Specifically, I want to open two prompts and execute one of these two transactions in each of them and see if they block each other:
Transaction #1：
start transaction;
insert into t values (2,2);
Transaction #2：
start transaction;
select * from t where id>1 and id1<3 for update;
But I only got one SQL editor. I could open two browsers and bring up a SQL editor in each, but I am not sure they represent two prompts.
Okay, let me try to work through this question. The user is asking whether opening two SQL editors in different browsers would represent two separate transaction contexts in TiDB Serverless.
First, I need to recall how TiDB handles transactions and sessions. From the provided documents, TiDB supports both optimistic and pessimistic transactions. Each transaction is managed within a session. The key point here is understanding how sessions are managed when using the SQL Editor in TiDB Cloud.
Looking at the information about the SQL Editor in TiDB Cloud, specifically in the Serverless tier, the SQL Editor is a web-based interface. When a user opens the SQL Editor, they typically start a session. If they open another instance in a different browser, each browser would have its own session unless there’s some session sharing mechanism, like cookies or tokens that are shared across browsers. However, usually, different browsers maintain separate sessions because they don’t share cookies or local storage.
In the context of TiDB Serverless, each SQL Editor session would initiate its own connection to the database. Each connection is a separate session in the database, which means transactions in one browser’s SQL Editor wouldn’t interfere with those in another. For example, if you start a transaction in one browser, it’s isolated from the transaction in another browser because they’re different sessions.
The transaction isolation levels in TiDB, such as Read Committed and Repeatable Read, apply per transaction within a session. Since each SQL Editor instance in different browsers is a separate session, their transactions are independent. This is supported by the documentation on transaction isolation levels and session management. The sources mention that TiDB’s transactions are handled at the session level, and each session operates independently unless explicitly managed otherwise.
Moreover, the Serverless Driver documentation indicates that connections can be made via public endpoints, and each connection (like from different browsers) would be treated separately. The SQL Editor’s behavior would align with standard database session handling, where each editor instance is a new session.
Therefore, opening two SQL editors in different browsers would indeed represent two separate transaction contexts in TiDB Serverless. Each editor’s session is isolated, and transactions in one do not affect the other unless they explicitly interact through the database (like modifying the same data, leading to conflicts based on isolation levels).
In TiDB Serverless, opening two SQL Editor sessions in different browsers would represent two separate transaction contexts, provided they are not sharing the same authenticated session (e.g., cookies or tokens). Here’s the breakdown:
Session Isolation
Each SQL Editor instance in TiDB Cloud operates as an independent session. When opened in separate browsers:
BEGIN; ..COMMIT; in Browser A’s SQL Editor will not affect transactions in Browser B’s SQL Editor.Transaction Isolation Levels
TiDB Serverless supports READ COMMITTED and REPEATABLE READ isolation levels (source). These levels apply per transaction, ensuring:
SQL Editor Behavior
The TiDB Cloud SQL Editor acts as a client interface:
BEGIN; UPDATE t SET x = 1; (no commit yet).SELECT x FROM t; (in READ COMMITTED mode).
x until Browser A commits.This separation ensures transactional integrity across multiple client sessions.
Okay, let’s tackle this problem. The user wants a step-by-step guide to simulate and observe transaction blocking in a TiDB Serverless cluster using two specific transactions. They also want an explanation of the expected behavior based on TiDB’s transaction isolation level.
First, I need to recall how transactions work in TiDB, especially the Serverless version. TiDB supports different isolation levels, and the user mentioned using “for update” which relates to locking. Since TiDB Serverless is based on TiDB’s architecture, the transaction behavior should align with TiDB’s documentation.
The two transactions are:
I need to simulate these transactions running concurrently and see if TX2 blocks TX1 or vice versa. The key here is understanding how locks are acquired in TiDB, especially with pessimistic transactions, which are the default in newer versions.
Step 1: Setting up the test environment. The user would need a TiDB Serverless cluster. I should mention creating a table ‘t’ if it doesn’t exist, perhaps with an ‘id’ primary key. Using the SQL Editor in the TiDB Cloud console makes sense here.
Step 2: Executing the transactions. Since they need to be concurrent, using two separate sessions (like two SQL Editor tabs) is necessary. The order of operations matters. TX1 starts, inserts, but doesn’t commit. Then TX2 starts and tries to select for update. Depending on the isolation level, TX2 might block until TX1 commits or rolls back.
Step 3: Observing blocking. In TiDB, you can check information_schema.innodb_lock_waits or use SHOW PROCESSLIST to see if there’s a lock wait. Also, the second transaction might hang if it’s waiting on the lock from the first.
