Skip to main content

Canister logs

View this sample's code on GitHub

Prerequisites​

This example requires an installation of:

  • DFX version 0.19.0 or newer
  • Install the IC SDK.
  • Download the following project files from GitHub: git clone https://github.com/dfinity/examples/

You will need to have 3 terminal windows:

  • Terminal A: Running a DFX instance and separating its output from anything else
  • Terminal B: Deploying a canister and seeing its output
  • Terminal C: Reading logs interactively

Step 1: Navigate into the folder containing the project's files and start a local instance of the replica with the command:​

# Terminal A -- for running DFX and separating its output from anything else.
$ cd examples/motoko/canister_logs
$ dfx start --clean

# Terminal B -- for deploying the canister and calling its methods.
$ cd examples/motoko/canister_logs

# Terminal C -- for polling logs.
$ cd examples/motoko/canister_logs

Step 2: Deploy the canister:​

# Terminal B
$ dfx deploy

Step 3: Check canister logs:​

Expect to see logs from timer traps.

# Terminal B
$ dfx canister logs CanisterLogs
[0. 2024-05-23T08:32:26.203980235Z]: right before timer trap
[1. 2024-05-23T08:32:26.203980235Z]: [TRAP]: timer trap
[2. 2024-05-23T08:32:31.836721763Z]: right before timer trap
[3. 2024-05-23T08:32:31.836721763Z]: [TRAP]: timer trap

Step 4: Call print method and check the logs:​

# Terminal B
$ dfx canister call CanisterLogs print hi
()

# Expect to see new log entry.
$ dfx canister logs CanisterLogs
...
[8. 2024-05-23T08:32:46.598972616Z]: right before timer trap
[9. 2024-05-23T08:32:46.598972616Z]: [TRAP]: timer trap
[10. 2024-05-23T08:32:48.713755238Z]: hi
[11. 2024-05-23T08:32:51.623988313Z]: right before timer trap
[12. 2024-05-23T08:32:51.623988313Z]: [TRAP]: timer trap
...

Step 5: Start constantly polling logs:​

In order not to call dfx canister logs CanisterLogs after every canister call in a separate terminal window/pane C start a script that will constantly poll logs:

# Terminal C
$ ./poll_logs.sh
...
[8. 2024-05-23T08:32:46.598972616Z]: right before timer trap
[9. 2024-05-23T08:32:46.598972616Z]: [TRAP]: timer trap
[10. 2024-05-23T08:32:48.713755238Z]: hi
[11. 2024-05-23T08:32:51.623988313Z]: right before timer trap
[12. 2024-05-23T08:32:51.623988313Z]: [TRAP]: timer trap
...

Step 6: Call print, trap and other canister methods:​

# Terminal B
$ dfx canister call CanisterLogs print hi!
()

$ dfx canister call CanisterLogs print hello!
()

$ dfx canister call CanisterLogs print yey!
()

$ dfx canister call CanisterLogs trap oops!
Error: Failed update call.
Caused by: Failed update call.
  The replica returned a rejection error: reject code CanisterError, reject message Canister bkyz2-fmaaa-aaaaa-qaaaq-cai trapped explicitly: oops!, error code None

$ dfx canister call CanisterLogs memory_oob
Error: Failed update call.
Caused by: Failed update call.
  The replica returned a rejection error: reject code CanisterError, reject message Canister bkyz2-fmaaa-aaaaa-qaaaq-cai trapped explicitly: StableMemory range out of bounds, error code None

Observe recorded logs that might look similar to this:

# Terminal C
...
[19. 2024-05-23T08:33:11.319493785Z]: right before timer trap
[20. 2024-05-23T08:33:11.319493785Z]: [TRAP]: timer trap
[21. 2024-05-23T08:33:14.229855179Z]: hi!
[22. 2024-05-23T08:33:16.413512126Z]: right before timer trap
[23. 2024-05-23T08:33:16.413512126Z]: [TRAP]: timer trap
[24. 2024-05-23T08:33:18.622686552Z]: hello!
[25. 2024-05-23T08:33:21.519088681Z]: right before timer trap
[26. 2024-05-23T08:33:21.519088681Z]: [TRAP]: timer trap
[27. 2024-05-23T08:33:22.96101893Z]: yey!
[28. 2024-05-23T08:33:26.601860526Z]: right before timer trap
[29. 2024-05-23T08:33:26.601860526Z]: [TRAP]: timer trap
[30. 2024-05-23T08:33:28.039227914Z]: right before trap
[31. 2024-05-23T08:33:28.039227914Z]: [TRAP]: oops!
[32. 2024-05-23T08:33:31.634215234Z]: right before timer trap
[33. 2024-05-23T08:33:31.634215234Z]: [TRAP]: timer trap
[34. 2024-05-23T08:33:35.96761902Z]: right before memory out of bounds
[35. 2024-05-23T08:33:35.96761902Z]: [TRAP]: StableMemory range out of bounds
[36. 2024-05-23T08:33:36.712223153Z]: right before timer trap
[37. 2024-05-23T08:33:36.712223153Z]: [TRAP]: timer trap
...