Repository: walt-python-packages; walt-server code.

This section explains various things helpful when modifying the walt server code.

As a reminder, this code is in subdirectory server of repository walt-python-packages, and it must maintain a compatibility with python 3.9. See walt help show dev-walt-python-packages for information regarding the whole repository.

Systemd services

On a WALT server, systemd manages various services:

# systemctl list-units | grep walt | grep service | sed -e "s/.service//"
walt-server-dhcpd     loaded active running   WalT network DHCP daemon
walt-server-httpd     loaded active running   WalT server HTTP service
walt-server-lldpd     loaded active running   WalT LLDP server (based on lldpd)
walt-server-named     loaded active running   WalT network DNS service
walt-server-netconfig loaded active exited    WalT platform network management
walt-server-ptpd      loaded active running   WalT PTP server (based on ptpd2)
walt-server-snmpd     loaded active running   WalT SNMP server (based on snmpd)
walt-server-tftpd     loaded active running   WalT TFTP server (based on tftpd-hpa)
walt-server           loaded active running   WalT server daemon

The main daemon of the WALT platform is systemd unit walt-server. The underlying executable started by this unit is walt-server-daemon. We will describe it in details later.

When the OS boots or shutdowns, the network configuration specific to WALT is setup or removed by systemd unit walt-server-netconfig, according to the content of /etc/walt/server.conf. Under the hood, it calls walt-net-config up or walt-net-config down. The code is at server/walt/server/

The other listed systemd units are network services. Except walt-server-httpd which is a pure-python service, all those services internally rely on external software:

Unit name

Underlying binary

Related apt package



















When installing WALT, the default systemd service installed with each apt package is disabled, and the relevant WALT-specific service is installed instead: the unit file is copied from server/walt/server/setup/<unit>.service to /etc/systemd/system/. Each WALT-specific service actually just reuses the binary, but it does not touch the default configuration provided with the apt packet. Instead, configuration files specific to WALT are generated in /var/lib/walt/services/<service-name>/* and the binary is called with the appropriate option to find it there.

Some of these WALT services also redirect the working directory to /run/walt/<service-name>/, for instance to store a PID file.

Note: if we were modifying the default configuration files, it would complexify OS upgrades with questions such as “Should we overwrite this file you modified with the one embedded in the new version of the packet?”.

In the case of lldpd, ptpd, snmpd and tftpd, computing the exact command line options of the binary requires specific processing. For instance regarding lldpd we need to specify the IP of interface walt-net using option -m <ip> for the service to listen on it, and retrieving this IP requires reading /etc/walt/server.conf. In this case, the systemd unit WALT installs does not refer to the binary directly. Instead, it calls an executable walt-server-<service> (e.g., walt-server-lldpd), which computes the full options and then starts the binary using os.execvp(). The python code of those executables is at server/walt/server/services/.

Other python or shell commands

The python package walt-server installs various executables, as shown in sections console_scripts and scripts in dev/

Apart from the executables already mentioned in previous section, we have:




Install or Upgrade the WALT server


Trackexec replay tool (1)


Trackexec analysis tool (1)


walt-server-dhcpd -> walt-server notifications (2)


ExecStartPre directive of walt-server.service (3)

(1): See walt help show trackexec.

(2): In the file dhcpd.conf, we specify that for each new IP lease, the dhcpd service must run walt-dhcp-event (with device ip, mac, vendor class identifier, etc., indicated as parameters). This python executable connects to walt-server-daemon and calls SSAPI.register_device(<parameters>). This API is defined at server/walt/server/processes/main/api/ By this way, walt-server-daemon can detect new devices or nodes. Note: SS stands for “Server to Server”, since both processes are running on the server.

(3): If ever walt-server-daemon crashes, it could let the system in a dirty state, for instance with podman containers still running or walt images still mounted. On next call to systemctl start walt-server, walt-server-cleanup will try to clean things up before walt-server-daemon starts again.

