HarpCaller daemon


harpcallerd [options] start
harpcallerd [options] status
harpcallerd [options] stop
harpcallerd [options] reload
harpcallerd [options] dist-erl-start
harpcallerd [options] dist-erl-stop
harpcallerd [options] list
harpcallerd [options] info <job-id>
harpcallerd [options] cancel <job-id>
harpcallerd [options] queue-list
harpcallerd [options] queue-list <queue-name>
harpcallerd [options] queue-cancel <queue-name>
harpcallerd [options] hosts-list
harpcallerd [options] hosts-refresh
harpcallerd [options] prune-jobs
harpcallerd [options] reopen-logs


HarpCaller daemon is a service that hails remote harpd(8) instances and records procedures that were called, their arguments, and returned values, what allows to check the result of a call at a later time.

HarpCaller was intended mainly for being a call dispatcher and result database for a web application, where waiting for a remote procedure to finish in a HTTP request is unfeasible. Thus, HarpCaller essentially converts an asynchronous call that comes from an application into synchronous communication with harpd(8).

HarpCaller incorporates a flexible request queueing mechanism, which helps in avoiding overloading the servers or resources they use.



Erlang VM does not support unix signals well, so to communicate with daemon, some other channel is needed. This is achieved by using an administrative socket. It actually gives more possibilities than using signals, so HarpCaller offers much wider administrative command line than typical daemon.

Following option is common to all the commands in later sections:


Path to controlling socket, through which administrative commands can be sent. Defaults to /var/run/harpcaller/control.

Controlling the daemon

harpcallerd start [--debug] [--config=FILE] [--pidfile=FILE]

Start HarpCaller daemon.


Start the daemon with Erlang SASL application started. This prints Erlang boot progress to screen, which makes it easier to debug any problems with harpcaller application.


Path to HarpCaller’s configuration file. Defaults to /etc/harpcaller/harpcaller.toml.


File to write PID to. Since all communication is passed through controlling socket, this is mostly informative.

harpcallerd status [--wait [--timeout=SECONDS]]

Check HarpCaller daemon’s status ("running" or "not running"), possibly waiting for HarpCaller to start. If the controlling socket does not exist at this point yet and --wait was specified, command waits for it to appear (at most for SECONDS).


Wait for daemon to confirm successful start.


How long the command should wait for daemon to start. If not specified, command waits infinitely.

harpcallerd stop [--timeout=SECONDS] [--print-pid]

Shutdown the running daemon. The command may print daemon’s PID, so the caller can wait for it to terminate (e.g. using kill -0 $PID).


How long the command should wait for daemon to shutdown. If not specified, command waits infinitely.


If specified, PID reported by the daemon is printed to screen.

harpcallerd reload
Reload configuration file.
harpcallerd dist-erl-start

Start Erlang networking (Distributed Erlang).

For this command to succeed, epmd(1) must already be running and networking not be configured with VM options file.

harpcallerd dist-erl-stop

Shutdown Erlang networking (Distributed Erlang).

For this command to succeed, networking must not be configured with VM options file.

Controlling call jobs

harpcallerd list [--all] [--queue]

List jobs currently running or waiting for their turn in some queue.

Output is a list of JSON hashes, one per line. The hashes have following structure (broken down for reading convenience):

  "job": "9e03ca7a-bdcb-4bc1-8a56-0f17b310a556",
  "call": {
    "host": "web01.example.net",
    "procedure": "some.procedure",
    "arguments": [...]
  "time": {
    "submit": 1455282411,
    "start": 1455282411,
    "end": null

Job identifier ("job" value) is always in UUID string format.


List all recorded jobs, including terminated.


Along with the running job, list the queue it belongs to (under the "queue" field). If the field is null, the job doesn’t belong to any queue.

harpcallerd info <job-id>

List information about particular job, running or terminated.

Output is a single line with JSON of the same structure as harpcallerd list prints.

harpcallerd cancel <job-id>
Cancel specific job.
harpcallerd queue-list

List queues that have any job running or waiting.

Queue name is a JSON hash, so the output is a list of JSON hashes, one per line.

harpcallerd queue-list <queue-name>

List content of specific queue.

Output is similar to what harpcallerd list prints. Obviously, a job that was submitted but not started yet still waits in a queue.

NOTE: Given the queue name is a JSON, you may need to use single quotes in your shell '...' around the name.

harpcallerd queue-cancel <queue-name>

Cancel all the jobs in specific queue.

NOTE: Given the queue name is a JSON, you may need to use single quotes in your shell '...' around the name.

