| The resource adapter is the clearing house for managing connections,
transactions, and XAResources in a JDBC based resource manager living in
the distributed transaction processing environment.
There is one instance of ResourceAdapter per Resource Manager (database).
The ResourceAdapter is responsible for keeping track of all run time global
transactions and their state. The resource adapter only knows of run time
global transactions, i.e., it does not know of in-doubt global transactions
re-created by recovery.
The following is an overall design of the JTA implementation in cloudscape,
most of it has little to do with the ResourceAdapter interface itself.
Design Overview
The overriding design principle is that existing code should be disturbed
as little as possible. This is so that DTP code will not add to the bloat
and drag of a normal, local, embbeded system. The second design principle
is that as much of the JDBC 2.0 extension functionality is to be
implemented in the Connectivity layer and not in the underlying storage
system as possible. Ideally, the additional storage interface will
implement no more than what is necessary to support the XAResource
interface.
Language and replication code should not be touched, or have very
minimal API changes. The API changes are confined to passing XA calls down
to the store.
Some change will be made to existing Connectivity code and new XA
modules will be added. This collection of code is hereby referred to as
the "blob of mysterious connectivity code", or the "resource adapter", or
"RA" for short. In the JTA doc, the resource adapter is considered to be
part of the JDBC driver. This RA means "some connectivity code", it
doesn't mean the object that implements the ResourceAdapter interface.
The most important difference, in terms of implementation, between a
Connection that deals with a local transaction and a Connection that deals
with a global transaction is that in a global transaction, 2 or more
objects and threads can influence it - maybe concurrently. The normal JDBC
interaction goes thru the Connection, but transaction demarcation comes
from an XAResource object(s). The RA will channel all XAResource calls
that deal with a run time XA transaction (i.e., commit, end, forget,
prepare, start) thru the TransactionController that represents the real
transaction underneath. Furthermore, the RA will make sure that all calls
thru a Connection or thru any XAResource objects must pass thru some sort
of synchronized object before it can get to the underlying transaction
object. This is so that there is only one path to change the state of a
run time transaction and the transaction object and the context manager can
remain single thread access.
In-doubt transaction (i.e., transactions re-created by recovery)
management and concurrency control is the responsibiliy of store. Moreover,
since the RA does not know the identities of the list of in-doubt
transactions, store must deal with (throw exception) when someone wants to
start a transaction with the same Xid as an existing in-doubt transaction.
In terms of what this means to the app server that is calling us: if the
Connection and the XAResource that represents a global transaction is being
accessed by 2 different threads, they will access the database serially and
not concurrently. An in-doubt transaction gotten thru recovery has no
transaction object that is ever visible to the RA - because there is no
connection that was ever made to it. Therefore it is safe to influence the
state of an in-doubt transaction directly thru some store factory interface
- and have that go thru the transaction table underneath to find the actual
transaction object and context manager etc.
One new functionality of a Connection is the ability to switch around
with different transactions. Before JTA, the lifetime of a transaction is
bounded by a connection, and a transaction cannot migrate from one
connection to another. In JTA, a global transaction can be detached from a
Connection. A transaction can move around and be attached to different
connections and its lifetime is not confine to the connection that started
it. From the Connection's point of view, before JTA, a (local) transaction
is always started and ended in the same connection. With JTA, it needs to
"take on" existing global transactions that was started by some other
connections.
The RA will have the responsibility of
- setting up a Context with the appropriate transaction before calling
store to do work.
- handling error on the context.
- restoring a previous context if it was switched out due to an XAResouce
call to commit a transaction that is not what the XAResoruce is currently
attached to.
Because of all these switching around, a Connection may be in a
transaction-less state. This happens between an XAResource.end call that
detached the current global transaction from the Connection, and the next
XAResource.start call that attach the next global transaction with the
Connection.
An (inferior) implementation is for the Connection object to start a
local connection once it is detached from a global transaction. If the
user then uses the Connection immediately without a XAResource.start call,
then this Connection behaves just like it did before JTA, i.e., with a
local transaction. If, on the other hand, an XAResource.start call happens
next, then either the local transaction is "morphed" into a global
transaction, or, if the start call is to attach the connection to a
pre-existing global transaction, then the local transaction is thrown away
and the Connection will take on the pre-exising global transaction.
