arthas/en/_sources/async.md.txt

Arthas Async Jobs
===

Asynchronous jobs in arthas. The idea is borrowed from [linux jobs](http://man7.org/linux/man-pages/man1/jobs.1p.html).


## 1. Use & to run the command in the background

For example, execute the trace command in the background:

```bash
trace Test t &  
```

By doing this, the current command is put to the background to run,  you can continue to execute other commands in the console.

## 2. List background jobs

If you want to list all background jobs, you can execute the `jobs` command and the results are as follows:


```bash
$ jobs
[10]*
       Stopped           watch com.taobao.container.Test test "params[0].{? #this.name == null }" -x 2
       execution count : 19
       start time      : Fri Sep 22 09:59:55 CST 2017
       timeout date    : Sat Sep 23 09:59:55 CST 2017
       session         : 3648e874-5e69-473f-9eed-7f89660b079b (current)
```

You can see that there is currently a background job executing:

* job id is 10, `*` indicates that this job is created by the current session.
* status is `Stopped`
* execution count is the number of executions, which have been executed 19 times since the start.
* timeout date: timeout timestamp, when the time exceeds this timestamp, the job will be automatically timeout and exit.

## 3. Suspend and cancel job

When the job is executing in the foreground, for example, directly executing the command `trace Test t`, or executing the background job command `trace Test t &`, then putting the job back to the forground via `fg` command, the console cannot continue to execute other command, but can receive and process the following keyboard events:

* ‘ctrl + z’: Suspend the job, the job status will change to `Stopped`, and the job can be restarted by `bg <job-id>` or `fg <job-id>`
* ‘ctrl + c’: Stop the job
* ‘ctrl + d’: According to linux semantics this should lead to exit the terminal, right now Arthas has not implemented this yet, therefore simply ignore this keystroke.

## 4. fg/bg, switch the job from the foreground to the background, and vise verse

* When a job is executed in the background or in suspended status (use `ctrl + z` to suspend job), `fg <job-id>` can transfer the job to the foreground to continue to run. 
* When a job is in suspended status (use `ctrl + z` to suspend job), `bg <job-id>` can put the job to the background to continue to run.
* A job created by other session can only be put to the foreground to run by using `fg` in the current session.

## 5. Redirect the output

The job output can be redirect to the specified file by `>` or `>>`, and can be used together with `&`. By doing this, you can achieve running commands asynchronously, for example:

```bash
$ trace Test t >> test.out &
```

The trace command will be running in the background and the output will be redirect to `~/logs/arthas-cache/test.out`. You can continue to execute other commands in the console, at the same time, you can also examine the execution result from the output file.

When connect to a remote Arthas server, you may not be able to view the output file on the remote machine. In this case, Arthas also supports automatically redirecting the output to the local cache file. Examples are as follows:

```bash
$ trace Test t >>  &
job id  : 2
cache location  : /Users/gehui/logs/arthas-cache/28198/2
```

If output path is not given, Arthas will automatically redirect the output to the local cache. Job id and cache location will be shown on the console. Cache location is a directory where the output files are put. For one given job, the path of its output file contains PID and job id in order to avoid potential conflict with other jobs. In the above example, pid is `28198` and job id is `2`.

## 6. Stop job

If you want to stop background job, just `kill <job-id>`.

## 7. Others

* Support up to 8 commands at the same time to redirect the output to the log files.
* Do not open too many background jobs at the same time to avoid negative performance effect to the target JVM.