protocol change in 7.4

Neil Conway wrote:

There has been some previous discussion of changing the FE/BE protocol
in 7.4, in order to fix several problems. I think this is worth doing:
if we can resolve all these issues in a single release, it will lessen
the upgrade difficulties for users.

<snip>

If I've missed anything or if there is something you think we should
add, please let me know.

Is there any thought about changing the protocol to support
two-phase commit? Not that 2PC and distributed transactions
would be implemented in 7.4, but to prevent another protocol
change in the future?

Mike Mascari
mascarm@mascari.com

Mike Mascari <mascarm@mascari.com> writes:

Is there any thought about changing the protocol to support
two-phase commit? Not that 2PC and distributed transactions would be
implemented in 7.4, but to prevent another protocol change in the
future?

My understanding is that 2PC is one way to implement multi-master
replication. If that's what you're referring to, then I'm not sure I
see the point: the multi-master replication project (pgreplication)
doesn't use 2PC, due to apparent scalability problems (not to mention
that it also uses a separate channel for communications between
backends on different nodes).

Cheers,

Neil

--
Neil Conway <neilc@samurai.com> || PGP Key ID: DB3C29FC

I'm now implementing 2PC replication and distributed transaction. My 2PC
needs some supports in startup packet to establish a replication session
or a recovery session.

BTW, my 2PC replication is working, and I'm implementing 2PC recovery now.

I would like to here more about your implementation. Do you have some
documentation that I
could read?

If not, perhaps (if you have the time) you could put together a post
describing your work. Like
Is it an internal or external solution. Are you sending SQL or tuples
in your update messages.
How are you handling failure detection? Is this partial or full
replication?

Please forgive me for asking so many questions, but I'm rather intrigued
by database
replication.

Darren

Neil Conway wrote:

Mike Mascari <mascarm@mascari.com> writes:

Is there any thought about changing the protocol to support
two-phase commit? Not that 2PC and distributed transactions would be
implemented in 7.4, but to prevent another protocol change in the
future?

My understanding is that 2PC is one way to implement multi-master
replication. If that's what you're referring to, then I'm not sure I
see the point: the multi-master replication project (pgreplication)
doesn't use 2PC, due to apparent scalability problems (not to mention
that it also uses a separate channel for communications between
backends on different nodes).

Actually, I was thinking along the lines of a true CREATE
DATABASE LINK implementation, where multiple databases could
participate in a distributed transaction. That would require the
backend in which the main query is executing to act as the
"coordinator" and each of the other participating databases to
act as "cohorts". And would require a protocol change to support
the PREPARE, COMMIT-VOTE/ABORT-VOTE reply, and an ACK message
following the completion of the distributed COMMIT or ABORT.

Mike Mascari
mascarm@mascari.com

Hi all,

Mike Mascari <mascarm@mascari.com> wrote:

Is there any thought about changing the protocol to support
two-phase commit? Not that 2PC and distributed transactions
would be implemented in 7.4, but to prevent another protocol
change in the future?

I'm now implementing 2PC replication and distributed transaction. My 2PC
needs some support in startup packet to establish a replication session
and a recovery session.

BTW, 2PC replication is working, and I'm implementing 2PC recovery now.

--
NAGAYASU Satoshi <snaga@snaga.org>

On Mon, Nov 04, 2002 at 08:10:29PM -0500, Mike Mascari wrote:

Actually, I was thinking along the lines of a true CREATE
DATABASE LINK implementation, where multiple databases could
participate in a distributed transaction. That would require the
backend in which the main query is executing to act as the
"coordinator" and each of the other participating databases to
act as "cohorts". And would require a protocol change to support
the PREPARE, COMMIT-VOTE/ABORT-VOTE reply, and an ACK message
following the completion of the distributed COMMIT or ABORT.

Right, you need TPC in order for pgsql to participate in transactions
that span anything outside the DB proper. A DB link is one example,
or an external transaction manager that coordinates DB and filesystem
updates, for example. Zope could use this, to coordinate the DB with
it's internal object store.

Ross

Hi,

Mike Mascari <mascarm@mascari.com> wrote:

Is there any thought about changing the protocol to support
two-phase commit? Not that 2PC and distributed transactions
would be implemented in 7.4, but to prevent another protocol
change in the future?