Another (superior) implementation is to make it possible for a
Connection to be transaction-less. When a Connection is first created by
XAConnection.getConnection, or when a XAResource.end call detached a global
transaction from the Connection, it is left in a transaction-less state.
If a XAResource.start call happens next, then the Connection either start a
new global transaction or it takes on an existing one. If a call is made
directly on the Connection before XAResource.start call happens, then the
Connection starts a new local transaction. This only affects Connections
that was gotten thru the XAConnection.getConnection(). Connections gotten
thru the DriverManager or a DataSource will have a local transaction
automatically started, as is the behavior today. When a Connection with a
local transaction commits, the transaction is still around but it is chain
to the next one - this is the current behavior. This behavior is very
desirable from a performance point of view, so it should be retained.
However, a local transaction cannot "morph" into a global transaction,
therefore when this Connection is attached to a global transaction, the
local transaction is thrown away and a global one started
The RA will need to keep track of all global transactions. This is done
by (yet another) transaction table that lives in the RA. This transaction
table maps Xid to the ContextManager of the global transaction and whatever
else a connection needs to talk to the transaction - I assume the
Connection object currently have tendrils into various contexts and objects
and these are things that need to be detached and attached when a
Connection is hooked up with another transaction. The reason for yet
another transaction table instead of the one in store is because the one in
store keeps track of local and internal transactions and is really quite
overworked already.
Detailed design
First some ugly pictures. Some links are not shown to reduce
clutter. Externally visible object is in bold.
When user ask for an XAConnection via a XADataSource, the following objects
exists
|-------------|
|======= produces=>| XAResource |
|| |-------------|
|| |
|| has A
|| |
|| |---------------------
|| V
|--------------| produces |--------------|
| XADataSource |=========>| XAConnection
|--------------| |--------------|
| |
extends extends
| |
| |-----------------------| |----------------------|
| | DB2jPooledConnection |==>| BrokeredConnection |
| |-----------------------| |----------------------|
| | ^ |
| has A | has A
| | | |
|-----------------| | --------------------
| EmbeddedDataSource | |
|-----------------| |
| |
has A |
| |
V V
|------------| |----------------------| |-----------------------|
| JDBCDriver |=produces=>| DetachableConnection |==>| XATransactionResource |
| LocalDriver| |----------------------| | |
|------------| | | points to : |
| |XATransactionController|
| | ContextManager |
| | LCC |
| | .. etc .. |
| |-----------------------|
| |
extends extends
| |
|-----------------| |-----------------------|
| EmbedConnection |-- ?-->| TransactionResource |
|-----------------| |-----------------------|
When user ask for a PooledConnection via a PooledDataSource, the following
objects exists
|-------------------------------|
| EmbeddedConnectionPoolDataSource |
|-------------------------------|
| ||
| ||
extends produces
| ||
| \/
| |-----------------------| |----------------------|
| | DB2jPooledConnection |==>| BrokeredConnection |
| |-----------------------| |----------------------|
| | ^ |
| has A | has A
| | | |
|-----------------| | --------------------
| EmbeddedDataSource | |
|-----------------| |
| |
has A |
| |
V V
|------------| |----------------------| |-----------------------|
| JDBCDriver |=produces=>| EmbedConnection |==>| TransactionResource |
| LocalDriver| |----------------------| |-----------------------|
|------------|
When user ask for a (normal) Connection via a DataSource, the following
objects exists. The EmbeddedDataSource is just a wrapper for the JDBCDriver.
|-----------------|
| EmbeddedDataSource |
|-----------------|
|
has A
|
V
|------------| |-----------------| |-----------------------|
| JDBCDriver |==produces=>| EmbedConnection |- ?->| TransactionResource |
| LocalDriver| |-----------------| |-----------------------|
|------------|
XADataSource inherits DataSource methods from EmbeddedDataSource. It also
implements ResourceAdapter, whose main job is to keep track of run time
global transactions. A global transaction table maps XIDs to
XATransactionResource. XADataSource also has a XAResourceManager, which
implements XAResource functionality in the Store.
