Replication Snapshot Agent Blocking

The snapshot agent is used to initialize the subscription in transaction replication, and it requires a very short duration schema modification (SCH-M) lock on all the articles of the publication. If the articles are being queried thousands per second or queried for a long time then they will block the snapshot agent because SCH-M lock is not compatible with any other lock, once the SCH-M lock is waiting, everybody else will wait.

If blocking for few seconds is an issue in your system then you can initialize when there is low activity or initialize without a snapshot documented in books online.

Below I will show you how to reproduce this blocking scenario using a long running query.

 

-- on publisher: create publication database

use master
create database publisher_test_snapshot_agent_blocking
go

use publisher_test_snapshot_agent_blocking
create table paul_test1(a int primary key, b int)
create table paul_test2(a int primary key, b int)

-- on subscriber: create subscription database

use master
create database subscriber_test_snapshot_agent_blocking
go

-- on publisher: create publication and subscription

use publisher_test_snapshot_agent_blocking
exec sp_replicationdboption @dbname = N'publisher_test_snapshot_agent_blocking', @optname = N'publish', @value = N'true'
exec sp_addpublication @publication = N'test_snapshot_agent_blocking', @description = N'test', @sync_method = N'concurrent', @retention = 0, @allow_push = N'true', @allow_pull = N'true', @allow_anonymous = N'false', @enabled_for_internet = N'false', @snapshot_in_defaultfolder = N'true', @compress_snapshot = N'false', @ftp_port = 21, @ftp_login = N'anonymous', @allow_subscription_copy = N'false', @add_to_active_directory = N'false', @repl_freq = N'continuous', @status = N'active', @independent_agent = N'true', @immediate_sync = N'false', @allow_sync_tran = N'false', @autogen_sync_procs = N'false', @allow_queued_tran = N'false', @allow_dts = N'false', @replicate_ddl = 1, @allow_initialize_from_backup = N'false', @enabled_for_p2p = N'false', @enabled_for_het_sub = N'false'
exec sp_addpublication_snapshot @publication = N'test_snapshot_agent_blocking', @frequency_type = 1, @frequency_interval = 0, @frequency_relative_interval = 0, @frequency_recurrence_factor = 0, @frequency_subday = 0, @frequency_subday_interval = 0, @active_start_time_of_day = 0, @active_end_time_of_day = 235959, @active_start_date = 0, @active_end_date = 0, @job_login = null, @job_password = null, @publisher_security_mode = 1
exec sp_addarticle @publication = N'test_snapshot_agent_blocking', @article = N'paul_test1', @source_owner = N'dbo', @source_object = N'paul_test1', @type = N'logbased', @description = N'', @creation_script = N'', @pre_creation_cmd = N'drop', @schema_option = 0x000000000803509F, @identityrangemanagementoption = N'none', @destination_table = N'paul_test1', @destination_owner = N'dbo', @status = 24, @vertical_partition = N'false', @ins_cmd = N'CALL [sp_MSins_dbopaul_test1]', @del_cmd = N'CALL [sp_MSdel_dbopaul_test1]', @upd_cmd = N'SCALL [sp_MSupd_dbopaul_test1]'
exec sp_addarticle @publication = N'test_snapshot_agent_blocking', @article = N'paul_test2', @source_owner = N'dbo', @source_object = N'paul_test2', @type = N'logbased', @description = N'', @creation_script = N'', @pre_creation_cmd = N'drop', @schema_option = 0x000000000803509F, @identityrangemanagementoption = N'none', @destination_table = N'paul_test2', @destination_owner = N'dbo', @status = 24, @vertical_partition = N'false', @ins_cmd = N'CALL [sp_MSins_dbopaul_test2]', @del_cmd = N'CALL [sp_MSdel_dbopaul_test2]', @upd_cmd = N'SCALL [sp_MSupd_dbopaul_test2]'
exec sp_addsubscription @publication = N'test_snapshot_agent_blocking', @subscriber = N'(subscriber)', @destination_db = N'subscriber_test_snapshot_agent_blocking', @subscription_type = N'Push', @sync_type = N'automatic', @article = N'all', @update_mode = N'read only', @subscriber_type = 0
GO

-- on a new query window: run this long running select on the publisher

use publisher_test_snapshot_agent_blocking
select *
from master..spt_values a
cross join master..spt_values b
cross join paul_test1

-- on a new query window: start the snapshot agent  on the distributor

use distribution
exec sp_MSstartsnapshot_agent @publisher = N'(publisher)', @publisher_db = N'publisher_test_snapshot_agent_blocking', @publication = N'test_snapshot_agent_blocking'

-- on a new query window: run this long running select on the publisher

use publisher_test_snapshot_agent_blocking
select *
from master..spt_values a
cross join master..spt_values b
cross join paul_test1

-- on a new query window: run this long running select on the publisher

use publisher_test_snapshot_agent_blocking
select *
from master..spt_values a
cross join master..spt_values b
cross join paul_test1

-- on a new query window: run sp_WhoIsActive

exec sp_WhoIsActive 
@get_outer_command = 1
,@get_plans = 1
,@get_transaction_info = 1
,@find_block_leaders = 1
,@get_locks = 1

You will see that the long running query (session id = 88) is blocking the snapshot agent (session id = 95) that is requesting SCH-M locks, which in turn is blocking the other 2 queries (session_ id = 52 and 86) requesting Intent Shared (IS) locks.