I'm now implementing 2PC replication and distributed transaction. My 2PC
needs some supports in startup packet to establish a replication session
or a recovery session.

BTW, my 2PC replication is working, and I'm implementing 2PC recovery now.

--
NAGAYASU Satoshi <snaga@snaga.org>

Darren Johnson <darren@up.hrcoxmail.com> wrote:

I would like to here more about your implementation. Do you have some
documentation that I
could read?

Documentation is not available, but I have some slides for my presentation.

http://snaga.org/pgsql/20021018_2pc.pdf

Some answers for your questions may be in these slides.

And a current source code is available from:
http://snaga.org/pgsql/pgsql-20021025.tgz

If not, perhaps (if you have the time) you could put together a post
describing your work. Like
Is it an internal or external solution. Are you sending SQL or tuples
in your update messages.
How are you handling failure detection? Is this partial or full
replication?

It is an internal solution. In 2PC, pre-commit and commit are required.
So my implementation has some internal modifications on transaction
handling, log recording and else.

--
NAGAYASU Satoshi <snaga@snaga.org>

I don't see why 2PC would require any protocol-level change. I would
think that the API would be something like

BEGIN;
issue some commands ...
PRECOMMIT;
-- if the above does not return an error, then
COMMIT;

In other words, 2PC would require some new commands, but a new command
doesn't affect the protocol layer.

regards, tom lane

Questions have arisen during discussions about errors relating
to how to support error codes without changing the FE/BE
protocols. (see TODO.detail/error)

Now that the protocol is up for revision, how about supporting
sql state strings, error codes, and other information directly in
the protocol.

Regards,
Grant

Neil Conway wrote:

Grant Finnemore <grantf@guruhut.co.za> writes:

Now that the protocol is up for revision, how about supporting
sql state strings, error codes, and other information directly in
the protocol.

Ah, thanks for pointing that out. Error codes would be another thing
we can ideally support in 7.4, and we'd need a protocol change to do
it properly, AFAICS. IIRC, Peter E. expressed some interest in doing
this...

Cheers,

Neil

--
Neil Conway <neilc@samurai.com> || PGP Key ID: DB3C29FC

Tom Lane wrote:

I don't see why 2PC would require any protocol-level change. I would
think that the API would be something like

BEGIN;
issue some commands ...
PRECOMMIT;
-- if the above does not return an error, then
COMMIT;

In other words, 2PC would require some new commands, but a new command
doesn't affect the protocol layer.

I think a precommit-vote-commit phase of 2PC can be implemented in
command-lavel or protocol-level.

In command-level 2PC, an user application (or application programmer)
must know the DBMS is clustered or not (to use PRECOMMIT command).

In protocol-layer 2PC, no new SQL command is required.
A precommit-vote-commit phase will be called implicitly. It means an
user application can be used without any modification. An application
can use a traditional way (BEGIN...COMMIT).

So I made my decision to use protocol-layer implementation.
It doesn't affect the SQL command layer.

--
NAGAYASU Satoshi <snaga@snaga.org>

<br><font size=2 face="sans-serif">On 11/05/2002 04:42:55 AM Neil Conway wrote:<br>
&gt; Mike Mascari &lt;mascarm@mascari.com&gt; writes:<br>
&gt; &gt; Is there any thought about changing the protocol to support<br>
&gt; &gt; two-phase commit? Not that 2PC and distributed transactions would be<br>
&gt; &gt; implemented in 7.4, but to prevent another protocol change in the<br>
&gt; &gt; future?<br>
&gt; <br>
&gt; My understanding is that 2PC is one way to implement multi-master<br>
&gt; replication. If that's what you're referring to, then I'm not sure I<br>
</font>
<br><font size=2 face="sans-serif">Another use of two-phase commit is in messaging middleware (MOM, message oriented middleware), were both the middleware and the database participate in the same transaction. Consider:</font>
<br>
<br><font size=2 face="sans-serif">- DB: begin</font>
<br><font size=2 face="sans-serif">- MOM: begin</font>
<br><font size=2 face="sans-serif">- DB: insert</font>
<br><font size=2 face="sans-serif">- MOM: send message</font>
<br><font size=2 face="sans-serif">- DB: prepare</font>
<br><font size=2 face="sans-serif">- MOM: prepare ==&gt; fails</font>
<br><font size=2 face="sans-serif">- DB: rollback</font>
<br><font size=2 face="sans-serif">- MOM: rollback</font>
<br>
<br><font size=2 face="sans-serif">just a simple example...</font>
<br>
<br><font size=2 face="sans-serif">Maarten</font>
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On Mon, Nov 04, 2002 at 07:22:54PM -0500, Neil Conway wrote:

