Concurrency Control

Concurrency control is a way to coordinate simultaneous transactions (briefcases) while preserving data integrity. Concurrency control is implemented in the code of an app and is based on the identity of a briefcase. Concurrency control should not to be confused with user access control. To make coordinated changes, an app must follow 3 basic rules:

  1. Reserve Codes before using them.
  2. Optionally lock models and elements before modifying them, depending on the iModel's concurrency control policy
  3. Pull and merge before pushing.

An iModel has a concurrency control policy that specifies how multiple briefcases may modify models and elements. The policy may stipulate that locks must be used, forcing transactions to be sequential (pessimistic), or it may specify change-merging with conflict-resolution to combine the results of simultaneous transactions (optimistic).

An app uses IModelDb and ConcurrencyControl to follow concurrency control rules.

Locks and code reservations are associated with a briefcase while it is making changes and are released when it pushes.

Background

This article assumes that you already know that:

  • An iModel is a multi-user database
  • An app works with a briefcase using the IModelDb class.
  • A briefcase has a unique identity that is issued and tracked by iModelHub.
  • Changes are captured and distributed in the form of ChangeSets.
  • ChangeSets are ordered in a sequence that is called the timeline of the iModel.
  • ChangeSets are stored in iModelHub
  • A Code is an identifier that is assigned to an element and is managed by a central Code Service.

Concurrency Glossary

Term Definition
Base ChangeSet B is based on ChangeSet A if B comes after A in the timeline.
Change-merging Same as merge.
Code Reservation The right to use a Code
Concurrency Control How to coordinate simultaneous transactions while preserving data integrity.
Concurrency Control Policy The rules that apps must follow when changing models and elements. May be optimistic or pessimistic.
Conflict Arises when two ChangeSets change the same object in different ways, where neither ChangeSet is based on the other.
Conflict-resolution Choosing how to resolve a conflict.
Lock The right to access a specific type of data with specific sharing permissions.
Merge Apply a ChangeSet to a briefcase.
Optimistic Concurrency Control A policy that allows apps to change models and elements without acquiring locks.
Pessimistic Concurrency Control A policy that requires apps to acquire locks before changing models or elements.
Push Upload a ChangeSet to iModelHub
Pull Download a ChangeSet from iModelHub. See IModelDb synchronization
Rebase Adjust a ChangeSet so that its pre-change state matches the post-change state of some other ChangeSet.
Tip The most recent version of an iModel. Also, the most recent ChangeSet in the timeline.
Transaction A set of changes that are committed or abandoned atomically, making up a unit of work. Multiple transactions to a briefcase are combined into a ChangeSet.
Version The state of an iModel as of a specific point in its timeline, that is, the result of the ChangeSets up to that point.

Code Reservation

An app must reserve all Codes that it plans to assign to elements during a local editing session. An app can reserve more Codes than it actually uses. For example, an app may reserve a range or block of Codes in a sequence to ensure that it can use them, but before it knows exactly how many it will need. When the local ChangeSet is finally pushed, the Code Service sorts out which reserved Codes were actually used and which were not. Unused Codes are returned to the pool, while used Codes are marked as used and unavailable. When an element is deleted, its Code may be returned to the pool.

See below for how to reserve codes.

The optimistic concurrency control policy does not apply to Codes.

Concurrency Control Policies

Preemptive locking is always required when importing Schemas or changing or inserting CodeSpecs. The APIs for those objects take care of locking automatically, so there is nothing special for an app to do.

For models and elements, there are two locking policy options: pessimistic and optimistic. The ConcurrencyControl class specifies the concurrency control policy for an iModel. The app must check the iModelDb.concurrencyControl property to learn the policy and then implement it.

Pessimistic Concurrency Control

To set up an iModel to use the pessimistic concurrency control policy, specify the ConcurrencyControl.PessimisticPolicy, as follows:

     iModel.concurrencyControl.setPolicy(new ConcurrencyControl.PessimisticPolicy());

The pessimistic concurrency policy requires that models and elements must be locked before local changes can be pushed to iModelHub. Locking prevents concurrent changes and forces briefcase transactions affecting the same models and elements to be sequential.

For reference, the pessimistic locking rules are as follows:

Operation LockType Lock Level
Insert element Model Shared
Modify element Element Exclusive
Delete element Element Exclusive
Insert model DgnDb Shared
Modify model Model Exclusive
Delete model Model Exclusive
" Elements in model Exclusive

An app will normally implement the pessimistic policy by using high-level APIs to build lock requests based on the intended operation, as explained below. These high-level APIs take care of both acquiring locks and reserving Codes.

