Pg_catalog.pg_locks and the pg_catalog.pg_stat_activity.
Postgres lock queue update#
This update will attempt to acquire a ShareLock on the toys table. We will next try to update the same record from our This will not block an AccessShareLock,īut it will block an update.
Using the following command ActiveRecord::ansaction do In one of the rails consoles we will be creating a lock on the Toys table See these locks in action we will need to first add a lock to the database that The locks acquired in PostgreSQL can be viewed using the psql client to queryįor this example we will need to open three connections to our rails console. : SELECT "toys".* FROM "toys" WHERE "toys"."id" = $1 LIMIT $2 FOR UPDATEįrom (irb):75:in `block (2 levels) in irb_binding' Process 1978 waits for ShareLock on transaction 82070 blocked by process 1976.ĬONTEXT: while locking tuple (0,29) in relation "toys" Toy Load (1002.7ms) SELECT "toys".* FROM "toys" WHERE "toys"."id" = $1 LIMIT $2 FOR UPDATE, ]ĪctiveRecord::Deadlocked: PG::TRDeadlockDetected: ERROR: deadlock detectedĭETAIL: Process 1976 waits for ShareLock on transaction 82071 blocked by process 1978. Toy Load (1.4ms) SELECT "toys".* FROM "toys" WHERE "toys"."name" = $1 LIMIT $2, ] Toy Load (0.7ms) SELECT "toys".* FROM "toys" WHERE "toys"."id" = $1 LIMIT $2 FOR UPDATE, ] Toy Load (0.8ms) SELECT "toys".* FROM "toys" WHERE "toys"."name" = $1 LIMIT $2, ] Person Load (0.8ms) SELECT "people".* FROM "people" WHERE "people"."name" = $1 LIMIT $2, ] Occur when transaction A tries to acquire a lock on the same record transactionī has already acquired a lock for. If both of these transactions are run at the same time you will see a dead lock List includes a couple of the same toys the first transaction is going to update. Transaction B sends a request to update the user on several toys as well. Names = %w(bike yo-yo racecar truck rocket) Transaction A sends a request to update the user on several toys.
Resolve the dead lock by aborting one of the transactions involved. When this occurs neither process can proceed. These locks please check out the PostgreSQL documentation here.Ī deadlock is where 2 or more transactions attempt to acquire a lock the other holds. ActiveRecord::Migration.add_column :toys, :counter, :integerĪdding a column to a table will acquire an ACCESS EXCLUSIVE lock.įor more information on the other database level locks and what commands create The command ALTER TABLE can also acquire anĪCCESS EXCLUSIVE. This lock is acquired by the DROP TABLE, TRUNCATE, REINDEX, CLUSTER, VACUUM FULL,Īnd REFRESH MATERIALIZED VIEW. The ACCESS EXCLUSIVE lock will block all other locks from being acquired. This method is usedīy developers who want to use a Pessimistic locking strategy. In Rails you can acquire this lock by using the lock! method. This lock is created by the user or the application that isĬonnected to PostgreSQL. The SHARE ROW EXCLUSIVE is a lock that is not automatically acquired by any of the Share Row Exclusive ShareRowExclusiveLock Selection of a single row will not block the selection of the same row by another These locks only there while the query is running. The find_by method on an ActiveRecord object is an easy way to acquire theĪCCESS SHARE lock. Any table referencesįor a query can create a lock on that referenced table. Requested Lock ModeĪccess share is the most common lock and is only blocked by an ACCESS EXCLUSIVE.Ī simple select on any table will acquire the ACCESS SHARE lock. This table shows the which locks on a table (columns) will block a requested lock (rows). # Create the tablesĬlass CreatePeople < ActiveRecord::MigrationĬlass CreateToys < ActiveRecord::Migration Setupįor the examples in this post we will be using the following data setup with a new Rails application. Using ActiveRecord we can create these database level locks. Locks can also be acquired manuallyĪctiveRecord provides the interface between the Rails application and the database. Locks are automatically acquired by most Postgres commands to make sure tables are notĭropped or modified while a command is executed. Postgresql provides different locking modes to control concurrency within the database.
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