# Where is error message on the remote worker?
**URL:** https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898
**Category:** General Usage
**Tags:** question, parallel
**Created:** [September 18, 2019, 3:42pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898 "2019-09-18T15:42:22Z")
**Posts on this page:** 15
**Page:** 1
### Author:  [@Chong\_Wang](https://discourse.julialang.org/u/Chong_Wang)
#### Post date: [September 18, 2019, 3:42pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/1 "2019-09-18T15:42:22Z")
The following code should give an error since `foo` is not defined on process 2.
```julia
using Distributed
addprocs()
function foo()
return 1
end
remote_do(foo, 2)
```
I expect some error message somewhere, but the above code just runs fine. Where can I get the error message?
---
### Author:  [@zgornel](https://discourse.julialang.org/u/zgornel)
#### Post date: [September 19, 2019, 8:20am UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/2 "2019-09-19T08:20:06Z")
`remote_do` will just trigger the remote computation but will not fetch the exception.
`remotecall_fetch` will.
---
### Author:  [@Chong\_Wang](https://discourse.julialang.org/u/Chong_Wang)
#### Post date: [September 19, 2019, 10:58pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/3 "2019-09-19T22:58:40Z")
I understand that `remotecall_fetch` will return the error. However, the function that is going to be executed on the remote process will never return and will always be waiting for tasks from the master process:
```julia
mutable struct ParallelFunction
jobs::RemoteChannel
results::RemoteChannel
nreceived::Int64
ntaken::Int64
end
function do_work(f, jobs, results, args...)
while true
x = take!(jobs)
result = f(x, args...)
put!(results, (x, result))
end
end
function ParallelFunction(f, args...; len=128)
jobs = RemoteChannel(()->Channel{Any}(len))
results = RemoteChannel(()->Channel{Any}(len))
for p in workers()
remote_do(do_work, p, f, jobs, results, args...)
end
return ParallelFunction(jobs, results, 0, 0)
end
function give!(pf::ParallelFunction, x)
put!(pf.jobs, x)
pf.nreceived += 1
end
function get!(pf::ParallelFunction)
result = take!(pf.results)
if result[2] isa ErrorException
throw(result[2])
end
pf.ntaken += 1
return result
end
```
Suppose a function `f` has four arguments `x`, `a`, `b` and `c`. The purpose is to evaluate the function `f` at several different `x`s with fixed argument `a`, `b` and `c`.
A `ParallelFunction` can be constructed as
```julia
pf = ParallelFunction(f, a, b, c; len=128)
```
To evaluate the function `f` at `x1`, `x2` and `x3`, the following statement can be used
```julia
give!(pf, x1)
give!(pf, x2)
give!(pf, x3)
```
The function will be evaluated in parallel on any possible processes. To obtain the result:
```julia
for i in 1:3
println(get!(pf))
end
```
`get!(pf)` returns a tuple, the first element of the tuple is the argument `x` and the second argument is the corresponding result.
However, it is possible that the user forget to define the function `f` on the remote process. In this case, the `get!(pf)` will hang indefinitely, which is not a desirable result. I want to throw an error if this is the case.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [September 20, 2019, 1:19am UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/4 "2019-09-20T01:19:55Z")
why it will hang indefinitely? The thing is, the workers won’t know if the function is defined or not unless actually trying to execute them, which defies the purpose of `future-fetch` model.
---
### Author:  [@zgornel](https://discourse.julialang.org/u/zgornel)
#### Post date: [September 20, 2019, 5:57am UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/5 "2019-09-20T05:57:23Z")
The program hangs because no result is put on the remote channel and taking a value blocks the main thread. You could wrap the call to the function `f` in `do_work` in a `try ... catch` and return any caught exception as the result and put it on the channel…
---
### Author:  [@Chong\_Wang](https://discourse.julialang.org/u/Chong_Wang)
#### Post date: [September 20, 2019, 5:34pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/6 "2019-09-20T17:34:50Z")
I tried the following implementation
```julia
function do_work(f, jobs, results, args...)
while true
x = take!(jobs)
result = try
f(x, args...)
catch e
ErrorException(sprint(showerror, e))
end
put!(results, (x, result))
end
end
```
But this `do_work` will not catch any exception is `f` is simply not defined on the remote process. What I was asking is simply how to catch the undefined `f` error.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [September 20, 2019, 5:39pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/7 "2019-09-20T17:39:16Z")
as I said, you should NOT be able to before `fetch`, because you won’t know if a function is defined for a worker unless that worker has tried to execute the function.
