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Independent deployment is a central aim of microservices. Besides, the deployment has to be automated because manual deployment or even just manual corrections are not feasible due to the large number of microservices.

Deployment Automation

There are different possibilities for automating deployment:

  • Installation scripts can be used that only install the software on the computer. Such scripts can, for instance, be implemented as shell scripts. They can install necessary software packages, generate configuration files, and create user accounts. Such scripts can be problematic when they are called repeatedly. In that case the installation finds a computer on which the software is already installed. However, an update is different from a fresh installation. In such a situation a script can fail, for example, because user accounts or configuration files might already be present and cannot easily be overwritten. When the scripts are supposed to handle updates, development and testing the scripts get more laborious.
  • Immutable servers are an option to handle these problems. Instead of updating the software on the servers, the server is completely deployed anew. This facilitates not only the automation of deployment but also the exact reproduction of the software installed on a server. It is sufficient to consider fresh installations. A fresh installation is easier to reproduce than an update, which can be started from many different configuration states and should lead to the same state from any of those. Approaches like Docker29 make it possible to tremendously reduce the expenditure for installing software. Docker is a kind of lightweight virtualization. It also optimizes the handling of virtual hard drives. If there is already a virtual hard drive with the correct data, it is recycled instead of installing the software anew. When installing a package like Java, first a virtual hard drive is looked for that already has this installation. Only when one does not exist is the installation really performed. Should there only be a change in a configuration file when going from an old to a new version of an immutable server, Docker will recycle the old virtual hard drives behind the scenes and only supplement the new configuration file. This does not only reduce the consumption of hard drive space, but also profoundly speeds up the installation of the servers. Docker also decreases the time a virtual team needs for booting. These optimizations turn immutable server in conjunction with Docker into an interesting option. The new deployment of the servers is very fast with Docker, and the new server can also rapidly be booted.
  • • Other possibilities are tools like Puppet,30 Chef,31 Ansible,32 or Salt.33 They are specialized for installing software. Scripts for these tools describe what the system is supposed to look like after the installation. During an installation run the tool will take the necessary steps to transfer the system into the desired state. During the first run on a fresh system the tool completely installs the software. If the installation is run a second time immediately afterwards, it will not change the system any further since the system is already in the desired state. Besides, these tools can uniformly install a large number of servers in an automated manner and are also able to roll out changes to a large number of servers.
  • • Operating systems from the Linux area possess package managers like rpm (RedHat), dpkg (Debian/Ubuntu), or zypper (SuSE). They make it possible to centrally roll out software onto a large number of servers. The file formats used are very simple, so that it is very easy to generate a package in a fitting format. The configuration of the software poses a problem, though. Package managers usually support scripts that are executed during installation. Such scripts can generate the necessary configuration files. However, there can also be an extra package with the individual configurations for each host. The installation tools mentioned under the last bullet point can also use package manager for installing the actual software so that they themselves only generate the configuration files.
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