Working with multiple DB Connections

THIS PAGE IS WORK IN PROGRESS AND DESCRIBES FEATURES THAT ARE NOT YET IN TRUNK !!!

What is described here is subject to change without notice until this is in trunk !!!

Working with multiple DB Connections: Overview

The Limitations That it Overcomes

• Multiple Threads can run arbitrary DB commands against a single DB Connection in a safe way due to the introduction of the Co-ordinated DB Access (Thread-safe DB Access) feature, but there are limitations to this:
  • Each Thread must acquire a DB Transaction with the GetNewOrExistingXXXXTransaction Methods as it usually not known which of the Threads will start the DB Transaction first and which one will 'piggy-back on it';
  • Two Threads cannot take out/piggy-back on a DB Transaction when the IsolationLevels that are required don't match (as the Threads are sharing the same DB Transaction in a thread-safe way);
  • 'Parallel' DB access is not achievable (as the Threads are sharing the same DB Transaction in a thread-safe way by running DB commands one-after-the-other instead of truly parallel);
  • Handling the outcome of a Thread 'losing out' on obtaining Co-ordinated DB Access when a time-out occurs is either done automatically, which results in 'stock messages' being shown to the user (asking the user to retry the action that [s]he wanted to take a bit later), or - to avoid this - needs to be done intentionally by the software engineers. As the timing in which such situations happen cannot easily be foreseen and not easily be 'forced to happen' during development this can be a cumbersome aspect of the Co-ordinated DB Access approach;
• Historically, only a single DB Connection per connected OpenPetra Client was possible. (This 'Default DB Connection' gets established automatically when a Client connects and gets closed automatically when a Client disconnects or crashes.)
  • While an OpenPetra software engineer could make an attempt to open a separate DB Connection, start a DB Transaction on that separate DB Connection and try to run DB commands against that DB Connection this failed with a whole variety of Exceptions due to several bugs in the 'OpenPetra DB Access and Abstraction Layer' (in the TDataBase Class and related Classes).

From trunk commit Rev. XXXX (September xx, 2015) onwards it has become possible for OpenPetra software engineers to...

• reliably open (a) DB Connection(s) that is/are truly separate from the 'Default DB Connection' which exists for each Client connection (opening of DB Connections is OK even when it should occur in parallel in multiple Threads!);
• reliably start a new DB Transaction with the BeginXXXXTransaction commands (or GetNewOrExistingXXXXTransaction commands) on such a separate DB Connection, with any IsolationLevel;
• reliably run an arbitrary number of arbitrary SQL Commands that are enlisted in such a DB Transaction;
• Commit or Rollback such a DB Transaction completely independent from DB Transactions on other DB Connections (also from the 'Default DB Connection');
• start and end an unlimited number of DB Transactions on such a separate DB Connection;
• Close such a separate DB Connection completely independent from other DB Connections (also from the 'Default DB Connection').

True parallel DB Access from multiple Threads where it is desired/needed has become possible!

The Solution

The primary solution was to find bugs in the TDataBase Class (OpenPetra's Database Access and Abstraction Layer) and related Classes that prevented reliable independent DB Connections, DB Transactions and DB Commands, and to fix them. This was done by extending our existing NUnit Tests for the TDataBase Class with multi-threaded NUnit Tests (found in the csharp/ICT/Testing/lib/Common/DB/tests.Multithreading.cs class file) and through those new NUnit Tests probing what ought to work, finding out through those what didn't work, and then addressing the bugs that prevented what ought to have worked. (The feature Bug for this solution is Bug 4364).

Relation to Co-ordinated DB Access (Thread-safe DB Access)

The ability to work with multiple parallel DB Connections does not away with the need for the Co-ordinated DB Access (Thread-safe DB Access) approach. The latter is needed everywhere where we don't deliberately start a new Thread for parallel DB Access (that is, in most situations in OpenPetra)! Rather, the ability to work with multiple parallel DB Connections opens up the way of true parallel DB command execution, and not just pseudoparallel execution, and for independent DB Transactions (both independent in their IsolationLevels and the timings of starting, committing and rolling back).