(1) Add an optional textual message to NOTIFY

(2) Remove the hard-coded limits on database and user names
(SM_USER, SM_DATABASE), replace them with variable-length
fields.

(3) Remove some legacy elements in the startup packet
('unused' can go -- perhaps 'tty' as well). I think the
'length' field of the password packet is also not used,
but I'll need to double-check that.

(4) Fix the COPY protocol (Tom?)

(5) Fix the Fastpath protocol (Tom?)

(6) Protocol-level support for prepared queries, in order to
bypass the parser (and maybe be more compatible with the
implementation of prepared queries in other databases).

(7) Include the current transaction status, since it's
difficult for the client app to determine it for certain
(Tom/Bruce?)

(8) Error codes (maybe needn't change protocol)
- without this is PostgreSQL useless in real DB aplication

(9) Think about full dynamic charset encoding (add new encoding on
the fly)

Karel

--
Karel Zak <zakkr@zf.jcu.cz>
http://home.zf.jcu.cz/~zakkr/

C, PostgreSQL, PHP, WWW, http://docs.linux.cz, http://mape.jcu.cz

Satoshi Nagayasu kirjutas T, 05.11.2002 kell 08:05:

Tom Lane wrote:

I don't see why 2PC would require any protocol-level change. I would
think that the API would be something like

BEGIN;
issue some commands ...
PRECOMMIT;
-- if the above does not return an error, then
COMMIT;

In other words, 2PC would require some new commands, but a new command
doesn't affect the protocol layer.

I think a precommit-vote-commit phase of 2PC can be implemented in
command-lavel or protocol-level.

In command-level 2PC, an user application (or application programmer)
must know the DBMS is clustered or not (to use PRECOMMIT command).

In protocol-layer 2PC, no new SQL command is required.
A precommit-vote-commit phase will be called implicitly. It means an
user application can be used without any modification. An application
can use a traditional way (BEGIN...COMMIT).

If application continues to use just BEGIN/COMMIT, then the protocol
level must parse command stream and recognize COMMIT in order to replace
it with PRECOMMIT, COMMIT.

If the communication library has to do that anyway, it could still do
the replacement without affecting wire protocol, no ?

------------------
Hannu

Hannu Krosing <hannu@tm.ee> wrote:

I think a precommit-vote-commit phase of 2PC can be implemented in
command-lavel or protocol-level.

In command-level 2PC, an user application (or application programmer)
must know the DBMS is clustered or not (to use PRECOMMIT command).

In protocol-layer 2PC, no new SQL command is required.
A precommit-vote-commit phase will be called implicitly. It means an
user application can be used without any modification. An application
can use a traditional way (BEGIN...COMMIT).

If application continues to use just BEGIN/COMMIT, then the protocol
level must parse command stream and recognize COMMIT in order to replace
it with PRECOMMIT, COMMIT.

If the communication library has to do that anyway, it could still do
the replacement without affecting wire protocol, no ?

In my implementation, 'the extended(2PC) FE/BE protocol' is used only in
the communication between the master and slave server(s), not between a
client app and the master server.

libpq <--Normal FE/BE--> (master)postgres <--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres

A client application and client's libpq can work continuously without
any modification. This is very important. And protocol modification
between master and slave server(s) is not so serious issue (I think).

--
NAGAYASU Satoshi <snaga@snaga.org>

On Tue, Nov 05, 2002 at 08:54:46PM +0900, Satoshi Nagayasu wrote:

Hannu Krosing <hannu@tm.ee> wrote:

In protocol-layer 2PC, no new SQL command is required.
A precommit-vote-commit phase will be called implicitly. It means an
user application can be used without any modification. An application
can use a traditional way (BEGIN...COMMIT).

