use for SNI proxy management

  • sni_ca set the ca file to use for SNI proxy management

  • algo (uWSGI 2.1) set the load balancing algorithm to use (they are pluggable, included are wrr, lrc, wlrc and iphash)

  • proto (uWSGI 2.1) the protocol to use, by default it is ‘uwsgi’

  • backup (uWSGI 2.1) set the backup level (change meaning based on algo)


When you subscribe to a server, you can ask it to “acknowledge” the acceptance of your request.

Just add --subscription-notify-socket <addr> pointing to a datagram (Unix or UDP) address, on which your instance will bind and the subscription server will send acknowledgements to.

Mountpoints (uWSGI 2.1)

Generally you subscribe your apps to specific domains.

Thanks to the mountpoint support introduced in uWSGI 2.1, you can now subscribe each node to a specific directory (you need to specify how much levels you want to support):

First of all you need to tell the subscription server to accept (and manage) mountpoint requests:

uwsgi --master --http :8080 --http-subscription-server --subscription-mountpoints 1

Then you can start subscribing to mountpoints.

uwsgi --socket --subscribe2 server=,key=mydomain.it/foo
uwsgi --socket --subscribe2 server=,key=mydomain.it/bar
uwsgi --socket --subscribe2 server=,key=mydomain.it/foo
uwsgi --socket --subscribe2 server=,key=mydomain.it

The first and the third instance will answer to all of the requests for /foo, the second will answer for /bar and the last one will manage all of the others.

For the secured subscription system, you only need to use the domain key (you do not need to generate a certificate for each mountpoint).

If you want to support mountpoints in the form /one/two instead of /one, just pass ‘2’ to –subscription-mountpoints and so on. For performance reason you need to choose how much elements your path can support, and you cannot mix them (read: if –subscription-mountpoints is 2 you can support /one/two or /foo/bar but not /foobar)