Details of the Implementation

Requirements For Safe Multi-threaded DB Access

Introduction

In every 'deliberate multi-threading scenario' (that is, a scenario that we create on purpose and which we control) in which some form of DB Access is required, the creation of a private, independent TDataBase instance and working solely with that instance (and with no other TDataBase instance!) throughout the Threads' lifetime is required.

The reason for this is that ADO.NET providers (and specifically the PostgreSQL ADO.NET Provider 'npgsql') are not thread-safe.

To be able to find violations of those rules early, our TDataBase Class blocks attempts to take out / work with parallel DB Transactions and raises specific Typed Exceptions (see Automatic (and Fully Transparent): Multi-threading and multi-connection 'Sanity Checks' in the TDataBase Class). Whenever the TDataBase Class raises such an Exception, various information is logged in the server log file, too. (See Verbose Logging for influencing the verbosity of the logging information.) Would such Exceptions not be raised, a whole variety of Exceptions from various sources would be thrown 'further down the line' when parallel DB access through the same TDataBase instance would be attempted (see Bug 4364).

Rules That Need to be Adhered to

• Server-side code that runs in a Thread that got started in a 'deliberate multi-threading scenario' (that is, a scenario that we create on purpose and which we control) must perform any DB access only through an instance of TDataBase that got created in that Thread (see next paragraph)!
• Server-side code that doesn't run in a deliberately started Thread:
  • If the server-side code got invoked from the client side it DOES get run in a separate Thread which .NET Remoting creates for us 'under the hood'! When that server-side code access the DB it usually uses the 'globally available' DBAccess.GDBAccessObj TDataBase instance.
    • Co-ordinated DB Access (Thread-safe DB Access) will be at work in this case and will provide pseudoparallel thread-safe DB access to that TDataBase instance.

Server-side Code That Runs in a Deliberately Started Thread

The creation of a private, independent TDataBase instance and solely working with that from within that Thread throughout the Threads' lifetime is required.

In any deliberately started Thread, ...

• Create a private, independent TDataBase instance and store a reference to it in a Variable. If you store a reference to it in a Field you must make sure it isn't accessed from anywhere outside of that Thread, or if that should be needed, it must be accessed in a completely thread-safe manner.
• Open a private, independent DB Connection for this Thread by calling the EstablishDBConnection Method on that Threads' TDataBase instance.
  • Supply a unique name for the DB Connection with the optional 'AConnectionName' Argument of the EstablishDBConnection Method as this can be a big help in debugging (see Verbose Logging)!
  • Example: \csharp\ICT\Petra\Server\lib\MCommon\Main.cs, Method 'EstablishDBConnection'.
• Start a private, independent DB Transaction on that Threads' TDataBase instance.
  • You can use any of our usual methods for that and you can choose any IsolationLevel!
  • Supply a unique name for the DB Transaction with the optional 'ATransactionName' Argument of the Method you are using for this as this can be a big help in debugging (see Verbose Logging)!
  • An arbitrary number of private, independent DB Transactions can be started and ended on that Threads' TDataBase instance!
• Run an arbitrary number of arbitrary SQL Commands on the private, independent DB Transaction(s) on that Threads' TDataBase instance.
• Commit or Rollback the private, independent DB Transaction(s) on that Threads' TDataBase instance.
• Close the private, independent DB Connection by calling CloseDBConnection on that Threads' TDataBase instance.

That way what that specific Tread that needs to execute commands against the DB is completely independent of what any other running Thread(s) might be doing in any (of their) DB Transaction(s) (with whatever IsolationLevel) against the same DB (using the commong DBAccess.GDBAccessObj or its/their own separate TDatabase instance) at roughly the same time! However, a limiting factor for the 'data throughput' and the parallelism achieved could be DB Table Locks as they could impede multi-threading performance and parallelism.