---
### Author:  [@Chong\_Wang](https://discourse.julialang.org/u/Chong_Wang)
#### Post date: [September 20, 2019, 5:42pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/8 "2019-09-20T17:42:38Z")
The thing is, how can I fix such a problem for my program. I tried the code in the [here](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/6) and was wishing that the error can be catched, but it wasn’t. Maybe there is some middleware somewhere but I cannot find it in the documentation.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [September 20, 2019, 5:48pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/9 "2019-09-20T17:48:40Z")
for the record, when do you want the `error` msg, when you `@spawnat` or when you `fetch()`?
---
### Author:  [@Chong\_Wang](https://discourse.julialang.org/u/Chong_Wang)
#### Post date: [September 20, 2019, 5:51pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/10 "2019-09-20T17:51:27Z")
I can accept that the error is thrown to me anywhere as long as I get access to it. As I explained [here](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/3), my worker is designed to wait on the remote process for input from the master process forever, and should not be fetched.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [September 20, 2019, 5:57pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/11 "2019-09-20T17:57:05Z")
well in that case you can let your remote workers to put the `error` into your result queue, so your `get!` will pop an error msg from the queue?
---
### Author:  [@Chong\_Wang](https://discourse.julialang.org/u/Chong_Wang)
#### Post date: [September 20, 2019, 5:58pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/12 "2019-09-20T17:58:34Z")
That was my plan, see [here](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/6), but it didn’t work. The error was not catched if `f` is not defined.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [September 20, 2019, 6:11pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/13 "2019-09-20T18:11:29Z")
you can certainly catch the error, as you already know, so something happened in your program when putting the thing into the results queue.
```julia
julia> er = try
remotecall_fetch(hello, 2)
catch e
e
end
RemoteException(2, CapturedException(UndefVarError(Symbol("#hello")), Any[(deserialize_datatype at Serialization.jl:1186, 1), (handle_deserialize at Serialization.jl:775, 1), (deserialize at Serialization.jl:722, 1), (handle_deserialize at Serialization.jl:782, 1), (deserialize_msg at Serialization.jl:722, 1), (#invokelatest#1 at essentials.jl:790 [inlined], 1), (invokelatest at essentials.jl:789 [inlined], 1), (message_handler_loop at process_messages.jl:183, 1), (process_tcp_streams at process_messages.jl:140, 1), (#105 at task.jl:268, 1)]))
```
---
### Author:  [@zgornel](https://discourse.julialang.org/u/zgornel)
#### Post date: [September 20, 2019, 6:17pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/14 "2019-09-20T18:17:08Z")
That’s the way to do it. Although I am not replicating all of your code
```julia
r=try
bar() # bar is not defined
catch e
e
end
```
results in an `UndefVarError(:bar)` being assigned to `r`. The problem is probably somewhere else… Try reducing a bit the code to the local worker (thread) and debug it.
---
### Author:  [@Chong\_Wang](https://discourse.julialang.org/u/Chong_Wang)
#### Post date: [September 20, 2019, 6:28pm UTC](https://discourse.julialang.org/t/where-is-error-message-on-the-remote-worker/28898/15 "2019-09-20T18:28:15Z")
Thank you, but I don’t think these will work. I believe (not sure though) the issue is that in calling the function
```julia
function do_work(f, jobs, results, args...)
...
end
```
with
```julia
remote_do(do_work, worker_id, f, jobs, results, args...)
```
`f` is evaluated before being fed into the function `do_work`. Therefore I cannot catch the error inside the function `do_work`.