ciao-launcher is an ssntp agent that runs on compute and network nodes. Its primary purpose is to launch and manage containers and virtual machines. For more information on installing and running ciao-launcher see for more information.


ciao-launcher tries to take advantage of Go's concurrency support as much as possible. The intention here is that most of the work involved in launching and manipulating VMs is mostly self contained and IO bound and so should in theory lend itself well to a concurrent design. As a consequence, ciao-launcher is highly concurrent and performant. The concurrent nature of ciao-launcher can make it a little difficult to understand for new comers, so here are a few notes on its design. ciao-launcher can be thought of as a collection of distinct go routines. These notes explain what these go routines are for and how they communicate.


Main is the go routine that starts when ciao-launcher is itself launched. The code for this is in main.go. It parses the command line parameters, initialises networking, ensures that no other instance of ciao-launcher are running and then starts the server go routine. Having done all this, the main go routine waits for a signal, e.g., SIGTERM, from the OS to quit. When this signal is retrieved it instructs all child go routines to quit and waits for their exit. Note that it only waits for 1 second. If all child go routines have failed to exit in 1 second, ciao-launcher panics. The panic is useful as it prints the stack trace of all the running go routines, so you can see which ones are blocked.

The Server go routine

Manages the connection to the SSNTP server and pre-processes all commands received from this server. The code for this go routine is also in main.go, at least for the time being. ciao-Launcher establishes a connection to the ssntp server via the ssntp.Dial command. This creates a separate go routine, managed by the ssntp library. Any subsequent ssntp events that occur are handled by the ciao-launcher function CommandNotify. CommandNotify is called in the context of the ssntp go routine. To avoid blocking this go routine, ciao-launcher parses the YAML associated with the command, sends a newly created internal command down a channel to the server go routine and returns. The command is then processed further in the server go routine.

Most commands are operations on instances, e.g., create a new instance, restart an instance, and are ultimately processed by a go routine dedicated to the particular instance to which they pertain. These instance go routines are managed by another go routine called the overseer which will be discussed in more detail below. Before the server go routine can forward a command to the appropriate go routine it needs to ask the overseer for a channel which can be used to communicate with the relevant instance go routine. This is done by sending an ovsAddCmd or an ovsGetCmd to the overseer via the overseer channel, ovsCh. ovsAddCmd is used when starting a new instance. ovsGetCmd is used to process all other commands.

The overseer go routine is started by the server go routine. When the server go routine is asked to exit by the main go routine, i.e., main go routine closes the doneCh channel, the server go routine closes the channel it uses to communicate with the overseer. This instructs the overseer to close, which it does after all the instance go routines it manages have in turn exited. The server go routine waits for the overseer to exit before terminating.

The Overseer

The overseer is a go routine that serves three main purposes.

1.  It manages instance go routines that themselves manage individual vms.
2.  It collects statistics about the node and the VMs it hosts and
    tranmits these periodically to the ssntp server via the STATS and
    STATUS commands.
3.  It Rediscovers and reconnects to existing instances when ciao-launcher is started.

Overseer launches new instances via the startInstance function from instance.go. This function starts a new go routine for that instance and returns a channel through which commands can be sent to the instance. The overseer itself does not send commands down this channel. It cannot as this would lead to deadlock. Instead it makes this channel available to the server go routine when it is requested via the ovsAddCmd or the ovsGetCmd commands.

The overseer passes each instance go routine a reference to the a single channel, childDoneCh. This channel is closed when the overseer starts shutting down. Closing this channel serves as a broadcast notification to each instance go routine, indicating that they need to shutdown. The overseer waits until all instance go routines have shut down before exiting. This is achieved via a wait group called chilWg.

The overseer maintains a map of information about each instance called instances. The map is indexed by the uuid of instances. It contains information about the instances, namely their running state and their resource usage. This information is used when sending STATs and STATUS commands.

Information about the instances ultimately comes from the instance go routines. However, these go routines cannot access the overseer's instance map directly. To update it they send commands down a channel, which the overseer passes to startInstance, for example, ovsStatsUpdateCmd or ovsStateChange. The overseer processes these commands in the processCommand function.

The Instance Go routines

ciao-launcher maintains one go routine per instance it manages. These go routines exist regardless of the state of the underlying instance, i.e., there is generally one go routine running per instance, regardless of whether that instance is pending, exited or running.

The instance go routines serve 3 main purposes:

1. They accept and process commands from the server go routine down their
   command channel.  These commands typically come from the ssntp server,
   although there are some occasions where the commands originate from inside
   ciao-launcher itself.
2. They monitor the running state of VMs.
3. They manage the collection of instance statistics, which they report
   to the overseer.

The nice thing about this design is that almost all instance related work can be performed in parallel. Stats can be computed for one instance at the same time as a separate instance is being powered down. ciao-Launcher can process any number of commands to start new instances in parallel. There is no locking required apart from a synchronised access to the overseer map made by the server go routine when the command is first received and a small check related to the image from which the instance will be launched, in the case where instances are being started. An additional synchronisation point is required for docker instances to ensure that the relevant docker network has been created before the container.

Note that although commands submitted to different instances are in executed in parallel, the instance go routines serialise commands issue against a single instance. This is necessary to avoid instance corruption.

Right now the command channels that the server go routine uses to send command to instances are not buffered. This might need to change as currently it could be possible for a SSNTP server to kill ciao-launcher's parallelism by repetively sending commands to the same instance over and over again.

The code for the instance go routines is in instance.go. However, the code that executes most of the commands has been placed in separate files named after the commands themselves, e.g., start.go, delete.go, restart.go. It should be noted that the code in these files runs in the context of an instance go routine. Finally, some of the code used to process instance commands is in payloads.go. This is for legacy reasons and in the future this file will probably go away and its contents will be redistributed. It should not be assumed as of the time of writing that all the code in payloads.go runs in an instance go routine. payloads.go needs cleaning up (

The virtualizer

The instance go routines need to talk to qemu and docker to manage their VMs and containers. However, they do not do so directly. Rather they do so via a virtualizer interface.

The virtualizer interface is designed to isolate ciao-launcher, and in particular, functions that run in the instance go routine, from the underlying virtualisation technologies used to launch and manage VMs and containers. All the methods on the virtualizer interface will be called serially by the instance go routine. Therefore there is no need to synchronised data between the virtualizers methods.

qemu.go contains functions to start and stop a VM, to create a qcow2 image, to collect statistics about the instance running in the hypervisor, such as its memory and cpu usage, and to monitor the instance, i.e., to determine whether the VM is actually running or not.

docker.go contains methods to manage docker containers.

For more information about the virtualizer API, please see the comments in

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