If application continues to use just BEGIN/COMMIT, then the protocol
level must parse command stream and recognize COMMIT in order to replace
it with PRECOMMIT, COMMIT.

If the communication library has to do that anyway, it could still do
the replacement without affecting wire protocol, no ?

No, I think Satoshi is suggesting that from the client's point of view,
he's embedded the entire precommit-vote-commit cycle inside the COMMIT
command.

In my implementation, 'the extended(2PC) FE/BE protocol' is used only in
the communication between the master and slave server(s), not between a
client app and the master server.

libpq <--Normal FE/BE--> (master)postgres <--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres

A client application and client's libpq can work continuously without
any modification. This is very important. And protocol modification
between master and slave server(s) is not so serious issue (I think).

Ah, but this limits your use of 2PC to transparent DB replication - since
the client doesn't have access to the PRECOMMIT phase (usually called
prepare phase, but that's anothor overloaded term in the DB world!) it
_can't_ serve as the transaction master, so the other use cases that
people have mentioned here (zope, MOMs, etc.) wouldn't be possible.

Hmm, unless a connection can be switched into 2PC mode, so something
other than a postgresql server can act as the transaction master.

Does your implementation cascade? Can slaves have slaves?

Ross

Neil Conway wrote:

(6) Protocol-level support for prepared queries, in order to
bypass the parser (and maybe be more compatible with the
implementation of prepared queries in other databases).

Let me add
(6b) Protocol level support for query parameters. This would actually
make (6) more powerful and speed up non prepared (but similar)
queries via the query cache (which is already there IIRC).
[I talk about <statement> USING :var ... ]

(n) Platform independant binary representation of parameters and
results (like in CORBA). This can _really_ speed up
communication with compiled programs if you take the time to
implement it. This was previously planned for a future
CORBA fe/be protocol, but this does not seem to come any time
soon.

(n+1) Optional additional Result qualifiers. E.g. dynamic embedded
sql has a
flag to indicate that this column is a key. Previously it was
impossible to set this flag to a meaningful value. Also
the standard has additional statistical information about the
size of the column etc. If it's unclear what I'm talking about
I will look up the exact location in the standard (it's embedded
sql, dynamic sql, get descriptor)

Yours
Christof

"Ross J. Reedstrom" <reedstrm@rice.edu> wrote:

If application continues to use just BEGIN/COMMIT, then the protocol
level must parse command stream and recognize COMMIT in order to replace
it with PRECOMMIT, COMMIT.

If the communication library has to do that anyway, it could still do
the replacement without affecting wire protocol, no ?

No, I think Satoshi is suggesting that from the client's point of view,
he's embedded the entire precommit-vote-commit cycle inside the COMMIT
command.

Exactly. When user send the COMMIT command to the master server, the
master.talks to the slaves to process precommit-vote-commit using the
2PC. The 2PC cycle is hidden from user application. User application
just talks the normal FE/BE protocol.

In my implementation, 'the extended(2PC) FE/BE protocol' is used only in
the communication between the master and slave server(s), not between a
client app and the master server.

libpq <--Normal FE/BE--> (master)postgres <--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres

A client application and client's libpq can work continuously without
any modification. This is very important. And protocol modification
between master and slave server(s) is not so serious issue (I think).

Ah, but this limits your use of 2PC to transparent DB replication - since
the client doesn't have access to the PRECOMMIT phase (usually called
prepare phase, but that's anothor overloaded term in the DB world!) it
_can't_ serve as the transaction master, so the other use cases that
people have mentioned here (zope, MOMs, etc.) wouldn't be possible.

Hmm, unless a connection can be switched into 2PC mode, so something
other than a postgresql server can act as the transaction master.

I think the client should not act as the transaction master. But if it
is needed, the client can talk to postgres servers with the extended 2PC
FE/BE protocol.

Because all postgres servers(master and slave) can understand both the
normal FE/BE protocol and the extended 2PC FE/BE protocol. It is
switched in the startup packet.

