System overview


HarpCaller is a system that allows calling procedures on remote machines and collects their results of various types. Procedures to be called are part of system’s configuration, so they can do many administrative tasks. This is much more robust than a set of shell scripts called through SSH, especially when the procedures need to be parametrized and/or return some structured data.

Main intended consumer of HarpCaller is a web application, so HarpCaller needs to work in asynchronous manner: call request returns job identifier, so the job’s status and returned value can be obtained at later time.

From the called machine’s perspective, HarpCaller system is synchronous, keeping a single connection (one per request) through which a request is sent one way and all the results are sent in the opposite direction.

Beside the usual RPC mechanisms, HarpCaller supports returning a stream of partial results, which can be read in real time or replayed later, what comes handy for debugging procedures that track the changes in system status.

Components of the system

HarpCaller system is divided into three parts: daemon, dispatcher, and client library.


harpd(8) daemon is a service running on every server that can be a target for RPC call. It is meant to carry out any procedure that is called and send the value that the procedure returned as a response to RPC call. The procedures available to daemon are supplied as daemon’s configuration.


HarpCaller (request dispatcher) is a single central service tasked with connecting to daemons to pass them call requests and receive call results, and to store these results on disk for later access.

Given the queues are independent from any part of call requests, dispatcher is the place where queueing occurs.

Dispatcher is also the service that client library talks to directly in typical use.

Client library

Python harp module is a client implementation of the protocol to talk to HarpCaller and to harpd(8) services (note: harpd(8) uses slightly different protocol). The primary use case for this interface was to allow issuing commands to servers from within a web application, but it should be equally convenient for other uses.

Module documentation: Python interface

Example usage: Synopsis

Using HarpCaller

HarpCaller works asynchronously, what means that client receives an acknowledgement with job ID immediately after sending a call request. This job ID can be later used to check the job’s status, read its result (or follow streamed partial results), or wait for the job to finish. All can be done in the same or different process, or even in multiple processes simultaneously.

HarpCaller maintains a list of known hosts that can be used as call targets. This list, called hosts registry, contains target address and port (address is either a domain name or an IP) and credentials for each of the hosts. The hosts are identified by their names, which can be arbitrary strings. For calling code host names are the only available way to tell where to run a procedure.

import harp
rpc = harp.HarpCaller("localhost")

job ="").run_foo()
print # output: 8ec71d6b-28d1-4ce2-aa23-f3ea7887beda
# job ID, being a string, can be stored in a database for later use

# recall a job ID and check job's status
job = rpc.job("8ec71d6b-28d1-4ce2-aa23-f3ea7887beda")
print "job {host}:{procedure}(...)".format(
    host =,
    procedure = job.procedure(),

if job.result() is harp.CALL_NOT_FINISHED:
    print "job still running"
    print "job terminated"

print job.start_time() # unix timestamp
print job.end_time()   # unix timestamp or `None' if still running

print job.result(wait = True) # NOTE: errors are returned, not raised

End result

There are two methods for retrieving job’s return value:

  • job.get(), which waits for the job to terminate (this behaviour can be changed with wait=False parameter) and raises an exception on error
  • job.result(), which doesn’t wait for the job to terminate (this behaviour can be changed with wait=True parameter) and returns an exception object on error

The recommended way to check if the job is still running is to compare value returned by job.result() with harp.CALL_NOT_FINISHED constant.

Stream result

Remote procedure can return a stream of partial results, end result, or both. Returning a stream is a feature useful for writing debugging procedures, which could run for longer time and report state changes immediately as they happen.

Stream of partial results is recorded beside the end result and job metadata, so it can be retrieved at a later time.

import harp
rpc = harp.HarpCaller("localhost")
job = rpc.job("8ec71d6b-28d1-4ce2-aa23-f3ea7887beda")

# print partial results collected up until now
for msg in = 0):
    print msg

# print partial results collected from now on until the job terminates
# NOTE: we may have missed some results that came between job.since()
# and job.follow() calls
for msg in job.follow(recent = 0):
    print msg

The difference between job.follow() and is similar to the difference between job.get() and job.result(): the former runs until the job terminates, while the latter doesn’t.