This command is not an atomic operation, so if a job is submitted to the queue in the same moment queue-cancel was called, the queue may end up not being deleted and re-created. This may affect queue’s concurrency level.

Hosts registry

harpcallerd hosts-list

List hosts known to the hosts registry, and thus available to RPC call requests.

Output is a list of JSON hashes, one per line, which look like this:

{"hostname": "web01.example.net", "address": "", "port": 4306}

Note that while this output is similar to registry filler script’s, but it lacks credentials.

harpcallerd hosts-refresh
Order the HarpCaller to refresh its hosts registry outside the schedule.

Log handling/rotation

harpcallerd prune-jobs [--age=DAYS]

Remove information about jobs older than DAYS (default: 30 days).

This command is mainly intended to work under cron(8) or logrotate(8).

harpcallerd reopen-logs

Close erlang.log_file and reopen it. No-op if the option was not set.

This command is mainly intended for logrotate(8).


Configuration file

The configuration file (default: /etc/harpcaller/harpcaller.toml) is in TOML format.

First, example config:

# network
listen = ["*:3502"]
#ca_file = "/etc/harpcaller/ca_certs.pem"
known_certs_file = "/etc/harpcaller/known_certs.pem"

# jobs
stream_directory = "/var/lib/harpcaller/stream/"
default_timeout = 600
max_exec_time = 600

# hosts registry
host_db_script = "/etc/harpcaller/update-hosts"
host_db = "/var/lib/harpcaller/hosts.db"
host_db_refresh = 900

# logging
log_handlers = ["harpcaller_syslog_h"]

node_name = "harpcaller"
name_type = "longnames"
#cookie_file = "/etc/harpcaller/cookie.txt"
distributed_immediate = false
#log_file = "/var/log/harpcaller/erlang.log"

The config has two sections: main and [erlang]. Parameters in main section control the daemon behaviour. Section [erlang] is responsible for configuring Erlang/OTP, an addition to Erlang VM configuration.

Main section

List of addresses to listen on for requests. A listen address has form of "<bind-address>:<port>", with <bind-address> being a hostname, IP address, or * to bind to any addresses.
ca_file, known_certs_file

These two parameters control how HarpCaller will verify called harpd(8).

If ca_file is specified, harpd(8) certificate needs to be signed properly by one of the CAs from the file (or a sub-CA, with proper certificate chain). commonName attribute is not verified yet.

If known_certs_file is specified, harpd(8) certificate needs to be whitelisted in this file.

If both files are specified, a certificate satisfying any of the above criteria is accepted. If neither is specified, any certificate is accepted.

Directory to store information about call jobs and their results (stream results and end results).
Default timeout (seconds) for waiting for job’s activity (either end result or next packet from streamed result). Request may specify longer timeout if needed.
Maximum time (seconds) the job can take. Any job longer than this will be aborted. Request may specify different execution time, but can’t make it higher than set in config.
Maximum age (hours) of jobs that are remembered. If not specified, jobs are not automatically removed and operator needs to call harpcallerd prune-jobs.
Path to a file where hosts registry will store information about known hosts, collected from running host_db_script.
Script to fill hosts registry. It should print JSON hashes, one per line, each containing address and port to communicate with a host. See Hosts registry filler script to for expected output format.
Frequency (seconds) of running host_db_script to refresh hosts registry.

List of Erlang modules to handle log messages generated by HarpCaller.

HarpCaller comes with two such modules: harpcaller_stdout_h, which prints the logs to STDOUT, and harpcaller_syslog_h, which sends the logs to local syslog.

Erlang VM configuration

Parameters of Erlang virtual machine can be supplied in /etc/harpcaller/erlang.args. It’s the same command line parameters as for erl command, and in fact, this is achieved by including a file with -args_file.

In most uses it should not be necessary to fill this file.

Hosts registry filler script

Registry filler script is executed in regular intervals to fill the database of hosts that are available for RPC calls. This script is supposed to write JSON hashes with information about hosts, one JSON per line.

Filler script can be written in any language (e.g. in Python or shell), as long as it can be executed as a command. It can safely assume that it won’t be called such that two instances would run at the same time (it can take longer than host_db_refresh to execute the script). Any not recognized line will be ignored.

If the script exits with non-zero code, hosts registry will not be updated.

The information the script prints should contain name of the host, its IP address and port, and credentials (user and password) to authenticate request. A single JSON hash could look like this (broken down for reading convenience):

  "hostname": "web01.example.net",
  "address": "",
  "port": 4306,
  "credentials": {
    "user": "rpc-system",
    "password": "caixaudakuPus6yo"

See Also

  • harp(3)
  • harpd(8)