See 10 page.
http://snaga.org/pgsql/20021018_2pc.pdf

I embeded 'the connection type' in the startup packet to switch postgres
backend's behavior (normal FE/BE protocol or 2PC FE/BE protocol).

In current implementation, if the connection type is 'R', it is handled
as the 2PC FE/BE connection (replication connection).

Does your implementation cascade? Can slaves have slaves?

It is not implemented, but I hope so. :-)
And I think it is not so difficult.

--
NAGAYASU Satoshi <snaga@snaga.org>

Satoshi Nagayasu kirjutas K, 06.11.2002 kell 04:15:

"Ross J. Reedstrom" <reedstrm@rice.edu> wrote:

If application continues to use just BEGIN/COMMIT, then the protocol
level must parse command stream and recognize COMMIT in order to replace
it with PRECOMMIT, COMMIT.

If the communication library has to do that anyway, it could still do
the replacement without affecting wire protocol, no ?

No, I think Satoshi is suggesting that from the client's point of view,
he's embedded the entire precommit-vote-commit cycle inside the COMMIT
command.

Exactly. When user send the COMMIT command to the master server, the
master.talks to the slaves to process precommit-vote-commit using the
2PC. The 2PC cycle is hidden from user application. User application
just talks the normal FE/BE protocol.

But _can_ client (libpq/jdbc/...) also talk 2PC FE/BE protocol, i.e. act
as "master" ?

In my implementation, 'the extended(2PC) FE/BE protocol' is used only in
the communication between the master and slave server(s), not between a
client app and the master server.

libpq <--Normal FE/BE--> (master)postgres <--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres
<--Extended(2PC)FE/BE--> (slave)postgres

A client application and client's libpq can work continuously without
any modification. This is very important. And protocol modification
between master and slave server(s) is not so serious issue (I think).

Ah, but this limits your use of 2PC to transparent DB replication - since
the client doesn't have access to the PRECOMMIT phase (usually called
prepare phase, but that's anothor overloaded term in the DB world!) it
_can't_ serve as the transaction master, so the other use cases that
people have mentioned here (zope, MOMs, etc.) wouldn't be possible.

Hmm, unless a connection can be switched into 2PC mode, so something
other than a postgresql server can act as the transaction master.

I think the client should not act as the transaction master. But if it
is needed, the client can talk to postgres servers with the extended 2PC
FE/BE protocol.

Because all postgres servers(master and slave) can understand both the
normal FE/BE protocol and the extended 2PC FE/BE protocol. It is
switched in the startup packet.

Why is the protocol change neccessary ?

Is there some fundamental reason that the slave backends can't just wait
and see if the first "commit" command is PRECOMMIT or COMMIT and then
act accordingly on for each transaction ?

-----------------
Hannu

Christof Petig wrote:

Neil Conway wrote:

(6) Protocol-level support for prepared queries, in order to
bypass the parser (and maybe be more compatible with the
implementation of prepared queries in other databases).

Let me add
(6b) Protocol level support for query parameters. This would actually
make (6) more powerful and speed up non prepared (but similar)
queries via the query cache (which is already there IIRC).
[I talk about <statement> USING :var ... ]

(n) Platform independant binary representation of parameters and
results (like in CORBA). This can _really_ speed up
communication with compiled programs if you take the time to
implement it. This was previously planned for a future
CORBA fe/be protocol, but this does not seem to come any time
soon.

After one night's sleep I think that perhaps a CORBA based protocol
might be less work (but I have no idea about a decent authentification
schema, I'd tend to reuse the already authentificated stream). A
corbaized query-only interface might easily cover these issues and be
less work than a full corba backend access. JDBC (I don't know much
about it) might give a reasonable interface design (perhaps combined
with a libpq[++|xx] like interface if there's benefit to it).

(n+1) Optional additional Result qualifiers. E.g. dynamic embedded
sql has a
flag to indicate that this column is a key. Previously it was
impossible to set this flag to a meaningful value. Also
the standard has additional statistical information about the
size of the column etc. If it's unclear what I'm talking about
I will look up the exact location in the standard (it's embedded
sql, dynamic sql, get descriptor)

This does not need an implementation soon. But the new protocol should
allow future things like this.

