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install anyadditional packages andbringuptheservice. This technique isknownas frying
the image, because the image is cooked up while you wait.
Automated Baking
Baked images can be installed more rapidly because all the configuration is done ahead of
time. When tuning up hundreds of machines, this can be a significant win. Unfortunately,
maintainingmanyimagescanbeaburden.Installingapatchonmanyimages,forexample,
is labor intensive. When installation is done this way, there is no version control, which is
bad.
The solution is to automate the creation of baked images. Software frameworks for cre-
ating baked images include Vagrant, Docker, and Netflix Aminator. All of these options
provide languages for describing how to build the image from scratch, by specifying the
base OS release, packages, file settings, and so on. The image is then created from this de-
scription. The description can be kept under version control, such that the service delivery
platform can be used to build, test, and deploy images.
Baked images can be used for building a new service as well as upgrading an existing
one. For example, if there are 10 web servers behind a load balancer, upgrading them in-
volves taking each one out of the load balancer's rotation, deleting it, and re-creating it
from the image. All 10 web servers running the new release will then have a well-under-
stood configuration.
Persistent Data
Not all machines can simply be wiped and reinstalled in this way. For example, a database
server or file server has irreplaceable data that is not part of the build or configuration pro-
cess. The solution in such a case is to put such data on virtual disks, mounted from a stor-
age area network (SAN) or other remote storage system. The boot disk is replaced, but on
boot-up it mounts its virtual disk, thereby reattaching the system to the data it needs. By
decoupling where storage is provided from where it is used, machines become more dis-
posable.
Baked versus Fried
Configurationmanagementisoftenfasterthanupgradingamachinebyinstallinganimage.
Configurationmanagementisalsolessdisruptive,becauseitmakesjusttheminimumnum-
ber of changes needed to achieve the desired configuration. Those same 10 web servers
needingtobeupgradedcansimplybeindividuallyrotatedoutoftheloadbalancerandonly
the desired package upgraded. With this technique, the machines are not wiped, so no data
is lost.
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