Changing (Library) Methods so That They Can Work With the Default DB Connection or a Separate DB Connection

When Such a Method Has Got a TDBTransaction instance available

Replace any occurrence of DBAccess.GDBAccessObj with a call to DBAccess.GetDBAccessObj(ATransaction), passing the reference of the TDBTransaction instance that the Method has got available.

The overloaded DBAccess.GetDBAccessObj(...) Method acts as a 'set of points' (using figurative railway language here) and will yield the 'globally available' DBAccess.GDBAccessObj in case a null value for the ATransaction gets passed, otherwise it will yield the TDataBase instance which started the DB Transaction that got passed in in its TDBTransaction ATransaction Argument.

When Such a Method Has NOT Got a TDBTransaction instance available

• Either...
  • add a new optional Argument that allows the passing-in of a TDataBase instance;
    • you are encouraged to use 'TDataBase ADataBase = null' at the very end of the Methods' argument list for the sake of consistency;
  • or add a new optional Argument that allows the passing-in of a TDBTransaction instance;
    • you are encouraged to use 'TDBTransaction ATransaction = null' at the very end of the Methods' argument list for the sake of consistency.
• Then replace any occurrence of DBAccess.GDBAccessObj with a call to either DBAccess.GetDBAccessObj(ADataBase) or DBAccess.GetDBAccessObj(ATransaction).

The overloaded DBAccess.GetDBAccessObj(...) Method acts as a 'set of points' (using figurative railway language here) and will yield the 'globally available' DBAccess.GDBAccessObj in case a null value for the ADataBase or ATransaction gets passed, otherwise it will either yield the TDataBase instance passed in in its TDataBase ADataBase Argument, or the TDataBase instance which started the DB Transaction that got passed in in its TDBTransaction ATransaction Argument (depending on which overload of the Method is used).

Standardised XML Comments

For the sake of consistency you are encouraged to use the following XML Comments when adding an Argument as described above to existing Methods:

/// <param name="ADataBase">An instantiated <see cref="TDataBase" /> object, or null (default = null). If null
/// gets passed then the Method executes DB commands with the 'globally available'
/// <see cref="DBAccess.GDBAccessObj" /> instance, otherwise with the instance that gets passed in with this
/// Argument!</param>

and

/// <param name="ATransaction">An instantiated <see cref="TDBTransaction" /> object, or null (default = null).
/// If null gets passed then the Method executes DB commands with the 'globally available'
/// <see cref="DBAccess.GDBAccessObj" /> instance, otherwise with the the <see cref="TDataBase" /> instance
/// which started the DB Transaction that gets passed in with this Argument!</param>

Writing new (Library) Methods That Can Work With the Default DB Connection or a Separate DB Connection

Refer to the section Changing (Library) Methods so That They Can Work With the Default DB Connection or a Separate DB Connection on how to achieve this and use the overloaded DBAccess.GetDBAccessObj(...) Method to work with either the Default DB Connection or a separate DB Connection right from the start.

Extra Things that You Need to Know

Using 'Central Server-side' Methods With Separate DB Connections

Using Methods of our TTypedDataAccess Class and Auto-generated 'Typed DataStore' Methods With Separate DB Connections