Expected behavior: Since TX1 has an exclusive lock on the inserted row (id=2), TX2’s SELECT FOR UPDATE should block until TX1 commits. This is because, under the Repeatable Read isolation level with pessimistic locking, the FOR UPDATE clause locks the existing rows. However, the insert in TX1 is a new row. Wait, in TiDB’s pessimistic transactions, does the insert lock the key immediately, even before commit? Yes, because in pessimistic mode, the locks are acquired during the transaction, not just at commit. So when TX1 inserts (2,2), it places a lock on that row. Then TX2’s SELECT FOR UPDATE for id>1 and id<3 would try to lock the same row, leading to a block until TX1 commits or rolls back.
But wait, the SELECT FOR UPDATE in TX2 is looking for existing rows. If TX1 hasn’t committed yet, the new row (2,2) isn’t visible to TX2 because of the isolation level. However, in pessimistic transactions, the locks are still acquired even on uncommitted data. So TX2’s SELECT FOR UPDATE would attempt to lock the range, but since TX1 has a lock on id=2, which is within that range, TX2 would block.
The expected outcome is that TX2’s SELECT FOR UPDATE will wait for TX1’s transaction to complete. If TX1 commits, TX2 will see the new row and the lock will be acquired after TX1’s commit. If TX1 rolls back, the lock is released, and TX2 can proceed without the row.
I should also mention the relevant isolation levels. TiDB’s default is Repeatable Read, which in pessimistic mode prevents phantom reads by locking the ranges. The “FOR UPDATE” in TX2 would lock any existing rows in that range, and the insert in TX1 would conflict if it’s within the same range.
Need to reference the TiDB documentation on transaction isolation levels and locking. The source_uri from the provided data includes https://docs.pingcap.com/tidb/v8.4/transaction-isolation-levels and https://docs.pingcap.com/tidb/v8.4/pessimistic-transaction, which are relevant.
Also, when setting up the environment, using the SQL Shell or a client like MySQL CLI connected to the Serverless cluster is necessary. Mentioning the connection steps briefly, but since the user is using Serverless, the exact connection method (like using the web SQL Editor) is appropriate.
In the observation step, if TX2 is blocked, the user can check the locks. In TiDB, querying the INFORMATION_SCHEMA.INNODB_TRX and INFORMATION_SCHEMA.INNODB_LOCKS (or the equivalent in TiDB) would show the waiting transactions. However, TiDB’s implementation might differ from MySQL here, so need to use the correct system tables. Alternatively, using SHOW PROCESSLIST to see the state of the transactions.
Finally, explaining the expected behavior based on TiDB’s isolation levels. Under Repeatable Read with pessimistic transactions, the locks are held until the end of the transaction. So TX1’s insert places a lock on id=2, and TX2’s SELECT FOR UPDATE attempts to lock the same row, leading to a block until TX1 commits or rolls back.
Make sure to structure the answer with clear steps: Setup, Execution, Observation, Expected Behavior. Include the necessary SQL commands and references to the documentation where applicable.
Here’s a detailed guide to simulate and observe transaction blocking in TiDB Serverless:
a. Create Table (if not exists):
CREATE TABLE t (
id INT PRIMARY KEY,
value INT
);
b. Verify Cluster Connection:
Use TiDB Cloud’s SQL Editor or connect via MySQL client:
mysql --connect-timeout 15 -u root -h <host> -P 4000 -p
Session 1 (TX1):
START TRANSACTION;
INSERT INTO t VALUES (2, 2); -- Locks row immediately in pessimistic mode
-- DO NOT COMMIT YET
Session 2 (TX2):
START TRANSACTION;
SELECT * FROM t WHERE id>1 AND id<3 FOR UPDATE;
-- This will wait for TX1's lock
a. Lock Monitoring:
SHOW PROCESSLIST; -- Look for 'Wait' states
SELECT * FROM INFORMATION_SCHEMA.INNODB_TRX
WHERE trx_state = 'LOCK WAIT';
b. Expected Blocking Behavior:
trx_state: RUNNINGtrx_state: LOCK WAITBased on TiDB’s pessimistic transaction model:
Lock Acquisition:
FOR UPDATE attempts to lock same range (1-3)Isolation Level Impact:
Resolution Scenarios:
COMMIT: TX2 proceeds with new visible versionROLLBACK: TX2 acquires lock immediatelyinnodb_lock_wait_timeout)-- Commit/Rollback in both sessions
COMMIT; -- or ROLLBACK
-- Verify final state
SELECT * FROM t;
FOR UPDATE in TX2 creates range lock contentionFor deeper understanding of TiDB’s locking mechanism, see Transaction Isolation Levels documentation.
This guide should help you effectively simulate and observe transaction blocking behavior in your TiDB serverless cluster using the two transactions you specified.
Hi @erickaufmann16 , To test transactions accurately, it’s better to use the MySQL CLI or any standard MySQL client instead of the SQL Editor. The SQL Editor runs as a service, so you don’t have full control over how connections are handled. Since you’re testing multi-transaction scenarios, having two clearly separated connections is important. Any regular MySQL client should work just fine for this purpose.
I see, thanks.
Feel free to let me know if you have any other questions. I’m happy to help.