Real-time Action Output Streaming


This feature was added and is available in StackStorm v2.5.0 and above. Initially it was disabled by default. From v2.6.0, it is enabled by default. This can be changed with the actionrunner.stream_output config option in st2.conf.

How it Works

By default when a user runs a StackStorm action, they don’t see the actual output produced by the action until the execution has finished (either failed, succeeded or timed out).

In v2.5.0 we introduced new functionality which allows users to see output in real-time, as it is produced by the action. This is especially useful with long running actions and in CI/CD scenarios where you want to see the output in an incremental manner as soon as it’s available.

Output streaming functionality is currently available for the following runners:

  • local command runner

  • local script runner

  • remote command runner

  • remote script runner

  • python runner


Output streaming only works with the remote runner when you run the command/script on a single host at once (when you pass a single host for the host parameter - one host per action execution).

If you want to run a command/script on multiple hosts and you want real-time action output, you should create one execution per host as shown below:

st2 run examples.my_remote_action host=host1
st2 run examples.my_remote_action host=host2
st2 run examples.my_remote_action host=host3

# instead of
st2 run examples.my_remote_action host=host1,host2,host3

Inside the runners we explicitly disable stdout and stderr output buffering. Some scripts and programs use their own internal buffer which means that in some cases output might be slightly delayed, depending on the size of the buffer used by the underlying script/program.

Accessing Real-time Action Output

Real-time streaming action output can be accessed using one of the approaches described below, after an execution has been scheduled.

1. Via CLI

The easiest way to access the output is to use the st2 execution tail <execution id> command:

st2 execution tail <execution id> [--type=stdout/stderr/other]
st2 execution tail last  # "last" is a special convenience keyword which will automatically
                         # retrieve and use ID of the last execution which has been scheduled.

This command listens for new data on the StackStorm event stream API and prints it as it becomes available.


st2 execution tail Only displays output for simple actions and workflows which are not nested (e.g. workflow which calls a simple action).

If you have multiple levels of nested workflows (e.g. workflow which calls a workflow which calls an action) and you want to see output of an action which is called by a nested workflow, you need to directly tail execution of a nested workflow.

Keep in mind that this command utilizes the stream API endpoint so it will only print any new data which comes in after you ran the command.

If you want to view output of an execution which has completed, you can use the execution output API endpoint (see below) or use the executions API endpoint (GET /v1/executions/<execution id>) and access the result attribute on the returned object.

# Tailing running execution
$ st2 execution tail last
stderr -> Line: 7
stdout -> Line: 8
stderr -> Line: 9

$ st2 execution tail last --include-metadata
[2017-08-31T11:51:06.961844Z][stderr] stderr -> Line: 7
[2017-08-31T11:51:07.462199Z][stdout] stdout -> Line: 8
[2017-08-31T11:51:07.963102Z][stderr] stderr -> Line: 9

# Tailing execution which has finished
stderr -> Line: 7
stdout -> Line: 8
stderr -> Line: 9
stdout -> Line: 10

Execution 59a7f8260640fd686303e628 has completed.

2. Via the StackStorm API

Output can also be accessed using the StackStorm API:

  • GET /v1/executions/<execution id>/output[?output_type=stdout/stderr/other]

$ curl ""
stderr -> Line: 1
stdout -> Line: 2
stderr -> Line: 3
stdout -> Line: 4
stderr -> Line: 5
stdout -> Line: 6
stderr -> Line: 7
stdout -> Line: 8
stderr -> Line: 9
stdout -> Line: 10

This API endpoint returns data produced by the execution so far and closes the connection. If the execution has already completed, it will return all data produced by that execution.

Similar to the CLI command, you can also use last for the execution id, and the ID of the execution which has been scheduled last will be used.

If you are interested in a real-time output as it comes in, you should use one of the stream API endpoints documented below.

3. Via the general purpose StackStorm Stream API endpoint

If you are interested in real-time output as it’s produced by the execution, you can access it using the event stream API.

This API endpoint follows the server-sent events specification (JSON messages delimited by a new line - \n\n) and is also used for other events.

