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Service Discovery and configuration with schema and rules
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Layer cake is a suite of tools for building, maintaining and configuring containers. Container configuration is designed to be formalized around various service discovery sources with schemas to validate incoming data for correctness and rules to decide how to configure the container.

  • disco Docker Inspired Slice of COmpute - Discovery Service
  • cake Container Assemble Kit Extraordinaire

Use various service discovery sources and handlers to configure a container at runtime. Disco will poll/watch various configured sources, validate the configuration data according to various schema and then match various rules to dispatch handlers within the container. If all the rules have been completed successfully disco will exit by exec'ing the containers primary command.


Layercake depends on Python 3.5+

pip3 install layercake


Sources are various systems for service discovery, consul and etcd for example. These provide data which is then validated according to schemas.

  • Consul
  • Etcd
  • Flat Files

Sources will map backend data, usually at some fixed location in the remote sources keyspace, to a map of YAML described data.


You may include any number of .rules files in your container. These files should be YAML formatted with the following structure:

format: 1
    - rule:
        op: any | all
        when: <interface:str> | [<interface:str>, ]
        do: <handler>

'When' targets an interface for which there should be a matching schema. The when rule triggers by default when all the data from composed sources include a key under which enough valid data exists to satisfy the schema. When may be a list of interface names or a single interface as a string.

'all' is the default operation (op) for comma delimited interface names (thus the last two examples are identical). All means all interfaces must validate before the rule can trigger.

'any' means at least one of the listed interfaces must validate with proper data.

'do' should specify a handler. See the handlers section for calling conventions.


Handlers are passed a JSON serialized bag of data containing all the matched (validate) interfaces from their 'when' trigger. This is written to STDIN of the handler process. The handler then only need exit normally (0) for the rule to succeed. If the handler fails a number of times (see disco.fail_limit) we assume the handler is broken and terminate. We take this approach because we have some assurances that the data is valid based on the schema and rules.


Disco is configured with the DISCO_CFG environment variable and can take a number of flags this way. DISCO_CFG is a '|' delimited set of key/value pairs where the keys are namespaces via a '.'. Subsystems (disco and the various backends) then have values in their namespace available to them at runtime.

  disco.path: (str) directory in container to use for schema, rules and
  handlers, path itself maybe ': delimited'

  disco.interval: (int:1) time in seconds to sleep between reading various
  service backends or dispatching handlers

  disco.fail_limit: (int:5) the number of times a handler can be invoked
  with validated data before we assume it won't exit successfully

  <source>.<key>: a mapping of all keys under source will be available to
  the source 


  flat.file: (path) Configure the flat file source with a configuration file in YAML

Consul (str) http://addr:port

Etcd (str) addr
  etcd.port: (int) port


Once you have an idea about how to configure your container it would be nice if you could share and collaborate around the code required to do this. A layer, in this context, are the rules, schema handlers and assets needed to configure some aspect of a container.

For some container application the configuration is custom, for example, how to configure yourself as a client of mysql, but then layer might still include useful code you can use in your handler. It can always include verification that your being passed the agreed upon data by setting up the proper schema and rules.

Great applications are composed of many concerns, each of these can be modeled in a layer and then the shared bits can be collaborated around so you can always ensure a tasteful experience with your container.

Along those lines we include a tool called 'cake', 'Container assembly kit extraordinaire'. This can be used to resolve layers from a public, central index and easily include them into your container at build time.

For example in a Dockerfile you might say

RUN cake layer <layer-name>

Layers can be found at

Layers should include a layer.yaml in their top directory with the following format:


    name: layername
    author: Name
    repo: git repo
    repopath: <optional subpath under git repo to treat as the layer>


The 'cake' command has a 'build' directive that will take a cake.conf (yaml) file and assist you in automatically updating a Dockerfile to include all the proper directives to run disco. This will include pulling down any layers used in the container, installing them and setting up the ENTRYPOINT to use disco.


This simple YAML file should take the format

    layers: [ layername ]


For now, examples assume you've installed tox and can run from inside its testing environment.

$ tox
$ . .tox/py35/bin/activate
$ env DISCO_CFG="flat.file=tests/mysql.yaml|disco.path=tests"" disco echo "doit"

This should load the mysql data from a flat file in the tests dir, which will pass the schema, trigger a handler (also in the test dir) and then echo the command

$ curl -X PUT -d ''
$ curl -X PUT -d 'test'
$ curl -X PUT -d 'youshallnot'
$ curl -X PUT -d 'test'
$ env DISCO_CFG="|disco.path=tests|disco.interval=5" disco echo "doit"

After after changing IP address in the above command to your consul this will pull the configuration information from consul like the previous example. With the consul ui ( in this example) you can do into the KV store delete and add keys and see the changing behavior of disco.

Disco is intended to replace the CMD in your Dockerfile, this can be done by prefixing the cmd with disco.

CMD = disco <original cmd>


If disco isn't working as expected you might try calling it as 'disco -l DEBUG' which will show more detailed operations including handler failure.

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