Strand dump tool

The Ballerina runtime can have unexpected behaviors due to user code errors, bugs, or issues with the running environment. These will result in memory leaks, CPU spinning, runtime hangs, performance degradation or crashing with various errors. Ballerina provides a tool to dump the status of currently running strands.

Strand dump

Ballerina strand dump provides information on the available strands and strand groups during the execution of a Ballerina program. This can be used to:

  • troubleshoot runtime errors

  • find data races, race conditions, livelocks, and deadlocks

  • inspect strand and strand group status

Note: Currently, this ability is only available in the operating systems in which the SIGTRAP POSIX signal is supported (SIGTRAP is not available on Windows).

Get the strand dump

To get the strand dump when a Ballerina program is running, you need to know the process ID (PID) of the Ballerina program. For that, you can use the jps tool. Then, you need to send the SIGTRAP signal to the process. The strand dump will be produced to the standard output stream in the text format.

For example, consider the following Ballerina program.

import ballerina/lang.runtime;
import ballerina/io;

public function main() {
    future<int> addResult = start addnum(1, 2);
    int|error addition = wait addResult;

function addnum(int num1, int num2) returns int {

    worker sender {
        num1 -> receiver;

    worker receiver returns int {
        int firstNum = <- sender;
        return num2 + firstNum;

    int intResult = wait receiver;
    return intResult;

Run this Ballerina package using the bal run command.

$ bal run
Compiling source

Running executable

Obtain its PID while the program is running.

$ jps
3408 Main
28851 Jps
28845 $_init

You get the PID for this program as 28845 because $_init is the main class of the Ballerina program.

Note: If you run the tests in a Ballerina package or a file using the bal test command, you need to get the PID of the process denoted by the BTestMain classname.

To get the strand dump, send the SIGTRAP signal to that process. You can use the following CLI command.

$ kill -SIGTRAP 28845


$ kill -5 28845

Then, the dump of the runtime strands will be emitted to the standard output stream of the Ballerina program. For example, see the sample below.

Ballerina Strand Dump [2022/10/12 12:08:02]

Total strand group count        :       5
Total strand count              :       5
Active strand group count       :       2
Active strand count             :       4

group 4 [QUEUED]: [1]
        strand 2 "main" [demo.strandDump.0:main] [WAITING]:
                at      demo.strandDump.0.1.0:main(main.bal:6)

group 5 [QUEUED]: [3]
        strand 3 "addResult" [demo.strandDump.0:main][2] [WAITING]:
                at      demo.strandDump.0.1.0:addnum(main.bal:22)

        strand 4 "sender" [demo.strandDump.0:addnum][3] [BLOCKED]:
                at      ballerina.lang.runtime.0.0.0:sleep(runtime.bal:61)

        strand 5 "receiver" [demo.strandDump.0:addnum][3] [BLOCKED ON WORKER MESSAGE RECEIVE]:
                at      demo.strandDump.0.1.0:$lambda$_1(main.bal:18)


Output format and available details

The strand dump contains the following information.

  • the date and the time when the strand dump was obtained

  • the total number of strand groups and strands created in the program

  • the active number of strand groups and strands in the program

The details on the active strand groups and strands are given in the following format.

Strand dump output format

Strand group IDA unique ID given to a particular strand group. A strand group comprises a set of strands that run on the same thread.
Strand group stateCurrent state of the strand group. For the available states, see Strand group states.
The current number of strands in the strand groupA strand group consists of one or more strands. Only one of them runs on a thread at a time.
Strand IDA unique ID given to a particular strand.
Strand nameName of the strand associated with the strand ID. This is optional and will be omitted if not available.
Strand initiated moduleName of the module, which created the strand.
Strand initiated functionName of the function, which created the strand.
Parent strand IDID of the parent strand. This will be omitted if there is no parent strand.
Strand stateCurrent state of the strand. For the available states, see Strand states.
Strand yielded location stack traceThe stack trace, which points to the location where the strand is blocked (yielded). This is omitted if the state is RUNNABLE or DONE. A line in the stack trace is given by the format:
module name:function name(filename:line number)

Strand group states

RUNNABLEStrand group is ready to run or is currently running.
QUEUEDStrand group execution is blocked or completed or it comprises a new set of strands that are not yet scheduled to run.

Strand states

WAITING FOR LOCKStrand is waiting to acquire a lock.
BLOCKED ON WORKER MESSAGE SENDStrand is blocked due to the sync send action.
BLOCKED ON WORKER MESSAGE RECEIVEStrand is blocked due to the receive action.
BLOCKED ON WORKER MESSAGE FLUSHStrand is blocked due to the flush action.
WAITINGStrand is blocked due to the wait action.
BLOCKEDStrand is blocked due to any other reason than the above. E.g., sleep, external function call, etc.
RUNNABLEStrand is ready to run or is currently running.
DONEStrand execution is completed.