All these proposals are motivated by (future) ecpg [C/C++] needs. So
IMHO the ODBC, JDBC, libpqxx people might be interested in many of these
issues, too. We definitely should make sure to have asked them.

Yours
Christof

Hannu Krosing <hannu@tm.ee> wrote:

Exactly. When user send the COMMIT command to the master server, the
master.talks to the slaves to process precommit-vote-commit using the
2PC. The 2PC cycle is hidden from user application. User application
just talks the normal FE/BE protocol.

But _can_ client (libpq/jdbc/...) also talk 2PC FE/BE protocol, i.e. act
as "master" ?

Not for now. The current libpq/jdbc can talk only normal FE/BE protocol.
But it can be implemented.

Because my (experimantal)libpq can talk 2PC FE/BE protocol. :-)

I think the client should not act as the transaction master. But if it
is needed, the client can talk to postgres servers with the extended 2PC
FE/BE protocol.

Because all postgres servers(master and slave) can understand both the
normal FE/BE protocol and the extended 2PC FE/BE protocol. It is
switched in the startup packet.

Why is the protocol change neccessary ?

Because the postgres backend must detect a type of incomming connection
(from the client app or the master).

If it is comming from the client, the backend relays the queries to the
slaves (act as the master).

But if it is comming from the master server, the backend must act as a
slave, and does not relay the queries.

How the backend detect them in the multi-master replication? Detecting
inside the start packet is a simple way.

My implementation is working without protocol modification, because the
session type information is embeded in the 'unused' field now. So the
backend can understand both the normal FE/BE protocol and the extended
2PC FE/BE protocol.

But if the unused field is removed in 7.4, my replication will not work.

I think there are several types of connection in the sync replication or
the distributed transaction. Especially, the bulk transfer of tables or
indexes will be neccesary for the distributed query in future.

So, I think embedding the connection type information in the startup
packet is a good idea.

Is there some fundamental reason that the slave backends can't just wait
and see if the first "commit" command is PRECOMMIT or COMMIT and then
act accordingly on for each transaction ?

Are two "commit" commands required on the clustered postgres?
And is one "commit" command required on the single postgres?

I think it will confuse the application programmer.

--
NAGAYASU Satoshi <snaga@snaga.org>

On Wed, Nov 06, 2002 at 05:02:14PM +0900, Satoshi Nagayasu wrote:

Hannu Krosing <hannu@tm.ee> wrote:

Exactly. When user send the COMMIT command to the master server, the
master.talks to the slaves to process precommit-vote-commit using the
2PC. The 2PC cycle is hidden from user application. User application
just talks the normal FE/BE protocol.

But _can_ client (libpq/jdbc/...) also talk 2PC FE/BE protocol, i.e. act
as "master" ?

Not for now. The current libpq/jdbc can talk only normal FE/BE protocol.
But it can be implemented.

Because my (experimantal)libpq can talk 2PC FE/BE protocol. :-)

<snip>

Because the postgres backend must detect a type of incomming connection
(from the client app or the master).

If it is comming from the client, the backend relays the queries to the
slaves (act as the master).

But if it is comming from the master server, the backend must act as a
slave, and does not relay the queries.

So, your replication is an all-or-nothing type of thing? you can't
replicate some tables and not others? If only some tables are replicated,
then you can't decide if this is a distributed transaction until it's
been parsed.

Also, if we want to cascade, then one server can be both master and slave,
as it were. For full-on-2PC, I'm not sure cascading is a good idea, but
it's something to consider, especially if there's provisions for partial
replication, or 'optional' slaves.

I think there are several types of connection in the sync replication or
the distributed transaction. Especially, the bulk transfer of tables or
indexes will be neccesary for the distributed query in future.

So, I think embedding the connection type information in the startup
packet is a good idea.

Is there some fundamental reason that the slave backends can't just wait
and see if the first "commit" command is PRECOMMIT or COMMIT and then
act accordingly on for each transaction ?

Are two "commit" commands required on the clustered postgres?
And is one "commit" command required on the single postgres?

