Docker Swarm strategies
The Docker Swarm scheduler features multiple strategies for ranking nodes. The strategy you choose determines how Swarm computes ranking. When you run a new container, Swarm chooses to place it on the node with the highest computed ranking for your chosen strategy.
To choose a ranking strategy, pass the --strategy flag and a strategy value to the swarm manage command. Swarm currently supports these values:
spreadbinpackrandom
The spread and binpack strategies compute rank according to a node’s available CPU, its RAM, and the number of containers it has. The random strategy uses no computation. It selects a node at random and is primarily intended for debugging.
Your goal in choosing a strategy is to best optimize your cluster according to your company’s needs.
Under the spread strategy, Swarm optimizes for the node with the least number of containers. The binpack strategy causes Swarm to optimize for the node which is most packed. Note that a container occupies resource during its life cycle, including exited state. Users should be aware of this condition to schedule containers. For example, spread strategy only checks number of containers disregarding their states. A node with no active containers but high number of stopped containers may not be selected, defeating the purpose of load sharing. User could either remove stopped containers, or start stopped containers to achieve load spreading. The random strategy, like it sounds, chooses nodes at random regardless of their available CPU or RAM.
Using the spread strategy results in containers spread thinly over many machines. The advantage of this strategy is that if a node goes down you only lose a few containers.
The binpack strategy avoids fragmentation because it leaves room for bigger containers on unused machines. The strategic advantage of binpack is that you use fewer machines as Swarm tries to pack as many containers as it can on a node.
If you do not specify a --strategy Swarm uses spread by default.
Spread strategy example
In this example, your cluster is using the spread strategy which optimizes for nodes that have the fewest containers. In this cluster, both node-1 and node-2 have 2G of RAM, 2 CPUs, and neither node is running a container. Under this strategy node-1 and node-2 have the same ranking.
When you run a new container, the system chooses node-1 at random from the Swarm cluster of two equally ranked nodes:
$ docker tcp://<manager_ip:manager_port> run -d -P -m 1G --name db mysql f8b693db9cd6 $ docker tcp://<manager_ip:manager_port> ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES f8b693db9cd6 mysql:latest "mysqld" Less than a second ago running 192.168.0.42:49178->3306/tcp node-1/db
Now, we start another container and ask for 1G of RAM again.
$ docker tcp://<manager_ip:manager_port> run -d -P -m 1G --name frontend nginx 963841b138d8 $ docker tcp://<manager_ip:manager_port> ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 963841b138d8 nginx:latest "nginx" Less than a second ago running 192.168.0.42:49177->80/tcp node-2/frontend f8b693db9cd6 mysql:latest "mysqld" Up About a minute running 192.168.0.42:49178->3306/tcp node-1/db
The container frontend was started on node-2 because it was the node the least loaded already. If two nodes have the same amount of available RAM and CPUs, the spread strategy prefers the node with least containers.
BinPack strategy example
In this example, let’s says that both node-1 and node-2 have 2G of RAM and neither is running a container. Again, the nodes are equal. When you run a new container, the system chooses node-1 at random from the cluster:
$ docker tcp://<manager_ip:manager_port> run -d -P -m 1G --name db mysql f8b693db9cd6 $ docker tcp://<manager_ip:manager_port> ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES f8b693db9cd6 mysql:latest "mysqld" Less than a second ago running 192.168.0.42:49178->3306/tcp node-1/db
Now, you start another container, asking for 1G of RAM again.
$ docker tcp://<manager_ip:manager_port> run -d -P -m 1G --name frontend nginx 963841b138d8 $ docker tcp://<manager_ip:manager_port> ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 963841b138d8 nginx:latest "nginx" Less than a second ago running 192.168.0.42:49177->80/tcp node-1/frontend f8b693db9cd6 mysql:latest "mysqld" Up About a minute running 192.168.0.42:49178->3306/tcp node-1/db
The system starts the new frontend container on node-1 because it was the node the most packed already. This allows us to start a container requiring 2G of RAM on node-2.
If two nodes have the same amount of available RAM and CPUs, the binpack strategy prefers the node with most containers.
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