• TTypedDataAccess Class: All Methods of the TTypedDataAccess Class that access the DB now work with the DB Connection that started the DB Transactions that gets passed into those Methods. That means that you can utilise those Methods also on separate DB Connections that got started on a separate Thread. (These Methods used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)
• Auto-generated 'Typed DataStore' Methods: All Methods of the auto-generated 'Typed DataStore' now work with the DB Connection that started the DB Transactions that gets passed into those Methods. That means that you can utilise those Methods also on separate DB Connections that got started on a separate Thread. Example for such a Method: PPartner.LoadByPrimaryKey(...) in Partner.Access-generated.cs. (These Methods used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)
• Auto-generated SubmitChanges Methods of 'Typed DataSets': All these auto-generated Methods now have an optional Argument TDataBase ADataBase = null. Pass in the separate DB Connection that got started on a separate Thread to make these Methods execute on that DB Connection. (These Methods used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)
• Auto-generated GetCacheableRecordReferenceCount / GetNonCacheableRecordReferenceCount Methods: All these auto-generated Methods now have an optional Argument TDataBase ADataBase = null. Pass in the separate DB Connection that got started on a separate Thread to make these Methods execute on that DB Connection. (These Methods used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)
• Auto-generated server-side code for Cacheable DataTables: All server-side Methods of the auto-generated Cacheable DataTable code now have an optional Argument TDataBase ADataBase = null. Pass in the separate DB Connection that got started on a separate Thread to make these Methods execute on that DB Connection. (These Methods used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)
  • *Cacheable.ManualCode.cs files: The hand-written Methods in such files need to be adapted manually so that the DB Connection that started ATransaction runs the commands and no longer the hard-coded DBAccess.GDBAccessObj's DB Connection! Example: \csharp\ICT\Petra\Server\lib\MPartner\Partner.Cacheable.ManualCode.cs.

Using TCommonDataReader.GetData and SaveData Methods With Separate DB Connections

An overload has been added to both the TCommonDataReader.GetData and TCommonDataReader.SaveData Methods. That overload has got an Argument TDataBase ADataBase. Pass in the separate DB Connection that got started on a separate Thread to make these Methods execute on that DB Connection. (These Methods used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)

Using TSequenceWebConnector.GetNextSequence With Separate DB Connections

An overload of the TSequenceWebConnector.GetNextSequence Method has got an optional Argument TDataBase ADataBase. Pass in the separate DB Connection that got started on a separate Thread to make these Methods execute on that DB Connection. (That Method used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)

Using TPagedDataSet.ExecuteQuery With Separate DB Connections

The TPagedDataSet.ExecuteQuery Method now has got an optional Argument TDataBase ADataBase = null. Pass in the separate DB Connection that got started on a separate Thread to make that Methods execute on that DB Connection. (The ExecuteFullQuery Method that it calls used to be hard-coded to always use GDBAccess.DBAccessObj but this is no longer the case.)

Using FastReport Reports Server-side Code With Separate DB Connections

A given FastReport can now optionally be run on a separate DB Connection. To do so, a Report must create an instance of TReportingDbAdapter with the Argument 'ASeparateDBConnection' set to true. This must be enabled separately for each Report as using this feature for a given Report requires thorough checking of all DB access the FastReport Report performs (also through any library-like function calls!) and only once this is established it should be enabled for the individual Fast Report!

Automatic (and Fully Transparent): Multi-threading and multi-connection 'Sanity Checks' in the TDataBase Class

TODO

Locking of DB Tables

A 'side effect' of multiple Threads having their own DB Connections and running DB Transactions independently from each other and DB Commands potentially in parallel is that DB Locks are taken out, potentially in parallel. For the RDBMS this scenario is essentially the same as the scenario where multiple OpenPetra users are connected (each having their own 'globally available' DB Connection) and each of them running DB Transactions and DB Commands independently and potentially in parallel. In both scenarios DB Locks are taken out, according to the IsolationLevels of the DB Transactions involved, and in both situations DB Locking needs to be taken into account.

In the scenario where multiple Threads run DB Transactions and in which DB Commands can potentially get executed in parallel we need to get used to thinking about DB Locking and the effects it can have, for the other scenario (multiple users having separate DB Connections) that thinking should be ingrained in us already.

One effect of DB Locking that may well be felt when multiple Threads run multiple DB Commands in parallel is that the DB Locking might reduce the performance of the multiple DB-accessing Threads because the concurrency that the multiple Threads provide might be impeded by DB Locking. In the worst case DB Locks could reduce the outcome of our multi-Threading attempts to serial processing instead of parallel processing, resulting in what could essentially look like the outcome of Co-ordinated DB Access (Thread-safe DB Access) because the RDBMS might allow only one Thread to perform DB queries at once due to the DB Tables involved and DB Locking being at work!!!

