Is asynchronous jdbc call possible?

问题:

I wonder if there is a way to make asynchronous calls to a database?

For instance, imagine that I've a big request that take a very long time to process, I want to send the request and receive a notification when the request will return a value (by passing a Listener/callback or something). I don't want to block waiting for the database to answer.

I don't consider that using a pool of threads is a solution because it doesn't scale, in the case of heavy concurrent requests this will spawn a very large number of threads.

We are facing this kind of problem with network servers and we have found solutions by using select/poll/epoll system call to avoid having one thread per connection. I'm just wondering how to have a similar feature with database request?

Note: I'm aware that using a FixedThreadPool may be a good work-around, but I'm surprised that nobody has developed a system really asynchronous (without the usage of extra thread).

** Update **
Because of the lack of real practical solutions, I decided to create a library (part of finagle) myself: finagle-mysql. It basically decodes/decodes mysql request/response, and use Finagle/Netty under the hood. It scales extremely well even with huge number of connections.

回答1:

I don't understand how any of the proposed approaches that wrap JDBC calls in Actors, executors or anything else can help here - can someone clarify.

Surely the basic problem is that the JDBC operations block on socket IO. When it does this it blocks the Thread its running on - end of story. Whatever wrapping framework you choose to use its going to end up with one thread being kept busy/blocked per concurrent request.

If the underlying database drivers (MySql?) offers a means to intercept the socket creation (see SocketFactory) then I imagine it would be possible to build an async event driven database layer on top of the JDBC api but we'd have to encapsulate the whole JDBC behind an event driven facade, and that facade wouldn't look like JDBC (after it would be event driven). The database processing would happen async on a different thread to the caller, and you'd have to work out how to build a transaction manager that doesn't rely on thread affinity.

Something like the approach I mention would allow even a single background thread to process a load of concurrent JDBC exec's. In practice you'd probably run a pool of threads to make use of multiple cores.

(Of course I'm not commenting on the logic of the original question just the responses that imply that concurrency in a scenario with blocking socket IO is possible without the user of a selector pattern - simpler just to work out your typical JDBC concurrency and put in a connection pool of the right size).

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Looks like MySql probably does something along the lines I'm suggesting --- http://code.google.com/p/async-mysql-connector/wiki/UsageExample

回答2:

It's impossible to make an asynchronous call to the database via JDBC, but you can make asynchronous calls to JDBC with Actors (e.g., actor makes calls to the DB via JDBC, and sends messages to the third parties, when the calls are over), or, if you like CPS, with pipelined futures (promises) (a good implementation is Scalaz Promises)

I don't consider that using a pool of threads is a solution because it doesn't scale, in the case of heavy concurrent requests this will spawn a very large number of threads.

Scala actors by default are event-based (not thread-based) - continuation scheduling allows creating millions of actors on a standard JVM setup.

If you're targeting Java, Akka Framework is an Actor model implementation that has a good API both for Java and Scala.

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Aside from that, the synchronous nature of JDBC makes perfect sense to me. The cost of a database session is far higher than the cost of the Java thread being blocked (either in the fore- or background) and waiting for a response. If your queries run for so long that the capabilities of an executor service (or wrapping Actor/fork-join/promise concurrency frameworks) are not enough for you (and you're consuming too many threads) you should first of all think about your database load. Normally the response from a database comes back very fast, and an executor service backed with a fixed thread pool is a good enough solution. If you have too many long-running queries, you should consider upfront (pre-)processing - like nightly recalculation of the data or something like that.

回答3:

Perhaps you could use a JMS asynchronous messaging system, which scales pretty well, IMHO:

• Send a message to a Queue, where the subscribers will accept the message, and run the SQL process. Your main process will continue running and accepting or sending new requests.

• When the SQL process ends, you can run the opposite way: send a message to a ResponseQueue with the result of the process, and a listener on the client side accept it and execute the callback code.

回答4:

There is no direct support in JDBC but you have multiple options like MDB, Executors from Java 5.

"I don't consider that using a pool of threads is a solution because it doesn't scale, in the case of heavy concurrent requests this will spawn a very large number of threads."

I am curious why would a bounded pool of threads is not going to scale? It is a pool not thread-per-request to spawn a thread per each request. I have been using this for quite sometime on a heavy load webapp and we have not seen any issues so far.

