Handling Pessimistic Locking with JPA on Oracle, MySQL, PostgreSQL, Apache Derby and H2

July 24, 2020

One of the key properties for correct pessimistic locking handling and testing is LockTimeout. Unfortunately its JPA support by different RDBMS providers is far from complete. In this article I will show you how you may overcome this limitation on Oracle, MySQL, PostgreSQL, Apache Derby and H2. Moreover, I will give you some suggestions about when pessimistic locking might be the correct solution and when not.

But before that, let's take a closer look at what pessimistic locking is all about.

Pessimistic Locking Explained

... the pessimistic locking could be the right choice!

🔔 Please note that this article is focused on exclusive row-level locks. If you are interested on other pessimistic locking aspects please check direct the JPA and RDBMS provider's documentation.

Before you decide to embrace the pessimistic locking solution you should first have an answer to the following questions:

While concurrency avoidance is out of scope of this article, I will provide you some hints for the other 4 questions:

Concurrent Database Transactions Problem

case-two-req-no-locking

In most RDBMS the default transaction isolation level is at least read committed. It solves dirty reads phenomena, which guarantees that the transaction reads only committed data by other transactions.

Because of the isolation level and of the concurrent nature of the transactions, the conflict could happen when two transactions have been processed simultaneously:

🔔 In this case the optimistic locking handling would be a better solution because the concurrency is low.

But just for a demonstration purpose, I'll show you how you could solve it with a pessimistic locking solution:

Exclusive Pessimistic Locking In Action

case-two-req-psm-locking-notimeout

While the waiting time for acquiring the lock for the second transaction in this concrete case is lower than the defined LockTimeout, there is no exception thrown.

What Happens When The LockTimeout's Boundary Has Been Reached?

case-two-req-psm-locking-timeout

🔔 Handling a LockTimeoutException through internal retry might not be the most adequate mechanism. If the LockTimeout was not enough, instead of catching the exception and retrying to obtain it, it could make more sense just to double the duration of LockTimeout.

Now that I showed you how pessimistic locking works, let's find some example where pessimistic locking is actually a better solution than optimistic locking.

High-Level Writing Concurrency Problem

case-four-req-no-locking

I will show you what could happen if you implement optimistic locking handling with 4 concurrent transactions:

Optimistic Locking For Solving High-Level Writing Concurrency Problem

case-four-req-opt-locking

I will show you very roughly how you could calculate the minimal baseline of the amount of retries needed for handling optimistic locking:

concurrency level minimum retries for transaction minimum total retries
1 0 0
2 0 + 1 1
3 0 + 1 + 2 3
4 0 + 1 + 2 + 3 6
5 0 + 1 + 2 + 3 + 4 10

From these numbers, the following formula can be empirically extracted:

minTotalRetries(concurrencyLevel) = F(x) = x * (x-1)/2

🔔 Well, as you see with so many rollbacks by high level of concurrency, it doesn't make much sense to use optimistic locking handling.

Will pessimistic locking come to the rescue?

Exclusive Pessimistic Locking For Solving High-Level Writing Concurrency Problem

case-four-req-psm-locking

🔔 Some transactions will reach the "LockTimeout's" boundary, will generate LockTimeoutException and will require handling mechanism. This could sound like extra development effort, but it could save your system from exhausted resources, especially if you have lower LockTimeout and a good pessimistic locking handling strategy!

How Long Should The LockTimeout Be?

Why Is It Not A Good Idea To Implement Pessimistic Locking With NOWAIT?

🔔 If you don't use the "blocking feature" of pessimistic locking, then most probably you could solve the same problem with optimistic locking handling.

What Could Be A Good Way To Handle Lock Timeout Exception?

Let me first explain why I would not suggest using internal retry mechanism:

🔔 On the other hand, an external mechanism for handling LockTimeoutExceptions with retries could be a valuable investment for your architecture: For example a dedicated error handling microservice.

What About Deadlocks?

🔔 Don't forget to adjust deadlock timeout (the timeout after which the database checks for deadlocks): Their values should be in sync with LockTimeout!

Implementing Pessimistic Locking With Different Providers

Till now, I demonstrated to you the importance of LockTimeout for a correct implementation of exclusive pessimistic locking.

In the following paragraphs I will show you:

The Comparison Results Between Different Providers Have Been Achieved Through The Following Setup:

and:

provider maven groupId maven artifactId:version database server
Oracle com.oracle.jdbc ojdbc8:12.2.0.1 oracle-xe-11g
MySQL mysql mysql-connector-java:8.0.20 mysql:8.0.20
PostgreSQL org.postgresql postgresql:42.2.14 postgres:12.3
H2 com.h2database h2:1.4.200 in-memory
Apache Derby org.apache.derby derby:10.14.2.0 in-memory

🔔 I didn't find any complete and reliable information on the internet about LockTimeout and the jpa support of it by different vendors. That's why I decided to invest some time, to test all features first-hand and to summarize them bellow!