XAConnection is the one thing that unites a global connection and the
XAResource that delineates the global transaction. This is where the real
XAResource functionality is implemented. All XAResource calls to the
XAResource object as well as Connection call to the BrokeredConnection
channels thrus the XAConnection, which makes sure only one thread can be
accessing the DB2jPooledConnection at any given time.
XAResource and BrokeredConnection[23]0 are the two objects we give back
to the TM and the user application respectively to control a distributed
transaction. According to the XA spec, the app server is supposed to make
sure that these objects are not used the same time by multiple threads, but
we don't trust the app server. Therefore, we channel everthing back to the
XAConnection.
The MT consideration is actually more complicated than this,
because a XAResource is allowed to control any transaction, not just the
one its XAConnection is current attached to. So it is not sufficient to
just synchronized on XAConnection to guarentee single thread access to the
underlying transaction context. To control some arbitrary global
transaction, the TM can call XAResource to prepare any Xid. To do that,
the XAResource pass the request to the XAConnection, the XAConnection ask
the XADataSource to find the XATransactionResource, sets up the thread's
context, and call ask the XATransactionResource to prepare. The
XATransactionResource is synchronized to prevent some other thread from
attaching, commiting, and in any way calling on the the same transaction
context. If any error is thrown, it is handled with the context of the
transaction being prepared. After the error is handled, the old context
(the one where the XAResource is really attached to), is restored. While
this monkey business is going on, the thread that holds the connection the
XAConnection is supposed to be attached to is blocked out. It can resume
after its XAConnection restored its context. (Here is where I am not
really sure what happens since that thread obviously doesn't know about all
these hanky panky caused by the thread holding the XAResource commiting,
preparing and rolling back some other irrelavant transactions, so how would
its context be affected in any way?).
DB2jPooledConnection implements PooledConnection, is hands out these
connection handles which allows some app server to do connection pooling.
This is a very thin layer. A connection handle implements a Connection by
passing thru all calls to the underlaying connection. In this case, it
passes Connection call thru the DB2jPooledConnection to the
DetachableConnection underneath.
EmbeddedDataSource implements JNDI and is a replacement for Driver.
The LocalDriver can now produce a DetachableConnection as well as a
EmbedConnection (which is the pre-JTA Connection that cannot detach and
attach to different transactions). The way the LocalDriver knows to create
a DetachableConnection versus a EmbedConnection is thru some extremely
hackish URL settings. This thing is very ugly and a more elegant way can
(and should) no doubt be found.
DetachableConnection is a connection which can detach and attach to
different XATransactionResource, and can be totally unattached to any
transaction.
XATransactionResource is a bundle of things that sets up a connection
with all the stuff it needs to actually talk to the database, do error
handling, etc. It is also the object that lives in the transaction table
managed by the ResourceAdapter (XADataSource). A XAResource (which may or
may not be attached to a transaction) can commit, prepare, or rollback any
global transaction that is not attached to an XAConnection. To do that,
the ResourceAdapter fishes out the XATransactionResource, set up the
context, and do the commit processing/error handling on the current
thread.
Local Connection is the same old local Connection except one
difference. Pre-JTA, a localConnection uses itself (or a root Connection)
as the object to synchronized upon so that multiple threads getting hold of
the same Connection object cannot simultaneously issue calls to the
underlying transaction or context (since those things must be single thread
access). With JTA, the object of synchronization is the
TransactionResource itself. This part has not been well thought through
and is probably wrong.
TransactionResource is a base class for XATransactionResource. For a
local transaction which cannot be detached from a connection, there is no
need to encapsulate a bundle of things to set up a connection, so a
TransactionResource (probably misnamed) has nothing and is used only for
synchronization purposes. This part has not been well thought throught and
is probably wrong.
The non-XA PooledConnection is just a thin veneer over the normal
connection. I now have it over a Detachable connection just to simplify
the inheritence (XAConnection need to extend PooledConnection and XAConnect
needs to be detachable. However, PooledConnection itself need not be
detachable). It could be changed around to have LocalDriver producing
either EmbedConnection or XAConnection, and have the XAConnection
implements detachable. But the current way is simpler.
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