phantom playbooks

for the key that previously appeared in the map, the Put method returns null, and the Remove method returns false for key set, value collection, and entry Set checks, the number of elements generated is decreasing.3. Each key object in the Weakhashmap is stored indirectly as an indication object of a weak reference. Therefore, the key is removed automatically, either within the map or outside the map, only after the garbage collector clears the weak reference for a key.(ii) Four categories of r

sub-things, and thing B also has a number of sub-things, and things A and B in high concurrency, there will be a variety of problems. "A variety of problems," summed up the main is five: the first category of missing updates, the second category of missing updates, dirty read, non-repeatable read, Phantom read. Of the five, the first category of missing updates, the second category of missing updates is not important, do not talk about dirty reading,

Mysql lock mechanism usage summary, mysql Lock Mechanism 1. isolation level (1) Read uncommitted (RU) At this isolation level, transactions are completely isolated and dirty reads are generated. uncommitted records can be read, which is not used in actual cases. (2) Read committed (RC) Only committed records can be read. At this isolation level, phantom read occurs. Phantom read refers to executing the same

( //try last COMMIT Transaction} catch () {conn.rollback ();/ROLLBACK TRANSACTION} Transaction ISOLATION Level 1, transaction concurrent read problem dirty read: Read to another transaction uncommitted data (cannot be allowed to appear) Things) cannot be read repeatedly: Two read inconsistent Phantom read (virtual Read): Read to another transaction committed data 2, concurrent transaction problem because concurrent transactions cause problems with ro

the same transaction may have new commits during the instance processing, so the same select may return different results.Repeatable Read (can be reread) This is the default transaction isolation level for MySQL, which ensures that multiple instances of the same transaction will see the same rows of data while concurrently reading the data. In theory, however, this can lead to another tricky problem: Phantom Reading (

Note: This article is reproduced from http://www.hollischuang.com/archives/900Read behavior is a condition that can be encountered when multiple transactions are executing concurrently, while reading data. Understanding them first helps you understand the meaning of each isolation level. These include dirty reads, non-repeatable reads, and Phantom reads.Dirty ReadDirty read also known as invalid data readout, refers to in the database access, transact

operations are rolled back.Non-repeatable read (non-repeatable Reads): One transaction repeats two reads of the same row of data, but it gets different results. For example, in the middle of two reads, there is another transaction that modifies the row's data and submits it.Two update issues (Second lost updates problem): A special case that cannot be read repeatedly. There are two concurrent transactions that read the same row of data at the same time, and one of them modifies the commit, and

, however, this can lead to another tricky problem: Phantom Reading (Phantom read). To put it simply, Phantom reads when a user reads a range of data rows, another transaction inserts a new row within that range, and when the user reads the data row of that range, a new phantom row is found. The InnoDB and Falcon stora

commit the current transaction, is the change to a permanent change rollback: ROLLBACK canTo rollback the current transaction, cancel its change 2, directly using set to change the MySQL auto-commit mode MySQL default is automatically submitted, that is, you submit a query, it is directly executed!We can use set Autocommit=0 to disable autocommit set autocommit=1 turn on auto commit to implement transaction processing. But note that when you use set autocommit=0, all of your later SQL will be t

created:soft 0Just created:soft 1Just created:soft 2Just created:soft 3Just created:soft 4Just created:soft 5Ju St Created:soft 6Just created:soft 7Just created:soft 8Just created:soft 9Just created:weak 0Just created:weak 1Just Created:weak 2Just created:weak 3Just created:weak 4Just created:weak 5Just created:weak 6Just created:weak 7Just C Reated:weak 8Just created:weak 9Finalizing ... Weak 7Finalizing ... Weak 8Finalizing ... Weak 9Finalizing ... Weak 4Finalizing ... Weak 5Finalizing ... W

between transactions, with four isolation levels. First you need to explain a few conceptual definitions related to isolation: (1) Dirty reading: Refers to the transaction read dirty data, the so-called dirty data, refers to the incorrect data, such as the transaction during the execution of the modification of a record, and then rollback, if the other transaction read the record in the middle of the modified value, dirty read. (2) Non-repeatable reads: A transaction reads the same existing rec

-repeatable read (nonrepeatable Read): When a transaction reads the data it has read before, the discovery data has been modified or deleted by other committed transactions.there is no read (Phantom Read): short for "phantom read". When a transaction is re-queried by a previous condition, the returned result set contains new data that satisfies the conditions that were inserted by other committed transactio

ReviewOf the many storage engines in MySQL, only InnoDB supports transactions, and all of the transaction isolation levels described here refer to the transaction isolation level under InnoDB.READ UNCOMMITTED: One transaction can read uncommitted modifications to another transaction. This can lead to dirty reading, phantom reading, non-repeatable reading problems. (Basic useless)Read committed: One transaction can read only the modifications that have

two query process to update the original data, so the same select might return different results.Repeatable Read (can be reread)This is the default transaction isolation level for MySQL, which ensures that multiple instances of the same transaction will see the same rows of data while concurrently reading the data. In theory, however, this can lead to another tricky problem: Phantom Reading (Phantom read).

levels (snapshot of the read-committed isolation level based on the row version).Different isolation levels to control dirty read, non-repeatable read, Phantom Reading has a certain degree of control, also can have a certain extent of influence,The lower the isolation level, the higher the concurrency, but the more likely it is to produce dirty reads, non-repeatable reads, and Phantom reads; With the submi

solve the database transaction concurrency problem. Copyright Notice and Acknowledgements: This article original nerd RicoAuthor Blog address: http://blog.csdn.net/justloveyou_/ Examples of the explanations for dirty reads, non-repeatable reads, and Phantom reads are from the blog post, "Database transaction isolation level."I. Overview of transactionsIn general, each request of a user corresponds to a business logic method, and each business logic m

this case, non-repeated read occurs. (A transaction rereads data it has previusy read and finds that another committed transaction has modified or deeted the data .) phantom read: The same query is performed multiple times in the same transaction. Because of the insert operations performed by other commit firms, different result sets are returned each time. In this case, a phantom read occurs. (A transacti

the consistency of each data read/write operation. The standard SQL92 Specification defines four isolation levels and three Phenomena (Phenomena) to be banned ). The higher the isolation level, the stronger the guarantee of data consistency, but the worse the database concurrency. Problems caused by concurrent database operations include: 1. Dirty read (Dirty read ). Transaction A reads data that has not been committed in transaction B. If transaction B rolls back, A reads the wrong data. 2. Un

same transaction may have a new commit during the processing of this instance, so the same select may return different results. Repeatable Read (repeable)This is the default transaction isolation level of MySQL. it ensures that multiple instances of the same transaction will see the same data rows when reading data concurrently. However, theoretically, this will lead to another tricky problem: Phantom Read ). In short,

context.Isolation levelThe second aspect of a declarative transaction is the isolation level. The isolation level defines the degree to which a transaction may be affected by other concurrent transaction activity activities. Another way to think about the isolation level of a transaction is to think of it as the selfish degree to which the transaction deals with the data.In a typical application, multiple transactions run concurrently, often manipulating the same data to complete their work. Co