JPA Pessimistic Locking Support By Different Providers

jpa feature Oracle MySQL PostgreSQL H2 Apache Derby
LockModeType.PESSIMISTIC_WRITE x x x x x
javax.persistence.lock.timeout x
javax.persistence.lock.timeout=0 (nowait) x x
handling of LockTimeoutException x x x x

As you see, the jpa support for LockTimeout by different providers listed in the table is not that great:

🔔 Note that if you use Spring Framework instead of LockTimeoutException you have to handle CannotAcquireLockException.

Native LockTimeout Support by Different Providers

LockTimeout Oracle MySQL PostgreSQL H2 Apache Derby
unit of measure sec sec ms ms sec
nowait (or similar) 0 0 1 1 0
min 0 0 1 1 0
max 2.147.483 oo (no internal limit) 2.147.483.647 2.147.483.647 oo (no internal limit)
infinite wait -1 N/A 0 N/A N/A
default -1 50 0 4000 60

🔔 As you see in the table, not setting LockTimeout for a pessimistic locking could cause resource exhaustion for your production system: Some databases, like Oracle and PostgreSQL, have a LockTimeout of infinite!

🔔 As you notice there are many differences between different vendors: the meaning of -1 and 0, the different min values, the different measure units! Maybe this explains partially the reason why they don't have good jpa support for LockTimeout!

Setting LockTimeout With JPA & Oracle

@Lock(LockModeType.PESSIMISTIC_WRITE)
@QueryHints({@QueryHint(name = "javax.persistence.lock.timeout", value = "3000")})
Optional<Item> getItemById(UUID id);

🔔 This jpa query will try to acquire a pessimistic lock with LockTimeout set to 3 seconds (3000 ms). For more information check Oracle documentation here and here.

🔔 As you see Oracle has great jpa support for setting LockTimeout. No native code needed!

📢 A blog post with working example how to test a pessimistic locking handling with SpringBoot and Oracle is on its way! Stay tuned!

Setting LockTimeout With JPA & MySQL

select @@innodb_lock_wait_timeout
set session innodb_lock_wait_timeout = 3

🔔 This native query takes effect for the current transaction only. You could now acquire a pessimistic lock through jpa query, and it will set LockTimeout to 3 seconds. For more information check MySQL documentation here.

🔔 This native query has a small side effect: It sets as well the default LockTimeout for subsequent transactions which are part of the current session. If you decide to use it do not rely anymore on default LockTimeout, but be sure to set LockTimeout for all queries which need to acquire a pessimistic lock!

📢 A blog post with working example how to test a pessimistic locking handling with SpringBoot and MySQL is on its way! Stay tuned!

Setting LockTimeout With JPA & PostgreSQL

show lock_timeout
set local lock_timeout = 3000

🔔 This native query takes effect for the current transaction only. You could now acquire a pessimistic lock through jpa query, and it will set LockTimeout to 3 seconds (3000 ms). For more information check PostgreSQL documentation here or check here.

🔔 No side effects here!

📢 A blog post with working example how to test a pessimistic locking handling with SpringBoot and PostgreSQL is on its way! Stay tuned!

Setting LockTimeout With JPA & H2 (In-Memory Database)

spring:
  datasource:
    url: jdbc:h2:mem:test;DB_CLOSE_DELAY=-1;LOCK_TIMEOUT=3000

🔔 Adding LOCK_TIMEOUT parameter to the database connection string will set the default LockTimeout to 3 seconds (3000 ms). In my experience there is no way to set it for a single transaction (or at least I was not able to find a way 😉)! For more information check H2Database documentation here.

🔔 According to the documentation to avoid closing database with the last connection you need to set DB_CLOSE_DELAY to -1. Unfortunately this will not solve the "Connection is closed" problem, which you obtain after LockTimeoutException and which make impossible to test your pessimistic locking handling strategy.

🔔 In this database the LockTimeout parameter is not very accurate. For example if you set it to 2000 ms (2 seconds), the actual LockTimeoutException comes in 4 seconds!

After this unfortunate experience with H2 database 😫, I decided to look for another in-memory database more suitable for a pessimistic locking integration testing...

...I tried HsqlDB as well, but the situation hadn't improved much...

...and I was about to give up when I finally found it...

📢 Okay, here it comes!

Setting LockTimeout With JPA & Apache Derby (In-Memory Database)

VALUES SYSCS_UTIL.SYSCS_GET_DATABASE_PROPERTY('derby.locks.waitTimeout')
CALL SYSCS_UTIL.SYSCS_SET_DATABASE_PROPERTY('derby.locks.waitTimeout',  '3')

🔔 This native query will set the default LockTimeout to 3 seconds (3000 ms). For more information check Apache Derby documentation here.

🔔 Execute this native query at start-up only and within dedicate transaction. It will take effect in the subsequent transactions only.

🔔 As you can see Apache Derby is a first-class solution for a pessimistic locking handling integration testing!

Summary

About the author: Andrey Zahariev Stoev

Loves software craftsmanship and systems thinking. Passionate about travel, languages and cultural diversity exploration.

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