Visor

The scope of Visor is to provide support for reliable, long-running and fault tolerant applications written in Julia.

Visor is a tasks supervisor that provides:

• a workflow for ordered shutdown of tasks.
• a set of policies to control the logic of tasks restart.

Visor is influenced by Erlang Supervisor design principles with the aims to fit as "naturally" as possible into the Julia task system.

On Unix hosts Visor manages SIGINT signal in a deterministic way and shutdown tasks in an orderly fashion.

Getting started

using Pkg
Pkg.add("Visor")

demo.jl:

using Visor

# The only requirement for a task driven by Visor is that the first argument is
# a descriptor representing the supervised task, in this case `pd` is the 
# descriptor of the `printer` process.
function printer(pd, device)
    println("(re)starting printer driver")
    receive(pd) do msg
        device.jobcount += 1
        print("processing $msg ... ")
        if device.jobcount === 3
            # unexpected printer failure 
            error("printer service panic error: $msg not printed")
        end
        println("done")
    end
end

function controller(pd)
    # from returns a Visor task object from process name.
    # The default name is the name of process task.  
    printer_process = from("printer")
    for i in 1:8
        # send a one-way message to the printer process.
        cast(printer_process, "job-$i")
    end
end

# needed for simulating a printer failure:
# when jobcount is equal to 3
mutable struct PrinterStatus
    jobcount::Int
end

supervise([process(printer; args=(PrinterStatus(0),)), process(controller)])

myhost:~/$ julia demo.jl
(re)starting printer driver
processing job-1 ... done
processing job-2 ... done
processing job-3 ... ┌ Error: [printer] exception: ErrorException("printer service panick error: job-3 not printed")
└ @ Visor .../Visor.jl:1107
(re)starting printer driver
processing job-3 ... done
processing job-4 ... done
processing job-5 ... done
processing job-6 ... done
processing job-7 ... done
processing job-8 ... done
^C
myhost.~/$

The types of nodes that compose a supervision tree are:

• Process: a node running a task.

• Supervisor: a node that supervises other nodes.

The supervision tree is controlled with the methods:

• supervise: start a supervision tree.

• startup: start a node.

• shutdown: terminate a node.

A supervised task function needs to have as a first argument a task descriptor td for inter-task communication and shutdown requests.

In the above example the printer function is the supervised task function of the Process defined by process(printer; args=(PrinterStatus(0),)).

Applications messages sent to the task and the Shutdown control message are received from the td.inbox Channel.

To check for a termination request use the method isshutdown:

function mytask(td)
    for msg in td.inbox
        if isshutdown(msg)
            break
        end
        # process application messages
        # ...
    end
end

In alternative the receive method processes the messages until a shutdown is received.

function mytask(td)
    receive(pd) do msg
        # process application messages
        # ...
    end
end

If the task is only interested in checking for shutdown request then the method isshutdown(td) suffice, but in this case remember that other messages in the process inbox are silently discarded.

simple-process.jl:

using Visor

function worker(td; steps=15, check_interrupt_every=Inf)

    @info "making $steps steps and checking for SIGINT every $check_interrupt_every steps"
    try
        for i in 1:steps
            @info "[$(td.id)]: doing $i ..."
            sleep(0.5)
            if i % check_interrupt_every == 0
                if isshutdown(td)
                    return
                end
            end
        end
    catch e
        @error "worker: $e"
        rethrow()
    end
end

forever([
    process(worker, namedargs=(steps=15,check_interrupt_every=5)),
])

myhost:~/$ julia simple-process.jl
[ Info: making 15 totals steps and checking for interrupt requests every 5 steps
[ Info: [worker]: doing 1 ...
[ Info: [worker]: doing 2 ...
^C CTRL-C
[ Info: [worker]: doing 3 ...
[ Info: [worker]: doing 4 ...
[ Info: [worker]: doing 5 ...
myhost:~/$

To send application messages the standard Channel api may be used or the utilities provided by Visor:

• cast(td, "path.to.target", message): send a message to the node with full name path.to.target.
• call(td, "path.to.target", request): send a request to and wait for a response.
• isrequest(message): check if message is a request.
• reply(message, response): send the response to the call method that issued the message request .

The following example shows the mechanics of inter-task communication. Note that the order of tasks shutdown is the reverse of startup order.

# app.jl:

using Visor

struct CounterMsg
    count::Int
end

struct ControllerMsg
    query::String
end 

function db_service(td)
    @info "[$td] starting"
    for msg in td.inbox
        if isshutdown(msg)
            break
        elseif isa(msg, CounterMsg)
            @info "[$td] got message: $msg"
        elseif isrequest(msg)
            if isa(msg.request, ControllerMsg)
                reply(msg, :on)
            end
        end
    end
    @info "[$td] shutting down"
end

function app_counter(td)
    @info "[$td] starting"
    n = 0

    # get the db_service process handle
    db = from("db_service")

    while true
        sleep(2)
        if isshutdown(td)
            break
        end
        # send a data message to db_service process
        cast(db, CounterMsg(n))
        n += 1
    end
    @info "[$td] shutting down"
end

function app_controller(td)
    @info "[$td] starting"

    # get the db_service process handle
    db = from("db_service")

    while true
        sleep(2)
        if isshutdown(td)
            break
        end

        # send a request to db_service process 
        response = call(db, ControllerMsg("select status from ..."), -1)
        @info "[$td] current status: $response"
    end
    @info "[$td] shutting down"
end

forever([process(db_service), process(app_counter), process(app_controller)])

$ julia app.jl

[ Info: [db_service] starting
[ Info: [app_counter] starting
[ Info: [app_controller] starting
[ Info: [db_service] got message: CounterMsg(0)
[ Info: [app_controller] current status: on
^C[ Info: [db_service] got message: CounterMsg(1)
[ Info: [app_controller] shutting down
[ Info: [app_counter] shutting down
[ Info: [db_service] shutting down