Delphi Xe2 + ZeosLib 7.0.3 Stable + Firebird 1.0
I am doing updates to the several tables and data is retaining on memory. It is not reflected on the database in a way that other applications can see it.
I have tried using auto-commit only and did not work.
I have also used explicit transaction control, ZConnection.StartTransaction and ZConnection.Commit and did not work either.
I am updating data in a webserver created on delphi with Indy httpServer. I get post requests then act reading or updating the database. The connection is stateless, however it is maintained a list of client apps that are connected and an a instance of ZConnection for each client to have isolation since the requests are threaded.
Besides that I am having the problem of not saving having only one client connected and doing one request per time, no overlapping or re-entrance.
I need to hear advice on using this scenario of Firebird, What I should do to make the commit work.
Related
Are there major disadvantages using the embedded Firebird 3 in a multi-user application server (Delphi Webbroker) instead of the full blown server install?
The application usually has very short transactions with low data volume.
As far as I am informed accessing one database file with multiple threads through the embedded server is not problematic but user security is not available. As the application server does the rights stuff I do not need Firebird security.
But will I loose performance or things like garbage collection?
Firebird Embedded provides all the features (except network access and authentication) that a normal Firebird server provides. However, because it is in-process, any problems that cause your application to crash, will take Firebird with it and vice versa.
Other possible downsides:
Garbage collection will - as far as I know - always use the 'cooperative' model (where the connection to find old record versions, is the one that cleans it up),
You can't use other tools to access your database remotely which may make administration harder,
You can't put your database on a separate server from your web application (think of security requirements).
Personally, I would only choose Firebird Embedded if the situation calls for it. In all other situations, I will use Firebird Server.
I'm in process of upgrading our client-server ERP app to multi-tier. We want to offer our customers the possibility having their databases in cloud(hosted in our server).So, clients are written in Delphi, server is a http IOCP server written also in Delphi (from mORMot framework), for dbs we use Firebird embedded.
Our customers(let's say 200), can have 25-30 Firebird databases (total 5000-6000 databases), accessed by 4-5 user per each customer. This is not happening all at once. One user can work in one db, other 2 users can work in another db, but all the dbs should be available and online. So, I can have 800-1000 users working at 700-900 dbs. Databases are not big, typically 20-30 MB each but can go to 200 MB.
This is not data sharding so please don't suggest to merge all databases together, I really need them individually with possibility to backup/restore/replace each one of them.
So, I need multiple pools of connections - for every database I need a pool of let's say 2 connections. I read about Firedac connection pooling. It seems that TFDManager should be perfect for me. I define multiple "ConnectionDef"s with "Pooled=true" and it can maintain multiple pools of connections (each connection lasting until some minutes of inactivity).
Questions:
I have to create all "ConnectionDef"s before server starts serving requests?
Can TFDManager "handle" requests (and time-out connections on inactivity), while in other thread I need to create a new database , so automatically I need to create a new pool of connections and start serving requests from newly created database. Practically can I call FDManager.AddConnectionDef(..) while other pools are in use?
AFAIK Firebird embedded does not have any "connection". As its name states, it is embedded within the same process, so there is no connection pool needed. Connection pools are needed when several clients connect/disconnect to the same DB over the network, whereas here all is embedded, and you would have direct access to the Firebird engine.
As such, you may:
Define one "connnection" per Firebird embedded database;
Protect your SOA code via a mutex (aka critical section) per DB. In fact, mORMot's HTTP IOCP server would run the incoming requests from a thread pool, so ensure that all DB access is safely made.
Ensure you use at least Firebird 2.5 since the embedded version is told to be threadsafe only since revision 2.5 (see the release notes).
Instead of FireDAC, consider using ZDBC/Zeos (in latest 7.2/7.3 branch), which has nice features, together with the native mORMot SynDB libraries.
Looking at Firedac sources, seems that all about adding connection definitions and acquiring connections in pooled mode is thread-safe.
Adding a connection definition or matching one is guarded by a TMultiReadExclusiveWriteSynchronizer and acquiring a connection from the pool is guarded by a TCriticalSection.
