Where lies the truth?

Ben Bradlee, the former editor of the Washington Post died recently. A famous speech of his from 1997 contains some words that mean something for me. It starts like this

"Newspapers don't tell the truth under many different, and occasionally innocent, scenarios. Mostly when they don't know the truth. Or when they quote someone who does not know the truth.

And more and more, when they quote someone who is spinning the truth, shaping it to some preconceived version of a story that is supposed to be somehow better than the truth, omitting details that could be embarrassing.

And finally, when they quote someone who is flat out lying...."

and summarises with

"Where lies the truth? That's the question that pulled us into this business, as it propelled Diogenes through the streets of Athens looking for an honest man. The more aggressive our search for the truth, the more people are offended by the press. The more complicated are the issues and the more sophisticated are the ways to disguise the truth, the more aggressive our search for the truth must be, and the more offensive we are sure to become to some. So be it."

before ending

"I take great strength from that now, knowing that in my experience the truth does emerge. It takes forever sometimes, but it does emerge. And that any relaxation by the press will be extremely costly to democracy."

Who would have thought that his words apply so well to PostgreSQL and the cost of data integrity? Yes, referential integrity does have an additional performance cost to make it work right, but how else can we be sure that we are passing valid data around? Surely the purpose of a database needs to be primarily a home for the truth, verified to be so by cross checks and constraints. (to be continued)

Reducing Contention with Foreign Key Locks

PostgreSQL's Foreign Key support underpins our ability to enforce data quality in the database.

A feature known as "Referential Integrity"allows us to link relational tables together to ensure the values in each table match.

Each lookup requires a lock to be applied to the row. In 9.2 and earlier that lock prevented writes to the row that was being checked. Which meant that UPDATEs against that row would be blocked, reducing performance in some applications. Worse, it also meant that deadlocks could occur in some cases.

The solution was a relatively simple idea: just lock the key value, not the whole row. That allows UPDATEs to proceed and application contention to be reduced in many cases.

Álvaro Herrera just committed a patch to make this happen. The simple idea required many new concepts and changes to make it work. The patch was so large it touched more than 100 files and thousands of changed lines.

Development took more than 2 years, with reviews from many different hackers to bring things to a state where it can be committed. So many people and companies were involved in this its hard to mention one without missing others. It's best to call this a true community effort in support of Álvaro and his work.

So in release 9.3, available in Sept 2013 we expect to see significant improvements in the performance of many applications. And it will make it easier to avoid those problems for new applications.

PostgreSQL: The Multi-Model Database Server

I'd like to change the way we describe PostgreSQL.

Calling PostgreSQL and Object Relational database is misleading and years out of date. Yes, PostgreSQL is Relational and the project follows the SQL Standard very closely, but that's not all it does.

PostgreSQL supports all of the following:

* Relational
* Object Relational
* Nested Relational (record types)
* Array Store
* Key-Value Store (hstore)
* Document Store (XML, JSON)

and 9.2 adds

* Range Types

So what do we call it?

We support multiple models, so I guess we should call it a "Multi-Model Database".

The good thing here is that we support them all, in one platform, allowing you to join data together no matter what shape the data is held in.

Which means that PostgreSQL is a great General Purpose database and a great default choice for use by applications. Stonebraker has spoken out against the idea of a General Purpose database, but his interest is in bringing VC-funded startups to market, not in supporting production systems and catering to a range of business requirements with flexibility and speed. The reality is that if you pick a specialised database that fits your current requirements you're completely stuck when things change, like they always do.

Alan Turing's Legacy

It's Alan Turing's 100th Anniversary today.

Interesting for me, since I live near Bletchley Park where he worked during the war. I recommend a visit there, its becoming something of a tourist attraction now after years of obscurity.

Born out of the needs of that war, Turing laid the ground work for both modern computation and cryptanalysis. Many other people helped that effort and have been forgotten, something not helped by people re-writing history in films like U-571 and other sensationalist material.

Originally, computers had just one use - mathematical computation. We might think he foresaw many things, but I don't think he even dreamed of the worlds of databases or graphics. The level and complexity of "persistent computing" we now have in PostgreSQL would astound him, though he'd probably not make the distinction between the different layers of software and hardware that we do, only see the whole system working together as being amazing.

I like to think he'd be impressed with what we've done, but I'm also thinking he might observe how much more there is yet to achieve.