The name of the event is st2.execution.output__create:

  • GET /v1/stream?events=st2.execution.output__create

$ curl

event: st2.execution.output__create
data: {"timestamp": "2017-09-12T13:31:28.608095Z", "runner_ref": "remote-shell-cmd", "output_type": "stderr", "action_ref": "examples.remote_command_runner_print_to_stdout_and_stderr", "data": "stderr line 1\n", "id": "59b7e1b00640fd119d798359", "execution_id": "59b7e1ae0640fd0f72fdc746"}

event: st2.execution.output__create
data: {"timestamp": "2017-09-12T13:31:28.836387Z", "runner_ref": "remote-shell-cmd", "output_type": "stdout", "action_ref": "examples.remote_command_runner_print_to_stdout_and_stderr", "data": "stdout line 2\n", "id": "59b7e1b00640fd119d79835a", "execution_id": "59b7e1ae0640fd0f72fdc746"}

event: st2.execution.output__create
data: {"timestamp": "2017-09-12T13:31:28.863368Z", "runner_ref": "remote-shell-cmd", "output_type": "stderr", "action_ref": "examples.remote_command_runner_print_to_stdout_and_stderr", "data": "stderr line 3\n", "id": "59b7e1b00640fd119d79835b", "execution_id": "59b7e1ae0640fd0f72fdc746"}

event: st2.execution.output__create
data: {"timestamp": "2017-09-12T13:31:29.100242Z", "runner_ref": "remote-shell-cmd", "output_type": "stdout", "action_ref": "examples.remote_command_runner_print_to_stdout_and_stderr", "data": "stdout line 4\n", "id": "59b7e1b10640fd119d79835c", "execution_id": "59b7e1ae0640fd0f72fdc746"}

This endpoint will only return new events which have been generated after you have established a connection to the endpoint. If you are also interested in the output which has been generated before you have established a connection to this endpoint, you should use a special purposed execution output stream API endpoint documented below.

Keep in mind that this feature is still behind a feature flag and that’s why you need to explicitly pass ?events=st2.execution.output__create query param to the API endpoint to make sure you also receive these events.

4. Via the special purpose execution output StackStorm Stream API endpoint


This stream API endpoint has been added in StackStorm v2.9.0.

  • GET /stream/v1/executions/<execution id>/output[?output_type=stdout/stderr/other]

In addition to the general purpose stream API endpoint, you can also utilize special purpose execution output stream API endpoint which also follows server-sent events specification.

The main difference between this endpoint and the one above is that this one operates on a single execution and in addition to the real-time data produced by the execution, it also returns data which has been produced by the execution so far (before the connection to the endpoint has been established).

$ curl

event: st2.execution.output__create
data: {"timestamp": "2018-08-02T10:08:32.473143Z", "runner_ref": "local-shell-cmd", "output_type": "stdout", "action_ref": "core.local", "data": "1\n", "id": "5b62d820962d74784ef53da3", "execution_id": "5b62d815962d747771af2596"}

event: st2.execution.output__create
data: {"timestamp": "2018-08-02T10:08:37.474653Z", "runner_ref": "local-shell-cmd", "output_type": "stdout", "action_ref": "core.local", "data": "2\n", "id": "5b62d825962d74784ef53da4", "execution_id": "5b62d815962d747771af2596"}

event: st2.execution.output__create
data: {"timestamp": "2018-08-02T10:08:42.480517Z", "runner_ref": "local-shell-cmd", "output_type": "stdout", "action_ref": "core.local", "data": "3\n", "id": "5b62d82a962d74784ef53da5", "execution_id": "5b62d815962d747771af2596"}

event: st2.execution.output__create
data: {"timestamp": "2018-08-02T10:08:47.481681Z", "runner_ref": "local-shell-cmd", "output_type": "stdout", "action_ref": "core.local", "data": "6\n", "id": "5b62d82f962d74784ef53da6", "execution_id": "5b62d815962d747771af2596"}

event: EOF
data: ''

To signal that the execution has finished and no more events will be produced on this endpoint and that the client doesn’t need to re-establish a connection, it sends a special EOF event before closing the connection.

Security Implications

This functionality can be restricted via RBAC. To access the execution stdout and stderr API endpoint, EXECUTION_VIEW permission type is required.

Depending on your actions and what kind of output they produce, the output may contain sensitive data. Because of that you are strongly encouraged to only grant this permission to users who explicitly require it. In addition to that, you are also strongly encouraged to modify your actions to mask/hide any potentially sensitive data inside the action output if it’s not needed for further processing inside StackStorm.

For more information about masking and securely passing secrets between the actions, please see Secrets Masking page.

Keep in mind that action output data is the same data which is available via execution result attribute through /v1/executions/<execution id> API endpoint (this API endpoint also requires EXECUTION_VIEW RBAC permission).

Garbage Collection

If your actions produce a lot of output, enabling real-time output streaming for each action execution can result large amounts of data being passed around and stored in the database. This data is stored in the special write ahead database collections (action_execution_stdout_output_d_b, action_execution_stderr_output_d_b).

Because of that, garbage collection is enabled by default for execution stdout and stderr objects - they are deleted automatically after 7 days.

If you want to disable garbage collection for those objects (unwise) or change the default TTL, you can do that by setting the garbagecollector.action_executions_ttl config option. This option is the TTL in days. Setting it to 0 disables garbage collection.

For more information on setting up garbage collection, please refer to the Purging Old Operational Data documentation page.