So, answers:
I don't have to create all "ConnectionDef"s before server starts serving requests.
Yes, I can call safely FDManager.AddConnectionDef(..) while other pools are in use.
Using Firedac, acquiring a connection for any of those databases will be guarded by one TCriticalSection. The solution proposed by #Arnaud Bouchez presents a more grained access by creating one TCriticalSection per database and I think will scale better, but you should be aware of a bug when using multiple TCriticalSection, especially that all will be initiated at once:
https://www.delphitools.info/2011/11/30/fixing-tcriticalsection/
In that article present a very simple fix for this bug.
I built an Web API with Delphi using FireDac and an HTTPServer component: The application is using a dbms powered by firebird.
Everything was working fine until I the moment I started to simulate multiple requests to the same API endpoint. This is causing internal server exceptions reporting that a second trasaction is being opened when there are already a transaction opened.
I know that all connections are being closed after being used and objects are being destroyed in order to prevent memory leaks but I couldn't understand why do the application triggers the exception.
Any help or toughs that might drive me to a solution?
Multiple requests will be processed concurrently by the HTTP server.
So if two clients try to access the same resource (URL) at the same time, the server will need two sets of database connections and data access components.
If your application uses distinct objects - one set per client - and does this in a thread safe way, both connection should work fine.
However if you use only one datamodule to serve all incoming HTTP requests, proper serialization is required. It does not help to close connections after use, the connections must be used only from one thread at a time.
So to understand the potential reason of the error, more information about the actual design of your server is needed.
What is the difference between ADOTable and ClientDataSet?
Both components are capable of performing Batch Update, why add the extra overhead of having 2 additional components like ClientDataSet and DataSetProvider.
The main difference is that ClientDataSet can operate without a connection to external database. You can use it as in-memory table or load it's contents from file.
In combination with DataSetProvider it is frequently used to overcome limits of unidirectional datasets and as a cache.
A ClientDataSet is an in-memory dataset, which has a lot of usefull additional functionallities.
One big advantage compared to Interbase/Firebird tables and queries is, that you don't need to keep a transaction alive, e.g. as long as you display the data in a grid.
Have a look at this article:
A ClientDataSet in Every Database Application
Client dataset is a generic implementation that works regardless of the underlying db access library. It can work (through the provider) with any TCustomDataset descendant, be it a dbExpress dataset, a BDE one, an ADO one, or any of the many libraries available for Delphi to allow for direct database access using the native client (i.e. ODAC, Direct Oracle Access, ecc. ecc.)
It can also work in a multi-tier mode where the data access dataset and provider are in a remote server application and the TClientDataset is in the client application, allowing for "thin client" deployment which doesn't require database clients or data access library like ADO installed on the client (the required midas.dll code can be linked to the application when using recent versions of Delphi, anyway only the midas.dll is required otherwise).
On top of that it can be used as an in-memory table able to store data in a local file. It allows for the "briefcase" model also, where a thin client can still work when not connected to the database, and then "sync" when a connection becomes available. That's was more useful in the past, when wireless access was not common.
As you can see, TClientDataset offers a lot more of a TADODataset.
The most important difference I can think of is resolving update conflicts. In fact, TClientDataSet exposes the handy ReconcileErrorForm dialog, which wraps up the process of showing the user the old and new records and allows them to specify what action to take, while with TADOTable for instance, you're basically on your own.
I have a Client/Server application written Delphi. Essentially all the application is doing is transferring xml data streams between a server application and connected clients. I am currently using the Indy TIdTCPServer component. But the server side application keeps crashing on some of my installments. And it has been extremely difficult to debug. So I am wondering if there is some "architecture" I should be utilizing which does all the tcp/ip connection management and database connection pooling, allowing me to concentrate on the business logic.
Here are more details:
clients must maintain a "persistent" connection. There are times when the server must notify and send data to all connected clients.
clients are connecting from laptop computers using wireless aircards. So network "drops" are pretty common.
Backend database is SqlServer.