On a related note, I saw something that pointed out that Blade Runner predicted flying cars yet all the cops did with them was land next to a static payphone. I guess imagining mobile phones was a stretch too far. Yet we have the mobile phones, but not the flying cars.

Predicting the future is easy. Building it is harder and slower than we like; funding the future is the hardest part.  Turing would have done nothing without a budget, and sheer persistence.

Replication Test Servers

As various people have already noted, IBM has set up some servers that will allow us to test software for robustness and performance and have kindly donated those to us for project use. Thanks very much to Mark Wong for arranging that as well as various IBM folk for assistance.

The general plan is for Rob Haas to get access to one server for performance testing, while I have access to 3 smaller virtual servers for testing replication. It's a pretty rough plan as yet and full access is still being fully configured.

This is good because it will allow us access right at the start of PostgreSQL 9.3 development cycle.

I'll post again when we've finished the 9.2 CommitFest and have had the chance to set these up in a useful way.

R is for Innovation

I'm pleased to note that Teradata just announced a plugin for the R language.

As many of you will know, PostgreSQL has supported server functions written in the R language for many years. So its good that Teradata has seen the light at last and by doing so has validated the innovations that PostgreSQL has made.

That means the list of databases that have responded directly to innovations in PostgreSQL, now extends to Oracle, Informix(Illustra), SQLServer, Sybase, DB2, Teradata. Of course, MySQL have been trying to catch up for a long time,

That pretty much is the complete set. Cool. Well, almost.

I'm intrigued as to what NoSQL vendors think will happen next. If their core values are simplicity then what new features can they add without going back on their core philosophy. Austerity isn't something you can have more of, is it? Let's wait and see what happens when the VC runs out.

PostgreSQL really is in a leadership position with regards to database innovation. And I'm happier than ever to be part of this phenomenon.

Cascading Replication

Cascading Replication is now part of PostgreSQL 9.2, thanks to Fujii Masao.

The idea is that a streaming replication standby can also stream data onto other standbys. This allows a complex network of interrelated servers to fulfil the roles of High Availability, High Durability, Distributed data access capacity and Reporting requirements.

You can set up chained configurations like A -> B -> C.

or more complex arrangements like
......A
....B....E
..C.D...F.G

This should make it much easier to reduce bandwidth for intercontinental replication.

Nice thing is that Hot Standby feedback works across the whole cluster, so you easily manage the interrelationships between servers.

CHAR(11) Conference Success

Finally recovered from attending CHAR(11) in Cambridge, UK. 2 complete days of Clustering, High Availability and Replication talks from various experts.

We had 15 talks from 14 speakers from US, Japan and from 8 European countries, including the keynote from Jan Wieck. Attendees came from US and all across Europe, many of whom could give detailed talks themselves. There's always next year...

The most amazing thing were the comments we received from attendees. Every talk was packed solid, and judging by the seats alone it seems almost everybody went to all the talks - for the whole talk. I don't recall a conference having such a good attendee rate, not even CHAR(10) last year.

Based on that, it looks pretty certain that we'll run CHAR(12) next year. We did discuss Japan for CHAR(12) but that's not going to be as easy as we'd hoped. Let's see how that goes.

I'm pleased with how everything ran, so a big thanks to the organising team.

Thanks very much to Koichi Suzuki for visiting again. The panel discussion between Postgres-XC, MGRID and Greenplum was very enlightening.

Thanks to all the speakers and attendees also.

Five Nines

In High Availability we talk about "Five Nines" meaning 99.999% availability. I like to joke that a badly configured system has "Nine Fives" availability or 55.555555% availability.

With a sensible architecture and good operational controls, data can be made "Five Nines" safe with PostgreSQL 9.1.

I was reminded today that "Five Nines" had another meaning in an earlier age. Wilfrid Owen's wartime poetry describes

And towards our distant rest began to trudge.
Men marched asleep. Many had lost their boots
But limped on, blood-shod. All went lame; all blind;
Drunk with fatigue; deaf even to the hoots
Of tired, outstripped Five-Nines that dropped behind.

meaning artillery shells falling away from the target of the front line troops.

The poem ends with an exhortation to learn from earlier mistakes

My friend, you would not tell with such high zest
To children ardent for some desperate glory,
The old Lie: Dulce et decorum est
Pro patria mori.

"How sweet and fitting it is to die for one's country"

I'm sure there's a modern message there, but I'll leave that up to you.

