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# Overview of Firebird Client Library Thread-Safety

The thread-safety properties of the standard Firebird client library vary according to the following:

- Firebird version
- operating system
- Firebird server architecture (SuperServer, Classic, Embedded)
- underlying connection protocol (embedded vs. local vs. IPC vs. remote)

Determining whether the client library you’re using is thread-safe can be quite confusing. This document aims to reduce that confusion by defining what thread-safety means in the context of the Firebird client library, then presenting a table that specifies which client library configurations are thread-safe.

Note that this document deals only with the standard Firebird client library that underlies the Firebird C API. It has no relevance to the clean-slate reimplementations in access libraries such as Jaybird.

## Definition of “Thread-Safety”

Currently, the highest level of concurrency supported by any version of the Firebird client library is thread-safety at the connection level.

When we say that the Firebird client library is thread-safe at the connection level, we mean that it is safe to use a particular connection in only one thread at a time, although the same connection can be manipulated by different threads in a serial fashion, and different connections can be manipulated by different threads in parallel.

For example, in a multithreaded application server, it is safe for a particular connection to be leased from a connection pool by Thread A, used, and returned to the pool for later lease by Thread B. It is not safe for Thread A and Thread B to use the same connection at the same time.

## Thread-Safety Table

FB Version | OS | FB Architecture | Remote / Local | Thread-Safe?
--- | --- | --- | --- | ---
1.0.3 | Windows | SuperServer | Local | No
Remote | Yes
Classic | Local | No such config.
Remote | No such config.
Embedded | Local | No such config.
Remote | No such config.
Linux | SuperServer | Local | No such config.
Remote | No
Classic | Local | No
Remote | No
1.5 | Windows | SuperServer | Local | No
Remote | Yes
Classic | Local | No such config.
Remote | Yes
Embedded | Local | Yes
Remote | No such config.
Linux | SuperServer | Local | No such config.
Remote | Yes
Classic | Local | No
Remote | No
2.0/2.1 | Windows | SuperServer | Local | Yes
Remote | Yes
Classic | Local | Yes
Remote | Yes
Embedded | Local | Yes
Remote | No such config.
Linux | SuperServer | Local | No such config.
Remote | Yes
Classic | Local | No
Remote | No

This document was written by David Rushby, with assistance from Dmitry Yemanov. Errors are attributable to Rushby rather than Yemanov.

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