The remaining executables mentionned in dev/ are helper scripts of walt-server-daemon.

In case you need to modify one of these commands, refer to dev/ to know where the code is.


walt-server-daemon is the most important process running on the server. It implements the core logic of a WALT server: it is responsible for mounting WALT images and exporting them through NFS, updating TFTP symlinks for the nodes to boot the appropriate image, saving platform and experiment logs, replying to requests from the walt client tool or its api object, interacting with the docker hub or a custom registry, communicating with network switches for network discovery or PoE reboots, and many other things.

Debugging options

For a quick debug session:

  1. Insert a line import pdb; pdb.set_trace() at the place you want to debug

  2. Update the venv with your modifications: run make quick-venv-update

  3. Get rid of systemd: run systemctl stop walt-server

  4. Start the service yourself: run walt-server-daemon

The service will stop with a pdb prompt at the place you inserted pdb.set_trace().

You can also enable trackexec to record the execution path of walt-server-daemon subprocesses and later analyse any unexpected behavior by using walt-server-trackexec-replay. See walt help show trackexec.

walt-server-daemon subprocesses

The code of walt-server-daemon is at server/walt/server/ It actually starts and connects the following 4 sub-processes:

  • server-main: the most important one, managing most features.

  • server-hub: the process receiving RPC calls (remote-procedure-calls) from clients, nodes, or other server processes, and transmitting them to server-main.

  • server-db: the process interacting with the WALT server database.

  • server-blocking: a process used for long running tasks.

server-hub is connected to server-main only. server-main is connected to the 3 others. Both server-main and server-blocking interact with server-db for managing the database.

We avoid multi-threading: each of these subprocesses handle tasks one at a time. Considering server-main, server-hub, and server-db, it is important to avoid blocking the process too long. Some long blocking tasks are delegated to server-blocking (for instance, the communication with network switches), or to other short-lived subprocesses, for this reason.

The code of each subprocess is at server/walt/server/processes/<name>.

Connectors between subprocesses

The connector between two sub-processes is a complex object, but its usage is straightforward.

It may be used synchronously, such as this code in processes/main/nodes/

for node in"devices", type="node"):

In this case, self.db is server-main’s connector for interacting with server-db.

This code is very readable, but be careful, it is more subtle than it seems: during this kind of synchronous call, the calling process (server-main here) returns to its event loop, for managing any unrelated task which may arrive in the meantime. So be careful with code reentrance.

Connectors may also be used asynchronously, such as in this code:


Since inserting a batch of logs may take a little time, and we do not have to wait for a return value, we specify do_async: server-main will not wait for this remote task to complete before jumping to the next instruction. Note however that since server-db is also running one task at a time, if server-main uses a synchronous db request shortly after this one, a small delay may occur.

Event loop

The daemon and each subprocess is managed by an event loop (source code at common/walt/common/

Basically the code works by plugging “listener objects” on the event loop. Each listener is responsible for managing a given file descriptor, which may be the socket connected to a client, a server socket waiting for connections, the underlying pipe of a connector between two subprocesses, etc. The listener provides a method fileno() which returns this file descriptor, a method handle_event() called when the event loop detects an event related to this file descriptor, and a method close() for cleaning up.

A new listener is plugged on the event loop by using: ev_loop.register_listener(<listener>). If the listener has completed its work or detected a problem, the call to its method handle_event() should return False. In this case, the event loop will call <listener>.close() and automatically remove <listener> from its set of listeners.

The event loop also handles planning events at a given time (look for calls to plan_event()), and simplifies running parallel shell or python commands (look for calls to and ev_loop.auto_waitpid()).

Other “pluggable” objects

Similarly to the event loop, the subclasses of GenericServer, such as TCPServer or UnixServer (both implemented in walt-common) accept various kind of requests by plugging appropriate “listener classes”. When a client connects to this kind of generic server, it sends a request ID which the server uses to know which listener class should be instanciated to process the request.