Gentlemen, Start your Engines

The racing season is upon us. We have both the Le Mans 24 hour race and the Indy 500 coming in the next month, both long distance, high speed motor racing events.
http://www.indianapolismotorspeedway.com/indy500/
http://www.lemans.org/

We also have the beta of PostgreSQL 9.1 and associated tools.

Just like motor sport, a 5 minute engine test proves very little. Only good solid usage at high levels of performance will prove whether the engine is good enough to be world class.

Just like a race, we have deadlines and we must remember we aren't the only people in the world producing database software. The deadline is more important this year because we are attempting to cut the time of the beta cycle down by weeks and months.

The PostgreSQL project needs you to start your engines. Start testing PostgreSQL 9.1 as soon as possible and take it to the very limits of durability and performance.

Make the tests run for 500 miles and/or 24 hours. Report the results, in detail.

Do it. Do it now.

Brand New Kayak

I've just bought a new Dagger GT max kayak on eBay, just collected it today.

It's shorter than my old Mountain Bat, with a flat bottom for surfing and rivers, and tramlines to allow you edge through turns better. Lots of padding and a back rest that actually works. And its bright yellow.

Like most things, it makes me think about databases in a new way.

First the buying experience: I'm ecstatic, but I've not been in the water yet. Why am I ecstatic? Well, its yellow and has got lots of features I'm interested in. And its yellow. From that I take it that look and feel is important with a new product in addition to the real usability features. Uh, yeh, err... just like psql...

I realise that this might be the best I ever feel about the kayak. If it has shortcomings, then I'll be disappointed. Imagine a boat with very few issues, with footrests that can be adjusted to make it just right. That sounds like I boat I'd like, and a database too.

I also note that it's taken me 18 years to buy a new kayak. From that I learn that annoyances with products do build up over a period of time and that useful new features are important in changing. But kayak salesmen need to be patient and respect the views and wishes of paddlers with prior experience of other craft.

What made me change? A friend bought one. Not just that - I watched him go down some whitewater that I'd had trouble on, but he edged it like it wasn't there. From that I learn that word of mouth and references are important, but demonstrations are even better.

Now back to my first thought: why did I buy the Dagger? It's been interesting to watch kayak development over the intervening years, with all sorts of specialist kayaks emerging. Sea kayaks, river kayaks, whitewater and playboats. My feeling was that these were all too specialist. I wanted a boat I could use for short sea trips and whitewater. This made me think about Stonebraker's recent years. Why the fascination with all these specialist databases? They are good for some situations, no question. But how do you know the conditions you'll be facing? How can you trust you haven't selected something too specialised?

What I'd really like is a comfortable canoe that can be configured according to the conditions I meet. I don't really want a seacanoe or a playboat because then I'd need lots of different boats, all sitting waiting for the right situation. I know I can't have a modifiable kayak because its hull is made of PBS. But I can get that with software, if its configurable enough to meet my needs. Not hundreds of adjustments, just a few important parameters to allow me to tailor it to the major points of the current solution. Speed, stability, comfort, safety and security.

So, some important lessons for databases: How do I make PostgreSQL bright yellow?

Feedback on the PostgreSQL development process

For the last few weeks, the PostgreSQL Hackers list has been discussing how to improve the PostgreSQL development process.

You might be forgiven for asking "Why? What is wrong with it?". Indeed, you might.

The process has changed many times down the years. Essentially, the process revolves around a few key people with the knowledge and time to contribute reviews of the submitted patches. All of those people have got views about what's right and wrong with the exact current system.

What would be useful is to hear from people who
* never submitted a patch for a definite blocking reason
* submitted a patch but had it rejected
* wrote a first patch but were dissuaded from doing that again

If you'd like to review patches for PostgreSQL then we're short of manpower there. We're short of manpower because PostgreSQL believes that peer review is an essential technique to producing good code. You'll need to spend some time getting to understand the review process and guidelines and you may also need assistance on some technical aspects. Apart from that, reviews consist of asking questions like "Won't that break ALTER TABLE?" and observing "there's not enough code comments here, and no docs".

If you have feedback, or you can help, please join the hackers list and speak out.

Busy Times

It's been 6 months since I found time to blog, which I guess shows how much I had been concentrating on getting Sync Replication finished.

Sync Replication is the raison d'etre for in-database replication. Only by bringing replication to the database layer can we control the replication process in a useful way. Did it have to be transaction log shipping replication? No, I guess it might have been possible to do sync rep using other mechanisms such as triggers or writesets but the transaction log seemed the most natural way to go, at least initially.