I think Hannu is suggesting that COMMIT could occur from either of two
states in the transaction state diagram: from an open transaction, or
from PRECOMMIT. There's no need to determine before that moment if
this particular transaction is part of a 2PC or not, is there? So, no
you don't _require_ PRECOMMIT/COMMIT because it's clustered: if a
'bare' COMMIT shows up, do what you currently do: hide the details.
If a PRECOMMIT shows up, report status back to the 'client'.

So, it seems to me that the minimum protocol change necessary to support
this model is reporting the current transaction status to the client.

I think it will confuse the application programmer.

I think your mental image of an application programmer needsto be
expanded: it should also include middleware vendors, who very much want
to be able to control a distributed transaction, one part of which may
be a postgresql replicated cluster.

Ross

"Ross J. Reedstrom" <reedstrm@rice.edu> wrote:

Because the postgres backend must detect a type of incomming connection
(from the client app or the master).

If it is comming from the client, the backend relays the queries to the
slaves (act as the master).

But if it is comming from the master server, the backend must act as a
slave, and does not relay the queries.

So, your replication is an all-or-nothing type of thing? you can't
replicate some tables and not others? If only some tables are replicated,
then you can't decide if this is a distributed transaction until it's
been parsed.

Yes. My current replication implementation is 'query based' replication,
so all queries to the database (except SELECT command) are replicated.
The database will be completely replicated, not partial.

I know this 'query based' design can't be used for a distributed
transaction. I think more internal communication between distributed
servers is required. We need 'the partial transfer of tables', 'the
bulk transfer of the index' or something like that for a distributed
transaction. I'm working for it now.

As I said, several connection types, a client application connection, an
internal transfer connection or a recovery connection, will be required
on replication and distributed transaction in near future. Embedding
connection types in the startup packet is a good way to decide how the
backend should behave. It is simple and extendable, isn't it?

If the backend can't understand the incoming connection type, the
backend will answer "I can't understand." and need only disconnect it.

Also, if we want to cascade, then one server can be both master and slave,
as it were. For full-on-2PC, I'm not sure cascading is a good idea, but
it's something to consider, especially if there's provisions for partial
replication, or 'optional' slaves.

Yes. There are several implementation designs for replication. Sync or
async, pre- or post-, full or partial, query-level or I/O-level or
journal-level. I think there is no "best way" for replication, because
applications have different requirements in different situations.

So the protocol should be more extendable.

I think Hannu is suggesting that COMMIT could occur from either of two
states in the transaction state diagram: from an open transaction, or
from PRECOMMIT. There's no need to determine before that moment if
this particular transaction is part of a 2PC or not, is there? So, no
you don't _require_ PRECOMMIT/COMMIT because it's clustered: if a
'bare' COMMIT shows up, do what you currently do: hide the details.
If a PRECOMMIT shows up, report status back to the 'client'.

After status is returned, what does the 'client' do?
Should the client talk the 2PC protocol?

For example, if the database is replicated in 8 servers,
does the client application keep 8 connections for each server?

Is this good?

--
NAGAYASU Satoshi <snaga@snaga.org>

Is exists patch for 7.4devel ?

regards
Haris Peco

There has been some previous discussion of changing the FE/BE protocol
in 7.4, in order to fix several problems. I think this is worth doing:
if we can resolve all these issues in a single release, it will lessen
the upgrade difficulties for users.

Here are a couple of other changes you might consider (maybe these changes
already exist and I just don't know about them):

a) Make much of the metadata sent to the client optional. When I execute
20 fetches against the same cursor, I don't need the same metadata 20
times. For narrow result sets, the metadata can easily double or triple
the number of bytes sent across the net. It looks like the protocol needs
the field count, but everything else seems to be sent for the convenience
of the client application.

b) Send a decoded version of atttypmod - specifically, decode the
precision and scale for numeric types.

b) Send a decoded version of atttypmod - specifically, decode the
precision and scale for numeric types.

I want decode type,length,precision and scale

Type is returned by PQftype(), length is returned by PQfsize(). Precision
and scale are encoded in the return value from PQfmod() and you have to
have a magic decoder ring to understand them. (Magic decoder rings are
available, you just have to read the source code :-)

PQftype() is not easy to use because it returns an OID instead of a name
(or a standardized symbol), but I can't think of anything better to return
to the client. Of course if you really want to make use of PQftype(), you
can preload a client-side cache of type definitions. I seem to remember
seeing a patch a while back that would build the cache and decode precision
and scale too.

