This error message, “[FireDAC][Phys][
FB] request synchronization error,” when accessing a Firebird database using Delphi FireDAC, indicates that there is a problem with the database synchronization process.
FireDAC (Firebird
ADO Connection Library) is a popular data
access component for Delphi and C++Builder applications that provides an interface to Firebird databases. The “request synchronization error” occurs when there is a conflict between data being updated concurrently in the database. This can happen when two or more transactions attempt to modify the same data at the same time.
To understand this error message in more detail, it’s important to know how Firebird handles concurrent transactions. Firebird supports both optimistic and pessimistic concurrency control. In optimistic concurrency control, each transaction assumes that other transactions will not modify the data it is using until it commits its changes. In contrast, pessimistic concurrency control locks the data being modified by a transaction to prevent other transactions from modifying it until the first transaction commits its changes.
When using optimistic concurrency control in FireDAC, if two transactions attempt to modify the same data and one of them commits before the other, the second transaction will receive a “request synchronization error.” This error message indicates that the second transaction needs to obtain the latest version of the data from the database before it can continue.
To resolve this issue, you have several options:
Use pessimistic concurrency control instead of optimistic concurrency control by setting IsolationLevel property of your TFDConnection object to tlxfFull or tlxfWaitRetried. This will lock the entire table or record during a transaction, preventing other transactions from modifying it until the first transaction commits its changes. However, this approach may lead to increased contention and decreased performance in multi-user environments.
Implement versioning or timestamping in your application logic to ensure that each transaction uses the latest version of the data before making modifications. This can be done by adding a version number or timestamp column to your tables and checking for conflicts before making modifications. For example, you can use Firebird’s SELECT FOR UPDATE OF ... INTO statement with a WHERE clause based on the version number or timestamp column to ensure that only the latest version of the data is modified.
Use Firebird’s built-in conflict resolution mechanisms such as last write wins (LWW) or base version wins (BVW). LWW allows the last committed transaction to win in case of conflicts, while BVW allows transactions with older versions of data to win over newer ones. These mechanisms can be configured at both application and database levels using various settings such as is_conflict_serializable, conflict_resolution, and conflict_resolution_timeout. However, these mechanisms may not be suitable for all use cases and can lead to unexpected results in some situations.
Implement application-level conflict resolution logic using techniques such as retrying failed transactions or merging conflicting changes manually. This approach requires more development effort but offers greater flexibility and control over how conflicts are resolved in your specific use case. For example, you can implement custom conflict resolution logic based on business rules or user input to determine which changes should take precedence in case of conflicts.
Use database triggers or stored procedures to implement complex conflict resolution logic directly in the database engine itself. This approach offers better performance and scalability compared to application-level conflict resolution but requires more database administration skills and may introduce additional complexity into your application architecture. For example, you can use Firebird’s built-in triggers such as BEFORE UPDATE or AFTER UPDATE to implement custom validation rules or conflict resolution logic based on specific table columns or conditions.
Consider upgrading your Firebird server or client components to newer versions that offer improved support for concurrent transactions and conflict resolution mechanisms such as multi-version concurrency control (MVCC) or snapshot isolation level (SIL). These features allow multiple transactions to read and write data without locking entire tables or records, improving overall performance and reducing contention in multi-user environments where frequent updates occur simultaneously from multiple clients or applications. For example, upgrading from Firebird 2.* series to Firebird 3.* series can provide significant improvements in handling concurrent updates with minimal impact on application code changes required due to backward compatibility guarantees provided by Firebird developers between major releases.
In summary, when encountering a “[FireDAC][Phys][
FB] request synchronization error” while accessing a Firebird database using Delphi FireDAC, it indicates that there is a problem with concurrent updates affecting conflicting data within your application environment.
To resolve this issue effectively, consider implementing one of several strategies discussed above such as pessimistic concurrency control, versioning/timestamping, built-in conflict resolution mechanisms like LWW/BVW, custom application-level conflict resolution logic using triggers/stored procedures, upgrading components/servers supporting MVCC/SIL features.
Authoritative References Used:
Embarcadero Technologies - Delphi Help: Concurrency Control
firebirdsql - Optimistic Concurrency Control
firebirdfaq - Request Synchronization Error