The uWSGI Legion subsystem¶
As of uWSGI 1.9-dev a new subsystem for clustering has been added: The Legion subsystem. A Legion is a group of uWSGI nodes constantly fighting for domination. Each node has a valor value (different from the others, if possible). The node with the highest valor is the Lord of the Legion (or if you like a less gaming nerd, more engineer-friendly term: the master). This constant fight generates 7 kinds of events:
- setup - when the legion subsystem is started on a node
- join - the first time quorum is reached, only on the newly joined node
- lord - when this node becomes the lord
- unlord - when this node loses the lord title
- death - when the legion subsystem is shutting down
- node-joined - when any new node joins our legion
- node-left - when any node leaves our legion
You can trigger actions every time such an event rises.
Note: openssl headers must be installed to build uWSGI with Legion support.
This is a very common configuration for clustered environments. The IP address is a resource that must be owned by only one node. For this example, that node is our Lord. If we configure a Legion right (remember, a single uWSGI instances can be a member of all of the legions you need) we could easily implement IP takeover.
[uwsgi] legion = clusterip 184.108.40.206:4242 98 bf-cbc:hello legion-node = clusterip 220.127.116.11:4242 legion-lord = clusterip cmd:ip addr add 192.168.173.111/24 dev eth0 legion-lord = clusterip cmd:arping -c 3 -S 192.168.173.111 192.168.173.1 legion-setup = clusterip cmd:ip addr del 192.168.173.111/24 dev eth0 legion-unlord = clusterip cmd:ip addr del 192.168.173.111/24 dev eth0 legion-death = clusterip cmd:ip addr del 192.168.173.111/24 dev eth0
In this example we join a legion named clusterip. To receive messages from the other nodes we bind on the multicast address 18.104.22.168:4242. The valor of this node will be 98 and each message will be encrypted using Blowfish in CBC with the shared secret hello. The legion-node option specifies the destination of our announce messages. As we are using multicast we only need to specify a single “node”. The last options are the actions to trigger on the various states of the cluster. For an IP takeover solution we simply rely on the Linux iproute commands to set/unset ip addresses and to send an extra ARP message to announce the change. Obviously this specific example requires root privileges or the CAP_NET_ADMIN Linux capability, so be sure to not run untrusted applications on the same uWSGI instance managing IP takeover.
To choose a Lord each member of the legion has to cast a vote. When all of the active members of a legion agree on a Lord, the Lord is elected and the old Lord is demoted. Every time a new node joins or leaves a legion the quorum is re-computed and logged to the whole cluster.
Choosing the Lord¶
Generally the node with the higher valor is chosen as the Lord, but there can be cases where multiple nodes have the same valor. When a node is started a UUID is assigned to it. If two nodes with same valor are found the one with the lexicographically higher UUID wins.
Even though each member of the Legion has to send a checksum of its internal cluster-membership, the system is still vulnerable to the split brain problem. If a node loses network connectivity with the cluster, it could believe it is the only node available and starts going in Lord mode.
For many scenarios this is not optimal. If you have more than 2 nodes in a legion you may want to consider tuning the quorum level. The quorum level is the amount of votes (as opposed to nodes) needed to elect a lord. legion-quorum is the option for the job. You can reduce the split brain problem asking the Legion subsystem to check for at least 2 votes:
[uwsgi] legion = clusterip 22.214.171.124:4242 98 bf-cbc:hello legion-node = clusterip 126.96.36.199:4242 legion-quorum = clusterip 2 legion-lord = clusterip cmd:ip addr add 192.168.173.111/24 dev eth0 legion-lord = clusterip cmd:arping -c 3 -S 192.168.173.111 192.168.173.1 legion-setup = clusterip cmd:ip addr del 192.168.173.111/24 dev eth0 legion-unlord = clusterip cmd:ip addr del 192.168.173.111/24 dev eth0 legion-death = clusterip cmd:ip addr del 192.168.173.111/24 dev eth0
As of 1.9.7 you can use nodes with valor 0 (concept similar to MongoDB’s Arbiter Nodes), such nodes will be counted when checking for quorum but may never become The Lord. This is useful when you only need a couple nodes while protecting against split-brain.
