Older blog entries for bagder (starting at number 776)

Better pipelining in libcurl 7.30.0

Back in October 2006, we added support for HTTP pipelining to libcurl. The implementation was naive and simple: it basically preferred to pipeline everything on the single connection to a given host if it could. It works only with the multi interface and if you do a second request to the same host it will try to pipeline that.

pipelines

Over the years the feature was bugfixed and improved slightly, which proved that at least a couple of applications actually used it – but it was never any particularly big hit among libcurl’s vast amount of features.

Related background information that gives details on some of the problems with pipelining in the wild can be found in Mark Nottingham’s Making HTTP Pipelining Usable on the Open Web internet-draft, Mozilla’s bug report “HTTP pipelining by default” and Chrome’s pipelining docs.

Now, more than six years later, Linus Nielsen Feltzing (a colleague and friend at Haxx) strikes back with a much improved and almost completely revamped HTTP pipelining support (merged into master just hours before the new-feature window closed for the pending 7.30.0 release). This time, the implementation features and provides:

  • a configurable number of connections and pipelines to each unique host name
  • a round-robin approach that favors starting new connections first, and then pipeline on existing connections once the maximum number of connections to the host is reached
  • a max-depth value that when filled makes the code not add any more requests on that connection/pipeline
  • a pipe penalization system that avoids adding new requests to pipes that are known to be receiving very large contents and thus possibly would stall subsequent requests for an extended period of time
  • a server blacklist that allows the application to specify a list of servers for which HTTP pipelining should not be attempted – real world tests has proven that some servers are too broken to be allowed to play the game

The code also adds a feature that helps out applications to do massive amount of requests in a controlled manner:

A hard maximum amount of connections, that when reached makes libcurl queue up easy handles internally until they can create a new connection or re-use a previously used one. That allows an application to for example set the limit to 50 and then add 400 handles to the multi handle but it will still only use 50 connections as a maximum so over time when requests get completed it will start new transfers on the requests that are waiting in line and thus shrinking the queue and keeping the maximum amount of connections until there’s less than 50 left to do…

Previously that kind of queuing had to be done by the application itself, but now with the much more extensive pipelining support it really isn’t as easy for an application to know when the new request can get pipelined or create a new connection so this logic is now provided by libcurl itself. It is likely going to be appreciated and used also by non-pipelining applications…

This implementation is accompanied with a bunch of new test cases for libcurl (and even a new HTTP test server for the purpose), and it has been tested in the wild for a while with libcurl as the engine in a web browser implementation (the company doing that has requested to remain anonymous). We believe it is in fairly decent state, but as this is a large step and the first release it is shipped with I expect there to be some hiccups along the way.

Two things to take note of:

  1. pipelining is only available for users of libcurl’s multi interface, and only if explicitly enabled with CURLMOPT_PIPELINING
  2. the curl command line tool does not use the multi interface and thus it will not use pipelining

Syndicated 2013-03-26 21:26:47 from daniel.haxx.se

Why no curl 8

no 8In this little piece I’ll explain why there won’t be any version 8 of curl and libcurl in a long time. I won’t rule out that it might happen at some point in the future. Just that it won’t happen anytime soon and explain the reasons why.

Seven point twenty nine, really?

We’ve done 29 minor releases and many more patch releases since version seven was born, on August 7 2000. We did in fact bump the ABI number a couple of times so we had the chance of bumping the version number as well, but we didn’t take the chance back then and these days we have much harder commitment and determinism to not break the ABI.

There’s really no particular downside with having a minor version 29. Given our current speed and minor versioning rules, we’ll bump it 4-6 times/year and we won’t have any practical problems until we reach 256. (This particular detail is because we provide the version number info with the API using 8 bits per major, minor and patch field and 8 bits can as you know only hold values up to 255.) Assuming we bump minor number 6 times per year, we’ll reach the problematic limit in about 37 years in the fine year 2050. Possibly we’ll find a reason to bump to version 8 before that.