External RPC APIs

For communicating with walt-server-daemon, clients, nodes and external server processes (such as walt-dhcp-event) can connect to TCP port 12345 for remote-procedure-call (RPC) type of communication.

TCP port 12345 is opened by server-hub. Requests are forwarded to server-main and are implemented in server/walt/server/processes/main/api/<api>.py.

Four RPC APIs are implemented:

  • Client to Server API (handling the many client requests)

  • Node to Server API (e.g., method sync_clock() is there)

  • Server to Server API (e.g., method register_device() called by walt-dhcp-event, a process running on the server too)

  • Virtual to Server API (handles calls from executables of package walt-virtual)

It is important to keep in mind the different compatibility requirements for these API files. Modifying an API call in is never a problem since the caller is also installed on the server, so we know that its code will be updated at the same time. Modifying an API call in is also fine because the client verifies that server and client WALT version are the same when connecting to the API. Modifying should be carefully thought because old WALT images embedding walt-node code must continue working; similarly, some of the API calls at are useful for VPN nodes already deployed.

For an example of how to use this RPC endpoint from client code, checkout walt help show dev-client.

Note that while processing those API calls, the server may itself perform API calls the other way (server to client), such as requester.stdout.write() or requester.get_username(). The client itself exposes a local RPC API to manage this at client/walt/client/

External TCP stream API

Clients and nodes can also connect to server TCP port 12347 for stream based communication features. TCP port 12347 is opened by server-main and is managed by a TCPServer object (see “Other pluggable objects” above): the remote peer writes a request ID just after connecting in order to let the server know which kind of processing is expected. This channel is used by the nodes to publish experiment logs for instance. It is also used by the client for file transfers (walt node cp, etc.), remote shells (for instance walt image shell), etc.

For an example of how to use this TCP stream API endpoint from client code, checkout walt help show dev-client.

Server database

The server database is managed by subprocess server-db, using library psycopg2. For more information about the database itself, see walt help show dev-server-db.

Workflow objects

Subprocess server-main makes frequent use of a class called Workflow. It is implemented in server/walt/server/processes/main/

For understanding the purpose of this class, let us consider the case of rebooting nodes. It is actually a complex process involving several kinds of callbacks called after very diverse wait conditions.

  • On node-side, “soft reboot” is based on a minimal busybox-based network service. The server relies on it to request a reboot, but communication should not block the whole process (so internally the function works with the event loop and a non-blocking socket).

  • “Hard rebooting” nodes using PoE involves sending a request to the switch to stop powering the PoE port (this is implemented as an async RPC call to server-blocking, with a callback in charge of continuing this procedure), then waiting for a little time (this involves a call to ev_loop.plan_event()), then restoring the power (again, by relying on server-blocking).

Without a Workflow object, going from one step to the next one would involve a set of callbacks and would be very hard to follow.

As shown in file server/walt/server/processes/main/nodes/, the Workflow class allows to clarify the suite of steps to perform:

wf = Workflow(

If needed, one of the steps involved may call wf.insert_steps() to further split the procedure in smaller logical pieces, without complexifying this high-level view.

Each workflow step is defined as a function (or method, with self added as first argument) written like this:

def wf_<func-name>(wf, <named-args-used-in-the-function,...>, **env):

The environment env is initialized when the Workflow is created (see above). Some workflow steps may alter the environment when needed by using wf.update_env().

The workflow continues with the next step when is called. It is the responsability of each workflow step to call, either directly, or by planning such a call later, by specifying as a callback somewhere. See function wf_poe_poweron in the same file for instance.

In case of an unrecoverable issue, a workflow step may use wf.interrupt() instead, which will explicitely stop the whole workflow.

If ever a call to or wf.interrupt() is missing, the Workflow object will print an error message when garbage collected, indicating how many remaining steps where missed, in order to help the developer fix the issue.