There can be upward of 100 computers simultaneously connected at a time.
When the server gets a new connection (TCPServer.OnConnect) I instantiate my own object containing it own SqlServer database connection. When tcp connections are dropped I in turn free these objects (and associated database connection).
Client application have a TTimer built into them. They routinely send heartbeats to the server. And if they "drop"/"lose" their connection they automatically establish a new connection once the network is back.
Anyone have any suggestions on the best approach/architecture here?
I presume the Indy component would work, but at the same time feel I am "reinventing the wheel" with respect to managing the connections.
Three component sets I am aware of that will take care of the nitty gritty technical aspects of client server applications for you:
kbmMW: http://components4developers.com/
Asta: http://www.astatech.com/index.asp
RemObjects: http://www.remobjects.com/
You may have to rework your applications to take advantage of the way these component sets work, but assuming you have properly separated layers that shouldn't be too much of a hassle and will buy you the advantage of well tested and widely used code for your client server work.
If you want some light TCP/IP components, take a look at our SynCrtSock unit.
You'll find low-level classes to create IP Client and Servers.
We implemented both TCP/IP and UDP/IP in one of our applications.
There is also a THttpServer class, which implement a HTTP/1.1 server. Therefore it follows the HTTP/1.1 connection management. There is also an optional compression, and using HTTP/1.1 on a port other than 80 is not a bad idea. And what is good with HTTP/1.1 is that it can pass through firewalls, and can be easily be VPNed or hosted on another HTTP server (like IIS or Apache) with a proxy. There is even a FastCGI class, if you need such a server under a linux-based solution.
Of course, a THttpClientSocket class does the same on the client class.
We use these classes to add HTTP/1.1 connection to our Open Source SQLite3 RESTful framework - http://synopse.info/forum/viewforum.php?id=2
See http://synopse.info/fossil/artifact?name=722e896e3d7aad1fe217b0e2e7903483e66d66d1 for the SynCrtSock unit. Open source, work from Delphi 7 to Delphi 2010.
Misha Charrett's CSI Application Framework covers pretty much exactly what you're asking for.
It's an open source Delphi framework that at its heart is a distributed message passing and threading framework that allows XML message passing from both client to server and server to client.
It can handle disconnections/reconnections, high client numbers and there's an optional virtual database library that will handle SQL server (or you could just use same SQL Server access you're using now).
It's not particularly well known yet but I can tell you that it's been actively developed over the last few years and that the author Misha is very keen to assist anyone who's interested in using it in their application.
Well, it would probably require a complete rewrite of much of your C/S code, but instead of using the Indy components, you could try to use a COM+ solution instead. Basically, you would create a COM+ component that will be installed on the server and your client applications will connect to this client and call the functions of this component directly. It will have transaction management which will be handled by Windows itself and the same is true about handling transactions. It's also technically possible to create events, which would allow the server to do callbacks to the client, although that would make things a bit more complicated.
I don't think this solution would work out for you, though, unless you have a lot of experience with COM development in Windows and/or you're brave enough to try something different.
In the past, I had a similar problem where hundreds of clients had to connect to a single server, doing all kinds of database transactions. It has a steep learning curve but me and my team managed to get things working and once we understood the technique, it resulted in a very stable and reliable solution which did manage to have up to 500 users simultaneously doing updates and other actions in a one-time extreme stress-test. But again, the learning curse is steep, so it might not be the solution you're looking for.
(Still, COM+ will use a lot of functionality that's build-in into Windows, like transaction management, database pooling and whatever more.)
If you use Indy each connection will equal a thread.
Anyway, I suggest for connecting to MSSQL to use SDAC from Devart http://www.devart.com/sdac/ and for the connection layer to use HPScktSrvr based on I/O Completion Port from http://www.torry.net/authorsmore.php?id=7131 (I don't know though what changes it will need for TThread changes in newer VCL).
You build your client class arround THPServerClient, you set your new class as the server ClientClass and the framework will create automatically new clients for you.
You may also want to have a look at the ICS/Midware combo: http://www.overbyte.be/