tidb_txn_mode don't default to pessimistic

[erikthorselius]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

docker run -d -p 4000:4000 pingcap/tidb:v4.0.8
mysql -h localhost -P 4000 -u root --protocol tcp -e "show global variables where Variable_name='tidb_txn_mode'

2. What did you expect to see? (Required)

+---------------+-------------+
| Variable_name | Value       |
+---------------+-------------+
| tidb_txn_mode | pessimistic |
+---------------+-------------+

3. What did you see instead (Required)

+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| tidb_txn_mode |       |
+---------------+-------+

4. What is your TiDB version? (Required)

| Release Version: v4.0.8
Edition: Community
Git Commit Hash: 66ac9fc31f1733e5eb8d11891ec1b38f9c422817
Git Branch: heads/refs/tags/v4.0.8
UTC Build Time: 2020-10-30 08:21:16
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |

[zyguan]

Thanks for the report, @erikthorselius , tidb_txn_mode defaults to pessimistic when the store is tikv, however, docker run -d -p 4000:4000 pingcap/tidb:v4.0.8 starts a tidb with mocktikv.

[erikthorselius]

It would be nice to have a mock that have similar configuration as to the real store or maybe make it configurable from start. We have a library that checks that the database have the right configuration before it can be uses. It works well when accessing a bigger installation (read dev/prod environments) of tidb but not when just running on a developers machine.

[zyguan]

Yes, you are right, we keep it unchanged when using mocktikv because many test code rely on it. Can set global tidb_txn_mode='pessimistic'; during provisioning resolve the problem?

[erikthorselius]

That's is how I solved it in the test's around the library right now. But it is a bit annoying because it takes around 2 seconds for the change to apply. It would be nice if it could be configured via configuration.

[zyguan]

@cfzjywxk @nullnotnil How do you think about this? Do we need to add a configuration item for it, or are there any other solutions?

[ghost]

@cfzjywxk @nullnotnil How do you think about this? Do we need to add a configuration item for it, or are there any other solutions?

I was not aware that the tests rely on optimistic, but that makes sense based on history. Without knowing how much work is involved to switch the tests, I agree with @erikthorselius that mocktikv should be using the same default. Otherwise there might be bugs in the default configuration which are not caught?

[erikthorselius]

Is it possible to take any decision? I'm just a humble user of your great software, I have a clunky workaround that I can live with. But maybe this could help other users.

[cfzjywxk]

Is it possible to take any decision? I'm just a humble user of your great software, I have a clunky workaround that I can live with. But maybe this could help other users.

@erikthorselius
I think it should be changed to the same, the problem is that unit test cases may fail becasuse they depend on the old default value and we may need to refactor these cases, as @nullnotnil said. It may need some time to check them all.
The set statment set global tidb_txn_mode = 'pessimsitic' could be used to change it currently before other statement executions.

[serbaut]

Fixed in #36578