Note that reading the partial results doesn’t raise any exceptions. To check for errors job.get() or job.result() needs to be called.

Time limits

A procedure can be called with time limits specified.

import harp
rpc = harp.HarpCaller("localhost")
job ="", max_exec_time = 60).run_foo()
job ="", timeout = 60).run_foo()

max_exec_time specifies how long (in seconds) the job can run in total before it is cancelled. timeout for jobs that return stream result specifies maximum time between consecutive partial results. For jobs that only return end result, timeout works in the same way as max_exec_time.

Call queues

Execution of remote calls can be organized in queues. A queue is identified by its name, which is a dictionary with arbitrary content supplied at call time. This way the jobs can be grouped by target host, procedure, or some (possibly unrelated to the call) data. A queue is created by the first call with the queue’s name and is deleted when the last job from the queue terminates.

By default only one job from a queue can be running at the sime time. This can be configured at queue’s creation time by setting concurrency level to the desired number of simultaneously running jobs. A queue retains the same concurrency level throughout its whole life.

A job can belong to one queue or to no queue at all. timeout and max_exec_time don’t count the time the job spent waiting for its turn in the queue.

import harp
rpc = harp.HarpCaller("localhost")
hostname = ""
queue = { "host": hostname, "command": "foo" } # per-host queue
# run at most 3 jobs in this queue
job =, queue = queue, concurrency = 3).run_foo()

print job.submit_time()
print job.start_time()  # `None' if the job still waits in the queue

Job information

User can retrieve from HarpCaller various information about a call job. This includes the names of target host and procedure (, job.procedure()), arguments (job.args()), and time of job submission (job.submit_time()), start (job.start_time()), and termination (job.end_time()).

User can also attach an additional information to a remote call. This information will not be interpreted by HarpCaller in any way and its meaning is at the user’s discretion.

import harp
rpc = harp.HarpCaller("localhost")
job_info = { ... } # JSON-serializable data
job ="", info = info).run_foo()

job = rpc.job("8ec71d6b-28d1-4ce2-aa23-f3ea7887beda")
print json.dumps(

Interpreting errors

TODO: Error reporting from HarpCaller needs to be cleaned up. There’s no unified way to identify the cause of an error for now.

Using harpd(8)

Sometimes running a request dispatcher may be an unnecessary overhead. For these cases, harp offers API for communicating with harpd(8) directly.

import harp
server = harp.HarpServer(
    host = "",
    user = "example_user",
    password = "example_password",
    ca_file = "/path/to/ca_certificates.crt",

value = server.run_foo()
print value

Since there’s no proxy that would save call results for later use, all calls are synchronous.

If the called procedure only returns an end result, value from above example will be a JSON-serializable object and no further work is needed to consume it. If the procedure returns partial results, value will be an iterable object (harp.HarpStreamIterator):

import harp
server = harp.HarpServer(...)

for record in server.run_stream():
    if isinstance(record, harp.Result):
        end_result = record
        print record
# do something with `end_result.value'

Unlike in the communication with HarpCaller, end result is included in the iterator (and similarly, errors are thrown from the iterator as well). To tell the partial results and end result apart, end result is wrapped in harp.Result container and is always the last value.

When an error is encountered, an exception (a subclass of harp.HarpException) is thrown. In particular, an exception thrown by the remote procedure is raised as harp.RemoteException.

NOTE: When calling a remote harpd(8) from a procedure under another harpd(8) instance, remember not to confuse the container classes harp.Result and harpd.proc.Result. They cannot be used interchangeably, though they have similar structure and symmetrical purposes.

See Also

  • harp(3)
  • harpd(8)
  • harpcallerd(8)