Now its done, I breathe a sigh of relief after 7 full years of work. The strange thing is that in order to fund such a task I needed to build a company, 2ndQuadrant. It's kind of like having to build the ramp up which the blocks of stone would travel for the pyramids of ancient Egypt. Anyway, its a good thing because it's brought together many contributors and opened up funding mechanisms to do the things we want to do with PostgreSQL.

Now it's finished, I see all the other tasks still to do, so I'll be busy a while longer yet. Feature complete, no way.

I'm pleased that I got all the essential features into sync rep that I was looking for. Transaction controlled replication, minimal bandwidth usage, shared memory queues ordered by xlog pointers, avoidance of complex configuration details and most importantly an approach everyone agrees is robust.

I hadn't realised it, but the sync rep implementation is actually better than MySQL's semi-synchronous replication. Don't think anybody set out to do that, just as usual the PostgreSQL approach to building things seems to end up with a rigorous design and implementation.

I'm thinking about replication because I've just been assembling the talk proposals for CHAR(11), the conference on Clustering, High Availability and Replication. The Call for Papers for the CHAR(11) conference is now closed, though we have a very cool lineup of speakers. Even better than CHAR(10) last year.

As ever, more on all of the above another time.

Extensions in 9.1+

One of the most exciting developments in 9.1 will be the new CREATE EXTENSION feature that Dimitri Fontaine has been designing. Dimitri has been working on them for some time now, proposing them in detail at the 2010 Developer Meeting. Recently he's been coding them and the first patch arrived a few days ago. Reviews should happen in November, more than enough time to happen for the next release.

What do extensions do? Make it easier to add and remove plugins, datatypes, functions etc with minimum fuss and without needing to compile things yourself. It turns out that there's a great many aspects to this and much more complex than I've made it sound.

Dimitri has himself designed a number of very cool add-ons for PostgreSQL and its from those that he's gained insight into what's required. I've been trying to follow it myself, but I'm a bit lost on some aspects of it just yet.

Anyway, looks like there will be a few websites and repositories from which you can download extensions, so the hope is that it will be much easier to both obtain and install add-in components in future versions of PostgreSQL.

We're especially lucky that funding has been made available from the 4CaaST project, a Cloud Computing project funded by the European Union (under FP7). More on that another time.

Cleaning your archive

Whether you're thinking of using PostgreSQL 9.0 yet or not, you maybe interested in a small new utility that is designed to help you clean out old WAL archive files.

pg_archivecleanup is a standalone program which simply removes all WAL files older than a certain filename. You could use it like this:

pg_archivecleanup /my/archive 000000010000DEAD0000BEEF

The good thing here is that the tool works just fine for any release of PostgreSQL, not just 9.0. Mostly useful for bash or Windows scripts, since people will no doubt confirm it's trivial to do this in Python or perl etc..

The utility was designed as a dual purpose tool, since with 9.0 pg_standby is no longer required, yet you still need some way to clear down an archive. So you can also include it in the archive_cleanup_command, like so

archive_cleanup_command = 'pg_archivecleanup /my/archive %r'

which gets called regularly with changing values of %r to clean out your archive directories.

Not big, but its useful.

PostgreSQL Admin Book: Last Edit

I've been working on a PostgreSQL Administration book for some time now and am very happy to report that I'm done with the last edit on the last chapter. Hannu finished his a few days earlier.

Phew!

The full title of the book is "PostgreSQL 9 Administration Cookbook" and has been written by Hannu Krosing and myself. We've written it together over the course of 9 months or so. I guess it's no surprise to some, but these things always take more time than you expect.

The book is in the form of short, punchy recipes that tell you what you need to know on a topic quickly with examples, then we describe how it works later. Some of the recipes are really basic to make sure we address the frequent questions, but around half are advanced topics. It's certainly taken some time to express both the basic and the advanced topics succinctly.

An advanced database system needs advanced books to show it off in the best light. I very much hope it is up to the high standards of the PostgreSQL community and moves us all forwards.

I'll write some more about it later. All I can really express now is relief!

You can pre-order the book here:

http://www.2ndquadrant.com/postgresql-books/

Hot Bugs & Cold Beer

2ndQuadrant is sponsoring a number of Bugs & Beer events at PostgreSQL user groups around the world.