回答5:

It looks like a new asynchronous jdbc API "JDBC next" is in the works.

See presentation here

You can download the API from here

回答6:

As mentioned in other answers JDBC API is not Async by its nature.
However, if you can live with a subset of the operations and a different API there are solutions. One example is https://github.com/jasync-sql/jasync-sql that works for MySQL and PostgreSQL.

回答7:

Ajdbc project seems to answer this problem http://code.google.com/p/adbcj/

There is currently 2 experimental natively async drivers for mysql and postgresql.

回答8:

An old question, but some more information. It is not possible to have JDBC issue asynchronous requests to the database itself, unless a vendor provides an extension to JDBC and a wrapper to handle JDBC with. That said, it is possible to wrap JDBC itself with a processing queue, and to implement logic that can process off the queue on one or more separate connections. One advantage of this for some types of calls is that the logic, if under heavy enough load, could convert the calls into JDBC batches for processing, which can speed up the logic significantly. This is most useful for calls where data is being inserted, and the actual result need only be logged if there is an error. A great example of this is if inserts are being performed to log user activity. The application won't care if the call completes immediately or a few seconds from now.

As a side note, one product on the market provides a policy driven approach to allowing asynchronous calls like those I described to be made asynchronously (http://www.heimdalldata.com/). Disclaimer: I am co-founder of this company. It allows regular expressions to be applied to data transformation requests such as insert/update/deletes for any JDBC data source, and will automatically batch them together for processing. When used with MySQL and the rewriteBatchedStatements option (MySQL and JDBC with rewriteBatchedStatements=true) this can significantly lower overall load on the database.

回答9:

You have three options in my opinion:

1. Use a concurrent queue to distribute messages across a small and fixed number of threads. So if you have 1000 connections you will have 4 threads, not 1000 threads.
2. Do the database access on another node (i.e. another process or machine) and have your database client make asynchronous network calls to that node.
3. Implement a true distributed system through asynchronous messages. For that you will need an messaging queue such as CoralMQ or Tibco.

Diclaimer: I am one of the developers of CoralMQ.

回答10:

A solution is being developed to make reactive connectivity possible with standard relational databases.

People wanting to scale while retaining usage of relational databases are cut off from reactive programming due to existing standards based on blocking I/O. R2DBC specifies a new API that allows reactive code that work efficiently with relational databases.

R2DBC is a specification designed from the ground up for reactive programming with SQL databases defining a non-blocking SPI for database driver implementors and client library authors. R2DBC drivers implement fully the database wire protocol on top of a non-blocking I/O layer.

R2DBC's WebSite

R2DBC's GitHub

Feature Matrix

回答11:

The Java 5.0 executors might come handy.

You can have a fixed number of threads to handle long-running operations. And instead of Runnable you can use Callable, which return a result. The result is encapsulated in a Future<ReturnType> object, so you can get it when it is back.

回答12:

Here is an outline about what an non-blocking jdbc api could look like from Oracle presented at JavaOne: https://static.rainfocus.com/oracle/oow16/sess/1461693351182001EmRq/ppt/CONF1578%2020160916.pdf

So it seems that in the end, truly asynchronous JDBC calls will indeed be possible.

回答13:

Just a crazy idea : you could use an Iteratee pattern over JBDC resultSet wrapped in some Future/Promise

Hammersmith does that for MongoDB.

回答14:

I am just thinking ideas here. Why couldn't you have a pool of database connections with each one having a thread. Each thread has access to a queue. When you want to do a query that takes a long time, you can put on the queue and then one of threads will pick it up and handle it. You will never have too many threads because the number of your threads are bounded.

Edit: Or better yet, just a number of threads. When a thread sees something in a queue, it asks for a connection from the pool and handles it.

回答15:

The commons-dbutils library has support for an AsyncQueryRunner which you provide an ExecutorService to and it returns a Future. Worth checking out as it's simple to use and ensure you won't leak resources.

回答16:

If you are interested in asynchronous database APIs for Java you should know that there is a new initiative to come up with a set of standard APIs based on CompletableFuture and lambdas. There is also an implementation of these APIs over JDBC which can be used to practice these APIs: https://github.com/oracle/oracle-db-examples/tree/master/java/AoJ The JavaDoc is mentioned in the README of the github project.