Debugging

Verbose Logging

An option for very verbose logging got introduced that is helpful especially for pinning-down problems in multi-threading and multi-connection situations. To switch it on, turn the logging level ('DebugLevel') of the Server to 11 in the servers' config file.

Extra information you will get in the log files when it is turned on:

• Unique DB Connection Identifiers (GUIDs) for each DB Connection;
• Unique DB Transaction Identifiers (GUIDs) for each DB Transaction and the DB Connection Identifier of the DB Connection which the DB Transaction got started with;
• For BeginTransaction / BeginAutoTransaction:
  • IsolationLevel of the new DB Transaction (if it was started with a specific IsolationLevel);
  • DB Transaction Identifier (GUID) of the new DB Transaction;
  • DB Connection Identifier (GUID) of the DB Connection that started the DB Transaction;
  • StackTrace at the point of DB Transaction start and the Thread and AppDomain in which the DB Transaction got started;
• In case of GetNewOrExistingXXXXTransaction commands if 'piggy-backing' on an existing DB Connection occurs:
  • the DB Transaction Identifier (GUID) of the DB Transaction it is 'piggy-backing' on;
  • IsolationLevel of the DB Transaction it is 'piggy-backing' on;
  • Thread and AppDomain in which the DB Transaction that it is 'piggy-backing' on got started;
  • DB Connection Identifier (GUID) of the DB Connection that originally started the DB Transaction;
  • Thread and AppDomain in which the GetNewOrExistingXXXXTransaction command got called.
• For CommitTransaction or RollbackTransaction the following gets logged after the command got executed, but with information that was gathered before the command executed:
  • the DB Transaction Identifier (GUID) of the DB Transaction;
  • whether the DB Transaction was still valid (i.e. not already Committed / Rolled back);
  • IsolationLevel of the DB Transaction;
  • whether the DB Transaction was reused;
  • Thread and AppDomain in which the DB Transaction got started;
  • Thread and AppDomain in which the CommitTransaction or RollbackTransaction command got called.
• For any DB Command that gets executed:
  • Thread and AppDomain in which the DB Command gets executed;
  • DB Transaction Identifier (GUID) of the DB Transaction that the DB Command is enlisted in;
  • DB Connection Identifier (GUID) of the DB Connection that originally started the DB Transaction that the DB Command is enlisted in.

Tips:

1. If the (new) options to name DB Connections and DB Transactions are used then the name of each named DB Connection and named DB Transaction is included in the logging, too! This is helpful as GUIDs - though they are unique - are not very readable.
2. This very verbose logging is extremely helpful when one wants to ascertain that a certain function in OpenPetra, e.g. a Report, Extract, etc. (including any DB access that any library/helper functions that it might call!) really performs every DB access it does on the separate DB Connection in a separate Thread that you purposefully created for it! Method:
   1. Turn the very verbose logging on before executing the function in OpenPetra;
   2. Scrutinise the log output after the execution of the function has finished. Check whether all DB Commands that it ran were indeed run only on the separate DB Connection and on the separate Thread. (Naming of DB Connections and DB Transactions pays off here!)
   3. This is quite some work but the only proof that things are indeed happening as they ought to!

Caveat: This logging is indeed very verbose and hence will slow down execution of the program. It is therefore conceivable that multiple threads will run with different timings when logging is turned up that much than if logging wouldn't be turned up that much...

Finding Out Which Queries Are Run (in Parallel) on Which RDBMS Connection (with PostgreSQL)

It can be quite helpful to see from the RDBSMs' end what queries are running against which DB Connection. For PostgreSQL this is possible by issuing the following SQL query:

SELECT datname, usename, application_name, pid, client_addr, client_port, waiting, query_start, query
    FROM pg_stat_activity
    WHERE datname='demo' AND usename='petraserver' AND state LIKE '%in transaction%' 
    ORDER BY pid;

Replace 'demo' with the name of the DB that the PetraServerConsole instance is connected to and run this query in pgAdmin III repeatedly (press <F5> for that) while the function of OpenPetra that you want to monitor for its DB connection use is executing.