A briefcase must pull before it can lock an element or model if it is affected by a recently pushed ChangeSet.

An app that implements the pessimistic concurrency control policy follows the pull -> lock -> change -> push pattern.

pessimistic concurrency example workflow

Locks are normally released when the briefcase pushes its changes, or they may be released if the briefcase abandons its changes.

Optimistic Concurrency Control

To set up an iModel to use optimistic concurrency control, specify the ConcurrencyControl.OptimisticPolicy, as follows:

     iModel.concurrencyControl.setPolicy(new ConcurrencyControl.OptimisticPolicy());

An optimistic concurrency policy allows apps to modify elements and models in an iModel without acquiring locks. In this case, the app uses change-merging to reconcile local changes before pushing.

Working without locks opens up the possibility that other apps may add ChangeSets to the timeline while the local editing session is in progress. The briefcase must then merge before it can push.

Suppose, for example, that two briefcases were editing different properties of the same element at the same time. Suppose that the first briefcase pushed first, creating ChangeSet#1. Now, the second briefcase must pull and merge before it can push.

optimistic concurrency example workflow

Conflicts

Working without locks also opens up the possibility that local changes may overlap with in-coming ChangeSets. When ChangeSets are merged into the briefcase, the change-merging algorithm checks for conflicts. The algorithm merges changes and checks for conflicts at the level of individual element properties. In the example above, the two briefcases changed different properties of the same element. That is not a conflict. Likewise, it is not a conflict for two briefcases both to set a property to the same value, or for two briefcases both to delete an element. Conflicts arise if the two briefcases set the same property to different values, or if one briefcase modifies a property and the other deletes the element.

If conflicts are found, the change-merging algorithm applies the iModel's conflict-resolution policy. This can be accessed using the IModelDb.concurrencyControl property. The policy object includes a ConcurrencyControl.ConflictResolutionPolicy that specifies a conflict-handling policy for each combination of changes that could conflict. The handling operations are defined by ConcurrencyControl.OnConflict. The default conflict-resolution policies are:

Local Change RemoteChange Resolution
update update RejectIncomingChange
update delete AcceptIncomingChange (reject not support)
delete update RejectIncomingChange (accept not supported)

Property-level change-merging is very fine-grained, and so it allows many kinds of changes to be made simultaneously without conflicts. A schema may also specify rules to check for conflicts on a higher level.

Only models and elements may be changed optimistically. Locking is required when importing Schemas or changing or inserting CodeSpecs.

How to Acquire Locks and Reserve Codes

This section describes how an app reserves Codes and/or acquires locks. There are two options for when and how to do this during a local transaction: before making changes (preemptively) or after making changes (bulk mode).

Acquiring locks and/or codes preemptively

  1. Call Model.buildConcurrencyControlRequest and Element.buildConcurrencyControlRequest to discover what locks and codes would be needed before making local changes.

  2. Call ConcurrencyControl.request to request the locks and/or codes that the planned local operations will require. This may send a request to iModelHub.

  3. If the request fails, cancel the local operation.
  4. If the request succeeds, go ahead with the local operation, make the planned local changes, and then call IModelDb.saveChanges.

This approach is the safest way to avoid conflicts. It requires that the app must plan ahead before making local changes.

Note that sending a request to iModelHub is a relatively expensive operation. Therefore it is important to batch up requests for locks and/or Codes.

Acquiring locks and/or codes in bulk mode

  1. Insert or update models and elements.

  2. Call ConcurrencyControl.request to request the locks and codes that those local operations require.

  3. If the request fails, call IModelDb.abandonChanges to roll back the local transaction.
  4. If the request succeeds, call IModelDb.saveChanges to commit the local transaction.

    Using bulk mode is simpler than using the preemptive approach, but it carries the risk that you must abandon all of your changes in case of a locking or code-reservation conflict. Use this approach only if you know that your changes are isolated such that conflicts are unlikely.

ChangeSets and Schema Changes

The special schema lock must be acquired before importing a schema into a briefcase. Also, schema changes must be isolated in a dedicated ChangeSet, separate from other kinds of changes. This is true for all concurrency control policies. To import a schema, an app must:

  1. Pull and merge to synchronize with the tip.
  2. Push any local changes to iModelHub.
  3. Perform the schema import in a local transaction.
  4. Pull and merge to synchronize with the tip.
  5. Push the results of the schema import as a ChangeSet to iModelHub.

After applying a schema ChangeSet, the IModelDb must be closed and reopened.

Last Updated: 14 November, 2019