PQfsize() is entertaining, but not often what you really want (you really
want the width of the widest value in the column after conversion to some
string format - it seems reasonable to let the client applicatin worry
about that, although maybe that would be a useful client-side libpq function).

b) Send a decoded version of atttypmod - specifically, decode the
precision and scale for numeric types.

I want decode type,length,precision and scale

regards
Haris Peco

On Thursday 07 November 2002 09:50 pm, korry wrote:

b) Send a decoded version of atttypmod - specifically, decode the
precision and scale for numeric types.

I want decode type,length,precision and scale

Type is returned by PQftype(), length is returned by PQfsize(). Precision
and scale are encoded in the return value from PQfmod() and you have to
have a magic decoder ring to understand them. (Magic decoder rings are
available, you just have to read the source code :-)

PQftype() is not easy to use because it returns an OID instead of a name
(or a standardized symbol), but I can't think of anything better to return
to the client. Of course if you really want to make use of PQftype(), you
can preload a client-side cache of type definitions. I seem to remember
seeing a patch a while back that would build the cache and decode precision
and scale too.

PQfsize() is entertaining, but not often what you really want (you really
want the width of the widest value in the column after conversion to some
string format - it seems reasonable to let the client applicatin worry
about that, although maybe that would be a useful client-side libpq
function).

I want this in any catalog view

regards
Haris Peco

Is exists patch for 7.4devel ?

A full tarball is based on 7.3devel.
There is no patch for 7.4devel.

http://snaga.org/pgsql/

--
NAGAYASU Satoshi <snaga@snaga.org>

snpe kirjutas R, 08.11.2002 kell 03:49:

PQfsize() is entertaining, but not often what you really want (you really
want the width of the widest value in the column after conversion to some
string format - it seems reasonable to let the client applicatin worry
about that, although maybe that would be a useful client-side libpq
function).

I want this in any catalog view

But this will make such a view terribly slow, as ith has to do
max(length(field)) over the whole table for any field displayed

------------
Hannu

On Friday 08 November 2002 09:29 am, Hannu Krosing wrote:

snpe kirjutas R, 08.11.2002 kell 03:49:

PQfsize() is entertaining, but not often what you really want (you
really want the width of the widest value in the column after
conversion to some string format - it seems reasonable to let the
client applicatin worry about that, although maybe that would be a
useful client-side libpq function).

I want this in any catalog view

But this will make such a view terribly slow, as ith has to do
max(length(field)) over the whole table for any field displayed

Why not with this functions ?

I have added the following TODO item on protocol changes:

* Wire Protocol Changes
o Show transaction status in psql
o Allow binding of query parameters, support for prepared queries
o Add optional textual message to NOTIFY
o Remove hard-coded limits on user/db/password names
o Remove unused elements of startup packet (unused, tty, passlength)
o Fix COPY/fastpath protocol?
o Replication support?
o Error codes
o Dynamic character set handling
o Special passing of binary values in platform-neutral format (bytea?)
o ecpg improvements?
o Add decoded type, length, precision

---------------------------------------------------------------------------

snpe wrote:

On Thursday 07 November 2002 09:50 pm, korry wrote:

b) Send a decoded version of atttypmod - specifically, decode the
precision and scale for numeric types.

I want decode type,length,precision and scale

Type is returned by PQftype(), length is returned by PQfsize(). Precision
and scale are encoded in the return value from PQfmod() and you have to
have a magic decoder ring to understand them. (Magic decoder rings are
available, you just have to read the source code :-)

PQftype() is not easy to use because it returns an OID instead of a name
(or a standardized symbol), but I can't think of anything better to return
to the client. Of course if you really want to make use of PQftype(), you
can preload a client-side cache of type definitions. I seem to remember
seeing a patch a while back that would build the cache and decode precision
and scale too.

PQfsize() is entertaining, but not often what you really want (you really
want the width of the widest value in the column after conversion to some
string format - it seems reasonable to let the client applicatin worry
about that, although maybe that would be a useful client-side libpq
function).

I want this in any catalog view

regards
Haris Peco

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