Each one of the four phases of a legion can trigger an action. The actions system is modular so you can add new kinds of actions. Currently the supported actions are:
Run a shell command.
Raise a uWSGI signal.
Log a message. For example you could combine the log action with the alarm subsystem to have cluster monitoring for free.
Multicast, broadcast and unicast¶
Even if multicast is probably the easiest way to implement clustering it is not available in all networks. If multicast is not an option, you can rely on normal IP addresses. Just bind to an address and add all of the legion-node options you need:
[uwsgi] legion = mycluster 192.168.173.17:4242 98 bf-cbc:hello legion-node = mycluster 192.168.173.22:4242 legion-node = mycluster 192.168.173.30:4242 legion-node = mycluster 192.168.173.5:4242
This is for a cluster of 4 nodes (this node + 3 other nodes)
You can join multiple legions in the same instance. Just remember to use different addresses (ports in case of multicast) for each legion.
[uwsgi] legion = mycluster 192.168.173.17:4242 98 bf-cbc:hello legion-node = mycluster 192.168.173.22:4242 legion-node = mycluster 192.168.173.30:4242 legion-node = mycluster 192.168.173.5:4242 legion = mycluster2 188.8.131.52:4243 99 aes-128-cbc:secret legion-node = mycluster2 184.108.40.206:4243 legion = anothercluster 220.127.116.11:4244 91 aes-256-cbc:secret2 legion-node = anothercluster 18.104.22.168:4244
Each packet sent by the Legion subsystem is encrypted using a specified cipher, a preshared secret, and an optional IV (initialization vector). Depending on cipher, the IV may be a required parameter. To get the list of supported ciphers, run openssl enc -h.
Each node of a Legion has to use the same encryption parameters.
To specify the IV just add another parameter to the legion option.
[uwsgi] legion = mycluster 192.168.173.17:4242 98 bf-cbc:hello thisistheiv legion-node = mycluster 192.168.173.22:4242 legion-node = mycluster 192.168.173.30:4242 legion-node = mycluster 192.168.173.5:4242
To reduce the impact of replay-based attacks, packets with a timestamp lower than 30 seconds are rejected. This is a tunable parameter. If you have no control on the time of all of the nodes you can increase the clock skew tolerance.
Tuning and Clock Skew¶
Currently there are three parameters you can tune. These tuables affect all Legions in the system. The frequency (in seconds) at which each packet is sent (legion-freq <secs>), the amount of seconds after a node not sending packets is considered dead (legion-tolerance <secs>), and the amount of clock skew between nodes (legion-skew-tolerance <secs>). The Legion subsystem requires tight time synchronization, so the use of NTP or similar is highly recommended. By default each packet is sent every 3 seconds, a node is considered dead after 15 seconds, and a clock skew of 30 seconds is tolerated. Decreasing skew tolerance should increase security against replay attacks.
Lord scroll (coming soon)¶
The Legion subsystem can be used for a variety of purposes ranging from master election to node autodiscovery or simple monitoring. One example is to assign a “blob of data” (a scroll) to every node, One use of this is to pass reconfiguration parameters to your app, or to log specific messages. Currently the scroll system is being improved upon, so if you have ideas join our mailing list or IRC channel.
You can know if the instance is a lord of a Legion by simply calling
int uwsgi_legion_i_am_the_lord(char *legion_name);
It returns 1 if the current instance is the lord for the specified Legion.
- The Python plugin exposes it as uwsgi.i_am_the_lord(name)
- The PSGI plugin exposes it as uwsgi::i_am_the_lord(name)
- The Rack plugin exposes it as UWSGI::i_am_the_lord(name)
Obviously more API functions will be added in the future, feel free to expose your ideas.
The Legion information is available in the The uWSGI Stats Server. Be sure to understand the difference between “nodes” and “members”. Nodes are the peer you configure with the legion-node option while members are the effective nodes that joined the cluster.
The old clustering subsystem¶
During 0.9 development cycle a clustering subsystem (based on multicast) was added. It was very raw, unreliable and very probably no-one used it seriously. The new method is transforming it in a general API that can use different backends. The Legion subsystem can be one of those backends, as well as projects like corosync or the redhat cluster suite.