The idea is to get together with your friends and see if we can shake out any more bugs in Hot Standby prior to release.

Particular focus on these areas:
* transactions holding locks before/during/after base backup taken
* prepared transactions
* correctness of results
* VACUUMs on master
* any other ways you can find to break it

Please write to me at simon@2ndQuadrant.com if you'd like funding for a local event.

All the best,

What's a VLF?

I read a good blog called http://sqlfool.com/

The funniest thing about it for me was the artful blogger talked about having 87,000 VLFs, taking up 1.5TB, though didn't mention what a VLF was.

Anyway, I checked - VLFs are WAL files in SQLServer. In case, like me, you didn't know, VLF stands for Virtual Log File. So it's a virtual physical thing. Got it. :-\

Which means the blogged-about related to 1.5TB worth of WAL files in the equivalent of pg_xlog. Ouch!

Ah! That reminds me: on both SQLServer and DB2 the log contains undo information, which means that the transaction log can't reuse files while transactions are still active. So long running write transactions causes the log to grow in size and can eventually fill the disk. This can be a real pain for DBAs and requires workarounds. That effect doesn't occur with PostgreSQL, so pg_xlog never overflows for that reason. With PostgreSQL, undo information is effectively stored in the database itself in the form of previous row versions. So a database grows as changes occur. Long running write statements can have a similar effect in PostgreSQL, but not long running transactions. So, one more problem that us Postgres-people get to miss out on.

And just to complete the circle, I will avoid explaining what a WAL file is here.

Smoothing replication

At first, streaming replication looked pretty raw though I've been doing a thorough review of the code. I guess that's probably how Hot Standby looked at first as well. Now, I'm getting quite excited at the way things are shaping up.

Originally the WALSender process waited for 200ms after sending each chunk of data. That's now been fixed so that WALSender will stream continually until no outstanding WAL data remains.

Also, the max chunk size has been reduced to 128kB, which is now the same default size used by DRBD.

On Monday, I noticed that the WALSender was sending WAL data before it had been fsynced. So in case of a crash that could cause a problem. That only really matters if you're using synchronous_commit = off or fsync = off. (Though fsync = off and replication just have no business being configured simultaneously).

Lots of smaller changes have also come through in recent months, so now we have the ability to get log messages when replication connects or disconnects. We also now have special messages if the replication connection is refused by the primary via the pg_hba.conf.

From here, it's looking like streaming replication is fast, efficient and low latency. It's also easy to use and easy to troubleshoot. It's an impressive community effort.

I'm also happy to say that almost all of the features of pg_standby have been assimilated into core now, so the only external module you'll need is something called pg_archivecleanup which is new in this release.

Next few months we're going to see designs emerge for synchronous replication. That is going to require some subtle coding to get good. It sounds like a couple of people are planning prototypes of how that should work.

Bollywood Features

PostgreSQL 9.0 is in the can and is coming to the end of post-production. Still looking forward to the usual last minute special effects by ILM (Industrial Lane & Magic) to pull some extra zing into performance.

So what next? Well, three genres attract my attention: High performance ("OLTP"), Replication and Data Warehousing. The first two are action features, while the last one is more of a romance for me. Some of the stunts for the first two are similar, so we'll be looking to reduce production costs with some clever thinking.

No blockbusters this year, though hopefully some worthwhile features.

High performance stuff we hope for in 9.1 will be

* Group commit
* MERGE related stuff - production costs stalled the first attempt to bring this to the big screen, though some Euro sponsorship should make this viable. Hoping that Greg Smith will take the lead role on this cos its a big performance critical project, with some hard bits.
* Some surprise features!

Replication

* Synchronous replication
* Relay replication
* Fast switchover - an art house flick with a small, yet good following

Data Warehousing features for 9.1 will be

* Bit map indexes - has had its script rewritten a few times and the actors have changed as well, though it really needs to happen this time around cos its just so cool.
I also expect to keep my eye on Partitioning features to ensure it actually works for the use cases we care about, which is Big Data.

Dancing is a key element of Bollywood and the best bits are when everyone gets involved in huge Busby Berkeley numbers. Expecting the dancing to be particularly intense for Synchronous Replication though the dancers are also fairly well trained, so we're hoping for a happy ending with a cool soundtrack.

Unsure if its a promise I can keep, but no more features with the word "Hot" in the title, especially since the dance routines are so long.

Not going to Cannes this year, but I will be appearing at the Brits: www.char10.org