Prepare yourself for seven point an-increasingly-higher-number for a number of years coming up!

Is bumping the ABI number that bad?

Yes!

We have a compatibility within the ABI number so that a later version always work with a program built to use the older version. We have several hundred million users. That means an awful lot of programs are built to use this particular ABI number. Changing the number has a ripple effect so that at some point in time a new version has to replace all the old ones and applications need to be rebuilt – and at worst also possibly have to be rewritten in parts to handle the ABI/API changes. The amount of work done “out there” on hundreds or thousands of applications for a single little libcurl tweak can be enormous. The last time we bumped the ABI, we got a serious amount of harsh words and critical feedback and since then we’ve gotten many more users!

Don’t sensible systems handle multiple library versions?

Yes in theory they do, but in practice they don’t.

If you build applications they have the ABI number stored for which lib to use, so if you just keep the different versions of the libraries installed in the file system you’ll be fine. Then the older applications will keep using the old version and the ones you rebuild will be made to use the new version. Everything is fine and dandy and over time all rebuilt applications will use the latest ABI and you can delete the older version from the system.

In reality, libraries are provided by distributions or OS vendors and they ship applications that link to a specific version of the underlying libraries. These distributions only want one version of the lib, so when an ABI bump is made all the applications that use the lib will be rebuilt and have to be updated.

Most importantly, there’s no pressing need!

If we would find ourselves cornered without ability to continue development without a bump then of course we would take the pain it involves. But as things are right now, we have a few things we don’t really like with the current API and ABI but in general it works fine and there’s no major downsides or great pains involved. We simply do not have any particularly good reason to bump version number or ABI version. Things work pretty good with the current way.

The future is of course unknown and at some point we’ll face a true limitation in the API that we need to bridge over with a bump, but it can also take a long while until we hit that snag.

Syndicated 2013-03-23 12:52:37 from daniel.haxx.se

some missing github features

github-social-codingI think github is a lovely resource for collaborating on source code with my friends all over the globe. Among other things, we host the primary curl repository there and we’ve been doing so for almost three years now. This experience has led me to discover a bunch of things I miss in the service…

github is clearly aimed at repositories run by one person or a small set of persons, while in the projects I run I try to involve as many as possible in wide collaboration and I put efforts into informing everyone to get the widest possible attention and feedback. I may have created the account and “own” the repository, but I want the work to be done by a large team and I want everything that happens to it to be seen by a large audience. This is not always possible to do easily with the existing github services.

To further this spirit and to widen cooperation more, I would like to see the following improvements:

  • pull requests can’t be disabled nor can i control to which email address to send the notification. In our project I want all patches posted to the mailing list for review, archiving and discussions before I get a pull request, and I don’t use github’s merge feature since it is hardly ever good enough (I want fast-forward and I usually feel a need to edit the commit message ever so slightly etc). I want the pull request to get translated into a patch review submission to the mailing list.
  • similarly, I cannot redirect where notifications are sent when someone comments a commit or a source line and this is highly annoying since we merge a lot of outsiders’ patches etc and as they may still read the mailing list we want the discussion there! Many times the contributors don’t have github accounts and of course we don’t want to require that.
  • after the death of the CIA.vc service, the current IRC notification service offered by github is nothing but inferior. The stupid bot has to join, tell its message and leave again. It is not an IRC friendly behavior and I can’t make it announce exactly what I’d like it to say…
  • I wish it had much better email notification on commits that would allow me to customize what it sends out without forcing me to write a full blown replacement. I want a unified diff included!

I realize github has features that offer me to create an “organization” to host a repository instead of it being owned by me as a person, but I don’t think that should be a requirement to get this functionality. And I don’t know if github truly offers better group functionality then either.

Syndicated 2013-03-09 18:58:52 from daniel.haxx.se

curl and new technologies

At March 5 2013 we had another foss-sthlm meetup. The 12th one in fact, and out of the five talks during the event I spoke about curl and new technologies. Here are the slides from my talk: