Immediate_sync on vs off

This is one of the settings that can cause issues with growth of your distribution database or when running Snapshot. It is FALSE by default and it can be set on Publication level. You can check what setting you have by checking syspublications on published database or mspublications on Distributor and distribution database. Let me try to explain what impact have that setting to your replication setup. We are assuming your distribution retention is not “fixed set” like this @min_distretention=48, @max_distretention=48 but more like this @min_distretention=8, @max_distretention=48. More about how this works in the following post.

TRUE

Normal behavior when your retention is set to min=8 and max=48 will be that transactions that been delivered to subscribers and are older than 8 hour will be removed by distribution cleanup. However that changes with Immediate_sync set to ON. The transactions will be kept (even if delivered) until get older than 48 hours. Also it matters when you adding new subscribers. If Snapshot was generated within last 48 hours and you add subscriber it will use existing snapshot and deliver all the transactions that were marked for replication after snapshot generation. Here is an example.

We have Publication1 with TableA as an article and it is published to Subscriber, Subscriberdb database.

Initial_setting

Snapshot was generated within that 48 hour period and two rows added to TableA on Publisher.

Snapshot

Inserts

We are planning to add new subscription stream to same subscriber but to Subscriberdb_2 database. After the creation you do not need to run Snapshot again it will use old Snapshot and all transactions that been delivered afterwards.

New subscription_ON

You can see that it used old Snapshot as TableA has 0 rows (we inserted 2 rows before creating new subscription) and once you select from TAbleA on Subscriberdb_2 you will find two rows. These been delivered as normal replication transactions.

subsriber_select.JPG

From my perspective there is one downside of that setting, especially when you are planning to add new articles quite often. Every time you add new article and want to send a snapshot of these new articles down to subscribers the Snapshot Agent will generate a snapshot for ALL articles with that Publication. In other word last Snapshot has been invalidated.  Lets add TableB to that Publication1 and lets run snapshot.

adding_newarticles_on.JPG

The Snapshot will generate two articles:

new_snapshot_newarticle_on.JPG

However existing Subscribers will send down only new articles TableB:

Subscription_new_article_on.JPG

The generation of Snapshot for ALL tables could be problematic when you publishing hundreds of large tables already. Adding one article will generate new snapshot for all of these tables and from my experience it will generate some locks on Publisher especially if Publisher is very busy.

Lastly the snapshot will be generated no matter if you add new articles or not. If someone will kick off snapshot by mistake it will still do snapshot for entire publication.

Lets have a look how it looks if this setting will be set to FALSE.

FALSE

First thing will be that will be different is that you will need to generate full snapshot every time you adding new subscription.

Second thing that will change is when your retention is set to something like that @min=0 and @max=72. All transactions that have been delivered will be cleaned up on the next Distribution Cleanup job run.

Third difference is that when adding new articles to existing publication the next snapshot run will generate ONLY files for new articles. Lets have an example.

First we changing setting to FALSE by running sp_changepublication:

changingto_false

Now lets add TableC into Publication1:

addingTableC

and run snapshot:

snapshot_with_false

As you can see Snapshot has been generated only for one Table, TableC. That is quite convenient especially when you adding one table into publication that already have hundreds of other tables.

Which settings you should choose? As always it depends on your environment. How often you adding article into Publication? How much space you have on your distributor? How busy is your Publisher? Can you afford of running snapshot for entire Publication?

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