How to read the output of that query: PostgreSQL starts a separate DB Server process for each DB connection. Each DB Server process has got its own 'pid' (Process Identification). This means that if several DB commands are executing in parallel on several DB connections you will get a list of all the processes that are currently running DB queries - one result line for each process/DB Connection (the 'query' column shows the SQL statement of the query that is currently running against that DB Connection). If you intended the function of OpenPetra to run solely on a specific DB connection and you are seeing multiple lines at times it isn't doing this! In that case refer to the section Verbose Logging.

Finding out the Number of Currently Open DB Connections (with PostgreSQL)

One can use the combination of the static Methods TDataBase.ClearAllConnectionPools() and TDataBase.GetNumberOfDBConnections to get the number of currently open DB connections on a RDBMS (using just the latter Method on its own is not reliable because of connection pooling)! Use the static Method TDataBase.ClearConnectionPoolAndGetNumberOfDBConnections to call both Methods at once. For an example on how to use this see Unit Tests in \csharp\ICT\Testing\lib\Common\DB\test.Multithreading.cs!

Multi-threading: General Advice

General caution/advice: Programming with multiple threads is always much, much harder than programming with a single thread. When multiple threads access the same program data (Fields, static Fields, structs, etc.) - and indeed data from a DB - one can easily get intermittent problems which can be very hard to pin-point and to resolve. Extreme diligence needs to be employed when multiple threads should access some shared program data - and indeed data that resides in a DB - in a writing fashion! Debugging a multi-threaded program is also considerably more difficult than debugging a single-threaded program. Whole books are devoted to the topic of multi-threaded programming as multi-threading is a quite difficult discipline that is hard to master...

FAQ's

Instead of opening a new DB Connection for 'Thread A': Could I not simply 'make a peek' to see if a DB Transaction is running on a given DB Connection in another Thread and if so, use that?

No. While it is possible to inquire the 'Transaction' Property of a given TDataBase instance that one has got access to (e.g. of DBAccess.GDBAccessObj) (and that inquiry is thread-safe) and in doing so it is possible to find out whether a DB Transaction is running on that TDataBase instance (if the 'Transaction' Property yields null then no DB Transaction is running, otherwise there will be one running), it would not be safe to use that DB Transaction...

• as there is no guarantee that the other Thread wouldn't be committing/rolling back that very DB Transaction any time (which would have unknown consequences for 'Thread A'!);
• as it got started on another Thread, and ADO.NET is not thread-safe.

To prevent that if such an attempt should happen / occur, the TDataBase class will throw...

• an EDBAttemptingToWorkWithTransactionThatGotStartedOnDifferentThreadException if one tries to 'piggy-back' on its running DB Transaction by calling GetNewOrExistingXXXXTransaction from a different Thread, and
• an EDBAttemptingToCreateCommandThatWouldRunCommandOnDifferentThreadThanThreadOfTheTransactionThatGotPassedException if one tries to run a DB Command on a TDBTransaction instance that one obtained by using the TDBTransaction instance that is returned by the 'Transaction' Property of a TDataBase instance which got created in another Thread. (The 'Transaction' Property of the TDataBase Class hasn't got XML Comments that explicitly warn against doing anything on and with that Transaction for no reason...!)

Could calling GetNewOrExistingXXXTransaction every give 'Thread A' a running DB Transaction from another Thread?

No. When a Method in 'Thread A' calls GetNewOrExistingXXXTransaction it must do that on the TDataBase instance that got instantiated on this Thread otherwise a EDBAttemptingToWorkWithTransactionThatGotStartedOnDifferentThreadException gets thrown. That way 'Thread A' will never (be able to) get a DB Transaction that belongs to a TDataBase instance of a different Thread ('Thread B') using any of the GetNewOrExistingXXXTransaction Methods!

Conversely, 'Thread B' (e.g. an intentionally started 'worker thread') will never (be able to) get a DB Transaction with GetNewOrExistingXXXTransaction that belongs to a TDataBase instance of a different Thread ('Thread A', which might e.g. use the 'globally available' DBAccess.GDBAccess instance) because it must not call GetNewOrExistingXXXTransaction on the instance of 'Thread A' - otherwise a EDBAttemptingToWorkWithTransactionThatGotStartedOnDifferentThreadException gets thrown!

Could calling BeginXXXTransaction every give 'Thread A' a new DB Transaction from another Thread?

Yes, and you must make sure that this isn't happening by only ever calling BeginXXXTransaction on the separate instance of TDataBase that gets purposefully created for 'your' Thread!

Reason why TDataBase needs to allow this and cannot not throw an Exception should that happen: If server-side code got invoked from the client side it DOES get run in a separate Thread which .NET Remoting creates for us 'under the hood'! When that server-side code accesses the DB it usually uses the 'globally available' DBAccess.GDBAccessObj TDataBase instance. That instance got created when the user logged in to the OpenPetra server, and it got created on a different Thread than the auto-created Thread that .NET Remoting manages.

Could 'Thread A' (accidentally) Commit or Rollback a DB Transaction that got started on another Thread (and vice versa)?

No. We must not do this because ADO.NET is not thread-safe. To prevent that if such an attempt should happen / occur, the TDataBase class will throw an EDBAttemptingToWorkWithTransactionThatGotStartedOnDifferentThreadException.

Could 'Thread A' (accidentally) execute a DB Command that it enlists in a DB Transaction of its own on a DB Connection that is owned by another Thread?

No. We must not do this because (1) ADO.NET is not thread-safe, (2) it really doesn't make sense, if you think about it what the RDBMS should do with it! To prevent that if such an attempt should happen / occur, the TDataBase class will throw an EDBAttemptingToCreateCommandOnDifferentDBConnectionThanTheDBConnectionOfOfTheDBTransactionThatGotPassedException. (The check that the TDataBase class performs is even more general: it simply disallows the execution of a Command that is to be enlisted in a DB Transaction that got created on a different DB Connection.)

Could 'Thread A' (accidentally) close a DB Connection that is owned by another Thread?

No. We must not do this because ADO.NET is not thread-safe. To prevent that if such an attempt should happen / occur, the TDataBase class will throw an EDBAttemptingToCloseDBConnectionThatGotEstablishedOnDifferentThreadException.

How can a 'library-like' Method be written that could utilise a running DB Transaction of a certain TDataBase instance if it is currently not busy, but would create its own DB Transaction on a separate, new TDataBase instance if it is busy?

• While this is possible, this must be done
  • within the same Thread as ADO.NET is not thread-safe;
  • only when the IsolationLevel is exactly the one that is needed;
  • in a way that ensures that no other Thread could utilise the TDataBase instance that has started the DB Transaction in question while the DB access code in the 'library-like' Method runs.

The TDataBase Class has got the Method CheckRunningDBTransactionIsCompatible that performs checks to make sure that the first conditions are met and returns 'true' only if they both are met. If you can ascertain that the third condition will always be met then you can use that Method and use the DB Transaction in question, otherwise the 'library-like' Method must create a separate, new TDataBase instance, establish a separate DB Connection and start a new DB Transaction. (You could use thread-synchronisation methods such as utilising a ManualResetEvent to ensure that the third condition will always be met - see how it is used in \csharp\ICT\Testing\lib\Common\DB\test.Multithreading.cs. In addition - and only in addition - you should call the ConnectionReady Method of the TDataBase instance that has started the DB Transaction in question to be sure that the DB Connection isn't busy at the moment.)

Questions?

In case you have questions regarding working with multiple DB Connections please contact ChristianK, the developer that fixed the issues that prevented it from working in the past, introduced the advanced multi-threading and multi-connection 'sanity checks' in the TDataBase Class and who wrote this documentation.